Encycl Energy 2004 ICE Vehicles

8/13/2019 Encycl Energy 2004 ICE Vehicles

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Internal CombustionEngine Vehicles

K. G. DULEEP

Energy and Environmental Analysis, Inc.Arlington, Virginia, United States

1. Introduction

2. Vehicle Energy Efficiency

3. Improving Efficiency by Reducing Tractive

Energy Required4. Improvement of Engine Efficiency

5. Increasing the Efficiency of Spark Ignition Engines

6. Increasing the Efficiency of Compression Ignition(Diesel) Engines

7. Intake Charge Boosting

8. Alternative Heat Engines

Glossary

engine efficiency Amount of energy produced by theengine per unit of fuel energy consumed.

fuel economy Vehicle distance traveled per unit volume offuel consumed.

internal combustion engine vehicle Vehicle where primarymotive power is derived from an engine that convertsfuel energy to work using the air-fuel mixture as theworking fluid.

light duty on highway vehicles Cars and light trucks with afully loaded weight below 6000 k (13,200 lbs).

off-highway vehicle Vehicles designed to operate primarilyon unpaved surfaces.

The vast majority of vehicles used in the world are

powered by internal combustion engines (ICE).Other forms of propulsion such as electric motorsor external combustion steam engines are used inspecialized applications that account for a smallfraction of the total vehicle fleet. Most vehicles arenow powered by reciprocating piston engines thatuse the Otto cycle (also called the spark-ignitionengine) or the diesel cycle (also called the compres-sion-ignition engine). Gas turbines are used primarily

in marine vessels and aircraft and are not discussedhere. A small number of vehicles using the Wankelengine have also been sold. Internal combustionenginepowered vehicles typically account for one-

quarter to one-third of total energy consumption inmost countries, and their fuel consumption and fuelefficiency are issues of major concern.

1. INTRODUCTION

The on-highway fleet of vehicles accounts for over95% of all vehicles in operation worldwide (which isin excess of a billion vehicles). The remainder iscomposed of off-highway vehicles, equipment such

as forklifts or bulldozers, and motorcycles. Annualsales of on-highway vehicles exceeded 57.6 millionsunits worldwide in 2002 with 39.5 millions unitclassified as passenger cars and 18.1 million unitsclassified as trucks. The distinction between cars andtrucks is not always clear (especially for light trucks)but trucks are usually used for cargo hauling or forcarrying more than six passengers. Cars span thegross vehicle weight (GVW) range from 1 to 3 tons,while trucks typically span the GVW range of 2 to 40tons. Off-road and specialized vehicles can be muchheavier.

The majority of cars and light trucks (under five

tons GVW) are powered by spark ignition engines,while most trucks that weigh more than 5 tons GVWare powered by diesel engines. Since the early 1990s,diesel engines have become more popular for carsand light trucks in the European Union (EU). Thediesel engines share in the new car market was over50% in 2002 in countries such as France andAustria. In contrast, few diesel enginepowered carsand light trucks are sold in North America.

Encyclopedia of Energy, Volume 3. r 2004 Elsevier Inc. All rights reserved. 497


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2. VEHICLE ENERGY EFFICIENCY

Vehicle energy efficiency is generally defined in termsof fuel economy measured in miles per gallon (mpg)or kilometers per liter of fuel. It can also be measuredin terms of fuel consumption, which is the inverse of

fuel economy, in units of liters per 100 km or gallonsper 100 miles. The fuel economy of a vehicle isstrongly dependent on the vehicles overall weight,but is also dependent on the efficiency of the engine,as well as the matching of the engine characteristicsto the vehicles operational requirements. The fueleconomy of a particular vehicle is dependent on theload carried, the driving cycle, the ambient tempera-ture, and the characteristics of the road such as itsgradient and surface roughness. Hence, the fueleconomy of a specific vehicle can vary widelydepending on how and where it is used.

In most developed countries, on-road light vehi-

cles (cars and light trucks) are certified for emissionsand fuel economy by the government. The fuelefficiency rating is measured in a laboratory-con-trolled environment and on a specified driving cycle.In the United States and Canada, for example, lightvehicle fuel economy is measured on a city cyclewith an average speed of about 20mph and ahighway cycle with an average speed of about50 mph. All aspects of the fuel economy test, rangingfrom the ambient temperature to the specification ofthe fuel used, are tightly controlled and this results ina fuel economy measurement that is repeatable towithin 72%, typically. While this measured fueleconomy may differ significantly from the fueleconomy for the same vehicle in any specific use,the measured value provides a comparative bench-mark for vehicle fuel economy that is useful from avehicle buyers perspective and from an engineeringperspective.

The sales-weighted average test fuel economy ofall new vehicles sold in the United States is about28 mpg for cars and 21 mpg for light trucks. Much ofthis difference between car and light-truck fueleconomy is attributable to the fact that light trucksare larger and heavier than cars, but some of the

difference is also attributable to the fact that carsutilize higher levels of efficiency enhancing technol-ogy. Fuel economy levels in Australia and Canada aresimilar to the U.S. levels, but cars in the EU haveabout 25% higher fuel economy, on average. Thehigher fuel economy in the EU is partly due to thesmaller size and weight of cars sold and partly due tothe higher penetration of diesel engines, which aremore efficient than spark ignition engines.

Studies conducted by technical agencies haveconcluded that vehicle fuel economy can be increasedsubstantially from average values without anyreduction of attributes such as interior space ofcargo carrying ability. The sources of energy loss andthe technology available to reduce these losses are

described later.A simple model of energy consumption in

conventional automobiles provides insight into thesources and nature of energy losses. In brief, theengine converts fuel energy to shaft work. This shaftwork is used to overcome the tractive energyrequired by the vehicle to move forward, as well asto overcome driveline losses and supply accessorydrive energy requirements. The tractive energy can beseparated into the energy required to overcomeaerodynamic drag force, rolling resistance and inertiaforce. It is useful to consider energy consumption onthe U.S. city and highway test cycles, which are

reference cycles for comparing fuel economy.Denoting the average engine brake specific fuel

consumption over the test cycle as bsfc, we have fuelconsumption, FC, given by

FCbsfc

Zd

EREAEK bsfc EACGiti tb

where Zdis the drive train efficiency, ERis the energyto overcome rolling resistance, EA is the energy toovercome aerodynamic drag, EK is the energy toovercome inertia force, EAC is the accessory energyconsumption, Gi is idle fuel consumption per unit

time, and ti

, tb

are the time spent at idle and braking.The first term in the above equation represents thefuel consumed to overcome tractive forces. Since theFederal Test Procedure (FTP) specifies the city andhighway test cycle in terms of speed versus time, ER,EA, and Ek can be readily calculated as function ofthe vehicle weight, the tire rolling resistance, bodyaerodynamic drag coefficient, and vehicle frontalarea. Weight reduction reduces both inertia force androlling resistance.

It should be noted that not all of the inertia forceis lost to the brakes, as a vehicle will slow downwithout the use of brakes, at zero input power due to

aerodynamic drag and rolling resistance. Brakingenergy loss is approximately 35% in the city cycleand 7% on highway cycle. The fuel energy is usednot only to supply tractive energy requirements butalso to overcome transmission losses, accounting forthe transmission efficiency that is in the first term.

The second term in the equation is for the fuelconsumed to run the accessories. Accessory powerrequirements are required to run the radiator cooling

498 Internal Combustion Engine Vehicles


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fan, alternator, water pump, oil pump, and powersteering pump. Air conditioners also absorb powerbut are not reflected in official fuel economyestimates since they are not turned on during theFTP. Idle and braking fuel consumption are largely afunction of engine size and idle RPM, while

transmission losses are function of transmission type(manual or automatic) and design. The engineproduces no power during idle and braking butconsumes fuel, so that factor is accounted for by thethird term. Table I shows the energy consumption (asa percentage) by all of these factors for a typical U.S.midsize car of mid-1990s vintage, with a 3-literdisplacement s.i. engine, four-speed automatic trans-mission with lock-up, and power steering.

