1 CHAPTER 2 DRIVE TRAIN & POWER PLANT 2.1 POWERPLANT ALTERNATIVES IN MOTOR VEHICLES 2.1.1 Internal Combustion Engines 2.1.1.1 What is an internal combustion engine? Combustion is burning fuel with an oxidizer, to supply heat. If this heat energy is delivered to a working fluid i.e. if the working fluid is heated by combustion that takes place in an external source, this is called external combustion. The working fluid then, by expanding and acting on the mechanism of the engine, produces motion and usable mechanical work. Two examples of external combustion engines are shown in Figure 1. First one is a uni-flow steam engine: The poppet valves are controlled by the rotating camshaft at the top. High-pressure steam enters, red, and exhausts, yellow. Second example is an alpha type Stirling engine. There are two cylinders. The expansion cylinder (red) is maintained at a high temperature while the compression cylinder (blue) is cooled. The passage between the two cylinders contains the regenerator. Figure 1. (a) Uniflow Steam Engine (b) Stirling Engine Contrary to external combustion, if the heat energy is delivered to a working fluid i.e. consisting of, mixed with or contaminated with combustion products, this is called internal combustion. Therefore, internal combustion engine (ICE) is a heat engine where the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. Internal Combustion Engines are heat machines, converting fuel chemical energy into mechanical work as a result of a combustion process (open thermodynamic cycle). The term internal combustion engine usually refers to an engine in which combustion is intermittent, such as four-stroke and two-stroke piston engines. A second class of internal combustion engines use continuous combustion: gas turbines, jet engines and most rocket engines, each of which are internal combustion engines as well. In this class, the term ICE will refer to the four stroke ICE.
Transcript
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CHAPTER 2 DRIVE TRAIN & POWER PLANT
2.1 POWERPLANT ALTERNATIVES IN MOTOR VEHICLES
2.1.1 Internal Combustion Engines
2.1.1.1 What is an internal combustion engine?
Combustion is burning fuel with an oxidizer, to supply heat. If this heat energy is delivered to a working fluid i.e. if the working fluid is heated by combustion that takes place in an external source, this is called external combustion. The working fluid then, by expanding and acting on the mechanism of the engine, produces motion and usable mechanical work.
Two examples of external combustion engines are shown in Figure 1. First one is a uni-flow steam engine: The poppet valves are controlled by the rotating camshaft at the top. High-pressure steam enters, red, and exhausts, yellow.
Second example is an alpha type Stirling engine. There are two cylinders. The expansion cylinder (red) is maintained at a high temperature while the compression cylinder (blue) is cooled. The passage between the two cylinders contains the regenerator.
Contrary to external combustion, if the heat energy is delivered to a working fluid i.e. consisting of, mixed with or contaminated with combustion products, this is called internal combustion. Therefore, internal combustion engine (ICE) is a heat engine where the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit.
Internal Combustion Engines are heat machines, converting fuel chemical energy into mechanical work as a result of a combustion process (open thermodynamic cycle).
The term internal combustion engine usually refers to an engine in which combustion is intermittent, such as four-stroke and two-stroke piston engines. A second class of internal combustion engines use continuous combustion: gas turbines, jet engines and most rocket engines, each of which are internal combustion engines as well.
In this class, the term ICE will refer to the four stroke ICE.
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2.1.1.2 Basic Terminology for ICE
Despite the fact that this is not an ICE class, we are going to cover basic terminology and give a short overview of the operating principles of ICEs.
4 Stroke (2 up and down motion of the piston) Engines: In a 4 stroke reciprocating engine, it takes four strokes per power generation, i.e., there is one power generating stroke for 2 revolutions of the crankshaft (engine). The four strokes are: intake, compression, power/expansion, exhaust. The ignition occurs near the end of compression, either spontaneously (compression ignition CI) or by command (spark ignition SI).
2 Stroke (1 up and down) Engines: In a 2 stroke (reciprocating) ICE, it takes two strokes of the piston per power generation, i.e., there is one power generating stroke per engine revolution.
Rotary Engines (Wankel and others): In a rotary engine, there is no reciprocating motion of the piston, but a rotary motion of the piston. There is typically one power stroke per engine revolution.
Air – fuel mixture preparation: The primary difference between carburetors and fuel injection is that fuel injection atomizes the fuel through a small nozzle at the inlet port under high pressure, while a carburetor relies on suction created by intake air accelerated through a venturi tube to draw the fuel into the airstream.
Carburetors were the usual method of fuel delivery for most SI engines up until the late 1980s, when fuel injection became the preferred method. Why? This change was dictated by the requirements of catalytic converters; a catalytic converter requires much more precise control over the (fuel / air) mixture, to closely control the amount of oxygen in the exhaust gases.
Types of fuel injection:
1) Direct injection
2) Indirect injection
In a direct injection engine, fuel is directly injected into the combustion chamber. Ex: Common rail direct injection (CRDI). Most diesel engines use this method and since recently SI engines as well.
In direct injection engines, the combustion chamber consists of a dished piston.
In an indirect injection system
1) In a gasoline engine fuel injector delivers the fuel at some point before the intake valve. Ex: Multipoint fuel injection (MPFI).
2) In diesel engines there is a separate pre-combustion chamber (alternatives: swirl chamber, air cell chamber). Combustion begins in the pre-combustion chamber and then spreads into the main combustion chamber, assisted by turbulence created in the chamber. Turbulence of the mixture leads to a smoother combustion which yields a less vibrant engine operation. Injector pressures can be lower, about 100 bar, using a single orifice tapered jet injector.
In indirect injection engines, the combustion chamber is usually in the cylinder head.
Advantages of indirect injection (for diesel):
The injection pressure required is low, so the injector is cheaper to produce.
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Higher engine speeds can be reached, since burning continues in the pre-chamber.
Leads to lower nitrogen-oxide emissions
Disadvantages of indirect injection (for diesel)
Efficiency is 5–10% lower than the one of direct injection because of heat loss due to large exposed areas and pressure loss.
Glow-plugs (ısıtma bujisi) are needed for a cold engine start. It is a heating device used to aid starting diesel engines.
Because the heat and pressure of combustion is applied to one specific point on the piston as it exits the pre-combustion chamber or swirl chamber, such engines are less suited to high specific power outputs.
Most present-day diesel engines use a mechanical single plunger high-pressure fuel pump driven by the engine crankshaft. For each engine cylinder, the corresponding plunger in the fuel pump measures out the correct amount of fuel and determines the timing of each injection. These engines use injectors that are very precise spring-loaded valves that open and close at a specific fuel pressure.
In diesel engines, the injection of the fuel into the air does not happen until the end of the compression. This leaves a very short time for the mixture formation. Therefore, particle formation (i.e. unburned fuel) in the exhaust gas cannot be avoided. Furthermore, the ignition delay of about one millisecond between the injection of the fuel and the compression ignition process, limits engine speed build-up. Maximum speed of diesel engines (excluding the ones used for heavy commercial vehicles) are around 5000 rpm and 4500 rpm with indirect and direct injection methods, respectively.
Multipoint fuel injection (MPFI)
Multipoint fuel injection (also called as port fuel injection) injects fuel into the intake ports just upstream of each cylinder's intake valve, rather than at a central point within an intake manifold. MPFI systems can be-sequential, in which injection is timed to coincide with each cylinder's intake stroke; batched, in which fuel is injected to the cylinders in groups, without precise synchronization to any particular cylinder's intake stroke; or simultaneous, in which fuel is injected at the same time to all the cylinders. The intake is only slightly wet, and typical fuel pressure runs between 40-60 psi.
Many modern fuel injection systems utilize sequential MPI; however, in newer gasoline engines, direct injection systems are beginning to replace the sequential ones.
Gasoline direct injection
The gasoline is highly pressurized, and injected via a common rail fuel line directly into the combustion chamber of each cylinder, as opposed to conventional multi-point fuel injection that injects fuel into the intake tract, or cylinder port. Directly injecting fuel into the combustion chamber requires high pressure injection whereas low pressure is used injecting into the intake tract or cylinder port.
In some applications, gasoline direct injection enables a stratified fuel charge (ultra lean burn) combustion for improved fuel efficiency, and reduced emission levels at low load.
Direct injection may also be accompanied by other engine technologies such as turbocharging or supercharging, variable valve timing (VVT) or continuous variable
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cam phasing1, and tuned/multi path or variable length intake manifolding (VLIM, or VIM). Water injection or (more commonly exhaust gas recirculation EGR) may help reduce the high nitrogen oxides (NOx) emissions that can result from burning ultra-lean mixtures; modern turbocharged engines use continuous cam phasing in place of EGR.
Drawback of direct injection: Although Direct Injection provides more power and higher efficiency, a carbon build-up occurs in the intake valves that over time reduces the airflow to the cylinders, and therefore reduces power. Fuel contains various detergents and can keep the intakes clean. When fuel is no longer being sprayed in the intake valves as is the case in direct injection, small amounts of dirt from intake air cakes on the intake walls, even with air filters that prevent most of the dirt from entering the cylinder. This build-up can become severe enough that a piece can break off and has been known to burn holes in catalytic converters. It can also cause sporadic ignition failures. These problems have been known for some time and technologies have been improved to reduce the carbon build-up.
2.1.1.3 Classification of ICE
We can classify engines with respect to different criteria:
2.1.1.4 Basic ICE Operation
Basic SI ICE Operation (Figure 3)
1) Intake stroke: Suction occurs through the suction (inlet) valve. Air/fuel mixture enters the
cylinder through intake passage as piston moves down.
2) Compression stroke: Inlet valve closes and Air/fuel mixture is compressed in the
cylinder between the head and piston crown. Ignition (spark) occurs near end of
compression, slightly before top dead center (TDC), i.e. spark advance.
3) Power stroke: Air/fuel mixture burns in the combustion chamber, contributing to a
large rise in pressure and temperature. Ignition and combustion expands whereas the
1 In cam phasing the angle of a camshaft is rotated forwards or backwards (relative to the crankshaft). Thus the
- The chamber within cylinder is used for both combustion and gas-exchange. The control of the gas exchange takes place through inlet (1-3 per cylinder) and exhaust (1-2 per cylinder) valves.
Figure 4 shows the pressure and heat energy variation within the combustion chamber
of a 2 liters SI ICE during a working cycle, with respect to crankshaft angle.
Figure 4 the pressure and heat energy variation within the combustion chamber of a 2 liters SI ICE during a working cycle, with respect to crankshaft angle.4 BDC: Bottom dead center. CRA: Crankshaft angle
Basic CI ICE Operation5
There are a few differences between SI and CI engines6:
- There is no electrical spark plug in a diesel engine.
- The diesel fuel has a low octane rating, i.e. it is very flammable.
- The heat of compression raises the temperature of the air in the cylinder sufficiently to ignite the diesel when it is injected into the cylinder, right after the compression stroke.
Figure 5 shows a simplified sketch illustrating the four different strokes of a CI ICE. Figure 6 shows the pressure and heat energy variation within the combustion chamber of a CI ICE during a working cycle, with respect to crankshaft angle. Due to higher compression ratios*, higher peak pressures are reached in diesel engines in comparison to SI engines.
*Compression ratio is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.
4 Figure source: Reference 2. 5 A video illustrating basic operating principles of diesel engines can be found in
https://www.youtube.com/watch?v=bZUoLo5t7kg&index=2&list=PLuUdFsbOK_8rJsh_osoqVKfIRUkb8-rOg 6 A short comparison of SI and CI engines can be found in this link
Figure 5 Simplified sketch illustrating the four different strokes of a CI ICE. Note that the only difference from Figure 3 is the fuel injector and the absence of the spark plug.7
Figure 6 The pressure and heat energy variation within the combustion chamber of a 11,5 liters 6 Cylinder, CI ICE rated max 147 kW @ 2200 rpm & 716 Nm @ 1400 rpm, during a working cycle, with respect to crankshaft angle.8
The compression ratio is calculated by the following formula:
Vc = clearance volume. It is the volume of the combustion chamber (including head gasket). This is the minimum volume of the space at the end of the compression stroke, i.e. when the piston reaches TDC. Because of the complex shape of this space, it is usually measured directly rather than calculated.
For example, a cylinder and its combustion chamber with the piston at the bottom of its stroke may contain 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.
A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency.
Higher compression ratios will however make gasoline engines subject to engine knocking if lower octane-rated fuel (highly ignitable) is used.
Table 1 compares the SI and CI engines with respect to some basic criteria including the compression ratio. Table 2 outlines the maximum speed and efficiency ranges of CI and SI engines.
Air fuel ratio control Amount of air flow Amount of injected fuel
Limitations Knock Loss of efficiency at full load
Table 2 Maximum speed and efficiency ranges of engines for different applications.
There are many on-going sensing and control actions during operation of a today’s engine. These actions are not the subject of this course. Figure 7 illustrates a sample engine cut-out along with some of its main components labeled.
9 DI and II stand for direct and indirect injection, respectively. 10 Throttle valve is also called butterfly valve (marş kelebeği). An exception to this generalization is newer diesel
engines meeting stricter emission standards where a throttle is used to generate intake manifold vacuum, thereby
allowing the introduction of exhaust gas (EGR: exhaust gas recirculation) to lower combustion temperatures. This
minimizes NOx production.
Some vehicles today do not use a traditional throttle, instead relying on their variable intake valve timing system
to regulate the airflow into the cylinders. The end result is the same, but with less pumping losses.
