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Fuel Injection Systems in SI Engines
Carburetion
http://web.iitd.ac.in/~jpsm/ICE-ME345-ME411N/SI_Engine_Mixture_Preparation.ppt
Perhaps soon to be obsolete?
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Mixture Requirements
Engine induction and fuel system must preparea fuel-air mixture that satisfies therequirements of the engine over its entireoperating regime.
Optimum air-fuel ratio for an SI engine is thatwhich gives
1. required power output2. with lowest fuel consumption
3. consistent with smooth and reliable operation
2
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Mixture Requirements (Continued)
The constraints of emissions may dictate adifferent air-fuel ratio
also require recycling some exhaust gas(EGR)
Relative proportions of fuel and air that give theabove requirements depend on engine speedand load.
Mixture strength is given in terms of air-fuel orfuel-air ratio or equivalence ratio.
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Mixture Requirements (Continued)
Mixture requirements are different for full load (wide-open throttle or WOT) and for part-load operation.
At full load, complete utilization of inducted airto obtainmaximum power for a given displaced volume is the
critical issue.
At part-load at a given speed, efficient utilization of fuelis the critical issue.
As seen in the next slide, at WOT, maximum power for agiven volumetric efficiency is obtained at a mixtureslightly richer than stoichiometric, 1.1
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Mixture Requirements (Continued)At part-load (or part-throttle) it is advantageous to dilute the
fuel-air mixture with excess air or with recycled exhaustgas. This dilution improves fuel conversion efficiency forthree reasons:
1. The expansion stroke work is increased for a given
expansion ratio due to the change in thermodynamicproperties,
2. For a given mean effective pressure, the intake pressureincreases with increasing dilution, so pumping workdecreases,
3. Heat losses to the walls are reduced because the burnedgas temperatures are lower.
In the absence of strict NOxemission control, excess air is theobvious diluent at part load and the engine runs lean
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Requirements with emission control
For control of NO, HC and CO, operating the engine withstoichiometric mixture is advantageous so that a three-way catalystcan be used for emission control. In such acase, for further decrease in NO the diluent used is EGR.
Amount used will depend on the EGR tolerance of theengine at a given speed and load based on the details ofthe engine combustion process.
Increasing excess air or EGR will slow down thecombustion process and increase combustion variabilityso as load decreases, less dilution must be provided andat idle, no EGR may be used and mixture will have to bemade rich.
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What is carburetion?
The process of formation of a combustible fuel-airmixture by mixing the proper amount of fuel
with air before it is admitted into the enginecylinder.
Comes from the words car and burettebecause the carburetormetersthe appropriatequantity of liquid fuel (like a burette) and mixedit with air before sending the mixture into the
engine cylinder. 10
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Factors affecting Carburetion1. Engine speed. In a 4-stroke engine running at 3000
rev/min, the intake will take about 10 ms during which thefuel has to evaporate, mix with air and be inducted into theengine.
2. Vaporization characteristics of the fuel. Will require avolatile fuel for quick evaporation and mixing with air.
3. The temperature of the in coming air. Must be high enough
to be able to evaporate the fuel and yet not too high as toreduce mass of fresh charge.
4. Design of the carburetor. This will help in properintroduction of fuel into the air stream and provide proper
distribution of the mixture to the various cylinders. 11
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12
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Calculation of Air-fuel Ratio
Applying the steady flow energy equation tosections A-A and B-B per unit mass flow of air:
Here, q and w are the heat and work transfersfrom the entrance to the throat and h and Cstand for enthalpy and velocity respectively.
If we assume reversible adiabatic conditions, andthere is no work transfer, q=0, w=0, and ifapproach velocity C10 we get
2122122
1CChhwq
13
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212 2 hhC
212 2 TTcC
writecanwehenceTch
getwegasperfectabetoassumedisairIf
p
p
1
1
2121
1
1
2
1
2
1
p
pTTT
p
p
T
T
then
isentropicbetothroattoinletfromflowAssume
14
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Substituting for T1T2 from Eq. 5 in Eq. 3, we get
)6(12
1
1
212
ppTcC p
By the continuity equation we can write down the theoretical mass flow rate of air
)7(222111
.
CACAm a
where A1and A2are the cross-sectional areas at the air inlet (point 1)
and venturi throat (point 2).