The values in Table I can be utilized to derivesensitivity coefficients for the reduction of variousloads. For example, reducing the weight by 10% willreduce both rolling resistance and inertia weight

forces, so that tractive energy is reduced by(30.35 40.22) 0.1 or 7.06% on the compositecycle. Fuel consumption will be reduced by7.06% 0.6544, which is the fraction of fuel usedby tractive energy, or 4.6%. This matches the commonwisdom that reducing weight by 10% reduces fuelconsumption by 4 to 5%. However, if the engine isalso downsized by 10% to account for the weight loss,fuel consumption will be reduced by 5.8% since idleand braking fuel consumption will be reduced in pro-portion to engine size. In addition, there will be somereduction (0.5%) in transmission and drivetrain loss.

Fuel economy can be improved by two primarymethods: (1) by reducing the power required to propel

the vehicle and (2) by increasing the engine efficiency.To estimate the effects of different technologyimprovements that affect engine power required orthe efficiency of the engine, it is useful to keep certainvehicle attributes constant. Vehicle attributes ofinterest to consumers are passenger room, cargo

space or payload capability, acceleration perfor-mance, and vehicle comfort/convenience features.The impact of technology on fuel economy is typica-lly measured while keeping these attributes constant.

Reducing the power required to propel the vehiclereduces engine load and can be accomplished byreduction of weight, aerodynamic drag, rollingresistance, or accessory loads. Engine efficiencyincreases can be accomplished not only by enginetechnologies but also by improved drivetrain tech-nologies that improve the match between engineoperating point and vehicle power requirements.Spark ignition engines convert only about 20 to 25%

of fuel energy to useful work during typical drivingso that a doubling of engine efficiency is theoreticallypossible without changing the basic Otto cycle.

3. IMPROVING EFFICIENCYBY REDUCING TRACTIVEENERGY REQUIRED

Since vehicle weight is one of the most importantvariables determining fuel economy, weight reduc-tion is an important method of improving fueleconomy. The vehicles weight is distributed betweenthe body structure, the drivetrain, the vehicles

TABLE I

Energy Consumption as a Percentage of Total Energy Requirements for a Typical Midsize Cara

City Highway Compositeb

Percentage of total tractive energy

Rolling resistance 27.7 35.2 30.35

Aerodynamic drag 18.0 50.4 29.43

Inertia (Weight) force 54.3 14.4 40.22

Total 100 100 100

Percentage of total fuel consumed

Tractive energy 57.5 80.0 65.44

Accessory energy 10.0 6.5 8.76

Idle Braking consumption 15.0 2.0 10.41

Transmission Driveline loss 17.5 11.5 15.39

aMidsize car of inertia weight 1588kg, CD 0.33, A 2.1m2, CR 0.011, 3L OHV V-6, power steering, four-speed automatic

transmission with lock-up, air conditioning.bHighway fuel economy is 1.5 times city fuel economy, and composite figures are based on the U.S. EPA 55% city/45% highway fuel

consumption weighting.

Internal Combustion Engine Vehicles 499


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interior, and vehicle suspension/tires. The first twocomponent groups account for over 75% of avehicles weight. Weight can be reduced in all fourcomponent groups by improved structural design aswell as by the use of alternative materials.

Improved structural design and packaging has been

made possible through advanced computer simula-tions of structural strength, so that mater-ial use and shape can be optimized for the loadsencountered. Most modern cars feature unibodydesigns where the body panels carry the structuralloads, but several older models as well as many lighttrucks continue to use a separate chassis to carrystructural loads. Heavy trucks, however, almostalways utilize a separate chassis on which bodycomponents are mounted. A new architecture calledspace frame designs have emerged where structuralloads are carried on skeletal frame from which bodypanels are hung. Improved packaging by optimization

of component placement, body layout, and drivetrainlocation can also yield weight benefits. The placementof the engine transversely between the front wheelsand driving the front wheels provides significantpackaging benefits over front engine, rear-wheel-drivepackages for light-duty vehicles.

The use of alternative materials such as ultra-high-strength steel, aluminum, and plastic composites isanother way to reduce weight. Because of its lowcost, steel, and cast iron continue to be the materialof choice for body structures. Aluminum is alreadywidely used for engine blocks and cylinder heads,and it is also used in critical suspension components.Some luxury cars now feature all-aluminum bodies,which weigh 30 to 35% less than their steelcounterparts. Plastic composites are also widely usedin body closures such as fenders, hood, and decklidwith weight savings of 20 to 25% relative to steelparts. Such composites also see wide usage for light-weight interiors in the vehicle dashboard, seats, anddoor panels. Specially constructed prototypes max-imizing the use of lightweight alternative materialshave shown that weight reduction of 25 to 30%(relative to a conventional average steel vehicle) ispossible, although with higher cost and with

manufacturability constraints. The use of alternativematerials in heavy trucks may not reduce loadedweight but will permit a larger payload to be carried.

Aerodynamic drag can be reduced by styling thevehicles exterior shape and guiding the vehiclesinterior airflow. At the speeds experienced by atypical vehicle, low drag shapes are a result of carefulattention to airflow at the front of the vehicle, rearwheel wells and outside mirrors, and at the end of the

roof. A measure of the drag is the aerodynamic dragcoefficient, CD, which is defined as

CD Drag force

12rv

2A ;

where r is the density of air, v is the velocity of

airflow, and A is the vehicle frontal area.In the early 1980s, cars had drag coefficients of

0.45 to 0.5. By 2000, the most aerodynamic cars haddrag coefficient of 0.25 to 0.28. Trucks typically havehigher drag coefficients because of their boxy shapeand increased ground clearance, relative to cars.Prototype cars with drag coefficients as low as 0.15have been built, but such designs typically involvereduction of vehicle attributes such as reduced rearpassenger headroom, reduced rear visibility, orreduced cargo space. Nevertheless, drag reductionstill offers opportunities to reduce fuel consumption.

The tires rolling resistance is the third major

contributor to overall load. The tire rolling resistancecoefficient (CR) is a measure of tire energy loss, and isdefined as

CR T

L R;

where T is the torque required at any speed, R thetire radius, and L the vertical load on the tire.Typically, most modern tires have a CR in the rangeof 0.009 to 0.012.

The tire rolling resistance results from a combina-tion of tire-to-road friction and hysteresis. As thetire deforms, heat is dissipated in the tires sidewall

and tread due to the visco-elastic nature of rubber.In comparison, a steel wheel riding on steel railshas about one-tenth the rolling resistance of a rubbertire. Tire rolling resistance can be reduced by impro-ved design of the tire tread, shoulders, and belts.In addition, the tire material formulation can signi-ficantly reduce hysteresis loss. The use of silicacompounds mixed with rubber has been found toreduce rolling resistance, without affecting otherdesirable properties such as braking and wettraction. Design improvements and changes in beltmaterial are also capable of reducing CR with limi-ted or no reduction of desirable attributes. It appears

possible to reduce CRby 15 to 25% in the short termand by up to 40% over the long term (B25 years).

4. IMPROVEMENT OFENGINE EFFICIENCY

Engine efficiency on the driving cycle is the mostsignificant determinant of vehicle fuel economy for a



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vehicle of a specific weight. Heat engine efficiencycan be stated in several ways. One intuitivelyappealing method is to express the useful energyproduced by an engine as a percentage of the totalheat energy that is theoretically liberated by com-busting the fuel. This is sometimes referred to as the

first law efficiency, implying that its basis is the firstlaw of thermodynamics, the law of conservation ofenergy. Another potential, but less widely used,measure is based on the second law of thermody-namics, which governs how much of that heat can beconverted to work. Given a maximum combustiontemperature (usually limited by engine materialconsiderations and by emission considerations), thesecond law postulates a maximum efficiency basedon an idealized heat engine cycle called the Carnotcycle. The ratio of the first law efficiency to theCarnot cycle efficiency can be utilized as a measureof how efficiently a particular engine is operating

with reference to the theoretical maximum based onthe second law of thermodynamics. However, themost common measure of efficiency used by auto-motive engineers is termed brake specific fuelconsumption (bsfc), which is the amount of fuelconsumed per unit time per unit of power. In theUnited States, the bsfc of engines is usually stated inpounds of fuel per brake horsepower hour, whereasthe more common metric system measurement unit isin grams per kilowatt-hour (g/kwh). The term brakehere refers to a common method historically used tomeasure engine shaft power output. Of course, allthree measures of efficiency are related to each other.