Engine type Max speed [rpm] Efficiency (up to) [%]
Large CI 4200 45
Automotive CI 5000 42
Automotive SI 7500 35
Motorcycle 13500 32
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Figure 7 Sample engine cut-out11
2.1.2 Electric Machines
2.1.2.1 What is an electric machine?
An electric machine is a device that is used to convert mechanical forces into electrical signals, and vice versa, by means of the coupling provided by the energy stored in the magnetic field, as shown in Figure 8. There are two aspects of this coupling, both of which play a role in the operation of electric machines: 1) Magnetic attraction and repulsion forces generate mechanical torque. 2) Magnetic field can induce voltage in the machine windings through Faraday’s law.
Figure 8 Simple sketch showing the electric machine functionality
Electric machine acts as a generator if it converts mechanical energy from a prime mover, like an internal combustion engine, to electrical form. Electric machine is classified as a motor if it converts electrical energy to mechanical form. Both of these modes are shown in Figure 9.
11 Picture source Reference 3.
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Figure 9. Motor and generator action of an electric machine. T and m are the motor torque and speed, respectively. Tload is the load torque.
2.1.2.2 Fundamental laws of operation of an electric machine
1) The force on a current carrying wire that is placed in a magnetic field. (Lorentz force)
F = il x B (2)
where
F = force on the wire [N]
l = length vector along the wire [m]
i = current magnitude [Amp]
B= magnetic field [Tesla]
2) Magnetic field caused by a current carrying wire (Ampere’s Law)
. . or where B= enclosed enclosedB ds i H ds i H (3)
. B ds integral of the magnetic field around a closed path
enclosedi = net current enclosed by the path
H = magnetic field intensity [Amp/m]
= permeability constant [T.m/Amp]
3) Using Ampere’s law, the magnetic field inside a tightly wound circular coil of wire, i.e. solenoid, as illustrated in Figure 10:
B =in (4)
where
n=turns per unit length of the solenoid
4) Faraday’s law of induction. The magnitude of the electromotive force induced in a coil of N turns is equal to the rate at which the magnetic flux through the loop changes with time.
where .B
B
dN B dA
dt
12 (5)
12 Why is there a minus sign on the right side of Equation 5? Because the induced emf tends to oppose the flux
change.
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Figure 10 The stretched-out solenoid on the left. The magnetic field lines on the right.13
where
electromotive force [V]
B magnetic flux through the loop [Weber]
N coil of N turns
A area of the loop
5) The magnetization curve of magnetic material, and the concept of permeability
constant (Figure 11): Three important things happen when we wind a current carrying
coil around a ferromagnetic material:
The magnetic field generated inside the coil (given by Equation 4) is now made to
generate a much greater flux density.
The resulting electromagnetic structure forces the magnetic flux to be concentrated
within the coil.
If the shape of the magnetic material is appropriate, completely confines the flux
within the magnetic material.
Due to the non-linear characteristic of the curve, the exact value of can not be determined, however if the field intensity is in the linear region, then the slope of the
curve can be used, otherwise the value of can be interpreted as the average permeability for intermediate values of magnetic field.
Fundamental working principle of motor and generator:
In motor action, the rotational voltage generated in the armature opposes the applied voltage; this voltage is the back-emf. How? When current i flows through conductors placed in a magnetic field, a force is produced on each conductor; if these conductors are attached to a cylindrical structure, a torque is generated and if the structure is free to rotate, it will rotate at an angular velocity. As the conductors rotate, however, they move through a magnetic field generating an electromotive force in opposition to the excitation.
In generator action, the electromagnetic torque is a reaction torque that opposes rotation of the machine (like the regenerative braking torque in an HEV). How? If the rotating element of the machine is driven by a prime mover (the engine for instance or
13 Picture source: Reference 4
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Figure 11 How the magnetic field changes with respect to the field intensity obtained by letting current flow in the wire wound around different ferromagnetic materials.
the wheels of the vehicle directly), then an emf is generated across the coil that is rotating in the magnetic field (magnetic field is obtained by means of a magnetizing current provided to the coil or by permanent magnets). If a load is connected to the armature, (such as the battery) current will flow to the load. Current flow in the magnetic field causes a reaction torque on the armature that opposes the torque imposed by the prime mover.
Electric motor operation principles are explained in the video links at the foot note.14
The four example problems shown in the class to explain the basic operation principles of electric machines are
1) The first question of Homework 1. The solution will be posted on 7.3.2016.
2) The electromagnet example. It was taken from Reference 5, Example problem 16.9 of Chapter 16 titled “Principles of Electromechanics”.
3) Magnetic structure of electric motor example. It was taken from Reference 5, Example problem 16.4 of the same chapter.
4) It was taken from Reference 4, Problem 17 of Chapter 31 on “Induction and Inductance”.
2.1.2.3 Basic terminology for electric machines
Rotor and stator both are of cylindrical shape. The rotor rotates inside the stator. They consist of a magnetic core, some electrical insulation, and the windings necessary to establish a magnetic flux (unless this is created by a permanent magnet).
The rotor is mounted on a bearing-supported shaft, which can be connected to mechanical loads (if the machine is a motor) or to a prime mover (if the machine is a generator), by means of belts, pulleys, chains, or other mechanical couplings.
If the current serves the sole purpose of providing a magnetic field, and is independent of the load, it is called magnetizing, or excitation current, and the winding is termed field winding.
If the winding carries only the load current, it is called an armature. In synchronous machines, separate windings exist to carry field and armature currents.
In the induction machine, the magnetizing and the load current flow in the same winding, called the input winding, or primary, the output winding is called the secondary.
Picture and a simple sketch of an electric motor is shown in Figure 12. Either (but not
both) of stator field BS and rotor field BR could be generated by a current in a coil, or by a
permanent magnet. The motion and associated electromagnetic torque (its direction is shown in Figure 12 on the right) of an electric machine are the result of two magnetic fields that are trying to align with each other so that the south pole of one field attracts the north pole of the other.
Figure 12. DC motor along with its main components labeled on the left. Simple sketch of the motor on the right, along with an analogy to explain the direction of the torque.
2.1.2.4 Classification of Electric Machines
A variety of configurations exist depending on whether each of the fields is generated by a current in a coil or by a permanent magnet and whether the load and magnetizing currents are direct or alternating.
1. DC machines: DC in both stator and rotor.
2. Synchronous machines: AC in one winding, DC in the other.
3. Induction machines: AC in both.
Table 3 Classification of electric machines15 excluding special purpose machines.
15 Table source: Chapter 17, “Introduction to Electric Machines” of Reference 5.
Machine type Winding Winding type Location Current
DC Input & Output Armature Rotor
AC (winding) DC (at brushes)
Magnetizing Field Stator DC
Synchronous Input & Output Armature Stator AC
Magnetizing Field Rotor DC
Induction Input Primary Stator AC
Output Secondary Rotor AC
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If the current flowing into the stator is alternating, the direction of the flux will also
alternate, so the two poles will reverse polarity every time the current reverses
direction, that is every half cycle of the sinusoidal current. Further, since the magnetic
flux is approximately proportional to the current in the coil, as the amplitude of the
current oscillates in a sinusoidal fashion, so will the flux density in the structure. Thus
the magnetic field developed in the stator changes both spatially and in time.
This property is typical for AC machines, where a rotating magnetic field is established by energizing the coil with an alternating current.
In DC machines commutator makes it possible for the rotor and stator magnetic fields to always align at right angles to each other.
2.1.2.5 Electric Machines in Automobiles
The Starter (Marş Motoru)
Engines are feedback systems which once started, rely on the inertia from each cycle
to initiate the next cycle. In a four-stroke engine, the third stroke releases energy from
the fuel, powering the fourth (exhaust) stroke and also the first two (intake,
compression) strokes of the next cycle, as well as powering the engine's external load.
To start the first cycle of engine's run session, the first two strokes must be powered in
some other way in order to accelerate the engine to the idle speed The starter motor
is used for this purpose and is not required once the system starts running.
Before starter technology, in order to provide the required start-up speed drivers used
to push a handle to turn a flywheel for cranking the engine. However, this was not only
inconvenient, but given the size of the engines, exhausting and dangerous (the engine
can kick back, causing sudden reverse rotation).
Today’s starter (Figure 13) is used in cars equipped with engines up to 6 liters and 3.5
liters of displacement for passenger cars and commercial vehicles, respectively.
Figure 13. Illustration of the starter.16
16 Picture source: Reference 6. A video on disassembling the starter:
The first generators installed in motor vehicles were DC generator, i.e. dynamos. The wide availability of solid-state diodes since 1950s has made the AC generator (alternator) having a significantly higher electromagnetic power density, the successor of the dynamo.
Question: What has the alternator to do with a diode? In electronics, the most common function of a diode is to allow an electric current to pass in one direction (called the diode's forward direction), while blocking current in the opposite direction (the reverse direction); it has low (ideally zero) resistance to the flow of current in one direction, and high (ideally infinite) resistance in the other. Thus, the diode can be viewed as an electronic version of a check valve. This unidirectional behavior is called rectification, and is used to convert alternating current to direct current.
Since all electrical accessories in the vehicle (including the 12 V lead-acid battery) work with DC voltage, a rectifier is integrated into the alternator to convert AC current into DC current.
A Picture of the alternator and an equivalent magnetic circuit of it is shown in Figure 14.
Figure 14. A picture of the alternator and its simplified model. The field winding is on the rotor, and the rotor field is obtained by means of a direct current provided to the rotor winding. The rotor is then driven by the output shaft of the engine through a V-belt.
Three separate loops are used in the stator, which are twisted by an angle of 120° against each other. The voltage is therefore generated as three-phase.
The frequency of the voltage generated by the alternator is
2 60
p nf Hz (6)
where n is the mechanical speed in [rpm] and p is the number of poles.
The structure of the windings is the same whether the AC machine is a generator or a motor; the direction of power flow is different.
https://www.youtube.com/watch?v=znrN2mgtxoM, Video on working principle of the starter:
https://www.youtube.com/watch?v=8WD5Q_PF3pM 17 Video explaining alternators: https://www.youtube.com/watch?v=tiKH48EMgKE
1. Case 4. Field controller 6. Rectifier 2. Stator 5. Slip ring 7. Cooling fan 3. Rotor
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The alternator should be able to supply sufficient energy to the vehicle, even under most demanding conditions, such as short-distance operation in urban traffic at night with long stopping durations for traffic lights with all accessories on, in winter time, in order to prevent battery discharging.
The AC generator utilizes at least one third of its rated power during idling (Figure 15, left). The current increases as the speed increases, however the alternator efficiency decreases (Figure 15, right).
Figure 15. Alternator current and efficiency with respect to alternator speed18
Also it is seen from Figure 15, right that at partial throttle the efficiency is lower than in full throttle conditions.
Alternators are usually belt driven 2-3 times the crank-shaft speed, in commercial vehicles typically ratios are up to 5:1.
The Alternator-Starter19
The time period a starter cranks the engine is usually less than a few seconds. During the trip, the starter is shut and kept off until next time the engine is cranked again. At the same time, the starter stays in the engine compartment and takes space and weight.
One way to save space and weight is the combination of the starter and alternator, since they both work with similar principles.
The advantages of the alternator-starter are as follows:
- A starter drives the engine up until 600-2000 rpm, whereas the starter-alternator speeds-up the engine up until the idle speed is precisely reached. Only then the fuel is injected which prevents start emissions.
- Higher efficiency (60-90% in generator mode)
- No commutator.
- Can be used to assist the engine avoid running at inefficient operating points.
- Provides regenerative braking.
- For small size passenger cars it can be utilized to “boost” the engine power during ride.
18 Figure source: Reference 14. 19 Video illustrating the stop-start feature using the alternator starter:
https://www.youtube.com/watch?v=VJSEQaxAh4c
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Two examples of alternator-starter are shown in Figure 16.
There are three alternatives for the main traction machine in current EVs and HEVs:
1. Induction machines21
2. Permanent-magnet synchronous machines (named as brushless DC motor as well)22
3. Switched reluctance machines
The name “Brushless DC motor” is not due to the construction of the machine, but to
the fact that its operating characteristics resemble those of a shunt DC motor (can be
found in Chapter 17 of Reference 5) with constant field current. This characteristic can
be obtained by providing the motor with a power supply whose electrical frequency is
always identical to the mechanical frequency of rotation of the rotor.
To generate a source of variable frequency, DC-AC converters (inverter) made of a
bank of transistors that are switched on and off at a frequency corresponding to the
rotor speed are used23; thus the inverter converts a DC source to an AC source of
variable frequency (The inverters are highlighted in Figure 17). As far as the user is
concerned, then, the source of excitation of a brushless DC motor is DC, although the
current that actually flows through the motor windings is AC. Commutation is performed
electronically by switching the current to the motor, rather than by brushes, as in DC
motors.
20 Picture source: Reference 6 21 https://www.youtube.com/watch?v=LtJoJBUSe28 22 https://www.youtube.com/watch?v=bCEiOnuODac 23 This switching action is among the subjects of another series of classes called Power Electronics, EE 463, EE464.
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Figure 17. The architecture of a charge-sustaining HEV.24 A 10 kW belted starter alternator (bsa) is
connected to the engine at the front axle, whereas a 100 kW induction machine drives the rear axle.
The inverters between the electric machines and the high voltage (300 V nominal) battery are
highlighted.