To calculate the mass flow rate of air at the throat, we have assumed the flow to be
isentropic till the throat so the equation relating p and v (or )can be used.
)8(2211 Avpvp
)8(
2
2
1
1 Bpp
1
1
2
12
p
p
15
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)9(12
1
1
2
12
1
1
2
1
.
p
pTcA
p
pm
pa
For a perfect gas we have )9(1
11 A
RT
p
)10(12
1
1
2
12
1
1
1
1
2.
AppTcA
RTp
ppm
pa
Thus
and rearranging the above equation we have
)10(2
1
1
2
2
1
2
1
12.
Bp
p
p
pc
TR
pAm
pa
16
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Since the fluid flowing in the intake is air, we can put in the approximate
values of R = 287 J/kgK, cp= 1005 J/kgK and = 1.4 at 300K.
)11(1562.0
1562.0
1
12
71.1
1
2
43.1
1
2
1
12.
T
pA
p
p
p
p
T
pAm
a
where
71.1
1
2
43.1
1
2
p
p
p
p
Here, pressure p is in N/m2, area A is in m2,and temperature T is in K.
If we take the ambient temperature T1 = 300Kand ambient pressure
p1= 105N/m2, then
)12(8.901 2
.
Ama 17
E ti 11 i th th ti l fl t f i Th t l fl t
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Equation 11 gives the theoretical mass flow rate of air. The actual mass flow rate,
am., can be obtained by multiplying the equation by the coefficient of discharge
for the venturi, Cd,a. Thus
)13(1562.0
1
12
,
.
T
pACm ada
where )14(.
.
,
a
a
ad
m
mC
The coefficient of discharge and area are both constant for a given venturi, thus
)15(
1
1. T
pma
Since we have to determine the air-fuel ratio, we now calculate the fuel flow rate.
The fuel is a liquid before mixing with the air, it can be taken to be incompressible.
We can apply Bernoullis equation between the atmospheric conditions prevailing
at the top of the fuel surface in the float bowl, which corresponds to point 1 and
the point where the fuel will flow out, at the venturi, which corresponds to point 2.
Fuel flow will take place because of the drop in pressure at point 1 due to the
venturi effect. Thus 18
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)16(2
2
21 gzCpp f
ff
where fis the density of the fuel in kg/m3, Cfis the velocity of the fuel
at the exit of the fuel nozzle (fuel jet), and z is the depth of the jet exitbelow the level of fuel in the float bowl. This quantity must always be
above zero otherwise fuel will flow out of the jet at all times. The value
of z is usually of the order of 10 mm.
From Eq. 16 we can obtain an expression for the fuel velocity at the jet exit as
)17(2 21
gz
ppC
f
f
Applying the continuity equation for the fuel, we can obtain the theoretical
mass flow rate,.
fm
)18(221
.
gzppA
CAm
fff
ffff
19
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where Afis the exit area of the fuel jet in m2. If Cd,fis the coefficient of discharge
of the fuel nozzle (jet) given by
)19(.
.
,
f
f
fd
m
mC
then .
21, )20(2 gzppACm
ffffdf
Since)21(
.
.
f
a
m
m
F
A
Fuel
Air
)22(
21562.0
211
12
,
,
gzppTp
AA
CC
FA
ffffd
ad
If we put21
pppa , we get the following equation for the air-fuel ratio20
C
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)23(2
,
,
gzp
p
A
A
C
C
F
A
fa
a
f
a
ffd
ad
where
)24(
1
1
2
1
1
2
1
1
2
2
1
2
p
p
p
p
p
p
For the normal carburetor operating range, where 1.0
1
p
pa
, the effects of compressibility which reduce below 1.0 are small.