The efficiency of Otto and diesel cycle engines isnot constant but depends on the operating point ofthe engine as specified by its torque output and shaftspeed (revolutions per minute or RPM). Enginedesign considerations, frictional losses, and heatlosses result in a single operating point whereefficiency is highest. This maximum efficiency foran s.i. engine usually occurs at relatively high torqueand at low to mid-RPM within the operating RPMrange of the engine. At idle, the efficiency is zerosince the engine is consuming fuel but not producingany useful work. When considering efficiency in a

vehicle, the maximum efficiency need not, by itself,be an indicator of the average efficiency undernormal driving conditions, since engine speed andtorque vary widely under normal driving. Themaximum efficiency of an engine is of interest toautomotive engineers, but a more practical measureof efficiency is its average efficiency during normaldriving or during the official city and highway fueleconomy test.

4.1 Theoretical MaximumEngine Efficiency

The characteristic features common to all pistoninternal combustion engines are as follows:

1. Intake and compression of the air or air-fuelmixture

2. Raising the temperature (and hence, the pressure)of the compressed air by combustion of fuel

3. The extraction of work from the high-pressureproducts of combustion by expansion

4. Exhaust of the products of combustion

Combustion of the homogenous air-fuel mixture ina spark ignition engine takes place very quicklyrelative to piston motion and is represented inidealized calculations as an event occurring atconstant volume. According to classical thermody-namic theory, the thermal efficiency, Z, of an idealized

Otto cycle, starting with intake air-fuel mixturedrawn in at atmospheric pressure, is given by

Z 11=rn1; 1

wherer is the compression (and expansion) ratio andn is the ratio of specific heat at constant pressure tothat at constant volume for the mixture. The equationshows that efficiency increases with increasingcompression ratio.

Using an n value of 1.4 for air, the equationpredicts an efficiency of 58.47% at a compressionratio of 9:1. A value ofn 1.26 is more correct for

products of combustion of a stoichiometric mixtureof air and gasoline. A stoichiometric mixturecorresponds to an air-fuel ratio of 14.7:1, and thisair-fuel ratio is typical for most spark ignitionengines sold in the United States. At this air-fuelratio, calculated efficiency is about 43.5%. Actualengines yield still lower efficiencies even in theabsence of mechanical friction, due to heat transferto the wall of the cylinder and the inaccuracyassociated with assuming combustion to be instanta-neous. Figure 1 shows the pressure-volume cycle of atypical spark ignition engine and its departure fromthe ideal relationship.

Compression ratios are limited by the octanenumber of gasoline, which is a measure of its resis-tance to preignition or knock. At high compressionratios, the heat of compression of the air-fuel mixturebecomes high enough to induce spontaneous combus-tion of small pockets of the mixture, usually those incontact with the hottest parts of the combustionchamber. These spontaneous combustion events arelike small explosions that can damage the engine and



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can reduce efficiency depending on when they occurduring the cycle. Higher octane number gasolineprevents these events, but also costs more and requi-res greater energy expenditure for manufacture at therefinery. The octane number is measured using twodifferent procedures resulting in two different ratingsfor a given fuel, called motor octane and researchoctane number. Octane numbers displayed at thepump are an average of research and motor octanenumbers, and most engines sold in the United Statesrequire regular gasoline with a pump octane numberof 87. Typical compression rates for engines runningon regular gasoline are in the range of 9:1 to 10:1.

The diesel, or compression ignition, engine differsfrom the spark ignition engine in that only air, ratherthan the air-fuel mixture, is compressed. The dieselfuel is sprayed into the combustion chamber at theend of compression in a fine mist of droplets andthe diesel fuel ignites spontaneously upon contact withthe compressed air due to the heat of compression.The sequence of processes (i.e., intake, compression,

combustion, expansion, and exhaust) is similar to thatof an Otto cycle engine. However, the combustionprocess occurs over a relatively long period and isrepresented in idealized calculations as an event occur-ring at constant pressure (i.e., combustion occurs asthe piston moves downward to increase volume anddecrease pressure at a rate offsetting the pressure risedue to heat release). Figure 2 shows the pressure-volume cycles for a typical diesel engine and its rela-

tionship to the ideal diesel cycle. If the ratio of volumeat the end of the combustion period to the volume atthe beginning of the period is rc, or the cutoff-ratio,the thermodynamic efficiency of the idealized con-stant-pressure combustion cycle is given by

Z 1

rn1rnc 1

n rc 1

: 2

It can be seen that forrc 1, the combustion occurs atconstant volume and the efficiency of the diesel andOtto cycle are equivalent.

The term rc also measures the interval duringwhich fuel is injected, and it increases as the poweroutput is increased. The efficiency equation showsthat as rc is increased, efficiency falls so that theidealized diesel cycle is less efficient at high loads.The combustion process also is responsible for amajor difference between diesel and Otto cycleengines. In an Otto cycle engine, intake is air

throttled to control power while maintaining anear constant air-fuel ratio; in a diesel engine, powercontrol is achieved by varying the amount offuel injected while keeping the air mass inductedper cycle at near constant levels. In most opera-ting modes, combustion occurs with considerableexcess air in a c.i. engine, while combustion occursat or near stoichiometric air-fuel ratios in a moderns.i. engine.

Pressure, psia

1000

100

10

1 10Volume, cubic inches

100

900800700600

500

400

300

200

90807060

50

40

30

20

2 3 4 5 6 7 8 9 20 30 40 50 60708090

n = 1.3

n = 1.28

FIGURE 2 Pressure-volume diagram for a diesel engine.Compression ratio 1713.

Pressure, psia1000

100

10

1 10

Volume, cubic inches

100

900800700600

500

400

300

200

90807060

50

40

30

20

2 3 4 5 6 7 8 9 20 30 40 50 60708090

n = 1.25

n = 1.28

FIGURE 1 Pressure-volume diagram for a gasoline engine.Compression ratio 8.711.



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At the same compression ratio, the Otto cycle hasthe higher efficiency. However, diesel cycle enginesnormally operate at much higher compression ratios,since there are no octane limitations associated withthis cycle. In fact, spontaneous combustion of thefuel is required in such engines, and the ease of

spontaneous combustion is measured by a fuelproperty called cetane number. Most c.i. enginesrequire diesel fuels with a cetane number over 40.

In practice, there are two kinds of c.i. engines, thedirect injection type (DI) and the indirect injectiontype (IDI). The DI type utilizes a system where fuel issprayed directly into the combustion chamber. Thefuel spray is premixed and partially combusted withair in a prechamber in the IDI engine, before thecomplete burning of the fuel in the main combustionchamber occurs. DI engines generally operate atcompression ratios of 15 to 20:1, while IDI enginesoperate at 18 to 23: 1. The theoretical efficiency of a

c.i. engine with a compression ratio of 20:1,operating at a cutoff ratio of 2, is about 54% (forcombustion with excess air, n is approximately 1.3).In practice, these high efficiencies are not attained,for reasons similar to those outlined for s.i. engines.

4.2 Actual versus Theoretical Efficiency

Four major factors affect the efficiency of s.i. and c.i.engines. First, the ideal cycle cannot be replicateddue to thermodynamic and kinetic limitations of thecombustion process, and the heat transfer that occurs

from the cylinder walls and combustion chamber.Second, mechanical friction associated with themotion of the piston, crankshaft, and valves consumea significant fraction of total power. Since friction is astronger function of engine speed rather than torque,efficiency is degraded considerably at light load andhigh RPM conditions. Third, aerodynamic frictionallosses associated with airflow through the air cleaner,intake manifold and valves, exhaust manifold,silencer, and catalyst are significant, especially athigh airflow rates through the engine. Fourth,pumping losses associated with throttling the airflowto achieve part-load conditions in spark ignition

engines are very high at light loads. Note that c.i.engines do not usually have throttling loss, and theirpart load efficiencies are superior to those of s.i.engines. Efficiency varies with both speed and loadfor both engine types.