2.2 DRIVETRAIN COMPONENTS
2.2.1 Clutching Components
The clutch25 is placed after the power plant for a vehicle with manual transmission. Its main function is to disconnect the power plant from the rest of the drive train when required. A diaphragm spring clutch is shown in Figure 18a. Dry plate friction clutches may be manually or automatically actuated.
There are multi-plate clutches26 as well. This type of clutch has friction couples; it has a higher torque transmission capacity. They are used in motorcycles and vehicles equipped with automatic transmissions. They can be operated both dry and wet (named as “oil bath clutches as well”). How does a “wet” multi-plate clutch operate? A wet multi-plate clutch is immersed in oil; therefore, a solid seal for the transmission is not needed anymore for the coupling, as is the case for a dry clutch. A wet multi-plate clutch has many advantages. Firstly, the amount of wear is low compared to the dry clutch. Also the engagement happens smoother. The oil takes away the heat due to the friction. However, there are “churning”27 losses in a wet clutch. Furthermore, when the vehicle with this type of transmission is not driven for a long time, agglutination of the clutches is possible. A multi-plate (with two plates) clutch is shown in Figure 18b.
24 Picture source: Reference 8. 25 Introductory video on clutches: youtube.com/watch?v=TqEfaeAMFxw 26 Introductory video on multi-plate clutch: youtube.com/watch?v=TcYsV063lk8 27 Due to the rotation of shafts, bearings and gears are dragged in the lubricant oil. Energy losses when gears are
dragged in the lubricant are called churning losses and are affected by the oil level, viscosity of the lubricant and
rotational speed. Gear churning losses can be a major contributor to the total energy loss in a gearbox. Running
gearboxes with a lower oil level and lower viscosities can reduce these losses, but causes worse lubrication and
thus higher temperatures.
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Figure 18. (a) A diaphragm spring clutch along (b) A multi-plate clutch with two plates. with its main components labeled.
The engagement and disengagement of this two-plate clutch is illustrated in Figure 19.
Clutch engagement dynamics and further information on clutches will be visited in
Section 5 of this course.
Figure 19. The clutch pedal released; The clutch pedal pushed;28 i.e. engagement of the clutch i.e. disengagement of the clutch
There are electric powered metal clutches as well.
The torque converter is placed after the power plant for a vehicle with automatic transmission. Its main function is to amplify torque during the vehicle drive away phase.
There are different types of torque converters which will be visited in Section 5 as well, the one shown in Figure 20 is the fluid-filled three-element torque converter, which is a single-stage, two-phase device.29
The three elements are called 1) the impeller or pump, which is attached to the engine 2) the turbine, which is attached to the transmission input shaft 3) the reactor or stator, which is attached to the housing with a one-way clutch. The two phases refer to the
28 Picture source: Reference 2. 29 Video explaining how the fluid coupling woks: youtube.com/watch?v=z5G2zQ_3xTc
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torque converter or torque amplification phase, and the fluid coupling phase, where no torque amplification occurs.
Figure 20. A fluid-coupling type of torque converter. Impeller in green, reactor in blue, and the turbine in red.30
Whether it is a manual transmission or an automatic one, the main function of a clutching element is to match the vehicle speed to the minimum (idle) engine speed.
In this case the net effect of the clutching element is to provide a controlled slip while transmitting torque. A certain amount of power is absorbed by the device, which is proportional to the amount of slip and the torque transmitted. This can also be thought of as a loss of efficiency between input and output.
Torque transmission with slippage warms the clutch disk and results in wearing of the clutch lining. For example in Germany31 a vehicle needs to be certified with five starts from standstill within five minutes, on 12% grade, at a rated maximum gross vehicle weight; including trailer.
While the torque engagement event is relatively short (of the order of a second or less) a significant power loss is experienced during this transient.
30 Picture source: Reference 9. 31 Info taken from Reference 14.
21
2.2.2 Gearbox
The gearbox follows the clutch/torque converter and allows the selection of suitable torque multiplication demanded by the driving conditions. The issues the gearbox solves are:
1. The engine torque & power characteristic curves don’t match the road load demand (which you will see in the next section).
2. Driving backwards needs to be handled.
3. Speed reduction. Consider a vehicle with no transmission, if speed ratio between the tire and the engine were 1-1, 800 rpm idle to 6000 rpm max engine speed would correspond to 90 kph min to 679 kph max vehicle speed.
4. Engine torque alone is inadequate for rapid acceleration or overcoming significant grades. This may still be achieved with extremely over-dimensioned engines, but the resulting powerplant would be far too large, too heavy, and uneconomical.
There are three types of transmissions:32
1. Manual transmission33. Two versions, sliding-gear (old), and constant mesh transmission. Manual transmission has higher efficiency than automatic transmission. Minimal losses are due to meshing gears, friction between shift components, idling gears and dynamic seals for shafts and bearings, as well as oil splash. Efficiencies may go up as high as 99%. Today manual gearboxes in passenger cars have five gears minimum, possibly 6 or 7.
Newest trend is to include 3 shafts and 2 clutches to allow for power shifts and seamless transmission of torque. This technique is applied in DSG; double clutch transmission that will be explained in the next section.
In addition to the standard gear lever changing mechanism that you would see in the video, the present development trend is toward cable shifters with advantages in terms of space requirements, vibration decoupling between the engine/transmission package and the vehicle cabin, weight and assembly.
An important component of the manual transmission is the synchromesh34, which permits positive gear engagement only after speeds are matched and assures good shift feel, in terms of shift smoothness.
Manual transmissions have either coaxial input & output shafts (direct gearbox, as in a FERWD), or staggered shaft axes (indirect gearbox, as in a FEFWD). An indirect gearbox is illustrated in Figure 20.
32 Two videos in TR on comparison of different types of transmissions:
https://www.youtube.com/watch?v=4xDvNN8JIZ4, www.youtube.com/watch?v=PYhtwlb1ums 33 Introductory video on manual transmission: https://www.youtube.com/watch?v=wCu9W9xNwtI 34 Synchromesh animation at https://www.youtube.com/watch?v=0Bqs-oHBBQk
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Figure 21. A six speed manual transmission.35
In passenger car applications, helical cut gears are generally employed, with the exception of reverse gear. Why? Helical gears are hard to engage and disengage. Since the forward gears are always engaged (considering the constant-mesh type), it makes sense to use helical gearing which can handle more load. Since reverse uses an idler gear which needs to be engaged and disengaged, the spur gear is much easier to use.
The highest gear ratio in a manual transmission is used for climbing steep grades, rapid acceleration and generally, for starting from rest.
Questions to be answered in establishing the lowest gear ratio are:
1- Is the vehicle expected to reach its terminal velocity on level road in that gear?
2- Are good hill climbing ability and acceleration reserves in top gear important, or should fuel consumption be as low as possible?
Manual gearboxes are becoming increasingly automated. What does this mean?
2. Automatic transmission. Three versions: Automated Manual Transmission, Double
The disadvantage of manually shifted transmissions in general is the loss of traction during shifting, compared to power shift (load changeable) transmissions; i.e. the planetary gear transmissions. For a manual transmission shift time may be as high as 1.5 s. BMW SMG system that was mentioned in Table 4, is reported to bring the shift time down to 0.11 s.
Double clutch transmission combines the high efficiency of automated manual with the
uninterrupted shifts of the power shift transmission. Figure 24 illustrates the working
principle of the double clutch transmission with the simplified example of a two-speed
transmission. First gear is transmitted by clutch C1, while clutch C2 is engaged for
36 Figure source: Reference 14.
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second gear. Figure 24b shows the torque history of both clutches, transmission output
torque history, and engine speed history for an upshift under load. Gear changes are
carried out by means of an overlap shift. During this overlap phase the torque
transmitted through the clutch to be disengaged is decreased. Simultaneously torque
transmitted through the clutch to be engaged is increased. At the end of the overlap
phase the newly engaged clutch transmits the torque of the newly selected gear even
while the rotational engine speed associated with the previous gear is still present.
Further increase of torque transmitted by the engaged clutch synchronizes engine side
rotating masses to the speed dictated by the newly engaged gear (torque increase
phase). From the engine output torque trace it is apparent that during the upshift
process, torque drops to the level of the newly selected gear towards the end of the
overlap phase. Next in the torque increase phase, drive torque increases as energy
stored in engine-side rotating messes is transferred to the driveline. Once rotating
speeds are synchronized, the working engine speed and torque are matched to the
new gear. The engine torque trace during the torque increase phase may be controlled
by modulation of the newly engaged clutch and by means of the engine control system.
Other shifts are carried out similarly.
(a) Two speed full-load shifting transmission (b) Torque and speed history during a power shift
Figure 24. Working principle of the double clutch transmission37
A more realistic version of the double clutch transmission would look like the one in
Figure 25, along with a history of measured torques transmitted through both clutches.
Note how gears 1-3-5 are grouped on one gear set, and 2-4-6 on the other.
37 Figure source: Reference 14.
26
Figure 25. Sketch of the double clutch on the left. Measurement from LuK double clutch on the right.38
A production double clutch transmission is shown in Figure 26. This design with 3
shafts and 2 clutches allows seamless transmission of torque; shift time is claimed to
go down as low as 8 milli-seconds.
Figure 26. DSG39
38 Source: Automobil Revue 46, 2002. 39 Videos in TR comparing the DSG with CVT: https://www.youtube.com/watch?v=7AGibpJDwVE
https://www.youtube.com/watch?v=JIEGQcoyaag
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Other than the automated manual transmissions and the double clutch transmission,
automatic transmissions are generally configured as planetary gear sets40. The
reasons are:
• Ability to achieve multiple gear ratios with a single gearset.
• Coaxial mounting of core components. Input and output are located on a
common axis, simplifying mounting of rotationally symmetric shift and coupling
elements.
• Compact dimensions. Furthermore planetary gears do not generate external
radial forces.
• High efficiency.
Main components of automatic transmission set:
- Transmission housing
- Torque converter
- Planetary gear and clutch system
- Hydraulic system
Planetary gearset is the main enabling mechanism of an automatic transmission. A simple planetary gearset is shown in Figure 27, which has three different gears: The sun, the ring, and the carrier with pinions (or planets). Speed and torque relationship between these different gears are given below:
C S R S S R Rn n n n (7)
C S R
S R S R
T T T
n n n n
(8)
where , T and n stand for the speed, torque and the teeth number respectively, associated with each gear with the corresponding S (sun), R (ring) or C (carrier) subscript.
Figure 27. Illustration of a simple planetary (epicyclical) gear set41
These equations can be interpreted as two speeds determine the other, one torque
determines the other. Change in gear ratio is achieved by holding some members of
40 A short and a longer video explaining the automatic transmission
ion_483412259 , youtube.com/watch?v=u_y1S8C0Hmc 41 An article in [TR] on the world’s oldest epicyclic gear set: https://tr.wikipedia.org/wiki/Antikythera_düzeneği
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the planetary gear sets fixed while allowing others to rotate. Hydraulically actuated
friction elements such as clutches and bands are utilized to get hold of or release
members of the planetary gear sets.
3. Variable ratio (or continuously variable) transmission. Engine and vehicle
characteristics are matched optimally with this type of transmission because it has
infinitely many number of gear ratios. (How this optimal matching is achieved will be
explained after we go over resistances to motion in the next section). Furthermore,
continuously variable transmission offers a ride comfort level that is as good as the
automatic transmission, and may have a higher efficiency depending on its type.
Besides, optimal fuel economy can be achieved by operating the engine on the most
efficient operating points of the efficiency map. (We will clarify how this happens in the
next section as well)
There are two types of continuously variable transmission, Mechanical continuously variable transmission (will be acronymed as CVT), and Electrically Variable Transmission (EVT). CVT has different versions:
- Hydrostatic transmissions use a variable displacement pump and a hydraulic motor.
This type of transmission has been effectively applied to a variety of lawn mowers and
garden tractors. Some heavy equipment may also be propelled by a hydrostatic
transmission; e.g. agricultural machinery including foragers, combines, and some
tractors. A variety of heavy earth-moving equipment, e.g. compact and small wheel
loaders, track type loaders and crawler tractors, skid-steered loaders and asphalt
compactors use hydrostatic transmission. Hydrostatic CVTs are usually not used for
extended duration high torque applications because of the heat that is generated by
the flowing oil, although there are a variety of oil cooler designs to help counter this
problem.
The Honda DN-01 motorcycle is the first road-going consumer vehicle with hydrostatic
drive that employs a variable displacement axial piston pump with a variable-angle
swashplate.
- Toroidal or roller-based (extroid) CVTs are made up of discs and rollers that transmit power between the discs. A toroidal CVT is shown in Figure 28.
- V – Belt42 , is the most commonly used CVT system; there are two V-belt pulleys that are split perpendicular to their axes of rotation, with a V-belt running between them. The gear ratio is changed by moving the two sheaves of one pulley closer together and the two sheaves of the other pulley farther apart. Because of the V-shaped cross section of the belt, this causes the belt to ride higher on one pulley and lower on the other. Doing this changes the effective diameters of the pulleys, which in turn changes the overall gear ratio. The distance between the pulleys does not change, and neither does the length of the belt, so changing the gear ratio means both pulleys must be adjusted (one bigger, the other smaller) simultaneously in order to maintain the proper amount of tension on the belt. The V-belt needs to be very stiff in the pulley's axial direction in order to make only short radial movements while sliding in and out of the pulleys.
