46046turbocharging

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Turbocharging

Introduction


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Fundamentals

The turbocharger's basic functions havenot fundamentally changed since the timesof Alfred Bchi, who first patented the

exhaust-driven supercharger in 1905. A turbocharger consists of a compressor

and a turbine connected by a common

shaft. The exhaust-gas-driven turbinesupplies the drive energy for thecompressor.


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Components


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Compressor

Design and functionTurbocharger compressors are generally centrifugalcompressors consisting of three essential components:compressor wheel, diffuser, and housing. With the

rotational speed of the wheel, air is drawn in axially,accelerated to high velocity and then expelled in a radialdirection.

The diffuser slows down the high-velocity air, largelywithout losses, so that both pressure and temperature

rise. The diffuser is formed by the compressor backplateand a part of the volute housing, which in its turn collectsthe air and slows it down further before it reaches thecompressor exit.


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Compressor characteristics

Operating characteristics

The compressor operating behaviour is

generally defined by maps showing therelationship between pressure ratio and

volume or mass flow rate. The useable

section of the map relating to centrifugal

compressors is limited by the surge andchoke lines and the maximum permissible

compressor speed.


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Choke line

Choke lineThe maximum centrifugal compressorvolume flow rate is normally limited by the

cross-section at the compressor inlet.When the flow at the wheel inlet reachessonic velocity, no further flow rate increaseis possible. The choke line can berecognised by the steeply descendingspeed lines at the right on the compressormap.


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Turbine

The turbine wheel is

made from a high nickel

superalloy investment

casting. This method

produces accurate

turbine blade sections

and forms. Larger units

are cast individually. For

smaller sizes a foundrycan cast multiple wheels

using a tree configuration.


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Turbine side

Design and function

The turbocharger turbine, which consists of a

turbine wheel and a turbine housing, converts

the engine exhaust gas into mechanical energyto drive the compressor.

The gas, which is restricted by the turbine's flow

cross-sectional area, results in a pressure and

temperature drop between the inlet and outlet.This pressure drop is converted by the turbine

into kinetic energy to drive the turbine wheel.


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Turbine types

There are two main turbine types: axial andradial flow. In the axial-flow type, flow throughthe wheel is only in the axial direction. In radial-flow turbines, gas inflow is centripetal, i.e. in a

radial direction from the outside in, and gasoutflow in an axial direction.

Up to a wheel diameter of about 160 mm, onlyradial-flow turbines are used. This corresponds

to an engine power of approximately 1000 kWper turbocharger. From 300 mm onwards, onlyaxial-flow turbines are used. Between these twovalues, both variants are possible.


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Gas energy conversion

The radial-flow turbine is the most popular typefor automotive applications.

In the volute of such radial or centripetal

turbines, exhaust gas pressure is converted intokinetic energy and the exhaust gas at the wheelcircumference is directed at constant velocity tothe turbine wheel. Energy transfer from kinetic

energy into shaft power takes place in theturbine wheel, which is designed so that nearlyall the kinetic energy is converted by the time thegas reaches the wheel outlet.


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Behaviour

The turbine's characteristic behaviour isdetermined by the specific flow cross-section,the throat cross-section, in the transition area of

the inlet channel to the volute. By reducing thisthroat cross-section, more exhaust gas isdammed upstream of the turbine and the turbineperformance increases as a result of the higherpressure ratio. A smaller flow cross-section

therefore results in higher boost pressures.The turbine's flow cross-sectional area can beeasily varied by changing the turbine housing.


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Turbine design

Besides the turbine housing flow cross-

sectional area, the exit area at the wheel

inlet also influences the turbine's mass

flow capacity. The machining of a turbine

wheel cast contour allows the cross-

sectional area and, therefore, the boost

pressure, to be adjusted. A contourenlargement results in a larger flow cross-

sectional area of the turbine.


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Parameters

In practice, the operating characteristics ofexhaust gas turbocharger turbines aredescribed by maps showing the flow

parameters plotted against the turbinepressure ratio. The turbine map shows themass flow curves and the turbineefficiency for various speeds. To simplifythe map, the mass flow curves, as well asthe efficiency, can be shown by a meancurve .


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Efficiency

For a high overall turbocharger efficiency,

the co-ordination of compressor and

turbine wheel diameters is of vital

importance. The position of the operating

point on the compressor map determines

the turbocharger speed. The turbine wheel

diameter has to be such that the turbineefficiency is maximised in this operating

range.