Hence, production spark ignition or compressionignition engines do not attain the theoretical valuesof efficiency, even at their most efficient operatingpoint. In general, for both types of engines, the

maximum efficiency point occurs at an RPM that isintermediate to idle and maximum RPM and at alevel that is 60 to 75% of maximum torque. On-roadaverage efficiencies of engines used in cars and lighttrucks are much lower than peak efficiency, since theengines generally operate at very light loads during

city driving and steady-state cruise on the highway.High power is utilized only during strong accelera-tions, at very high speeds, or when climbing steepgradients. The high load conditions are relativelyinfrequent, and the engine operates at light loadsmuch of the time during normal driving.

During normal driving, the heat of fuel combus-tion is lost to a variety of sources and only a smallfraction is converted to useful output, resulting inthe low values for on-road efficiency. Figure 3provides an example of the heat balance for a typicalmodern small car with a spark ignition engine undera low speed (25 mph or 40 mph) and a high-speed

(62 mph or 100 mph) condition. At very low drivingspeeds typical of city driving, most of the heat energyis lost to the engine coolant. Losses associated withother waste heat include radiant and convectionlosses from the hot engine block and heat losses tothe engine oil. A similar heat loss diagram for a dieselc.i. would indicate lower heat loss to the exhaust and

QI

QI

40 km/h

100 km/h

Le

Le

Qw

Qw

Qex

Qex

Otherwaste heat19.8%

Waste heat ofexhaust gas24.8%

Engineoutput20.8%

Waste heat ofengine cooling

34.7%

Otherwaste heat26.3%

Engineoutput28.3%

Waste heat ofengine cooling

27.3%

Waste heat ofexhaust gas18.2%

FIGURE 3 Heat balance of a passenger car equipped with1500cc engine 17.



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coolant and an increased fraction of heat convertedto work, especially at the low speed condition.During stop-and-go driving conditions typical of citydriving, efficiencies are even lower than thoseindicated in Fig. 3 because of the time spent at idlewhere efficiency is zero. Under the prescribed U.S.city cycle conditions, typical modern spark ignitionengines have an efficiency of about 18%, modern IDIc.i. engines have an efficiency of about 21%, andmodern DI diesel have an efficiency of about 23%.

Another method of examining the energy losses isby allocating the power losses starting from thepower developed within the cylinder. The usefulwork corresponds to the area that falls between thecompression and expansion curve depicted in Figs 1and 2. The pumping work that is subtracted fromthis useful work, referred to as indicated work, is afunction of how widely the throttle is open and, to alesser extent, the speed of the engine. Figure 4 showsthe dependence of specific fuel consumption (or fuelconsumption per unit of work) with load, at constant(low) engine RPM. Pumping work represents only5% of indicated work at full load, low RPMconditions, but increases to over 50% at light loads

of less than two-tenths of maximum power.Mechanical friction and accessory drive power,on the other hand, increase nonlinearly with enginespeed but do not change much with the throt-tle setting. Figure 5 shows the contribution of thevarious engine components as well as the alternator,water pump and oil pump to total friction, expres-sed in terms of mean effective pressure, as a func-tion of RPM. The brake mean effective pressure is

a measure of specific torque, or torque per unit ofengine displacement; typical engine brake meaneffective pressure (bmep) of spark ignition enginesthat are not supercharged range from 8.5 to 10 bar.Hence, friction accounts for about 25% of totalindicated power at high RPM (B6000) but onlyfor about 10% of indicated power at low RPM(B2000) in spark ignition engines. Friction in a c.i.engine is higher because of the need to maintain aneffective pressure seal at high compression ratios,and the friction mean effective pressure is 30 to 40%higher than that for a dimensionally similar s.i.engine at the same RPM. Since the brake meaneffective pressure of a diesel is also lower than thatof a gasoline engine, friction accounts for 15 to 16%of indicated maximum power even at 2000 RPM.Typical bmep values for a naturally aspirated c.i.engine range from 6.5 to 7.5 bar.

5. INCREASING THE EFFICIENCYOF SPARK IGNITION ENGINES

5.1 Design Parameters

Engine valvetrain design is a widely used method toclassify spark ignition engines. The first spark

200

0 20 40

Percentage of brake load

60 80 100

300

400

500

Specificfuelconsump

tion(g/kW-h)

Brake

Indicated

Constant speed

and air-fuel ratio

Fri

ction

losses

Pumpinglosses

FIGURE 4 Specific fuel consumption versus engine load 18.

3.0

1.5

0.00 700 3000 6000

Speed RPM(Idle)

Enginefrictiondistribution

Friction mean effective pressure bar

Generato

r

Water

pump

Oilpu

mp

Valve

train

Pist

onring

s

Pist

on

Connectin

grod

Crankshaft bearing

FIGURE 5 Friction distribution as a function of engine speed19.



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ignition engines were of the side-valve type, but suchengines have not been used in automobiles for severaldecades, although some engines used in off-highwayapplications, such as lawn mowers or forklifts, con-tinue to use this design. The overhead valve (OHV)design supplanted the side-valve engine by the early

1950s, and continues to be used in many U.S. enginesin much improved form. The overhead cam engine(OHC) is the dominant design used in the rest of thedeveloped world. The placement of the camshaft inthe cylinder heads allows the use of simple, lightervalvetrain, and valves can be opened and closed morequickly as a result of the reduced inertia. Thispermits better flow of intake and exhaust gases,especially at high RPM, with the result that an OHCdesign typically can produce greater power at highRPM than an OHV design of the same displacement.

A more sophisticated version of the OHC engineis the double overhead cam (DOHC) engine where

two separate camshafts are used to activate theintake and exhaust valves, respectively. The DOHCdesign permits a very light valvetrain as the camshaftcan actuate the valves directly without any interven-ing mechanical linkages. The DOHC design alsoallows some layout simplification, especially inengines that feature two intake valves and twoexhaust valves (4-valve). The 4-valve engine hasbecome popular since the mid-1980s and Japanesemanufacturers, in particular, have embraced theDOHC 4-valve design. The DOHC design permitshigher specific output than an OHC design, with the4-valve DOHC design achieving the highest specificoutput, in excess of 70 BHP/liter of displacement.

5.2 Thermodynamic Efficiency

Increases in thermodynamic efficiency within thelimitations of the Otto cycle are obviously possibleby increasing the compression ratio. However,compression ratio is also fuel octane limited, andincreases in compression ratio depend on how thecharacteristics of the combustion chamber and thetiming of the spark can be tailored to prevent knockwhile maximizing efficiency.

Spark timing is associated with the delay ininitiating and propagating combustion of the air-fuelmixture. To complete combustion before the pistonstarts its expansion stroke, the spark must beinitiated a few crank angle degrees (advance) beforethe piston reaches top dead center. For a particularcombustion chamber, compression ratio, and air-fuelmixture, there is an optimum level of spark advancefor maximizing combustion chamber pressure and,

hence, fuel efficiency. This level of spark advance iscalled MBT for maximum for best torque. However,MBT spark advance can result in knock if fuel octaneis insufficient to resist preignition at the highpressures achieved with this timing. Hence, there isan interplay between spark timing and compression

ratio in determining the onset of knock. Retardingtiming from MBT reduces the tendency to knock butdecreases fuel efficiency. Emissions of hydrocarbonsand oxides of nitrogen (NOx) are also dependent onspark timing and compression ratio, so that emissionconstrained engines require careful analysis of theknock, fuel efficiency, and emission trade-offs beforethe appropriate value of compression ratio and sparkadvance can be selected.

Electronic control of spark timing has made itpossible to set spark timing closer to MBT relative toengines with mechanical controls. Due to productionvariability and inherent timing errors in a mechanical

ignition timing system, the average value of timing inmechanically controlled engines had to be retardedsignificantly from the MBT timing. This protects thefraction of engines with higher than average advancedue to production variability from knock. The use ofelectronic controls coupled with magnetic or opticalsensors of crankshaft position has reduced thevariability of timing between production enginesand also allowed better control during transientengine operation. Engines have been equipped withknock sensors, which are essentially vibrationsensors tuned to the frequency of knock. Thesesensors allow for advancing ignition timing to thepoint where trace knock occurs, so that timing isoptimal for each engine produced regardless ofproduction variability.