42 A video showing how CVT works: http://www.hondajazz.org/CVT_nasil_calisir.html
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Figure 28. Toroidal CVT used by Nissan.43
This this type of CVT when used with a metal V-belt (chain) is called Van-Doorne type.
EVT on the other hand is slightly different than CVT (Differences will be asked in a homework question). It is used in Toyota Prius Hybrid for instance.
2.2.3 Differential & Half-shafts
Final drive design details depend upon engine location and orientation. Transverse engines require a cylindrical gearset, longitudinal engines require hypoid bevel gearing.
Question: Why do two sides of the differential rotate at different speeds?
On road surfaces with dissimilar traction for the driven wheels, the wheel with the
lowest friction coefficient (split ) determines the maximum traction available to the vehicle. In the event of additional torque commanded by the driver, this wheel will spin. Loss of traction may be reduced by a limited slip differential44, or braking the slipping wheel, which will generate torque that ensures traction of the non-spinning wheel.
Half shafts of front wheel drive vehicles are subjected to especially demanding services. They must not only compensate for engine and wheel movement but also be capable of extreme angularity imposed by steering movements. Figure 29 shows a half
43 Picture source: Reference 1. A simple animation showing how this mechanism works is at:
http://auto.howstuffworks.com/cvt3.htm 44 Animation video on Limited slip differential:
shaft with the wheel side on the left, synchronous fixed joint, and the differential side on the right a tripod slip joint.
Figure 29. Schematic of a front half-shaft45
2.3 Drive Configurations
Front mounted engine represents nearly the entire passenger car market. Engine and transmission are mounted ahead of the passenger compartment.
Transverse mounting is used only on front-wheel drive vehicles. For rear wheel drive, longitudinal mounting is employed due to its simpler drivetrain and vibration advantages.
Advantages:
-Short lines to all auxiliary devices, radiators and heat exchangers.
-Powerplant noise can be isolated from the cabin my means of the front firewall.
-In case of a frontal accident, early contact of the powerplant block with the crash partner relieves loading of the body shell structure by powerplant inertia forces.
-Provides adequate space for the exhaust system (mufflers and catalytic converters) and fuel tank in the underbody area and at the rear of the vehicle.
-For FEFWD entire powertrain is realized as a compact preassembled unit including the front axle.
-The advantage of FERWD is its increasing traction potential with increasing rear axle loading under acceleration or while climbing up grades.
Disadvantages:
-Transverse engine installations, with few exceptions, are limited to a maximum of six cylinders.
-Transmission size is also severely restricted by installed length (i.e. Vehicle width) limitations.
Rear mounted engines were more commonly employed in the past. Ex: Porsche 911 Carrera, old Skoda.
Advantages.
-Engine location behind the rear axle results in high rearward weight distribution (>60%) and outstanding traction, which increases under acceleration and in ascending grades.
-No heating of the interior.
45 Picture source: Reference 2
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-No drive shafts exhaust pipes.
Disadvantages.
-High rear axle load necessitates sophisticated suspension concepts to achieve good vehicle dynamics.
-Long water lines are required in case of water cooling with front mounted radiators as well as for heating and air conditioning systems.
-Body variability at the rear of the car is very strongly restricted by powerplant space requirements.
There are also mid-mounted engines Ex: Longitudinal mid-mounted engine, rear wheel drive, Porsche Boxster, Formula 1 engines.
This is the classic sports car configuration with engine located ahead of the rear axle. The engine may be mounted longitudinally (transmission behind engine) or transversely. Due to the space requirement of the engine-transmission unit, only two seat designs are practical.
The vehicle’s front and rear axles ared riven from a centrally located differential, with 50/50 or asymmetrical front / rear torque split.
The transfer case is often configured with planetary spur gears and with locking differential, friction limited slip, or open differential action. In the latter case intelligent control of individual wheel braking may be used to achieve limited slip effects on axle differentials as well as on the center differential.
2.4 Modeling the Internal Combustion Engine
Figure 30 shows the simplest sketch of the function that an internal combustion engine
has. It can be thought as a system that converts air and fuel flows into torque and
exhaust gases; meanwhile developing torque that provides the tractive force for the
vehicle through a transmission and driveline.
Figure 30. Simple sketch showing the function of an internal combustion engine.
A more detailed view of basic engine functions, illustrating the difference between ideal and actual external inputs, is given in Figure 31: Flow of air and fuel are regulated by “valves” or “flow restriction devices”; namely throttle body and fuel injectors, respectively. Two specific subsystems are sources of internal dynamics; intake manifold for air and the evaporation dynamics for the injected fuel.
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Figure 31. A more detailed view of basic engine functions.
Exhaust gas oxygen (EGO) sensors make modern electronic fuel injection and emission control possible. They help determine, in real time, if the air–fuel ratio of a combustion engine is rich or lean. Lower than stoichiometric are considered rich. The stoichiometric mixture for a gasoline engine is the ideal ratio of air to fuel that burns all fuel with no excess air. For gasoline fuel, the stoichiometric air–fuel mixture is about 15:1; i.e. for every one gram of fuel, 15 grams of air are required.
Figure 32. Torque and speed curves of a 2.0 liters multi-point fuel injection on the left, and a 1.9 liters naturally aspirated diesel engine, on the right.
It is observed that the magnitude of the torque for the diesel engine is lower than the one of the SI. The reason of sacrificing the high torque potential is due to exhaust emission reasons; the diesel engine can be operated with excess air only, which limits torque. The TDI version of the diesel engine is compared to the standard one in Figure 33.
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Figure 33. SDI TDI comparison.
2.5 INTRODUCTION TO HYBRID ELECTRIC VEHICLES46
2.5.1 What & Why?
- Hybrid vehicles are vehicles powered by two (or more) different powerplants. Hybrid can mean hybrid electric, hydraulic hybrid, fuel cell hybrid, plug-in hybrid, etc. In this context, Hybrid Electric Vehicle (HEV) is a vehicle powered by an ICE and EM(s).
- In the next 50 years, global population is predicted to increase to 10 billion, and the number of vehicles in operation is predicted to increase from 700 million to 2.5 billion. If internal combustion engines are to propel all these vehicles, 1) Where will the oil come from? (Figure 34 illustrates the future projection for shortage of oil) 2) Where should the emissions be disseminated?
Figure 34. Figure shows the trend comparing the demand and the supply of oil.47
46 Please remember this is not a class dedicated to HEVs, therefore we will only touch base with some basic
subjects on HEVs in this section. 47 Picture source: Reference 3. Oil sands are a type of unconventional petroleum deposit, that is either loose sands
or partially consolidated sandstone containing a naturally occurring mixture of sand, clay, and
34
- What is meant by emissions in general? CO (carbon monoxides), HC (hydrocarbons), NOx (nitride oxide), particulate matters (partikül madde-kurum) are pollutants. Other gases like carbon-dioxide CO2, water vapor H2O, hydrogen H2 and nitrogen N2 are not considered as pollutants.
- There exist emission standards all around the world, in different countries. What does this mean? The amount of pollutants emitted from the exhaust pipe should not exceed certain amounts. These standards started mostly in 60s, and evolved since then and are very stringent today, in comparison to previous years. Table 5 compares the current standards with the older versions.
- What are we using in this country? Euro 6 standard since January 1st, 2015. As a matter of fact, we even have standards published by Bilim, Sanayi & Teknoloji Bakanlığı, on emission standards of HEVs exclusively.
- We measure emissions the vehicle produces for a certain drive cycle using a chassis dynamometer, while the exhaust pipe is connected to the measurement equipment, as shown in Figure 35.
2.5.2 More Mechanical Engineering Related Motivations for HEVs
- The engine size is determined in terms of the maximum requirements (vehicle top
speed, maximum acceleration, high power demanding drives such as towing, etc).
Table 5. The evolution of the emissions standards. Light commercial & passenger vehicles at the top.
Heavy commercial vehicles at the bottom. Note how the standards become more and more stringent
with every new standard.48
water, saturated with a dense and extremely viscous form of petroleum technically referred to as bitumen (or
informally tar due to its similar appearance, odor, and color). 48 Source: Reference 10.
35
Figure 35. Exhaust emissions measurement tools.
However, during urban or highway driving, engines normally operate in the lower left quadrant of the engine operating map, as illustrated in Figure 36. This has two consequences: 1) During most of the driving the engine is utilized to operate at relatively low efficiencies on average, significantly lower than the peak efficiency. 2) Engines are significantly oversized compared to the average power required.
Figure 36. The efficiency map of a 1.9 l common rail diesel turbocharged engine
With this background, hybridization can attempt to address the following points:
• Operate the engine nearer its best efficiency
36
• Downsize the engine and still meet the maximum power requirements
• Using regenerative braking (the motor becomes a generator) to restore energy during deceleration instead of dissipating it as heat in the brakes
• Eliminate or mitigate the idling losses by turning the engine on and off (Engine stop-start)
• Eliminate or mitigate the clutching losses by engaging the engine only after the speeds are matched and do not require any slip
• Use of continuously varying gear ratio (such as EVT in Prius)
• Power the accessories / auxiliaries electrically
• Having more control over the engine transients: This impacts emissions and drivability49
- Investigation of a HEV (refer to Figure 17) reveals that there are just a couple of
additional components in an HEV compared to a conventional vehicle.
1) Secondary energy storage unit. (Primary one is the fuel)
2) Traction drive (note that we call a vehicle equipped with a belted-starter alternator
(BSA) such as the ones in Figure 16 “mild” hybrid as well)
3) Power inverters for converting the DC from the high voltage battery to AC for the
traction drive
4) The algorithms (software) that would run in the supervisory controller needed to
carry out the energy management problem.
2.5.3 Secondary Energy Storage in HEVs
There are different types of secondary energy storage units developed for use in HEVs:
- Electro-chemical batteries
- Electro-mechanical flywheels
- Ultracapacitors
- Mechanical flywheels
- Hydraulic accumulators
Common terminology that is used for battery studies are:
Specific energy (energy density) [Watt.hr/kg]: The amount of energy that can be contained in a specific mass of fuel or battery.
Specific power (power density) [Watt/kg]: Maximum electrical power that can be supplied (in/out) of the battery per unit mass.
Battery Capacity (rated in Amp-hr), is the current which can be delivered by the battery under specified conditions. Capacity decreases as discharge current increases and temperature decreases.
49 We will not go into with what metrics is drivability quantified in this class.
37
State of Charge (SOC) is a dimensionless value that describes the amount of usable charge that remains in the battery pack compared to the total charge capacity (under some nominal conditions). SOC can be expressed as a percent of total and will be measured by monitoring the Coulombic charge quantity in and out the battery pack. This is performed by integrating current and comparing that value with the rated ampere-hour capacity of the entire pack.
The cycle life is a measure of how long the battery will last before it needs to be replaced. Batteries usually slowly degrade both in terms of energy capacity and power capacity.
Figure 37 compares these different secondary energy storage units in terms of specific energy and power. The comparison is tabulated in Table 6.
-In pure EV’s, energy density is important because it controls vehicle range. At the same time, power density is also important as it controls the vehicle performance (electric traction only). In HEV’s pure electric range is usually not a factor. On the other hand, regeneration as well as short bursts of drive assist (such as start/stops, vehicle launch, passing power, etc.) and power density is more important.
-The sizing of the electric machines, drivetrain topology and intended mode(s) of operation are all important in determining the characteristics of the battery pack.
-Battery useful life is usually prescribed as the point at which the energy capacity has degraded by 20%. This is the most difficult factor to evaluate, as battery life strongly depends on pattern of use, temperature conditioning, etc. Furthermore, battery packs are made of large number of cells in series and pack performance is limited by the weakest element in the chain.
-Battery life is a key issue in production HEV’s, where 10 years and 161,000 kms (100000 miles) is a minimum life expectancy for vehicles. Batteries today are probably the biggest liability in hybrid drivetrains.
Table 6. Tabulated comparison of different secondary energy storage units
Energy density Watt.hr/kg
Batteries
Sealed Lead acid 27
Nickel Cadmium 35
Nickel-Metal-Hydride 62
Zinc-air 150
Lithium-Sulphur 370
Liquid Fuels
Reformulated gasoline 11,900
No. 2 Gasoline 12,300
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Figure 37. Comparison of different secondary energy storage units.50
-A supercapacitor (SC) (sometimes ultracapacitor, formerly electric double-layer capacitor (EDLC)) is a high-capacity electrochemical capacitor with capacitance values much higher than other capacitors (but lower voltage limits) that bridge the gap between electrolytic capacitors and rechargeable batteries. They typically store 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries. They are however 10 times larger than conventional batteries for a given charge. The advantages of supercapacitors are:
-Higher efficiency
-Longer life
-High power density
-Thermally stable
The advantages of batteries over supercapacitors on the other hand are
-High energy density
-Convenient discharge characteristic
-Established technology
-Less complicated
-Cheaper
50 Picture source: Reference 3.
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Ultracapacitors are being developed as energy devices for power assist during acceleration and hill climbing, as well as recovery of braking energy.
Table 7. Comparison of different battery types.