The equivalence ratio, ,where
)25(
F
A
F
A
s
21
A
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is given by )26(12
1
2
,
,
a
f
f
a
ffd
ads
p
gz
A
A
C
CF
A
In Eq. 22, if we take T1= 300K and p1= 105N/m2then
)27(
28.901
21
2
,
,
gzppA
A
C
C
F
A
ffffd
ad
The coefficient of discharge defined in Eq 19 represents the effect of all deviations
from the ideal one-dimensional isentropic flow. It is influenced by many factors of
which the most important are: 1.Fluid mass flow rate,
2.Orifice length-to-diameter ratio,
3.Orifice area-to-approach area ratio,4.Orifice surface area,
5.Orifice surface roughness,
6.Orifice inlet and exit chamfers,
7.Fluid specific gravity,
8.Fluid viscosity, and
9.Fluid surface tension. 22
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The use of the orifice Reynolds number
as a correlating parameter for the
coefficient of discharge accounts for theeffects of mass flow rate, fluid densityand viscosity, and length scale to a goodapproximation. The discharge coefficient
of a typical carburetor main fuel-meteringsystem orifice increases smoothly withincreasing orifice Reynolds number, Reo.
)28(Re
oo
VD
23
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Air-fuel ratio neglecting
compressibility of air If we assume air to be incompressible,
then we can apply Bernoullis equation to
air flow also. Since initial velocity isassumed zero, we have
)29(
2
2
221 Cpp
aa
Thus
Thus)30(2 21
2
a
ppC
24
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Applying the continuity equation for the fuel, we can obtain the theoretical mass
flow rate,
.
am, from
)31(2212
22
.
ppA
CAm
a
aa
where A2is the venturi in m2. If Cd,ais the coefficient of discharge of the
venturi given by)32(
.
.
,
a
a
ad
m
mC
then .
212,
.
)33(2 ppACm aada
Since )34(.
.
f
a
m
m
F
A
Fuel
Air
25
AC
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)35(21
212
,
,
gzpp
pp
A
A
C
C
F
A
ff
a
ffd
ad
)35(21
212
,
,A
gzpp
pp
A
A
C
C
F
A
ff
a
ffd
ad
If we assume z = 0, then
)36(2
,
,
f
a
ffd
ad
A
A
C
C
F
A
26
Carburetor Performance
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Carburetor Performance In Eq. 26, the terms A1, A2, a, and f are all
constant for a given carburetor, fuel, and ambient
conditions. Also, for very low flows, pa fgz.However, the discharge coefficients Cd,aand Cd,fand , all vary with flow rate. Hence, theequivalence ratio delivered by an elementary
carburetor is not constant. Figure shows the performance of an
elementary carburetor. The top graph shows thevariation of Cd,aand Cd,fand with the venturi
pressure drop. For pa fgz, there is no fuelflow. Once fuel starts to flow, the fuel flow rateincreases more rapidly than the air flow rate. Thecarburetor delivers a mixture of increasing
equivalence ratio as the flow rate increases. 27
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28
Disc ssion of Fig e
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Discussion of Figure Suppose the venturi and fuel orifice (jet) are sized
to give a stoichiometric mixture at an air flow ratecorresponding to 1 kN/m2venturi pressure drop(middle graph of Fig). At higher flow rates, thecarburetor will deliver a fuel-rich mixture. At very
high flow rates the carburetor will deliver anessentially constant equivalence ratio. At lower airflow rates, the mixture delivered leans out rapidly.
Thus, the elementary carburetor cannot providethe variation in mixture ratio which the enginerequires over the complete load range at anygiven speed.
29
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Summary of the Deficiencies of
the Elementary Carburetor1. At low loads, the mixture becomes leaner; the engine requires
the mixture to be enriched at low loads. The mixture is richestat idle.
2. At intermediate loads, the equivalence ratio increases slightly asthe air flow rate increases; the engine requires an almost
constant equivalence ratio.3. As the air flow approaches the maximum (WOT) value, the
equivalence ratio remains essentially constant; the enginerequires an equivalence ratio of about 1.1 at maximum enginepower.
4. The elementary carburetor cannot compensate for transientphenomena in the intake manifold. It also cannot provide a richmixture during engine starting and warm-up.
5. It cannot adjust to changes in ambient air density due tochanges in altitude.
30
Modern Carburetor Design
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Modern Carburetor DesignThe changes required in the elementary carburetor so that it
provides the equivalence ratio required at various air flowrates are as follows.
1. The main metering systemmust be compensated to provide aconstant lean or stoichiometric mixture over 20 to 80% ofthe air flow range.
2. An idle system must be added to meter the fuel flow at idleand light loads to provide a rich mixture.
3. An enrichment systemmust be provided so that the engine
can get a rich mixture as WOT conditions is approached andmaximum power can be obtained.4. An accelerator pumpmust be provided so that additional fuel
can be introduced into the engine only when the throttle issuddenly opened.