High-swirl, fast-burn combustion chambers havebeen developed to reduce the time taken for the air-fuel mixture to be fully combusted. The shorter theburn time, the more closely the cycle approximatesthe theoretical Otto cycle with constant volumecombustion and the greater the thermodynamicefficiency. Reduction in burn time can be achievedby having a turbulent vortex within the combustionchamber that promotes flame propagation and

mixing. The circular motion of the air-fuel mixtureis known as swirl, and turbulence is also enhanced byshaping the piston so that gases near the cylinderwall are pushed rapidly towards the center in amotion known as squish. Improvements in flowvisualization and computational fluid dynamics haveallowed the optimization of intake valve, inlet port,and combustion chamber geometry to achievedesired flow characteristics. Typically, these designs



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have resulted in a 2 to 3% improvement inthermodynamic efficiency and fuel economy. Thehigh-swirl chambers also allow higher compressionratios and reduced spark advance at the same fueloctane number. The use of these types of combustionchambers has allowed compression ratio from about

8:1 in the early 1980s to 10:1 in the early 2000s, andfurther improvements are likely. In newer engines ofthe 4-valve DOHC type, the sparkplug is placed atthe center of the combustion chamber, and thechamber can be made very compact by having anearly hemispherical shape. Engines incorporatingthese designs have compression ratios of 10:1 whilestill allowing the use of regular 87 octane gasoline.Increases beyond 10:1 are expected to have dimin-ishing benefits in efficiency and fuel economy andcompression ratios beyond 12:1 are not likely to bebeneficial unless fuel octane is raised simultaneously.

5.3 Reduction in Mechanical Friction

Mechanical friction losses are being reduced byconverting sliding metal contacts to rolling contacts,reducing the weight of moving parts, reducingproduction tolerances to improve the fit betweenpistons and bore, and improving the lubricationbetween sliding or rolling parts.

Friction reduction has focused on the valvetrain,pistons, rings, crankshaft, crankpin bearings, and theoil pump. Valvetrain friction accounts for a largerfraction of total friction losses at low engine RPM

than at high RPM. The sliding contract between thecam that activates the valve mechanism through apushrod in an OHV design, or a rocker arm in anOHV design, can be substituted with a rolling contactby means of a roller cam follower. Roller camfollowers have been found to reduce fuel consump-tion by 2 to 4% during city driving and 1 to 2% inhighway driving. The use of lightweight valves madeof ceramics or titanium is another possibility for thefuture. The lightweight valves reduce valve traininertia and also permit the use of lighter springs withlower tension. Titanium alloys are also being consi-dered for valve springs which operate under heavy

loads. There alloys have only half the shear modulusof steel and fewer coils are needed to obtain the samespring constant. A secondary benefit associated withlighter valves and springs is that the erratic valvemotion at high RPM is reduced, allowing increasedengine RPM range and power output.

The pistons and rings contribute to approximatelyhalf of total friction. The primary function of therings is to minimize leakage of the air-fuel mixture

from the combustion chamber to the crankcase, andoil leakage from the crankcase to the combustionchamber. The ring pack for most engines is composedof two compression rings and an oil ring. The ringshave been shown to operate hydrodynamically overthe cycle, but metal-to-metal contact occurs often at

the top and bottom of the stroke. The outward radialforce of the rings are as a result of installed ringtension and contribute to effective sealing as well asfriction. Various low-tension ring designs wereintroduced in the 1980s, especially since the needto conform to axial diameter variations or boredistortions have been reduced by improved cylindermanufacturing techniques. Reduced tension ringshave yielded friction reduction in the range of 5 to10%, with fuel economy improvements of 1 to 2%.Elimination of one of the two compression rings hasalso been tried on some engines, and two-ring pistonsmay be the low friction concept for the future.

Pistons have also been redesigned to decreasefriction. Prior to the 1980s, piston had large skirtsto absorb side forces associated with side-to-sidepiston motion due to engine manufacturing inaccura-cies. Pistons with reduced skirts diminish friction byhaving lower surface area in contact with the cylinderwall, but this effect is quite small. A larger effect isobtained for the mass reduction of a piston withsmaller skirts and piston skirt size has seen contin-uous reduction since the 1980s. Reducing thereciprocating mass reduces the piston-to-bore load-ing. Secondary benefits include reduced engine weightand reduced vibration. Use of advanced materialsalso result in piston weight reduction. Lightweightpistons use hypereutectic aluminum alloys, whilefuture pistons could use composite materials such asfiber-reinforced plastics. Advanced materials can alsoreduce the weight of the connecting rod, which alsocontributes to the side force on a piston.

The crankshaft bearings include the main bearingsthat support the crankshaft and the crankpinbearings and are of the journal bearing type. Thesebearings contribute to about 25% of total friction,while supporting the stresses transferred from thepiston. The bearings run on a film of oil and detailed

studies of lubrication requirements has led tooptimization of bearing width and clearances tominimize engine friction. Studies on the use of rollerbearings rather than journal bearings in this applica-tion has shown further reduction in friction ispossible. Crankshaft roller bearings are used onlyin some two-stroke engines such as outboard motorsfor boat propulsion, but their durability in auto-motive applications has not been established.



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Coatings of the piston and ring surfaces withmaterials to reduce wear also contribute to frictionreduction. The top ring, for example, is normallycoated with molybdenum, and new proprietary coa-ting materials with lower friction are being intro-duced. Piston coatings of advanced high temperature

plastics or resin are used widely and are claimed toreduce friction by 5% and fuel consumption by 1%.

The oil pumps generally used in most engines areof the gear pump type. Optimization of oil flow ratesand reduction of the tolerances for the axial georotorclearance has led to improved efficiency, whichtranslates to reduced drive power. Friction can bereduced by 2 to 3% with improved oil pump designsfor a gain in fuel economy of about half of a percent.

Improvements to lubricants used in the engine alsocontribute to reduced friction and improved fueleconomy. There is a relationship between oilviscosity, oil volatility, and engine oil consumption.

Reduced viscosity oils traditionally resulted inincreased oil consumption, but the development ofviscosity index (VI) improvers had made it possibleto tailor the viscosity with temperatures to formulatemultigrade oils such as 10W-40 (these numbers referto the range of viscosity covered by a multigrade oil).These multigrade oils act like low-viscosity oilsduring cold starts of the engine, reducing fuelconsumption, but retain the lubricating propertiesof higher viscosity oils after the engine warms up tonormal operating temperature. The development of5W-20 oils and 5W-40 oils can contribute to a fueleconomy improvement by further viscosity reduc-tion. Friction modifiers containing molybdenumcompounds have also reduced friction withoutaffecting wear or oil consumption. Future syntheticoils combining reduced viscosity and friction modi-fiers can offer good wear protection, low oilconsumption, and extended drain capability alongwith small improvements to fuel economy in therange of 1 to 3% over current oils.

5.4 Reduction in Pumping Loss

Reductions in flow pressure loss can be achieved by

reducing the pressure drop that occurs in the flow ofair (air-fuel mixture) into the cylinder and thecombusted mixture through the exhaust system.However, the largest part of pumping loss duringnormal driving is due to throttling, and strategies toreduce throttling loss have included variable valvetiming and lean-burn systems.

The pressure losses associated with the intakesystem and exhaust system have been typically

defined in terms of volumetric efficiency, which is aratio of the actual airflow through an engine to theairflow associated with filling the cylinder completely.The volumetric efficiency can be improved by makingthe intake airflow path as free of flow restrictions aspossible through the air filters, intake manifolds, and

valve ports. The shaping of valve ports to increaseswirl in the combustion chamber can lead to reducedvolumetric efficiency, leading to a trade-off betweencombustion and volumetric efficiency.

More important, the intake and exhaust processesare transient in nature as they occur only overapproximately half a revolution of the crankshaft.The momentum effects of these flow oscillations canbe exploited by keeping the valves open for durationsgreater than half a crankshaft revolution. During theintake stroke, the intake valve can be kept openbeyond the end of the intake stroke, since themomentum of the intake flow results in a dynamic

pressure that sustains the intake flow even when thepiston begins the compression stroke. A similar effectis observed in the exhaust process, and the exhaustvalve can be held open during the initial part of theintake stroke. These flow momentum effects dependon the velocity of the flow which is directlyproportional to engine RPM. Increasing the valveopening duration helps volumetric efficiency at highRPM but hurts it at low RPM. Valve timing andoverlap are selected to optimize the trade-off betweenhigh and low RPM performance characteristics.