Battery Type Advantages Disadvantages
Lead-Acid
-Can be designed for high power
-Inexpensive
-Safe
-Reliable
-Poor cold temperature performance
-Short cycle life
Nickel-Cadmium51
-High specific energy
-Good cycle life compared with lead acid
-Does not deliver sufficient power
Nickel-Metal Hydride
-Reasonable specific energy
-Reasonable specific power
-Much longer cycle life than lead acid
-Safe
-Abuse-tolerant52
-High cost
-Heat generation at high temperatures
-Low cell efficiency
Lithium Ion53
-High specific energy
-High specific power
-High energy efficiency
-Good high temperature performance
-Low self-discharge
Needs improvement in:
-Cycle life
-Abuse tolerance
-Acceptable cost
-Higher degree of battery safety
Lithium Polymer54
-High specific energy
-Has potential in providing high specific power
-Safe
-Good cycle and calendar life
Viable only if:
-The cost is much lower
-Specific power is increased
2.5.4 Energy management control problem in a HEV
-A hybrid electric vehicle (HEV) has two or more sources of onboard power. A strategy is needed to control the flow of power and to maintain adequate reserves of energy in the storage devices.
51 Although nickel-cadmium batteries, used in many electronic consumer products, have higher specific energy
and better life cycle than lead-acid batteries, they do not deliver sufficient power and are not being considered for
HEV applications. 52 Abusing a battery means overcharging, over discharging, or short-circuiting it. 53 Most popular one among the HEVs used in the market today. 54 Hyundai and Kia uses Lithium Polymer batteries in some of their hybrid vehicle models.
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-It should be noted that the control algorithm should take into account real world driving cycles, not just the standard cycles, and also consider the driver behavior, which affects energy consumption and emission.
-The flexibility in HEV design comes from the ability of the control strategy to manage how much power is flowing to or from each component.
-This way, the components can be integrated with a control strategy to achieve the optimal design for a given set of design constraints.
-There are many (often conflicting) objectives desirable for HEVs. The primary ones are to:
- Maximize fuel economy,
- Minimize emissions,
- Minimize propulsion system cost,
- Do all of the above while maintaining or improving drivability, acceleration, range, handling, noise, etc.
-One of the world’s first mass-produced HEV product was the Toyota Prius, currently its 4th generation charge sustaining HEV, and 3rd generation PHEV versions are available in the market.
-In the near future HEV is expected to have wider applications though its key issue remains the optimization of multiple energy sources to obtain best performance at lowest cost.
-This claim shouldn’t underestimate EVs. The main criticism to EVs are the range, and the charging infrastructure problem. However, current EVs are doing much better compared to their previous versions in terms of the electric range. There are already EVs made today with 500 kms electric range.55 Many examples can be found in the footnote.56
CHAPTER 3 TRACTIVE EFFORT
Transmission Efficiency
-Automatic transmission efficiency for different gears is given in Table 8, and the dependency of automatic transmission efficiency on the lubricant temperature is shown in Figure 38.
-Because of which component in an automatic transmission is the efficiency lower in
comparison to manual transmission? Torque converter.
-What makes the efficiency higher at high gears? Torque converter lock up happens at high speeds, i.e. the fluid coupling phase is realized where the pump and the turbine speeds (and torques) are equal. That reduces losses.
-Therefore, it is desirable to apply lock-up clutch from an efficiency standpoint. As long
as there is no slip, we have lower losses. However, locking causes driveline vibrations and
noise.
55 Consider how much one drives every day, from home to work or school, and back the same way in the evening;
probably not more than 50 kms in a city like Ankara. This means fuel consumption would simply be zero in a
normal weekday if the driven vehicle is an EV. 56 https://en.wikipedia.org/wiki/List_of_electric_cars_currently_available
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Table 8. Automatic transmission efficiencies.
Figure 38. Variation of the transmission efficiency, at third gear, at an engine speed 2500 rpm, with respect to lubricant temperature, for an automatic transmission57
-CVT efficiency with a rubber belt and a metal chain is shown in Figure 39.
Figure 39. 50 Nm and 100 Nm transmission efficiencies for CVTs equipped with rubber and metal chain (shown on the right). Not that there are two trends observed from the figure: Metal chain has a higher transmission efficiency that the one of the rubber belt, since it is stiffer. Secondly the transmission efficiency increases at high torques.
Pneumatic Tire
-Pneumatic tire starts with a guy named Dunlop, making a tire for his son’s three wheeled bicycle. “Dunlop üzeri lastik solüsyonu ile kaplanmış çadır bezini tekerleklerin
57 Figure source: Reference 3.
GEAR EFFICIENCY [%]
1 60-85
2 60-90
3 85-95
4 90-95
5 85-94
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üzerine çivi ile çakarak ve içine hava doldurarak ilk adımı atmıştır.” In 1895, E. Michelin produced the first practical pneumatic tire for use on motor vehicles.58
-Tyre: British English
-The wheels of all modern motor vehicles are provided with pneumatic tires, which cover the wheel’s rim, support the vehicle and transfer the driving power through the wheel-ground contact. They also provide the lateral forces which are needed in order to control the trajectory of the vehicle. The rigid structure of the wheel, made by the disc and the rim, is surrounded by a compliant element, made by the tire, providing a flexible cushion that absorbs shock
What is tire made of?
- Synthetic rubber
- Natural rubber
- Fabric and wire
- Carbon black
- Other chemical compounds
-Synthetic rubber, is synthesized from petroleum byproducts. About 15 billion kilograms of rubbers are produced annually, and of that amount two thirds are synthetic. Where else in a vehicle are they used? Hoses, belts.
-Natural rubber (kauçuk). Compared to vulcanized59 rubber, uncured rubber has relatively few uses. It is used for cements; for adhesive, insulating, and friction tapes; and in insulating blankets and footwear.
-Why is it called rubber in Western World? Because in 18th century, in France, they noticed that the material extracted from a rubber tree in South America, was extremely good for rubbing off pencil marks. That is why it is called rubber. Later the plantation of rubber started in India, and also African countries started production of rubber.
-Soon after Goodyear, Dunlop installed steel wires inside the tire to increase the resilience and invented the inner tube (iç lastik). Tubeless tires (dubleks lastik, yanlış kullanım) are safer than iç lastik. Why is that? Because when there is a leak in a tubeless tire, the air leaks out of the tire very slowly, especially if the vehicle is stationary. Also their performance is better.60
-Most bicycle tires, many motorcycle tires, and many tires for large vehicles such as buses, heavy trucks, and tractors are designed for use with inner tubes. Inner tubes are torus-shaped balloons made from an impermeable material, such as soft, elastic synthetic rubber, to prevent air leakage. The inner tubes are inserted into the tire and inflated to retain air pressure.
58 Source: Reference 1. 59 The modern process of vulcanization (named after Vulcan, the Roman god of fire) was developed in the 19th
century by Charles Goodyear. Vulcanization is a chemical process for converting natural rubber into more durable
materials via the addition of sulfur. Why is this done? Vulcanized materials are less sticky and have superior
mechanical properties.
As a matter of fact, the curing of rubber has been carried out since prehistoric times in South America, 3600 years
ago Aztecs were already using extracted rubber for some of their products. 60 Nice introductory videos on pneumatic tires (in Tr): https://www.youtube.com/watch?v=PBWR8TgUERk (Tire
manufacture in En) https://www.youtube.com/watch?v=abtaO0i7J4U
43
Fabric and wire: Tire fabrics serve as a reinforcement for a diverse variety of tires, ranging from bicycle tires to heavy duty tires. These textile reinforcements include tire cords, tire chafers, tire breakers, bead wrap and liners for processing of these tires. These products deliver high modulus, low shrinkage and firm stability. Ensuring high performance reinforcements with minimal moisture absorption, these fabrics also enable secure grip, better handling and less flat spotting. Figure 40 illustrates some different types of fabrics.
Figure 40. Different type of fabrics.
-Carbon black (shown in Figure 30) is a material produced by the incomplete combustion of heavy petroleum products such as tar (katran), and a small amount from vegetable oil.
Figure 41. Sample of Carbon-black
Parts of a Tire (Shown in Figure 42)
-The tread (sırt, taban) is the part of the tire that comes in contact with the road surface. The portion that is in contact with the road at a given instant in time is the contact patch. The tread is a thick rubber, or rubber/composite compound formulated to provide an appropriate level of traction that does not wear away too quickly. (The tread pattern is characterized by the geometrical shape of the grooves, lugs, voids)
44
Figure 42. Parts of a tire61
-The bead (topuk) is the part of the tire that contacts the rim on the wheel. The bead is typically reinforced with steel wire and compounded of high strength, low flexibility rubber. The bead seats tightly against the two rims on the wheel to ensure that a tubeless tire holds air without leakage.
-The sidewall (yanak) is that part of the tire that bridges between the tread and bead. The sidewall is largely rubber but reinforced with fabric or steel cords that provide for tensile strength and flexibility. (The sidewall contains air pressure and transmits the torque applied by the drive axle to the tread to create traction but supports little of the weight of the vehicle, as is clear from the total collapse of the tire when punctured.)
-Plies (iplik) are layers of relatively inextensible cords embedded in the rubber to hold its shape by preventing the rubber from stretching in response to the internal pressure. The orientations of the plies play a large role in the performance of the tire and is one of the main ways that tires are categorized.
Filler: Filler material is usually silica and carbon black.
-There is also a valve stem (sibop) not shown in Figure 42, which is a tube made of metal or rubber, through which the tire is inflated, with a check valve, typically a Schrader valve on automobiles and most bicycle tires, or a Presta valve on high-performance bicycles. Valve stems usually protrude through the wheel for easy access. They mount directly to the rim, in the case of tubeless tires, or are an integral part of the inner tube.
-In some tires there is a sensor module that is mounted to the valve stem inside the tire. What sensors are these? Tire pressure monitoring systems (TPMS) are electronic systems that monitor the tire pressures on individual wheels on a vehicle, and alert the driver when the pressure goes below a warning limit. There are several types of designs to monitor tire pressure. Some actually measure the air pressure, and some
61 Picture source: Reference 1
45
make indirect measurements, such as gauging when the relative size of the tire changes due to lower air pressure.
-Bias tire (or cross ply) construction utilizes ply cords that extend diagonally from bead to bead, usually at angles in the range of 30 to 40 degrees, with successive plies laid at opposing angles forming a crossed pattern to which the tread is applied. The design allows the entire tire body to flex easily, providing the main advantage of this construction, a smooth ride on rough surfaces. However, this cushioning characteristic also causes the major disadvantages of a bias tire: increased rolling resistance (which degrades fuel economy) and less control and traction at higher speeds. This is outdated.
-Radial tire construction utilizes body ply cords extending from the beads and across the tread so that the cords are laid at approximately right angles to the centerline of the tread, and parallel to each other, as well as stabilizer belts directly beneath the tread. The belts may be cord or steel. The advantages of this construction include longer tread life, better steering control, and lower rolling resistance (better fuel economy). Disadvantages of the radial tire include a harder ride at low speeds on rough roads and lower grip ability at low speeds.
-The Tweel62 (combination of tire and wheel) is an airless tire design. Its significant advantage over pneumatic tires is that it cannot burst, leak pressure, or become flat. Instead, the Tweel assembly's inner hub connects to flexible polyurethane combs which are used to support an outer rim and these engineered compliant components assume the shock-absorbing role provided by the compressed air in a traditional tire.
-There is a concept called the "critical speed of a tire". The significance of the critical
speed is explained by vibratory phenomena which takes place in the tire at high speed,
as you can see in Figure 43. The tread band vibrates both in its plane and in the
direction of the axis of the wheel. The critical speed is influenced by many parameters
and it is one of the factors which must be taken into account in the choice of the tires
for a particular vehicle.
Figure 43. Here the tire is rolling against a drum. The standing waves which propagate along the
circumference of the tire from the contact zone are clearly visible. 63
62 Video showing Tweel performance on sand: https://www.youtube.com/watch?v=367zv6ARsEs 63 Picture source: Reference 11.
46
CHAPTER 4 RESISTANCES TO MOTION
Coast-down tests. Recall the coast down test procedure given on Page IV-15 of the Lecture Notes, for calculating the rolling resistance and drag coefficients, respectively. We considered a low and a high speed range for the procedure and then took their average for the calculations.
Actually there is a less approximate, more accurate way of doing this calculation, first given in Reference 12, which can be found explicitly in Reference 13. Please have a look at it (Reference 13 is available in the library as a pdf version).
CHAPTER 5 MAXIMUM SPEED & ACCELERATION PERFORMANCE
Velocity Loss During Gear Shift
-During the acceleration of a vehicle, there will be some velocity loss, every time the gear is shifted. This results from the fact that the engine is disconnected and, thus, the vehicle decelerates under the action of resistance forces during the gear change time. -It is possible to calculate this velocity loss from the equation given in the old lecture notes (Equation V.28 with a2
2-4a1a3<0) by trial and error:
2
12
231
211
2
231
12
4
2tan
4
2 V
V
eq
aaa
aVa
aaa
mtt
(9)
Given t, V1, and vehicle specifications and road parameters, one can find V2, and thus
V. However, an approximate solution will be preferred here as it is much simpler and
faster and will give reasonably accurate results if the velocity loss is small. If it is
assumed that the resistance forces remain constant during the gear change, then the
deceleration of the vehicle is given by
V '
gseq
R ΔVd =
Δtm (10)
Figure 44. Highlighted points correspond to the gear shifts.
