knjiga183_195

1100

183

Categorisation of combustion engines:

By mixture formation and ignition:● Spark-ignition engines. These are run preferably

on petrol and with external or even internal mix-ture formation. Combustion is initiated by exter-nally supplied ignition (spark plug).

● Diesel engines. These have internal mixture for-mation and are run on diesel fuel. Combustion inthe cylinder is initiated by auto-ignition.

By operating principle:● Four-stroke engines. These have a closed (sepa-

rate) gas exchange and require four piston strokesor two crankshaft revolutions for one power cycle.

● Two-stroke engines.These have an open gas ex-change and require two piston strokes or onecrankshaft revolution for one power cycle.

By cylinder arrangement (Fig. 1):● In-line engines ● Opposed-cylinder engines● V-engines ● VR-engines

By piston stroke:● Reciprocating engines● Rotary engines

By cooling:● Liquid-cooled engines● Air-cooled engines

10.1 Spark-ignition engine

The spark-ignition engine is an internal-combustionengine which converts chemical energy into thermalenergy by burning fuel and then converts the thermalenergy into mechanical energy via a piston.

DesignThe spark-ignition engine (Fig. 2) consists primarily offour assemblies and additional auxiliary installations:

● Engine case Cylinder-head cover, cylinderhead, cylinder, crankcase, oilsump

● Crankshaft drive Piston, connecting rod, crank-shaft

● Engine timing Valves, valve springs, rockerarms, rocker-arm shaft, cam-shaft, timing gears, timingchain or toothed belt

● Mixture-formation Injection system or carburettor,system intake pipe

● Auxiliary Ignition system, engine lubri-installations cation, engine cooling, ex-

haust system, if necessary supercharging system

1100 Design and operating principle of a four-stroke engine

In-lineengine

Opposed-cylinderengine

90°

V-engine

15°

OffsetVR-engine

Fig. 1: Categorisation by type of cylinder arrangement

Throttle-valve assembly

Intakemanifold

Inlet port

Spark plug

Piston

Gudgeonpin

Cylinderwithcrankcase

Connectingrod

Crankshaft

Trigger wheelOil-pumpstrainerOil sump

Speedsensor

Exhaustport

Cylinder-head cover

Cylinder head

Injectionnozzle

Camshaft

Rocker arm

Valve

Valve-clearancecompensationelement

TDCsensor

Fig. 2: Structure of a four-stroke spark-ignition engine


1100

184

2nd stroke – Compression 3rd stroke – Combustion 4th stroke – Exhaust

Operating principle of spark-ignition engine

IV = Inlet valve EV = Exhaust valve

IV EVopen

BDC

Induction1st revolution

TDC = Top dead centre

BDC = Bottom dead centre

closed

TDC

BDC

TDCIo

Ic

IV EVclosed

Compression

closed

BDC

TDC

IV EVclosed

Combustion

closed

BDC

TDC

Eo

2nd revolution

IV EVclosed

Exhaust

open

BDC

TDC

Eo

EcEc: EV closesEo: EV opens

Io: IV opensIc: IV closes

Fig. 1:The four strokes of a power cycle

The four strokes of the power cycle are induction, compression, combustion and exhaust (Fig. 1). Onepower cycle takes place in two crankshaft revolutions (720° crank angle).

As the piston movesdown the cylinder, the in-creased volume in thecylinder causes a pres-sure differential of – 0.1 bar to – 0.3 bar com-pared with the externalpressure. Since the pres-sure outside the engine isgreater than that insidethe cylinder, air is forcedinto the induction sys-tem. The ignitable fuel-airmixture is formed eitherin the intake port or di-rectly in the cylinderthrough the injection offuel. In order to admit asmuch intake air or fuel-airmixture as possible intothe cylinder, the inletvalve (IV) opens alreadyat up to 45° CA before topdead centre (TDC) andcloses only at 35° CA to90° CA after bottom deadcentre (BDC).

As the piston moves upthe cylinder, the fuel-airmixture is compressed toa 7th to a 12th of the orig-inal cylinder volume. Inthe case of direct injec-tion, air is compressedand the injection pointcan already begin shortlybefore TDC. The gas heatsto 400 °C to 500 °C. Be-cause it cannot expand atthe high temperature, thefinal compression pres-sure increases up to 18 bar. The high pressureencourages further carbu-ration of the fuel and itsinternal mixture with theair. This enables combus-tion to take place quicklyand completely in the 3rdstroke. The inlet and ex-haust valves are closedduring the compressionstroke.

Combustion is initiatedby an ignition spark jump-ing across the electrodesof the spark plug. Thelength of time betweenthe jumping of the sparkand the complete devel-opment of the flame frontis approx. 1/1,000 secondat a combustion velocityof 20 m/s. For this reason,the ignition spark mustjump across at 0° to ap-prox. 40° before TDC, de-pending on the enginespeed, so that the neces-sary maximum combus-tion pressure of 30 bar to60 bar is available shortlyafter TDC (4° CA ... 10° CA).The expansion of the gas-es heated up to 2,500 °Cforces the piston to bot-tom dead centre andthermal energy is con-verted into mechanicalenergy.