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Twin entry Turbines

The turbine is rarely subjected to constant exhaustpressure. In pulse turbocharged commercial dieselengines, twin-entry turbines allow exhaust gas pulsationsto be optimised, because a higher turbine pressure ratio

is reached in a shorter time. Thus, through theincreasing pressure ratio, the efficiency rises, improvingthe all-important time interval when a high, more efficientmass flow is passing through the turbine. As a result ofthis improved exhaust gas energy utilisation, the

engine's boost pressure characteristics and, hence,torque behaviour is improved, particularly at low enginespeeds.


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Twin entry

To prevent the variouscylinders from interfering witheach other during the chargeexchange cycles, cylindersare connected together into

one exhaust gas manifold.Twin-entry turbines then allowthe exhaust gas flow to be fedseparately through the turbine.For example, a six-cylinderengine will use two 3-into-1

manifolds, and each manifoldthen feeds one turbo entry.


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Boost regulation

Control by turbine-side bypassThe turbine-side bypass is the simplest form of boostpressure control. The turbine size is chosen such thattorque characteristic requirements at low engine speeds

can be met and good vehicle driveability achieved. Withthis design, more exhaust gas than required to producethe necessary boost pressure is supplied to the turbineshortly before the maximum torque is reached.Therefore, once a specific boost pressure is achieved,

part of the exhaust gas flow is fed around the turbine viaa bypass. The wastegate which opens or closes thebypass is usually operated by a spring-loadeddiaphragm in response to the boost pressure.


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Modulated control

Electronic boost pressure controlsystems are increasingly used inmodern engines. When comparedwith purely pneumatic control,which can only function as a full-load pressure limiter, a flexible

boost pressure control allows anoptimal part-load boost pressuresetting.

This operates in accordance withvarious parameters such ascharge air temperature, degree oftiming advance and fuel quality.The operation of the flapcorresponds to that of thepreviously described actuator. Theactuator diaphragm is subjected toa modulated control pressureinstead of full boost pressure.


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Control pressures

This control pressure is lower than the

boost pressure and generated by a

proportional valve. This ensures that the

diaphragm is subjected to the boost

pressure and the pressure at the

compressor inlet in varying proportions.

The proportional valve is controlled by theengine electronics.


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Manifold blow-off valves

Blowoff valves are used to prevent compressor surge.Compressor surge is a phenomenon that occurs whenlifting off the throttle of a turbocharged car (with a non-existent or faulty bypass valve). When the throttle plateon a turbocharged engine running boost closes, highpressure in the intake system has nowhere to go. It isforced to travel back to the turbocharger in the form of apressure wave. This results in the wheel rapidlydecreasing speed and stalling. In extreme cases thecompressor wheel will stop completely or even go

backwards.C

ompressor surge is very hard on thebearings in the turbocharger and can significantlydecrease its lifespan. In addition, the now slower movingcompressor wheel takes longer to spool up when throttleis applied.


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Blow-off valves

With the installation of either a bypass valve or a blow-offvalve the pressurized air ia allowed to escape toatmosphere, allowing the turbo to continue spinning.This allows the turbocharger to have less turbo lag when

power is demanded next. Blow-off valves are not suitable for carburettor-equipped

engines, where the turbo is fitted as part of a suck-through installation, as the manifold mixture beingvented to atmosphere will contain fuel.

Blow-off valves also tend to make a tweeting noise, oftenassociated with an imbecile behind the wheel. This isespecially true if the blow-off valve has been equippedwith a little trumpet to amplify the sound.


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Variable Geometry

A Variable Turbine Geometryturbocharger is also known asa variable geometryturbocharger, or a VariableNozzle Turbine (VNT). A

turbocharger equipped withVariable Turbine Geometryhas little movable vanes whichcan direct exhaust flow ontothe turbine blades. The vaneangles are adjusted via an

actuator. The angle of thevanes vary throughout theengine RPM range to optimizeturbine behaviour.


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Low speed operation

In this cut-through diagram,you can see the direction ofexhaust flow when the variablevanes are in an almost closedangle. The narrow passage of

which the exhaust gas has toflow through accelerates theexhaust gas towards theturbine blades, making themspin faster. The angle of thevanes also directs the gas to

hit the blades at the properangle.


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High speed operation

The diagram on the

right shows how the

VGT vanes look like

when they are open. This is better for high

speed / high exhaust

pressure operation,

as it preventsoverspeed.


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High speed operation

This cut-throughdiagram shows theexhaust gas flow

when the variableturbine vanes are fullyopen. The highexhaust flows at highengine speeds are

fully directed onto theturbine blades by thevariable vanes.