47
which yields
1'21 Veq
s Rm
tVVV
(11)
where V1, V2 = vehicle speed before & after gear shift,
tgs = time taken for changing the gear, meq' = effective mass of the vehicle excluding the contribution of parts rotating at engine speed, |∑R|V1 = resultant of resistance forces evaluated at V1. Some typical values for the gear shift time, tgs, in seconds for skilled drivers are listed in the table. Table 9. Shift times [s] for different types of transmissions
Without synchronizer 1.3 – 1.5
Synchromesh 0.2 – 0.5
Semiautomatic64 0.05 – 0.1
Dual clutch automatic (DSG) 0.008
Since the duration of gear shift is quite small, the distance travelled during gear change can, then be estimated from the linear approximation:
2
21 VVtS s
(12)
Ex: Estimate the velocity loss and distance travelled during an up-shift from third to fourth gear for the vehicle specified below, at a vehicle speed of 75 [kph] if gear shift time is 1.2 [s]. Vehicle mass : 1320 [kg] Drag coefficient : 0.43 Frontal area : 1.8 [m2] Tires : Textile belted radial Rolling tire radius : 0.286 [m] Differential ratio : 4.1 Moment of inertia of parts rotating at engine speed : 0.3 [kgm2] propeller shaft speed : 0.025 [kgm2] wheel speed : 6.2 [kgm2] Solution:
64 A semi-automatic transmission (SAT) (also known as a clutchless manual transmission, automated manual
Figure 45. Clutch pedal force vs clutch pedal travel. Curve 1 corresponds to disengagement of the clutch, whereas Curve 2 is showing its engagement. The two curves not being identical is a typical example of hysteresis.65
Expectations from a good clutch design: -Easy usage -Low power loss -Low package volume -Low weight -High reliability -Long life -Low cost The clutch should: -Be capable of transmitting maximum engine torque -Provide engagement with no judder66 -Damp the torsional vibrations
65 Figure source: Reference 14. 66 What is judder? The variation of the clutch clamping force due to discontinuity of the torque transmitted may
excite the 1st natural frequency of the driveline, which is very disturbing. What are the sources of discontinuity
for torque transmission? 1) Inclined or unparallel pressure plates 2) Unequal leaf spring coefficients
49
-Transmit torque in both directions -Be resilient against fatigue (average of 30 engage-disengages per km for buses) How are clutches actuated?
-Metal cable (old method) -Hydraulically (current method, better damping effect) -Servo motors (for DSG for instance.) Derivation of the mean radius for calculating the clutch friction torque.
o
i
r2
2 20 r o i
dF FrF rdF r dA r rdrd
dA r - r
(13) For most automobiles, the values of the parameters are given in the ranges:
= 0.2 – 0.45, N = 1000 – 2000 [N] R = 0.1 – 0.2 [m] -The friction coefficient depends on the speed difference. It increases as the speed difference decrease (sticking friction coefficient), and decreases as the speed difference increases (sliding friction coefficient). It depends on temperature as well.
Figure 46. The engine torque that can be transmitted with a clutch, depending on the clutch diameter. Advantages of small diameter: Packaging, weight, cost. Advantages of large diameter: Same clamping force with a lower pedal force, better damping effect.67
Below are some limit values for clutches used in automobiles and commercial vehicles: -Max permissible temp: 600 Celsius -Max pressure: 200-500 kPa -Max specific power: 500-1300 kW/m2 -Max specific energy: 2-5 MJ/m2
67 Figure source: Reference 14.
50
CHAPTER 6 TRACTION & BRAKING PERFORMANCE
6.1 Braking Performance
Components of a disc brake system is illustrated in the link at the footnote.68
Wheel brake size is primarily determined by vehicle weight and the targeted deceleration rate. These imply a brake torque that must be withstood by the effective radius of the brake disc and the clamping force of the brake caliper. The required clamping force and the chosen brake caliper piston area imply a system pressure which must be provided by the brake actuation mechanism. The volume of the brake system is determined by the volume of the actuation mechanism. Suspension geometry and wheel rim contours determine which combination of caliper, brake pad, and brake disc can be selected.
Brakes convert kinetic energy to heat by means of friction. This heat is in part stored in appropriately dimensioned brake components, and in part rejected to the environment through air cooling.
Thermal loading of brakes is therefore determined primarily by the maximum attainable vehicle speed. The faster the car, the more kinetic energy there is that must be converted to heat.
The second major criterion for thermal layout is long downgrades, which result in extreme brake heating. In this situation, potential energy is converted to heat over an extended period of time without permitting an adequate cooling of air supply to reject this heat to the surroundings. Because of low speeds encountered in descents, air cooling of wheel brakes is comparatively ineffective.
Brakes are designed so that the manufacturer’s established limiting rotor temperatures are not exceeded under realistic extreme conditions; the consequence would be reduced brake performance (fading) or even destruction of the brake itself.
Pedal Characteristics, or so-called “pedal feel” provide the driver with feedback regarding the braking process and condition of the brake system. These characteristics vary as part of a vehicle’s brand and model specific identity. The major parameters affecting pedal feel are:
- Activation force
- Pedal free play
- Threshold pressure (which the brake booster applies to preload the brake system, at the beginning of brake actuation. This eliminates the so-called dead pedal feel resulting from seal friction in the TMC and wheel cylinders)
- Amplification
- Hysteresis
- Pedal travel
- Pedal travel and pedal force at booster vacuum run-out point
- Pedal travel increase and pedal force increase during fading
- Response time
68 Main components of a disc brake: https://www.youtube.com/watch?v=MAuVDB-G-HQ
51
- Release time
Development of brake actuation systems today is tending toward reduced pedal free play, lower actuation force, and higher threshold pressures to achieve the most direct brake system response possible; and toward high amplification and short pedal travel for increased comfort.
6.2 Tandem Master Cylinder
To increase safety, most modern car brake systems are broken into two circuits, with two wheels on each circuit. This makes the cylinder relatively failsafe. If a fluid leak occurs in one circuit, only two of the wheels will lose their brakes and your car will still be able to stop when you press the brake pedal.
When you press the brake pedal, it pushes on the primary piston through a linkage. Pressure builds in the cylinder and lines as the brake pedal is depressed further. The pressure between the primary and secondary piston forces the secondary piston to compress the fluid in its circuit. If the brakes are operating properly, the pressure will be the same in both circuits. This process is illustrated in the video.69
When the first circuit leaks, the pressure between the primary and secondary cylinders is lost. This causes the primary cylinder to contact the secondary cylinder. Now the master cylinder behaves as if it has only one piston. The second circuit will function normally, but you can see from the animation70 that the driver will have to press the pedal further to activate it. Since only two wheels have pressure, the braking power will be severely reduced.
Splitting the brake system into separate circuits provides added insurance against total brake failure, in the event of fluid leakage in one part of the system. The vehicle can still be adequately braked with the remaining intact brake circuit.
The most common dual circuit brake system configurations are:
Front/rear brake split
Here, the front and rear wheels each represent a separate braking circuit. The advantages of this configuration are: No asymmetrical pull in the event one circuit fails and less brake tube complexity.
Diagonal brake split
In this configuration, diagonally opposed brakes are joined in a single brake circuit. The higher braking ability of at least one front wheel is used even if one circuit fails. Brake pulling in the event of one circuit failing may be compensated for by appropriate design such as a negative steering offset.
Both of these configurations are illustrated in Figure 47.
69 Normal brake operation for disc and drum brakes: https://www.youtube.com/watch?v=bMg_j5_AGMg 70 Master cylinder operation explained in: https://www.youtube.com/watch?v=bGKJOICWmFQ&nohtml5=False
52
Figure 47. The two dual circuit brake systems. On the left, one hydraulic circuit feeds both front brakes and the hydraulic circuit feeds both rear brakes. On the right, one hydraulic circuit feeds the front left and rear right brakes, the other hydraulic circuit feeds the front right and rear left brakes.
Depending on the configuration of the valve separating the brake fluid reservoir and the master cylinder, three TMC types can be classified:
- Central valve TMC: In this type of TMC, the chambers are connected to the brake fluid reservoir by means of so called central valves located in the pedal pushrod (primary) piston and the floating (secondary) piston. In their inactive condition, these valves are held open by stop pins. This type of TMC is illustrated in Figure 48.
When the brake pedal is actuated, the primary piston is forced forward by the brake booster pushrod. Simultaneously, with opened central valves, the secondary (floating) piston moves forward in response to pressure from a preloaded spring between the primary and secondary pistons.
The so called “closing travel” or “lost travel” is an important feature of the TMC design. It is the movement of the piston until the connection to the reservoir is closed. It should be as small as possible to achieve immediate brake system response, i.e. for optimal pedal feel.
The captive spring in a Central Valve TMC permits nearly simultaneous closing of both central valves, which may reduce the total closing travel of the TMC to half that of sequential closing design, which has a positive effect on pedal feel.
With the central valves closed, return flow from the TMC to the reservoir is blocked. Fluid volume displaced by additional piston movement flows through the hydraulic control unit to the wheel brakes.
- Vent port TMC: In a conventional vent port master cylinder illustrated in Figure 49, connections to the brake fluid reservoir consist of small holes (called vent holes, compensating ports or replenishing ports) in the cylinder bores. As the piston is stroked, cup seals pass over these holes and interrupt connections between the main cylinder chambers and reservoir chambers. Pressure build-up in the brake system is possible from this point onward. As the brake are released, the pistons return to their retracted positions and brake fluid flows from the wheel brakes back into the TMC. In this type of TMC, the closing travel is related to the compensation ports, as shown in the video at footnote 70. Three modes of the compensation port is shown in Figure 50.
53
Figure 48. Both pistons have a center valve that manipulates the flow from the reservoir to the cylinders.
Figure 49. Primary piston has compensation port that prevents flow of the fluid from the reservoir to the cylinder during brake pedal apply.
54
Figure 50. Different modes of the compensation port. In the brake pedal released position the first cylinder is filled, and there is no fluid flow. As the brake pedal is applied, i.e. the piston is stroked, cup seals pass over the port and interrupt connections between the cylinder and the reservoir. Pressure build-up in the brake system is possible from this point onward. As the brake are released, the pistons return to their retracted positions and brake fluid flows from the wheel brakes back into the TMC
- Plunger TMC: TMC in plunger configuration is preferred in applications where a short installed length for the actuating mechanism is required. Pressure springs, sealing, and guide elements are in part arranged concentrically, which has advantages in reducing the required master cylinder overall length. This type of TMC has also a lower weight and cost. A picture pf the plunger type TMC is shown in Figure 51.
Figure 51. Plunger TMC design. Used for TCS and ESC applications. Lost travel is optimized.
55
This valve mechanism is suitable for TCS and ESC applications. In the retracted position, valve ports in the pistons connect each TMC chamber with the corresponding reservoir chamber. As the cylinder is actuated by the pushrod, the passages located ahead of the cup seals, as seen in the direction of actuation, are pushed under the seal lips, thereby closing these passages. As actuation continues, these passages are again opened by the cup seal on the chamber side, which takes place without seal damage as in this condition the same hydraulic pressure is found on both sides of the sealing lips. This is especially important on brake release because depending on possible prefilling of the brake systems, for example by TCS or ESC controls, a high pressure condition may exist in the TMC in this position.
Figure 52 shows the evolution from central valve TMC to two versions of plunger TMC, the latter the newer.
Figure 52. Evolution from Central valve TMC to Plunger TMC
Figure 53 shows different types of TMC designs for different applications on the left, and the Generation 2 plunger TMC on the right side. On the other hand Figure 54 shows the plunger TMC operation modes.
56
Figure 53. Different types of TMC designs on the left. Plunger TMC Gen2 on the right side.
Figure 54. Figure shows different modes of Plunger TMC operation.
57
Table 10. Comparison of different TMC types.
6.3 Brake Fluid Reservoir
The reservoir (shown in Figure 55) is attached to the top of the tandem master cylinder by means of so-called reservoir plugs (but can also be mounted elsewhere in the engine compartment) and is customarily joined to the master cylinder by an additional fastening arrangement which permits high pressure filling during vehicle assembly. The main function of the reservoir is to supply brake fluid to the tandem master cylinder (TMC). Secondary functions of the reservoir are: - Supply fluid for a hydraulically actuated clutch or for an electronic stability control (ESC) preloading pump – Supply fluid for charging a hydraulic pressure accumulator.
Figure 55. Photo showing the TMC and the reservoir.
Main components of the reservoir are labeled in Figure 56. Different versions of the fluid level sensor are shown in Figure 57. Figure 58 on the other hand illustrates
58
different reservoir volumes labeled along with their relationship to each other below the figure.
Figure 56. Reservoir components labeled.
59
Figure 57. Different versions of fluid level sensors.
Figure 58. Different reservoir volumes labeled.
6.4 Vacuum Boosters
Vacuum-assisted brakes, also called power brakes, use a vacuum booster to assist the driver stopping the vehicle. The common system is mounted directly against the dash panel opposite to the driver’s foot. It is mounted between the brake pedal and the tandem master cylinder.
60
The differential in pressure across the booster or diaphragm with the vacuum or low pressure on the master cylinder side, and the atmospheric or high pressure on the input side is what provides the power assist which actuates the master cylinder. Figure 59 shows different booster designs used today in vehicles.
Figure 59. Different types of vacuum boosters.