Efficiency improvements can be realized by chan-ging the valve overlap period to provide less overlapat idle and low engine speeds and greater overlap athigh RPM. In DOHC engines, where separatecrankshafts actuate the intake and exhaust valves,the valve overlap period can be changed by rotatingthe camshafts relative to each other. Such mechan-isms have been commercialized engines show lowRPM torque improvements of 7 to 10% with nosacrifice in maximum horsepower attained in the5500 to 6000 RPM range. Variable valve overlapperiod is just one aspect of a more comprehensivevariable valve timing system.

The oscillatory intake and exhaust flows can allow

volumetric efficiency to be increased by exploitingresonance effects associated with pressure wavessimilar to those in organ pipes. The intake manifoldscan be designed with pipe lengths that resonate, sothat a high-pressure wave is generated at the intakevalve as it is about to close, to cause a superchargingeffect. Exhaust manifolds can be designed to resonateto achieve the opposite pressure effect to purgeexhaust gases from this cylinder. For a given pipe



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length, resonance occurs only at a certain specificfrequency and its integer multiples so that, histori-cally, tuned intake and exhaust manifolds couldhelp performance only in certain narrow RPMranges. The incorporation of a resonance tanksusing the Helmholtz resonator principle in addi-

tion to tuned length intake pipes has led to impro-ved intake manifold design that provide benefitsover broader RPM ranges. Variable resonancesystems have been introduced, where the intake tubelengths are changed at different RPM by openingand closing switching valves to realize smooth andhigh torque across virtually the entire enginespeed range. Typically, the volumetric efficiencyimprovement is in the range of 4 to 5% over fixedresonance systems.

Another method to increase efficiency is byincreasing valve area. A 2-valve design is limited invalve size by the need to accommodate the valves and

sparkplugs in the circle defined by the cylinder base.The active flow area is defined by the product ofvalve circumference and lift. Increasing the numberof valves is an obvious way to increase total valvearea and flow area, and the 4-valve system, whichincreases flow area by 25 to 30% over 2-valvelayouts, has gained broad acceptance. The valves canbe arranged around the cylinder bore and thesparkplug placed in the center of the bore to improvecombustion. Analysis of additional valve layoutdesigns that take into account the minimum requiredclearance between valve seats and the sparkpluglocation suggest that fivevalve designs (3 intake, 2exhaust) can provide an additional 20% increase inflow area, at the expense of increased valvetraincomplexity. Additional valves do not provide furtherincreases in flow area either due to noncentral pluglocations or valve-to-valve interference.

Under most normal driving conditions, the throt-tling loss is the single largest contributor to reductionin engine efficiency. In s.i. engines, the air is throttledahead of the intake manifold by means of a butterflyvalve that is connected to the accelerator pedal. Thevehicles driver demands a power level by depressingor releasing the accelerator pedal, which in turn

opens or closes the butterfly valve. The presence ofthe butterfly valve in the intake air stream creates avacuum in the intake manifold at part throttleconditions, and the intake stroke draws in air atreduced pressure, resulting in pumping losses. Theselosses are proportional to the intake vacuum anddisappear at wide open throttle.

Measures to reduce throttling loss are varied. Thehorsepower demand by the driver can be satisfied by

any combination of torque and RPM since

Power TorqueRPM:

The higher the torque, the lower the RPM tosatisfy a given power demand. Higher torque impliesless throttling, and the lower RPM also reduces

friction loss so that the optimum theoretical fuelefficiency at a given level of horsepower demandoccurs at the highest torque level the engine iscapable of. In practice, the highest level is neverchosen because of the need to maintain a largereserve of torque for immediate acceleration and alsobecause engine vibrations are a problem at low RPM,especially near or below engine speeds referred to aslugging RPM. Nevertheless, this simple concept canbe exploited to the maximum by using a smalldisplacement high specific output engine in combina-tion with a multispeed transmission with five or moreforward gears. The larger number of gears allows

selection of the highest torque/lowest RPM combina-tion for fuel economy at any speed and load, whilemaintaining sufficient reserve torque for instanta-neous changes in power demand. A specific torqueincrease of 10% can be utilized to provide a fueleconomy benefit of 3 to 3.5% if the engine isdownsized by 8 to 10%. In light vehicles, the numberof forward gears has been increasing from 3 to 5, andsix-speed transmissions are likely to be standard inthe future. The continuously variable transmission isother development that allow continuous change ofgear ratios over a specific range.

Lean-burn is another method to reduce pumpingloss. Rather than throttling the air, the fuel flow isreduced so that the air-fuel ratio increases, orbecomes leaner. (In this context, the c.i. engine is alean-burn engine.) Most s.i. engines, however, do notrun well at air: fuel ratios leaner than 18:1, as thecombustion quality deteriorates under lean con;di-tions. Engines constructed with high swirl andturbulence in the intake charge can run well at air:fuel ratios up to 21:1. In a vehicle, lean-burn enginesare calibrated lean only at light loads to reducethrottling loss, but run at stoichiometric or rich air:fuel ratios at high loads to maximize power. The

excess air combustion at light loads has the addedadvantage of having a favorable effect on thepolytropic coefficient, n, in the efficiency equation.Modern lean-burn engines do not eliminate throt-tling loss, but the reduction is sufficient to improvevehicle fuel economy by 8 to 10%. The disadvantageof lean burn is that such engines cannot yet usecatalytic controls to reduce emissions of oxide ofnitrogen (NOx), and the in-cylinder NOx emission



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control from running lean is sometimes insufficientto meet stringent NOx emissions standards. How-ever, there are developments in lean NOx catalyststhat could allow lean-burn engines to meet the moststringent NOxstandards proposed in the future.

Another type of lean-burn s.i. engine is the

stratified charge engine. Research is focused ondirect-injection stratified charge (DISC) engineswhere the fuel is sprayed into the combustionchamber, rather than into or ahead of the intakevalve. Typically, this enables the air: fuel ratio to varyaxially or radially in the cylinder, with the richest air:fuel ratios present near the sparkplug or at the top ofthe cylinder. Stratification requires very carefuldesign of the combustion chamber shape and intakeswirl, as well as of the fuel injection system.Advanced direct injection systems have been able tomaintain stable combustion at total air: fuel ratios ashigh as 40:1. Such engines have been commercialized

in 2000 in Europe and Japan.Variable valve timing is another method to reduce

throttling loss. By closing the intake valve early, theintake process occurs over a smaller fraction of thecycle, resulting in a lower vacuum in the intakemanifold. It is possible to completely eliminate thebutterfly valve that throttles air and achieve all partload settings by varying the intake valve openingduration. However, at very light load, the intakevalve is open for a very short duration, and this leadsto weaker in-cylinder gas motion and reducedcombustion stability. At high RPM, the throttlingloss benefits are not realized fully. Throttling occursat the valve when the valve closing time increasesrelative to the intake stroke duration at high speeds,due to the valvetrain inertia. Hence, throttling lossescan be decreased by 80% at light load, low RPMconditions, but by only 40 to 50% at high RPM,even with fully variable valve timing.

Variable valve timing can also provide a numberof other benefits, such as reduced valve overlap atlight loads/low speeds (discussed earlier) and max-imized output over the entire range of engine RPM.Fully variable valve timing can result in engineoutput levels of up to 100 BHP/liter at high RPM

with little or no effect on low-speed torque. Incomparison to an engine with fixed valve timing thatoffers equal performance, fuel efficiency improve-ments of 7 to 10% are possible. The principaldrawback has historically been the lack of a durableand low-cost mechanism to implement valve timingchanges. A number of new systems have beenintroduced that are ingenious mechanisms with therequired durability.