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Bearings


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Bearings

The turbocharger shaft and turbine wheelassembly rotates at speeds up to 300,000 rpm.Turbocharger life should correspond to that of

the engine, which could be 1,000,000 miles for acommercial vehicle, and can be upwards of250,000 miles for a passenger car engine. Raceengines generally have much shorter lifespans,but are usually under significantly more strain.

Only sleeve bearings specially designed forturbochargers can meet these high requirementsat a reasonable cost.


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Bearings

Radial bearing systemWith a sleeve bearing, the shaft turns without friction onan oil film in the sleeve bearing bushing. For theturbocharger, the oil supply comes from the engine oilcircuit. The bearing system is designed such that brassfloating bushings, rotating at about half shaft speed, aresituated between the stationary centre housing and therotating shaft. This allows these high speed bearings tobe adapted such that there is no metal contact betweenshaft and bearings at any of the operating points.

Besides the lubricating function, the oil film in the bearingclearances also has a damping function, whichcontributes to the stability of the shaft and turbine wheelassembly.


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Bearings

The one-piece bearing system is a special form of asleeve bearing system. The shaft turns within astationary bushing, which is oil scavenged from theoutside. The outer bearing clearance can be designedspecifically for the bearing damping, as no rotation takesplace.

The hydrodynamic load-carrying capacity and thebearing damping characteristics are optimised by theclearances. The lubricating oil thickness for the innerclearances is therefore selected with respect to the

bearing strength, whereas the outer clearances aredesigned with regard to the bearing damping. Thebearing clearances are only a few hundredths of amillimetre.


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Ball Bearings

More recently, Turbocharger shafts have

been supported by small ball-bearing

races. This reduces the drag on the shafts

caused by the lubricant used in a plain

bearing and allows faster spool-up times.


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Axial-thrust bearing system

Neither the fully floating bushing bearings nor the single-piece fixed floating bushing bearing system supportforces in axial direction. As the gas forces acting on thecompressor and turbine wheels in axial direction are of

differing strengths, the shaft and turbine wheel assemblyis displaced in an axial direction. The axial bearing, asliding surface bearing with tapered lands, absorbs theseforces. Two small discs fixed on the shaft serve ascontact surfaces. The axial bearing is fixed in the centre

housing. An oil-deflecting plate prevents the oil fromentering the shaft sealing area.


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Oil drain

The lubricating oil flows into the turbocharger at apressure of approximately 4 bar. As the oil drains off atlow pressure, the oil drain pipe diameter must be muchlarger than the oil inlet pipe. The oil flow through the

bearing should, whenever possible, be vertical from topto bottom. The oil drain pipe should be returned into thecrankcase above the engine oil level. Any obstruction inthe oil drain pipe will result in back pressure in thebearing system. The oil then passes through the sealing

rings into the compressor and the turbine. The oil willalso oxidise due to the high temperatures, causing shaftdeposits and inadequate lubrication.


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Sealing

The centre housing must be sealed against thehot turbine exhaust gas and against oil loss fromthe centre housing. A piston ring is installed in a

groove on the rotor shaft on both the turbine andcompressor side. These rings do not rotate, butare firmly clamped in the centre housing. Thiscontactless type of sealing, a form of labyrinthseal, makes oil leakage more difficult due to

multiple flow reversals, and ensures that onlysmall quantities of exhaust gas escape into thecrankcase.


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Part Two

Cooling


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Cooling considerations

If we are to force in a greater charge of

fuel and air, then, assuming we manage to

combust it properly, we shall release more

energy, in the form of heat, light and

sound, and hopefully we will manage to

convert a significant proportion of this

energy into Kinetic energy as a result ofthe increased force on the piston crowns.


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Cooling

However, if we are using a 4-stroke petrol (spark

ignition) system, we are only looking at an

efficiency of about 28%.

We can harness some of the rest of the energyreleased to drive the turbo, but a lot of heat is

generated that we must be able to disperse.

Failure to successfully conduct away the excess

heat will lower the efficiency at best, and cause

catastrophic engine failure at worst.


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Cooling

The hottest part of the engine is the

combustion chamber. Temperatures may

reach over 2000C for short periods, and

when pistons melt at about 800C,

obviously something drastic has to be

done to conduct away the heat very

quickly.


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How can we remove excess heat

from Pistons?

Discuss!!


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Heat Pathways

There are three ways by which we can take heataway conduction, convection and radiation.