The main reason for popularity of this pneumatic-mechanical mechanism in vehicles is basically the no cost availability of a vacuum source on most gasoline engines. A vacuum line connects the vacuum chamber to the suction manifold of the engine, or in the case of diesel engines where there is no vacuum (because there is no throttle valve) or electric vehicles where there is no engine at all a vacuum pump is used to evacuate vacuum chamber. Figure 60 shows a simplified illustration of a brake booster.
61
Figure 60. Simplified sketch of a vacuum booster. The check valve either connects the two chambers, as in pedal released mode, or the rear chamber to the ambient during pedal apply mode. Whether the former or the latter is served by the check valve is specified by the movement of the piston rod. Manifold valve on the other hand connects the vacuum chamber to the suction manifold of the engine.
In the pedal released condition (Figure 61a), the same pressure is applied to both sides of the diaphragm, i.e. vacuum and rear chambers are both at the same pressure level (which is very close to the vacuum level; i.e. 0 bars. The engine suction manifold pressure as a function of accelerator pedal position is shown in Figure 62). This is achieved with the check valve connecting the two chambers to each other, with the connection to ambient cut. The power piston return spring holds the diaphragm support plate in its retracted position.
During partial braking (Figure 61b), as the input pushrod is actuated by the brake pedal, the check valve closes the vacuum control port on the rear chamber; i.e. the connection between the two chambers are cut. As the push rod continues its motion, the atmosphere control port is opened, thereby permitting atmospheric pressure to build within the rear chamber. As a result, a pressure differential exists between rear and vacuum chambers, and this moves the membrane plate towards the TMC, thereby amplifying the driver’s foot pressure.
In the tandem master cylinder, hydraulic pressure builds in response to the forward movement of the piston. Given constant foot pressure, TMC piston, the input pushrod and the valve plunger come to rest at a displaced position determined by the foot pressure level. This is the holding position, where the connection of the rear chamber to ambient is cut, and the pressure in the rear chamber is balanced. The piston stops and the braking force is kept constant. Any further change in pedal pressure will cause an increase or decrease in the pressure difference between the two sides of the diaphragm support plate. Analogously to pedal pressure, hydraulic pressure in the brake system is raised or lowered, thereby setting the desired vehicle deceleration.
In the full braking (Figure 61c) position, the passage between the rear and vacuum chambers totally closed, and the atmosphere control port is constantly open; i.e. the check valve in the simplified Figure 49 connects the ambient and the rear chamber. Full atmospheric pressure is therefore applied to the diaphragm in the rear chamber (around 1 bar) which yields the maximum possible boost of the brake force. This condition is termed the booster vacuum runout point, which is shown in Figure 63. Any
62
additional increase in pressure on the tandem master cylinder piston can be achieved only by increased pedal pressure. The position of the output (vacuum runout) point and the gradient of the force graph depend on different design parameters of the system like the master brake cylinder surface area, the diaphragm surface area, pedal leverage, vacuum level etc. Normally, the data sheet of the system shows the parameters of the graph for a nominal vacuum chamber pressure. When the pedal is released (Figure 61d), two forces push the diaphragm back to towards the rear chamber: 1) The reaction force from the TMC 2) The input rod return spring force. As the diaphragm moves right, the check valve is activated again, it closes the connection between the rear chamber and the ambient, and reconnects vacuum and rear chambers. This causes pressurized air in the rear chamber to flow into the vacuum chamber that is at vacuum level, until equilibrium is reached until both chambers are at the same (vacuum) level.
Figure 61. Four different modes of booster operation
63
Figure 62. Engine manifold pressure as a function of accelerator pedal position. Note that in the case when the accelerator pedal and brake pedals are applied simultaneously, there won’t be any vacuum at the vacuum chamber. This will stiffen the brake pedal since the boosting function is lost.
Figure 63. The TMC pressure is a result of a combination of the force at the pedal and an auxiliary assist. The proportion represented by the assist increases steadily up to full boost (or the vacuum runout or the output point). At full boost the maximum pressure difference between outside air and vacuum has been reached. Additional augmentation of the output force is only possible via an unaccustomed increase in the force applied at the pedal.
Booster F-F Diagram
A more realistic version of the push rod force (output force) versus the piston force (input force) diagram is shown in Figure 64.
64
Figure 64. Booster F-F diagram.
As you can see from the diagram, the input pedal force needs to beat the “cut-in” force in order to generate output force. This is the force required to initially open the valve and begin producing an output force. It can be formulated as:
Fci=Firrs-P.Avp (14)
where Fci is the cut-in force, Firrs is the input rod return spring force, P is the pressure difference (ambient-vacuum) and Avp is the area of the valve piston, variables illustrated in Figure 65 as well. The difference between when the input force is initially applied and when the valve is opened (check valve connecting the rear chamber to the ambient) is referred to as “lost travel”. The cut-in value is determined by specifying an appropriate input rod return spring installed load; increasing the installed load of the return spring will increase the cut-in value and vice versa. Modifications to the cut-in value are limited by both the ability of the booster to fully return and the valve return spring design.
Figure 65. Cut-in force is the force required to initially open the valve and begin producing an output force.
When the pedal is released, the net force that pulls the piston rod back can be formulated as:
65
Freturn=Firrs-P.Adv-Fdvs-Fdv (15)
where Adv is the area of the disc valve, Fdvs is the disc valve spring force and Fdv is the disc valve force, illustrated in Figure 66.
Figure 66. Return force.
On the other hand, the force balance during boosting mode can be written as:
Fpush rod-Fpiston rod=Fbooster=(P.Ab).b-Frs (16)
where Fbooster is the difference between the input piston rod force and the output push
rod force, Ab is the affective area of the membrane, b is the booster efficiency, Frs is the return spring force.
The pressure equilibrium at run-out point can be written as:
Figure 67. Pressure balance.
2 2
pushrod pistonrod irrs
pr vp
F F F
d d
(17)
where dpr is the push rod diameter, and dvp is the valve piston diameter. The above equation yields the boosting ratio:
Fpush rod = i.Fpiston rod – i.Frs (18)
where i = dpr2/dvp
2
In addition to this conventional vacuum booster design, tie rod actuated brake boosters are also currently in use. In this design, tensile forces in actuation are not transmitted by the booster housing (power chamber halves) but by a tie rod which completely
66
penetrates the device, including membrane and membrane supporting plate. This design permits appreciably thinner walled or aluminum booster halves, with corresponding weight savings, without encountering the expansion found in conventional designs. Tie rod design and comparison with conventional booster is illustrated in Figure 68.
Figure 68. Comparison of conventional and tie rod booster designs
6.5 Vacuum Pump The available vacuum found in the intake manifold of the gasoline engine vehicles represents an economical energy source for power brake boosters. The vacuum is generated due to the up and down motion of the pistons, as mentioned before. However for most diesel engines that doesn’t have a throttle valve and some of the gasoline engines that are equipped with fuel injectors, the vacuum provided by the engine is no longer sufficient and therefore vacuum pumps are required. These are usually rotary vane pumps attached to the engine and driven by the camshaft. Because they are permanently driven, they result in increased fuel consumption. Furthermore mechanical vacuum pumps require a permanently “running engine” even at stand still. On the other hand, there are electric vacuum pumps as well that can be controlled electronically with the engine control unit or the ABS module as well, for applications for advanced engine technologies, or electric and hybrid vehicles. Such an electric vacuum pump is shown in Figure 69, along with its specifications shown underneath, in Table 10.
67
Figure 69. Electric vacuum pump. Picture source: Continental website.
Table 10. Electric vacuum pump specifications.
The controller regulates the pressure in such a way that the pump is turned on; i.e. it evacuates the vacuum chamber when the pressure in the vacuum chamber reaches a certain threshold, and likewise it turns off the pump when the vacuum level (almost vacuum, usually around 0.2 bars) is reached in the chamber. That pump on-off control is illustrated in Figure 70.
Figure 70. Pump on-off control.
68
Hydraulic Brake Boosters: Compared to vacuum brake boosters, hydraulic boosters offer the advantage of reduced installation space requirements and generally much higher booster runout point. Disadvantages include higher cost and the dead pedal feel (no threshold pressure). 6.6 Brake Connections High-pressure metal brake tubing, brake hoses and flexible lines are used to connect brake system hydraulic components. Brake tubing is used between rigid, non-moving points on the body. This consists of double-walled, brazed steel tube. As protection against environmental influences, tube surfaces are galvanized and additionally covered with a plastic coating. Brake hoses are used at transitions to dynamically loaded components such as wheel calipers or brake calipers. They ensure unimpeded transfer of fluid pressure to the brakes, even under extreme conditions. Along with mechanical strength, the most important characteristics include pressure capacity, minimal volume change under pressure, chemical resistance to such substances as oil, fuels, and salt water, and good thermal stability. Brake hose construction consists of an inner hose, a two ply woven layer to resist pressure layer against external influences. Flex lines like brake hoses, are used at transitions to dynamically loaded components.
6.7 Wheel Brakes
Wheel brakes are friction brakes. The defining characteristic of brakes is the ratio of generated tangential force at the rotor or drum to applied clamping force, the so called C* value. For a disc brake that is C* = 2.m.P.A / P.A = 2m for a drum brake it is the ratio of the total circumferential force to the applied spreading shoe force. Drum Brakes (Kampana) Until 60s brakes were almost exclusively configured as drum brakes. Considerably reduced C* variation as a function of varying friction coefficients and ability to withstand higher thermal loading have made disc brakes the predominant brake configuration in use today. The main disadvantage of drum brakes is overheating during braking, since the drum mechanism is a closed system that doesn’t let heat exchange much. When the drum brake overheats, the coefficient of friction between the drum and the shoes goes down, causing a reduction in the braking torque. For this reason, drum brakes are used at the rear axle as the rear brakes take less load considering the load transfer to the front axle during braking (and therefore deceleration) of the vehicle. Main components of a drum brake are shown in Figure 71.71
71 Drum brake operation is explained in these videos: https://www.youtube.com/watch?v=_X5HB0wDG10
https://www.youtube.com/watch?v=bnc3VnQ8kUY
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Figure 71. Main components of a drum brake72
Drum brakes are fitted with two inner brake shoes which are passed against the friction surface of the drum during a braking event by means of hydraulic actuated cylinders. Springs draw the brake shoes back to their rest position, resulting in play between the drum friction surface and brake linings. There are several types of drum brakes: 1) Simplex drum brake (Illustrated in Figure 72a): - Employed on the rear axle of passenger cars with maximum speeds no greater than
170 kph
- Brake torque sensitivity to friction coefficient variation is C* = 2.0 – 2.3
- The forward (in the direction of vehicle travel) brake shoe (primary brake shoe)
provides about 65% of the braking torque, while the rear (secondary) shoe provides
the remaining 35%.
- Either the primary lining is thicker or differing arc lengths (circumferential arc length
over which the lining contacts the drum) are chosen for the two shoes.
2) Duplex drum brake (Illustrated in Figure 72b): - Each of the identically sized brake shoes pivots against its own fixed point on the brake backing plate and is pressed against the drum by a single-acting blind hole cylinder.
72 Figure source: Reference 10
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- Both shoes act as primary during forward braking (therefore higher braking force), and secondary during reverse braking. - C* values are higher, about 2.5 – 3.5, result in more difficult modulation - Integration of a park brake is more complicated. 3) Duo duplex drum brake (Illustrated in Figure 72c) - Both brake shoes are actuated by a double acting drum brake cylinder - By means both shoes act as primary during either forward or reverse braking 4) Servo drum brake (Illustrated in Figure 72d) - The wedge pivot point slides in this type of drum brake - Both shoes act as primary during forward braking, whereas just one acts as primary during reverse braking 5) Duo servo drum brake (Illustrated in Figure 72e) - Floating pivot instead of sliding pivot is used - Both shoes are primary during either forward or reverse braking. - This configuration generates very high torques (C* 3.5 – 6.5) - Therefore preferred for vehicles with high payload, for example, small to medium sized commercial vehicles - Used as a mechanically actuated park brake, while the disc brake provides all service brake functions on the rear axle
Figure 72. Different types of drum brakes
Brake drums are generally manufactured from
• Cast iron • Composite casting • Aluminum/ceramic composite casting
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Aluminum brake drums have a low weight but are difficult to manufacture, and their performance range is limited due to their low melting point. Disc Brakes - An important advantage of disc brakes is that the tangential brake force changes linearly with friction coefficient - Ability to withstand greater thermal loads - Consistent response (reproducibility) - More uniform pad wear - Simple self-acting adjustment - Simple pad replacement
C* value of the disc brake (2.) is compared with the C* value of some of the aforementioned drum brake types, in Figure 73.
Figure 73. C* value for different types of drum brakes and disc brake
There are two different brake caliper designs: Fixed caliper and floating caliper. They are both shown in Figure 74.
Figure 74. Floating caliper on the left, fixed caliper on the right.
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Fixed calipers have pistons (minimum two) on both sides of the brake rotor. Floating calipers have pistons on one side only and are mounted in such a way as to permit lateral motion. Clamping forces in the brake caliper are applied in an axial direction to brake pads by means of hydraulic cylinders. Brake pads act on both sides of the flat friction surfaces of the brake rotor. Fixed front axle calipers are the most common form encountered on heavy rear drive passenger cars because these type of vehicles provide ample installation space. Floating and sliding brake calipers are installed deeper (farther outboard) into the wheel. The advantage of floating frame calipers is low fluid temperatures in the cylinder, as the large open pad slots permit good cooling airflow to the pads. Other advantages of sliding calipers are: Require less space Large pad area Optimum pad shape Low weight Small form factor
The C* factor for disc brakes is C* = 2..P.A / P.A = 2
The clamping force is assumed to be acting at the center of the piston.