5. A chokemust be added to enrich the mixture during coldstarting and warm-up to ensure that a combustible mixture isprovided to each cylinder at the time of ignition.
6. Altitude compensationis necessary to adjust the fuel flowwhich makes the mixture rich when air density is lowered.
7. Increase in the magnitude of the pressure drop available forcontrolling the fuel flow is provided by introducing boostventuris (Venturis in series) or Multiple-barrel carburetors(Venturis in parallel). 31
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Fuel injection system in SIengines
32
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The point or location of fuel injection is one way toclassify a gasoline injection system. A single-pointinjection system, also call throttle body injection (TBI), has the injector nozzles in a throttle bodyassembly on top of the engine. Fuel is sprayed into
the top center of the intake manifold.
33
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A multi-point injection system, also called portinjection, has an injector in the port (air-fuel passage)
going to each cylinder. Gasoline is sprayed into eachintake port and toward each intake valve. Thereby,the term multipoint (more than one location)
fuel injection is used.
34
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An indirect injection systemsprays fuel into theengine intake manifold.Most gasoline injectionsystems are of this type.
Direct injection forces fuel
into the engine combustionchambers. Diesel injection
systems are direct type.
SoGasoline electronic Direct Injection System
is Classified as: multi-point and Direct injection systems
35
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System component:
Fuel tank
Electric fuel pump
Fuel filter Electronic control unit
Common rail and Pressure sensor
Electronic Injectors fuel line
37
F l t k
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Fuel tank is safe container for flammable liquids and
typically part of an engine system in which thefuel is stored and propelled (fuel pump) orreleased (pressurized gas) into an engine.
Typically, a fuel tank must allow or provide the
following:* Safe (UL Approved) fuel storage, there is some
concern that UL (Underwriters Laboratories) isnot the final arbiter of safety.
* Filling (the fuel tank must be filled in a secureway) No Sparks.
* Storage of fuel (the system must contain a givenquantity of fuel and must avoid leakage and limit
evaporative emissions) 38
* Provide a method for determining level of fuel in tank
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Provide a method for determining level of fuel in tank,Gauging (the remaining quantity of fuel in the tankmust be measured or evaluated)
* Venting (if over-pressure is not allowed, the fuelvapors must be managed through valves)
* Feeding of the engine (through a pump)
* Anticipate potentials for damage and provide safesurvival potential.
39
l f l
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Electric fuel pump An electric fuel pump is used on engines with fuel
injection to pump fuel from the tank to the injectors.The pump must deliver the fuel under high pressure(typically 30 to 85 psi depending on the application)so the injectors can spray the fuel into the engine.
Electric fuel pumps are usually mounted inside the fuel
tank, Some vehicles may even have two fuel pumps (a
transfer pump inside the tank, and a main fuel pumpoutside).
40
Electric fuel pumps come in a variety of designs. Some
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Electric fuel pumps come in a variety of designs. Someolder applications use a positive displacement "rollercell" pump. This type uses rollers mounted on anoffset disc that rotates inside a steel ring. Fuel is
drawn into the spaces (cells) between the rollers andpushed along from the pump inlet to the outlet. Thistype of pump can generate very high pressure, andthe flow rate tends to be constant. But the output
comes in pulses, so a muffler is often mounted in thefuel line after the pump to dampen pressure pulses. Aroller cell pump may also be mounted outside the fueltank, and used with a second low pressure supply
pump mounted inside the fuel tank.
41
Most newer vehicles use a "turbine" style fuel pump A
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Most newer vehicles use a turbine style fuel pump. Aturbine pump has an impeller ring attached to themotor. The blades in the impeller push the fuel
through the pump as the impeller spins. This type ofpump is not a positive-displacement pump, so itproduces no pulsations, runs very smoothly andquietly. It is also less complicated to manufacture and
is very durable. Some aftermarket pump supplies usethis type of pump to replace the older designs.
42
F l filt
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Fuel filter The fuel filter is the fuel system's primary line of
defense against dirt, debris and small particles of rustthat flake off the inside of the fuel tank .
many filters for fuel injected engines trap particles as
small as 10 to 40 microns in size.
fuel filter normally made into
cartridges containing a filter paper.