Conduction from pistons takes place via the

rings and skirt into the cylinder walls, and thenby further conduction into the coolantsurrounding the liners. The hot gas is also indirect contact with the cylinder walls and head,so conduction will take place there as well, againinto the coolant jacket. The coolant spacesaround the exhaust ports especially are largeand have to conduct away a lot of excess heat.


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The rest of the crown?

Hmm?


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Piston cooling

The piston crowns come in for a lot of thermalshock, and are too far away from the cylinderwalls to be cooled by direct conduction into thewater jacket. Pistons are therefore usuallycooled by an oil jet which is directed at theunderside of the crown, or by supplying oil to agallery which is cast into the piston and is keptfull either by the spray jet, or by an oil spray from

the top of the connecting rod. The oil in thegallery circulates and is ejected, so cooling thepiston crowns.


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So where does the heat go?

Water and oil?

Radiation into the surroundingatmosphere?


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Cooling

Obviously, the extra heat which is beingtransferred to coolant and oil must also bedissipated. It is vital to ensure that radiators cancope with the heat output, and necessarycoolant flow. It is equally important to ensurethat airflow through the radiators is unimpededby any sort of restriction, in terms of bodywork,air-way restriction or aerodynamic stagnancy. A

high-pressure air zone behind the radiator willimpede the airflow through it, and will reduce thesystem effectiveness.


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Cooling

Equally, the coolant must be of a high enoughspecification, in terms of boiling point, ability to wet thesurfaces properly (which water, due to it's surfacetension, is not always good at doing), anti-corrosion

properties, anti-freeze properties (especially somewherelike the Rally ofFinland), and the system must be able tohold a high-enough pressure, and be accuratelythermostatically controlled to ensure that the enginereaches operating temperature quickly and holds this

temperature stably (especially if the vehicle is racing invery cold or very wet conditions).


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Oil coolers

The same is true of oil coolers. The oil must bekept below it's natural oxidisation temperature,and the system must circulate the oil fastenough, through a big enough cooler, to ensurethat the excess heat can be dissipated. Thesump (on wet-sump engines) also acts as an oilcooler, so care must be taken when frontsplitters / air-dams are being modified to ensure

that an adequate airflow around the bottom ofthe engine remains. If this is not possible,additional oil-cooling circuitry must be installed.


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Combustion moderation

How can we moderate (ie keep control of)

combustion chamber temperatures?

M

ixture strength? Incoming charge additives?

EGR?

Alternative / more exotic fuels?


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Temperature moderation

Discuss!


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Variables

If we are going to run higher cylinder pressures,as a result of forced induction, increased thermaleffects and a potential fuel-change, we mustlook at all operational variables.

Will the mechanical components be strongenough to withstand the increase?

Will valves, valve lift and cam-timing suit the newinduction system?

Will the fuel detonate? Can we adjust ignitiontiming to compensate, or must we change fuelsor lower the compression ratio?


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Valves

Exhaust valves, especially, are subjected

to extreme conditions every time they

open to allow the spent gases to exit the

system. These gases may be at a

temperature of over 1000C, and have

significant corrosive and erosive properties

depending on fuel composition.


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Valve cooling

Valves are cooled, fundamentally, by

contact with the valve seat, and by

conduction up the valve stem and through

the guide and cam follower. It is therefore

in the interests of the engine designer to

keep exhaust valves closed as long as is

feasible to try to keep temperatures atacceptable levels.


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Valve design

There is a wide variety of designs of valve,ranging from simple penny on a stickdesigns to a tulip valve, with wide

variations across this spectrum. Valve seat areas have to strike a balance

between being wide enough to allow heatto be conducted away, while still beingnarrow enough not to pose a restriction togas-flow


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Stems

We have to be mindful of the fact that a lotof heat will be conducted up the valvestem, so we must therefore be careful in

our selection of materials for both valveand guide, and clearances must beadequate enough to allow for expansionwithout increasing oil consumption,

especially on inlet valve stems. Many raceengines don't have exhaust valve seals,which reduces the problems considerably.


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Stem design.

Must be thick enough for strength and to

soak up excessive heat, while being light

enough to prevent valve float / bounce.

Ideally valves should be as short as

possible, to conduct the heat away as fast

as possible.

Sodium filling?


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Bearings

More cylinder pressure puts more load onbearings. How can we offset this effect?

Increased bearing area? Higher oil

pressure? Better oil cooling to keepviscosity up and prevent degradation?

All the above will also create more

dynamic drag, which will contribute tohigher engine temperatures and cylinderpressures!

Date post:	08-Apr-2018
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