The exhaust valve opensat 40° to approx. 90° beforeBDC; this encourages thedischarge of the exhaustgases and relieves the loadon the crankshaft drive.The pressure of 3 bar to 5 bar still available at theend of the power strokecauses the exhaust gasesheated up to 900 °C to beexpelled from the cylinderat the speed of sound. Asthe piston moves up thecylinder, the remaining ex-haust gas is discharged ata dynamic pressure ofroughly 0.2 bar. To encour-age the exhaust gases tobe discharged, the exhaustvalve closes only after TDCwhile the inlet valve is al-ready open. This overlap-ping of the valve times en-courages the draining andcooling of the combustionchamber and improvescylinder charge.

1st stroke – Induction


1100

185

10.2 Diesel engine

Like the spark-ignition engine, the diesel engine is al-so an internal-combustion engine.

DesignThe diesel engine (Fig. 1), like the spark-ignition en-gine, consists primarily of four assemblies and addi-tional auxiliary installations:

● Engine case

● Crankshaft drive

● Engine timing

● Fuel system with fuel-injection equipment, fuel-supply pump, fuel filter, high-pressure injectionsystem, e.g. – common-rail system– unit-injector system

● Auxiliary installationsEngine lubrication, engine cooling, exhaust sys-tem, if necessary supercharging system, e.g. withexhaust-gas turbocharger and intercooling, if nec-essary cold-starting system, e.g. preheating sys-tem

The diesel-vehicle engine is used as a fast-runningengine with speeds up to approx. 5,500 rpm in pas-senger cars and light commercial vehicles. It is usedas a slow-running engine (speeds up to approx. 2,200rpm) in heavy commercial vehicles.

10.3 General physical andchemical principles

10.3.1 Features of a diesel engine

● Running on diesel or biodiesel fuel.

● Internal mixture formationOnly air is admitted into the cylinder during the in-duction stroke. The fuel-air mixture is formed dur-ing the compression stroke by the injection of fuelunder high pressure into the cylinder.

● Auto-ignitionImmediately after being injected, the fuel is auto-matically ignited on the air, which has been ren-dered extremely hot by compression. The finalcompression temperature exceeds the ignitiontemperature.

● Quality regulationThe naturally aspirated engine is unthrottled, i.e.there is no throttle valve before the intake ports. Inthis way, the engine is supplied over the entirespeed range with an extensively constant air flowas the charge. Load control is effected by alteringthe quantity of fuel to be injected, which in turn al-ters the fuel-air mixture depending on the operat-ing state.

Internal mixture formationAfter the start of injection, the still liquid fuel must beconverted into an ignitable mixture. Table 1 sets outthe time that elapses from the start of injection untilauto-ignition. For internal mixture formation heat isremoved from the hot air so that this air cools. But theair temperature must always be above the auto-igni-tion temperature of the fuel.

The time between the start of injection and the startof combustion is known as the ignition lag.Common rail

Injector

Fig. 1: Diesel-vehicle engine for passenger cars

Fuel is injected in a fine-mist but still liquid

state into the hot air.

Fuel mist is heated to boiling

temperature.

Fuel evaporates at boiling temperature.

Fuel vapours mix with the

hot air.

Fuel vapours heat up to ignition

temperature.

Fuel-air mixture ignites.

Initiation of combustion.

Tim

e r

eq

uir

em

en

t "i

gn

itio

n la

g“

Rem

oval o

f h

eat

fro

m t

he h

ot

air

Table 1: Internal mixture formation and initiationof combustion

Diesel engines consume up to 30 % less fuelthan spark-ignition engines. Their efficiency canstretch up to 46 %.


1100

186

Operating principle of diesel engine

2nd stroke – Compression 3rd stroke – Combustion 4th stroke – Exhaust

Exhaustgas

Fuel injector

IVopen

EVclosed

Air

Induction Compression Combustion Exhaust

Fuel

IVclosed

EVclosed

IVclosed

EVclosed

IVclosed

EVopen

Fig. 1: The four strokes of a power cycle in a direct-injection engine

The four strokes of the power cycle are, as in a spark-ignition engine, induction, compression, combus-tion and exhaust (Fig. 1). One power cycle takes place in two crankshaft revolutions (720° crank angle).

As the piston moves downthe cylinder, the increasedvolume in the cylindercauses a pressure differ-ential pa of – 0.1 bar to – 0.3 bar compared withthe external pressure. Airis forced into the cylinderby the greater externalpressure. The air is admit-ted unthrottled becausethere is no throttle valve.In order to admit as muchintake air as possible intothe cylinder, the inlet valveopens at up to 25° CA be-fore TDC; it closes only atup to 28° CA after BDC inorder to facilitate a subse-quent flow of intake air.The air heats up to 70 °Cto 100 °C in the cylinder.