43
El t i t l it
http://www.aa1car.com/library/fuel_filters.htmhttp://www.aa1car.com/library/fuel_filters.htm8/14/2019 S.I. Engine Mixture Preparation.ppt
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Electronic control unit In automotive electronics, electronic control unit
(ECU) is a generic term for any embedded systemthat controls one or more of the electrical systems orsubsystems in a motor vehicle.
An engine control unit (ECU), also known aspower-train control module (PCM), or enginecontrol module (ECM)is a type of electronic controlunit that determines the amount of fuel, ignition
timing and other parameters an internal combustionengine needs to keep running. It does this by readingvalues from multidimensional maps which containvalues calculated by sensor devices monitoring the
engine. 44
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Working of ECU Control of fuel injection:ECU will determine the
quantity of fuel to inject based on a number ofparameters. If the throttle pedal is pressed furtherdown, this will open the throttle body and allow moreair to be pulled into the engine. The ECU will inject
more fuel according to how much air is passing intothe engine. If the engine has not warmed up yet,more fuel will be injected .
Control of ignition timing :A spark ignition enginerequires a spark to initiate combustion in thecombustion chamber. An ECU can adjust the exacttiming of the spark (called ignition timing) to providebetter power and economy.
45
Control of idle speed : Most engine systems have
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Control of idle speed: Most engine systems haveidle speed control built into the ECU. The engine RPMis monitored by the crankshaft position sensor which
plays a primary role in the engine timing functions forfuel injection, spark events, and valve timing. Idlespeed is controlled by a programmable throttle stop or
an idle air bypass control stepper motor.
46
C il d P
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Common rail and Pressure sensor
The term "common rail" refers to the fact that all of
the fuel injectors are supplied by a common fuel railwhich is nothing more than a pressure accumulatorwhere the fuel is stored at high pressure. Thisaccumulator supplies multiple fuel injectors with high
pressure fuel.
47
The fuel injectors are typically ECU-controlled.
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The fuel injectors are typically ECU controlled.When the fuel injectors are electricallyactivated a hydraulic valve (consisting of a
nozzle and plunger) is mechanically orhydraulically opened and fuel is sprayed intothe cylinders at the desired pressure. Since thefuel pressure energy is stored remotely and the
injectors are electrically actuated the injectionpressure at the start and end of injection isvery near the pressure in the accumulator(rail), thus producing a square injection rate. Ifthe accumulator, pump, and plumbing are sizedproperly, the injection pressure and rate will bethe same for each of the multiple injection
events. 48
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49
Electronic Injectors
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Electronic Injectors
The injectors can survive the excessive temperature
and pressure of combustion by using the fuel thatpasses through it as a coolant
50
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The electronic fuel injector is normally closed, and
opens to inject pressurized fuel as long as electricity isapplied to the injector's solenoid coil.
When the injector is turned on, it opens, sprayingatomized fuel at the combustion chamber .
Depending on engine operating condition ,injection
quantity will vary .
51
fuel line
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fuel line Fuel line hoses carry gasoline from the tank to
the fuel pump, to the fuel filter, and to the fuelinjection system. While much of the fuel linesare rigid tube, sections of it are made of rubberhose, which absorb engine and road vibrations.
There are two basic types of fuel hose: Fuel andoil hoses that meet the SAE 30R7 standard, andfuel injection hose that meets the requirementsof SAE 30R9.
52
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Gasoline direct injection
In internal combustion engines, gasolinedirect injectionis a variant of fuel injectionemployed in modern two- and four- stroke petrolengines. The petrol/gasoline is highlypressurized, and injected via a common rail fuelline directly into the combustion chamber of
each cylinder, as opposed to conventional multi-point fuel injection that happens in the intaketract, or cylinder port.
53
H t k
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How system work:
54
When the driver turns the ignition key on,
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g y ,the power train control module (PCM)energizes a relay that supplies voltage to
the fuel pump. The motor inside the pumpstarts to spin and runs for a few secondsto build pressure in the fuel system. Atimer in the PCM limits how long the pump
will run until the engine starts. Fuel is drawn into the pump through an
inlet tube and mesh filter sock
The fuel then exits the pump through aone-way check valve and is pushedtoward the engine through the fuel line