6. INCREASING THE EFFICIENCYOF COMPRESSION IGNITION(DIESEL) ENGINES

Compression ignition engines, commonly referred to

as diesel engines, are in widespread use. Most c.i.engines in light-duty vehicle applications are of theindirect injection type (IDI), while most c.i. enginesin heavy-duty vehicles are of the direct injection type.In comparison to s.i. engines, c.i. engines operate atmuch lower brake mean effective pressures of(typically) about 7 to 8 bar at full load. Maximumpower output of a c.i. engine is limited by the rate ofmixing between the injected fuel spray and hot air. Athigh fueling levels, inadequate mixing leads to highblack smoke, and the maximum horsepower isusually smoke limited for most c.i. engines. Naturallyaspirated diesel engines for light-duty vehicle use

have specific power outputs of 25 to 35 BHP per liter,which is about half the specific output of a moderns.i. engine. However, fuel consumption is signifi-cantly better, and c.i. engines are preferred over s.i.engines where fuel economy is important.

Due to the combustion process, as well as the highinternal friction of a c.i. engine, maximum speedis typically limited to less than 4500 RPM, whichpartially explains the lower specific output of c.i.engine. In light-duty vehicle use, an IDI enginecan display between 20 to 40% better fuel economydepending on whether the comparison is based onengines of equal displacement or of equal power

output in the same RPM range. The improvement islargely due to the superior part load efficiency of thec.i. engine, as there is no throttling loss. At highvehicle speeds (4120km/hr), the higher internalfriction of the c.i. engine offsets the reducedthrottling loss, and the fuel efficiency differencebetween s.i. and c.i. engine narrows considerably.

Most of the evolutionary improvements forcompression ignition engines in friction and pumpingloss reduction are conceptually similar to thosedescribed for s.i. engines, and this section focuseson the unique aspects of c.i. engine improvements.

6.1 Design Parameters

Note that c.i. engines have also adopted some of thesame valvetrain designs as those found in s.i. engines.While most c.i. engines were of the OHV type,European c.i. engines for passenger car use are of theOHC type. The c.i. engine is not normally run athigh RPM, so that the difference in specific output



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between an OHV and an OHC design is small. TheOHC design does permit a simpler and lightercylinder block casting, which is beneficial for over-coming some of the inherent weight liabilities. OHCdesigns also permits the camshaft to directly activatethe fuel injector in unit injector designs, which are

capable of high injection pressure and fine atomiza-tion of the fuel spray. Four-valve OHC or DOHCdesigns allow central placement of the fuel injector inthe cylinder, which enhances uniform mixing of airand fuel.

6.2 Thermodynamic Efficiency

The peak efficiency of an IDI engine is comparable toor only slightly better than the peak efficiency of ans.i. engine, based on average values for engines inproduction. The contrast between theoretical andactual efficiency is notable, and part of the reason is

that the prechamber in the IDI diesel is a source ofenergy loss. The design of the prechamber isoptimized to promote swirl and mixing of the fuelspray with air, but the prechamber increases totalcombustion time. Its extra surface area also results inmore heat transfer into the cylinder head.

Direct injection (DI) systems avoid the heat andflow losses from the prechamber by injecting the fuelinto the combustion chamber. The combustionprocess in DI diesels consists of two phases. The firstphase consists of an ignition delay period followed byspontaneous ignition of the fuel droplets. The second

phase is characterized by diffusion burning of thedroplets. The fuel injection system must be capable ofinjecting very little fuel during the first phase andprovide highly atomized fuel and promote intensivemixing during the second. Historically, the mixingprocess has been aided by creating high swirl in thecombustion chamber to promote turbulence. How-ever, high swirl and turbulence also lead to flow lossesand heat losses, thus reducing efficiency. The newestconcept is the quiescent chamber where all of themixing is achieved by injecting fuel at very highpressures to promote fine atomization and completepenetration of the air in the combustion chamber.

New fuel injection systems using unit injectors canachieve pressures in excess of 1500 bar, twice as highas injection pressures utilized previously. Quiescentcombustion chamber designs with high-pressure fuelinjection systems have provided to be very fuelefficient and are coming into widespread use inheavy-duty truck engines. These systems have theadded advantage of reducing particulate and smokeemissions.

DI engines have entered the light-duty vehiclemarket, but these engines still utilize swirl typecombustion chambers. In combination with turbo-charging (see Section 6), the new DI engines haveattained peak efficiencies of over 41%. Fuel economyimprovements in the composite cycle relative to IDI

engines are in the 12 to 15% range, and are up to40% higher than naturally aspirated s.i. engines withsimilar torque characteristics. It is not clear ifquiescent combustion chambers will be ever used inDI engines for cars, since the size of the chamber isquite small and fuel impingement on cylinder walls isa concern.

Although the efficiency equation shows thatincreasing compression ratio has a positive effecton efficiency, practical limitations preclude anysignificant efficiency gain through this method. Athigh compression ratios, the size of the combustionchamber is reduced, and the regions of dead air

trapped between the cylinder and piston edges andcrevices became relatively large, leading to poor airutilization, reduced specific output, and, potentially,higher smoke. Moreover, the stresses on the engineincrease with increasing compression ratio, makingthe engine heavy and bulky. The compression ratiosare already somewhat higher than optimal to provideenough heat of compression so that a cold start atlow ambient temperature is possible.

6.3 Friction and Pumping Loss

Most of the friction reducing technologies that canbe adopted in s.i. engines are conceptually similar tothose that can be adopted for diesels. There arelimitations to the extent of reduction of ring tensionand piston size due to the high compression ratio ofc.i. engines, but roller cam followers, optimizedcrankshaft bearings, and multigrade lubricants havealso been adopted for c.i. engine use. Since friction isa larger fraction of total loss, a 10% reduction infraction in a c.i. engine can lead to a 3 to 4%improvement in fuel economy.

Pumping losses are not as significant a contributorto overall energy loss in a c.i. engine, but tuned

intake manifolds and improved valve port shapesand valve designs have also improved volumetricefficiency of modern c.i. engines. Four-valve designs,in widespread use in the heavy truck market, haveappeared in passenger cars, but their benefitsare smaller in c.i. engine use due to the lowmaximum RPM relative to s.i. engines. Nevertheless,the 4-valve head with a centrally mounted injector isparticularly useful in DI engines since it allows for



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symmetry in the fuel spray with resultant good airutilization.

Variable valve timing or any form of valve controlholds little benefit for c.i. engines due to the lack ofthrottling loss, and lack of high RPM performance.Valve timing can be varied to reduce the effective

compression ratio, so that a very high ratio can beused for cold starts, but a lower, more optimal, ratiofor fully warmed up operation.

7. INTAKE CHARGE BOOSTING

Most c.i. and s.i. engines for light vehicle use intakeair at atmospheric pressure. One method to increasemaximum power at wide open throttle is to increasethe density of air supplied to the intake byprecompression. This permits a smaller displacementengine to be substituted without loss of power and

acceleration performance. The use of a smallerdisplacement engine reduces pumping loss at partload and friction loss. However, intake chargecompression has its own drawbacks. Its effect issimilar to raising the compression ratio in terms ofpeak cylinder pressure, but maximum cylinderpressure is limited in s.i. engines by the fuel octane.Charge boosted engines generally require premiumgasolines with higher octane number if the chargeboost levels are more than 0.2 to 0.3 bar overatmospheric in vehicles for street use. Racing cars useboost levels up to 1.5 bar in conjunction with a very

high octane fuel such as methanol. This limitation isnot present in a c.i. engine, and charge boosting ismuch more common in c.i. engine applications. Mostc.i. engines in heavy-duty truck applications usecharge boosting.

Intake charge boosting is normally achieved by theuse of turbochargers or superchargers. Turbochar-gers recover the wasted heat and pressure in theexhaust through a turbine, which in turn drives acompressor to boost intake pressure. Superchargersare generally driven by the engine itself and aretheoretically less efficient than a turbocharger. Manyengines that use either device also utilize an after-

cooler that cools the compressed air as it exits fromthe supercharger or turbocharger before it enters thes.i. engine. The aftercooler increases engine specificpower output by providing the engine with a denserintake charge, and the lower temperature also helpsin preventing detonation, or knock. Charge boostingis useful only under wide open throttle conditions ins.i. engines, which occur rarely in normal driving, sothat such devices are usually used in high-perfor-

mance vehicles. In c.i. engines, charge boosting iseffective at all speeds and levels.