Typical friction coefficient values for disc brakes fall between = 0.35 – 0.5 (i.e. C* 0.7 - 1) where m is defined as the average operating friction coefficient for any pad material type.
It varies by 10% in response to rotor temperature, vehicle speed, contact pressure, etc.
Brake Disc
About 90% of the energy converted by brakes first enters the disc, from which it is transferred to the surrounding air. The friction ring may reach temperatures up to 700 C, for example in descending a long downgrade. Internally ventilated disc brakes, illustrated in Figure 75, are enjoying increased application due to improved cooling action, and less wear. There are also cross-drilled or slotted brake rotors designed for improved cooling action and additionally for reducing sensitivity to water Disadvantage of these include higher cost and sometimes increased noise generation There are disc designs that have a continuous groove as well, and that are made of composite material as well.
Figure 75. Ventilated disc brake.
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Caliper Material
Sliding calipers are generally cast of ductile iron. For special lightweight requirements, bolted calipers are used, in which the cylinder side is made of high strength cast aluminum. For some applications the entire caliper consists of a single aluminum piece. Fatigue strength of a caliper is assured by hydraulic testing with pulsating pressures, between 0 and 100 bar, for 500000 cycles. Brake pistons are made of steel, cast iron, aluminum alloys or injection molded plastics. In the case of steel pistons deep drawing and extrusion are customary manufacturing methods. In order to assure the required surface finish grinding the outside diameter is imperative.
Friction Material
Friction material used in drum and disk brakes are composed primarily of
• Metals, in fiber or powder form (14%)
• Fillers (ex: inorganic fibers, 23%)
• Polymers (ex: resins, rubbers, organic fibers, and fillers, 35%)
• Solid lubricants (28%)
6.8 Brake Proportioning Valves
In order to achieve short braking distances, it is necessary to make the best possible use of rear brake power. However rear wheels should not be locked up before the front wheels. This requirement is met by smaller rear brake dimensions than those of the front wheels.
In general because of the non-linear nature of the ideal brake force distribution curve, smaller dimensioning alone is insufficient, so that a so called brake proportioning valve is employed.
The most basic version of this pressure limiting mechanism is the brake pressure
limiter. It limits the output side pressure transmitted to the wheel brakes to a pre-
determined cut-off pressure, such as shown in Figure 76a.
On the other hand, brake proportioning valves (also known as brake pressure
regulators) are applied to vehicles in which only minimal axle load changes are
expected.
They have a fixed inflection or split point above which the rear brake line pressure is
reduced by a fixed ratio compared to front line pressure, such as shown in Figure 76b.
Load-sensing (also known as height sensing) proportioning valves are often used in
vehicles that may encounter extreme changes in axle load due to Cargo load.
Installation of load sensing proportioning valves is also indicated for small cars with
short wheelbase and high center of gravity that exhibit strongly deceleration dependent
weight transfer effects. Vehicle loading is indirectly sensed by means of the vehicle
ride height (compression of the suspension springs). When loaded, the reduced space
between body and axle results in increased spring force and therefore a raised
inflection point of the proportioning curve. Figure 76c shows a simplified version of the
load sensing valve, whereas Figure 76d is an illustration of how the pressure is cut,
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and then released again, this time with a lower slope, in order to imitate the ideal brake
force distribution curve as good as possible.
(a) (b)
(c)
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(d)
Figure 76. The brake pressure limiter in (a) cutting, and the brake proportioning valve in (b) restricting the fluid flow to the rear wheel cylinders when the measured pressure in the hydraulic circuit exceeds a certain threshold.73 (c) is a simplified sketch of the load sensing valve. (d) is showing the valve control result illustrating the trial to match the ideal brake force distribution.
How else can we change the brake force distribution? Using the existing valves of the
ABS system. Especially upper class vehicles equipped with brake by wire systems are
equipped with improved solenoid valves that are suitable for continuous manipulation
of the fluid flow to the front and rear axles such that the ideal brake force distribution is
achieved. These types of systems are getting more popular especially among EVs and
HEVs of the last generation, and the aim of reaching an ideal braking force distribution
factor can be achieved in most of the driving conditions.
6.9 Brake Fluid
Within the hydraulic section of the brake system, brake fluid serves as the energy transmission medium between TMC or hydraulic control unit and Wheel brakes.
Additionally, the fluid is tasked with lubricating moving parts such as seals, pistons, and valves and providing these with corrosion protection
Brake fluid should have as low viscosity as possible, even at extremely low temperatures (to -40 C), to provide good brake actuation and release behavior as well as to permit good electronic control system functions. Moreover, brake fluid should have as high a boiling point as possible, so that even the most extreme thermal loading of the brake system does not result in formation of vapor bubbles.
As a result of the limited volumetric capacity of the TMC, compressibility of vapor bubbles would result in inability to build up sufficient brake pressure.
73 Picture source: Reference 15.
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Brake hydraulic is either conventional or silicone based.
Conventional hydraulic, based on polyglycol and polyglycol ether are hygroscopic; they absorb and bind water. In this way water entering the system is not separated and can not boil to form vapor bubbles.
Numerous international standards such as DOT3, DOT4 and DOT5 require that brake hydraulic, when exposed to water exhibit a certain minimum «wet boiling point»
Silicone brake hydraulics meeting the DOT5 standard are based on hydrophobic silicone oil, which can absorb only small traces of water.
Any undissolved water present may boil (formation of vapor bubbles) or lead to component corrosion.
CHAPTER 8 FUEL CONSUMPTION
Villan’s Line Model
This analytical model is used to estimate the fuel consumption map of an engine by scaling from the known map of another engine, by using the known stroke and displacement.
For instance, we know the data given in Table 11, and the bsfc contours belonging to a specific engine, which is shown in Figure 77.
Table 11. Known engine specifications.
Figure 77. The engine power and bsfc data that is known for a specific engine.
The objective of using the Villan’s line model is that by using the given data above for a specific engine, we can estimate the fuel consumption of another engine whose displacement and stroke is known.
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In the given homework problem, that is the engine whose specifications are given in Table 12.
Table 12. The engine that is the subject of the homework problem.
The given code can be modified to calculate and plot the fuel consumption map (in [g/s]) as a function of engine speed (in [rpm]) and torque. In addition, we can calculate and plot the engine efficiency, as a function engine speed and torque.
The required relationships are (also provided in the old notes)
= / (bsfc. H) (19)(VIII – 9 in the notes)
Fuel consumption = Pe . bsfc (20)(VIII – 2 in the notes)
Making use of these relationships, we can reach at the following figures:
Figure 78. The engine efficiency and the fuel consumption (in [g/s]) of the given engine.
Using the engine data provided, we can rescale the axis and plot the fuel consumption map as a function of the mean piston speed (in [m/s]) and mean effective pressure (BMEP, in [bar]).
We can relate mean piston speed to rpm of the engine knowing that one piston stroke
corresponds to radians.
cm = ( S / ) (21)
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Figure 79. Fuel consumption as a function of mean piston speed.
In Part 3, we are asked to calculate and plot the Willan’s line model for this engine, considering the speeds of 1200, 1500, 2000, 2500, 3000, 3500, and 4000 rpm. We are also asked to plot all lines on the same graph.
Willan’s line model is simply a linear model for efficiency. Engine efficiency can be formulated as follows:
.Te = .Pc = . m .Qlhv (22)
where Pc is the combustion power, and Te are the engine speed and torque, m is the fuel consumption of the engine, and Qlhv is the lower heating value of the fuel.
If we model engine power as a function of combustion power such as
Pe = e.Pc – Ploss (23)
Dividing both sides of the equality by engine speed yields
Te = e.Qlhv. m / - Tloss (24)
Using the following normalizations, the dependence on engine size can be avoided
Pme= 4..Te / Vd Pma= 4..Qlhv m / (.Vd) cm = (S/).
yields:
Pme = e.Pma - Pml (25)
where Pma can be defined as the input, or available, or fuel intrinsic mean effective pressure, and Pml can be defined as the mean effective pressure loss, formulated as
Pml = (4. / Vd).Tloss (26)
We are given Pme in the problem, then Pma = Pme /
Doing this for each mean piston speed level, using the command plot3 for generating a 3-D plot, we would reach at
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Figure 80. (a) Willan’s line for each mean piston speed. (b) View from the z-axis, for the 2000 rpm line
In Figure 80a, all the points are the actual points, and the red lines are the linear curve fit. “polyfit” command can be used for the curve fit, which would output two values for the linear fit: y = a.x + b with the first value being a, and the second b.
Willan’s line and the actual data corresponding to an engine speed of 2000 rpm is shown in Figure 80b.
After obtaining the fits for each speed, we can further obtain curve fits for the following quantities: The slope; i.e. “e” in Equation 23. Note that this quantity can be counted as the efficiency of the engine. The y-intercept; i.e. pml is the mean effective pressure loss, that is due to the engine internal mechanical friction. Below are the two trends for each of the quantities.
Figure 81. (a) Willan’s line slope (efficiency) values (b) Willan’s line mechanical loss values.
How would we interpret the trends? The slope trend can be interpreted as follows: The efficiency increases at a higher rate as the torque level (or the mean effective pressure) it generates increases, at medium engine speeds (2000 - 3500 rpm).
On the other hand, the y-intercept; i.e. engine mechanical friction increases as mean piston speed increases.
We can also look at how the maximum mean affective pressure changes as a function of mean piston speed. A cubic curve fit yields Figure 82 for this quantity.
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Figure 82. Maximum mean effective pressure as a function of mean piston speed.
Note that this curve represents the maximum torque curve of Figure 77 and Figure 78.
Now we are ready to estimate and plot the fuel consumption map (in [g/s]) as a function of engine speed and torque, using these approximated curve fits for the Willan’s line
model. All we need to do is to leave m alone in Equation 25. This yields:
D mme loss
lhv
V .cm p p
4slopeQ S (27)
Figure 71 shows the actual map with the Villan’s line approximation.
Figure 83. Actual and the approximated fuel consumption map of the engine.
The main discrepancy is the maximum torque curve. Other than this, the fuel consumption contours are not too bad.
Based on the approximate Villan’s line model, we can estimate and plot the fuel consumption map for an engine of similar characteristics (Table 12). In order to do that, we need to do the three scalings as follows:
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dnewD mme loss new
lhv d new
de new
d dnew
newm mnew m
V .SV .cm p p m m
4slopeQ S V S
V4bmep T bmep bmep
V V
SSc c c
S
(28)
Once we find all three new quantities, we can compute back the torque (from bmep) and engine speed (from mean piston speed) for the new engine, and plot the new fuel consumption map:
Figure 84. Approximated fuel consumption map of the new engine.
REFERENCES 1. ME-465 Lecture Notes, “Automotive Engineering”, by Prof. Y. Samim Ünlüsoy, Middle East Technical University, Mechanical Engineering Department, 2016. 2. Lecture “Kraftfahrzeuge 1”, taught by Prof Hermann Winner, Automotive Engineering Institute, Technische Universitat Darmstadt, 2006. 3. Lecture “Energy Modeling of Hybrid Electric Vehicles” taught by Prof. Yann Guezennec, Mechanical Engineering Department, The Ohio State University, Fall 2007. 4. Fundamentals of Physics, authored by Halliday, Resnick and Walker, 6th Extended International Edition, ISBN 0-471-33235-6. 5. “Principles & Applications of Electrical Engineering” authored by Prof. Giorgio Rizzoni, ISBN 9780071217729. 6. “Generatoren und Starter, Robert Bosch GmbH”, 2002, ISBN: 3865220258 and “Autoelektrik / Autoelektronik”, Robert Bosch GmbH, 2002, ISBN: 9783322915610. 7. Lecture “Kraftfahrzeuge 2”, taught by Prof Hermann Winner, Automotive Engineering Institute, Technische Universitat Darmstadt, 2006. 8. SAE Technical Paper 2008-01-0868. “Cleaner Diesel Using Model-Based Design
and Advanced Aftertreatment in a Student Competition Vehicle”, 2008.
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9. Lecture “Powertrain Dynamics”, taught by Assoc. Prof. Marcello Canova, Dr. Fabio Chiara and Prof. Krishnaswamy Srinivasan, Mechanical Engineering Department, The Ohio State University, 2008. 10. Motorlu Araçlar Teknolojisi, Egzoz Emisyon Kontrolü, 525MT0300, Milli Eğitim Bakanlığı, 2011. 11. Motor Vehicle Dynamics Modeling and Simulation, authored by Giancarlo Genta, 3rd Edition, World Scientific, Singapore. ISBN 9810229119. 12. White, R.A. and Korst, H.H., “The Determination of Vehicle Drag Contributions from Coastdown Tests,” SAE Transactions, Vol. 81, paper 720099, 1972. 13. Vehicle Dynamics and Control, authored by Rajesh Rajamani, 2nd Edition, published by Springer ISBN 978-1-4614-1432-2 e-ISBN 978-1-4614-1433-9. 14. Handbook of Automotive Engineering, edited by Hans Hermann Braess and Ulrich Sieffert, translated by Peter Albrecht., 2005, ISBN 0-7680-0783-6. 15. Electude website, available at electude.com.
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