7.1 Turbochargers

Turbochargers in automotive applications are of the

radial flow turbine type. The turbine extractspressure energy from the exhaust stream and drivesa compressor that increases the pressure of the intakeair. A number of issues affect the performance ofturbomachinery, some of which are a result ofnatural laws governing the interrelationship betweenpressure, airflow, and turbocharger speed. Turbo-chargers do not function at light load because there isvery little energy in the exhaust stream. At high load,the turbochargers ability to provide boost is anonlinear function of exhaust flow. At low enginespeed and high load, the turbocharger provides littleboost, but boost increases rapidly beyond a certain

flow rate that is dependent on the turbocharger size.The turbocharger also has a maximum flow rate, andthe matching of a turbochargers flow characteristicsto a piston engines flow requirements involves anumber of trade-offs. If the turbocharger is sized toprovide adequate charge boost at moderate enginespeeds, high RPM boost is limited and there is asacrifice in maximum power. A larger turbochargercapable of maximizing power at high RPM sacrificesthe ability to provide boost at normal drivingconditions. At very low RPM (for example, whenaccelerating from a stopped condition), no practical

design provides boost immediately. Moreover, theaddition of turbocharger requires the engine com-pression ratio to be decreased by 1.5 to 2 points (or 1to 1.5 with an aftercooler) to prevent detonation.The net result is that turbocharged engines havelower brake specific fuel efficiencies than engines ofequal size, but can provide some efficiency benefitwhen compared to engines of equal mid range or topend power. During sudden acceleration, the turbo-charger does not provide boost instantaneously dueto its inertia, and turbocharged vehicles can havenoticeably different acceleration characteristics thannaturally aspirated vehicles. New variable geometry

turbochargers have improved response and betterboost characteristics over the operating range.Turbochargers are much better suited to c.i.

engines since these engines are unthrottled and thecombustion process is not knock limited. Airflow at agiven engine load/speed setting is always higher for ac.i. engine relative to an s.i. engine, and this providesa less restricted operating regime for the turbochar-ger. The lack of a knock limit also allows increased



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boost and removes the need to cap boost pressureunder most operating conditions. Turbocharged c.i.engines offer up to 50% higher specific power andtorque, and about 10% better fuel economy thannaturally aspirated c.i. engines of approximatelyequal torque capability.

7.2 Superchargers

Most s.i. engine superchargers are driven off thecrankshaft and are of the Roots blower or positivedisplacement pump type. In comparison to turbo-chargers, these superchargers are bulky and weighconsiderably more. In addition, the superchargers aredriven off the crankshaft, absorbing 3 to 5% of theengine power output depending on pressure boostand engine speed.

The supercharger, however, does not have the lowRPM boost problems associated with turbochargers

and can be designed to nearly eliminate any time lagin delivering the full boost level. As a result,superchargers are more acceptable to consumersfrom a driveability viewpoint. The need to reduceengine compression ratio and the superchargersdrive power requirement detract from overall effi-ciency. In automotive applications, a superchargedengine can replace a naturally aspirated engine that is30 to 35% larger in displacement, with a netpumping loss reduction. Overall, fuel economyimproves by about 8% or less, if the added weighteffects are included.

Superchargers are less efficient in combinationwith c.i. engines, since these engines run lean even atfull load, and the power required for compressing airis proportionally greater. Supercharged c.i. enginesare not yet commercially available, since the turbo-charger appears far more suitable in these application.

8. ALTERNATIVE HEAT ENGINES

A number of alternative engines types have beenresearched for use in passenger cars but have not yetproved successful in the market place. A brief

discussion of the suitability of four engines forautomotive power plants is provided next.The Wankel engine is the most successful of the

four engines in that it has been in commercialproduction in limited volume since the 1970s. Thethermodynamic cycle is identical to that of a four-stroke engine, but the engine does not use areciprocating piston in a cylinder. Rather a triangularrotor spins eccentrically inside a Fig. 8shaped

casing. The volume trapped between the two rotoredges and the casing varies with rotor position, sothat the intake, comparison, expansion, and exhauststroke occur as the rotor spins through one revolu-tion. The engine is very compact relative to a pistons.i. engine of equal power, and the lack of reciprocat-

ing parts provides smooth operation. However, thefriction associated with the rotor seals is high, andthe engine also suffers from more heat losses than ans.i. engine. For these reasons, the Wankel enginesefficiency has always been below that of a moderns.i. piston engine.

The two-stroke engine is widely used in smallmotorcycles but was thought to be too inefficient andpolluting for use in passenger cars. One developmentis the use of direct injection stratified charge (DISC)combustion with this type of engine. One of themajor problems with the two-stroke engine is thatthe intake stroke overlaps with the exhaust stroke

resulting in some intake mixture passing uncom-busted into the exhaust. The use of a DISC designavoids this problem since only air is inducted duringintake. Advanced fuel injection systems have beendeveloped to provide a finely atomized mist of fueljust prior to spark initiation and to sustain combus-tion at light loads. The two-stroke engines of thistype are thermodynamically less efficient than four-stroke DISC engines, but the internal friction lossand weight of two-stroke engine is much lower thana four-stroke engine of equal power. As a result, theengine may provide fuel economy equal or superiorto that of a DISC (four-stroke) engine when installedin a vehicle. Experimental prototypes have achievedgood results, but the durability and emissionsperformance of advanced two-stroke engines is stillnot established.

Gas turbine engines are widely used to poweraircraft, and considerable research has been com-pleted to assess its use in automobiles. Such enginesuse continuous combustion of fuel, which holds thepotential for low emissions and multifuel capability.The efficiency of the engine is directly proportionalto the combustion temperature of the fuel, which hasbeen constrained to 12001C by the metals used to

fabricate turbine blades. The use of high-temperatureceramic materials for turbine blades coupled with theuse of regenerative exhaust waste heat recovery wereexpected to increase the efficiency of gas turbineengines to levels significantly higher than theefficiency of s.i. engines.

In reality, such goals have not yet been attainedpartly because the gas turbine components becomeless aerodynamically efficient at the small engine



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sizes suitable for passenger car use. Part loadefficiency is a major problem for gas turbines dueto the nonlinear efficiency changes with airflow ratesin turbomachinery. In addition, the inertia of the gasturbine makes it poorly suited to vehicle applica-tions, where speed and load fluctuations are rapid in

city driving. As a result, there is little optimism thatthe gas turbine powered vehicle will be a reality inthe foreseeable future.

Stirling engines have held a particular fascinationfor researchers since the cycle closely approximatesthe ideal Carnot cycle, which extracts the maximumamount of work theoretically possible from a heatsource. This engine is also a continuous combustionengine like the gas turbine engine. While the engineuses a piston to convert heat energy to work, theworking fluid is enclosed and heat is conducted in andout of the working fluid by heat exchangers. Tomaximize efficiency, the working fluid is a gas of low

molecular weight like hydrogen or helium. Prototypedesigns of the Stirling engine have not yet attainedefficiency goals and have had other problems, such asthe containment of the working fluid. The Stirlingengine is, like the gas turbine, not well suited toapplications where the load and speed change rapidly,and much of the interest in this engine has faded.

SEE ALSO THEFOLLOWING ARTICLES

Alternative Transportation Fuels: ContemporaryCase Studies Combustion and Thermochemistry

Fuel Cycle Analysis of Conventional and AlternativeFuel Vehicles Fuel Economy Initiatives: Interna-tional Comparisons Hybrid Electric Vehicles

Internal Combustion (Gasoline and Diesel) Engines Transportation Fuel Alternatives for HighwayVehicles Vehicles and Their Powerplants: EnergyUse and Efficiency

Further Reading

Amman, C. (1989). The Automotive EngineA Future Perspec-

tive. GM Research Publication GMR-6653.

Lichty, C. (1967). Combustion Engine Processes. John Wiley &

Sons.National Academy of Sciences (2001). Effectiveness and Impact

of Corporate Average Fuel Economy Standards. NationalAcademy Press.

Office of Technology Assessment (1995). Advanced Automotive

TechnologyVisions of a SuperEfficient Family Car. Reportto the U.S. Congress, OTA-ETI-638.

Weiss, M.A.(Ed.) (2000). On the Road in 2020. Massachusetts

Institute of Technology Report MIT-EC00-003.

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