As the piston moves upthe cylinder, the air iscompressed to a 14th to a24th of the original cylin-der volume. The air heatsup to 600 °C to 900 °C inthe process. Because theair cannot expand at thehigh temperature, the fi-nal compression pressureincreases to 30 bar to 55 bar. Engines with sec-ondary combustion cham-bers, such as a turbulencechamber for example,must be compressed to agreater extent becauseheat losses are generatedby the larger combustion-chamber surface. The inletand exhaust valves areclosed during the com-pression stroke.

Towards the end of thecompression stroke, atroughly 15° CA before TDCto 30° CA before TDC, fine-ly atomised diesel fuel isinjected under high pres-sure (up to 2,050 bar) intothe combustion chamber.The fuel evaporates in thehot air and mixes with theair. Combustion is initiateddue to the fact that thetemperature of the com-pressed air is higher thanthe diesel fuel's auto-igni-tion temperature of 320 °Cto 380 °C. The time be-tween the start of injectionand the start of combus-tion is known as the igni-tion lag. The high combus-tion pressure of up to 160 bar moves the pistontowards BDC. Thermal en-ergy is converted into me-chanical work in theprocess.

The exhaust valve opensat 30° to approx. 60° be-fore BDC; this encouragesthe discharge of the ex-haust gases and relievesthe load on the crankshaftdrive. The pressure of 4 barto 6 bar still available atthe end of the powerstroke causes the exhaustgases heated up to 550 °Cto 750 °C to be expelledfrom the cylinder. As thepiston moves up thecylinder, the remainingexhaust gas is dischargedat a pressure of 0.2 bar to0.4 bar. The exhaust valvecloses slightly before orafter TDC. The heat lossesare lower than in a spark-ignition engine due to thelower exhaust-gas tem-peratures (greater effi-ciency).

1st stroke – Induction

Indirect-injection enginesThe fuel is injected into secondary combustion cham-bers (turbulence, precombustion chambers). Be-cause the split combustion chambers give rise tolarge surface areas, the correspondingly higher heatdissipation during the compression stroke must becompensated by greater compression in order forthe ignition temperature of the diesel fuel to be safe-ly exceeded. The compression ratio ε of indirect-in-jection engines is between 19 and 30.

Direct-injection engines (DI engines)The fuel is injected directly into the combustionchamber. The air heated by compression to up to 900 °C dissipates little heat to the compact combus-tion-chamber surface, thus allowing lower compres-sion. Direct-injection engines have a compression ra-tio ε of between 14 and 20 for passenger cars and ofbetween 14 and 19 for commercial vehicles.


1100

187

10.3.2 Features of a spark-ignition engine

● Running on petrol or gas.

● Mixture formationExternal mixture formation. The fuel-air mixture isformed in the carburettor or in the intake manifoldoutside the cylinder.

Internal mixture formation. Initially only air is ad-mitted into the cylinder during the inductionstroke. The fuel-air mixture is formed during theinduction or compression stroke by the injectionof fuel into the cylinder.

● Externally supplied ignition● Constant-volume combustion

Combustion takes place in a virtually constant vol-ume thanks to the sudden combustion of the fuel-air mixture.

● Quantity regulationThe quantity of the fuel-air mixture is altered ac-cording to the position of the throttle valve (loadstate).

ChargeCharge refers to the mass of the gases (fuel-air mix-ture or air) flowing into the cylinder during the in-duction stroke.

Charge improvement. In order to improve chargeand with it power, it is possible to extend the open-ing times of the inlet valves from 180° crank angle(corresponding to the piston stroke) to up to 315°crank angle (CA). During the exhaust stroke, theburned gases expelled at high speed generate asuction effect. If the inlet valve is opened before thepiston has reached top dead centre, the mixture orthe intake air can flow against the movement of thepiston into the cylinder as a result of the vacuumpressure.

Valve overlap

If the inlet valve is left open until well into the com-pression stroke, the fuel/air mixture acceleratedduring induction to up to 100 m/s (360 km/h) cancontinue to flow into the cylinder on account of itsmass inertia. This supercharging effect is terminat-ed when the pressure generated by the upward-moving piston brakes the flowing-in mixture. Theinlet valve must be closed again no later than at thispoint.

In spite of the induction time being extended, thecylinder charge reaches a maximum of 80 % in non-supercharged engines.

Volumetric efficiency

In the case of internal mixture formation, the volu-metric efficiency is the ratio of air mass drawn in tothe theoretically possible air charge in kg.

λL Volumetric efficiencymz Drawn-in mass of fresh-air or fuel-air mixture in kgmth Theoretically possible mass of fresh-air or fuel-air mix-

ture in kg

In naturally aspirated engines, the volumetric effi-ciency ranges between 0.6 and 0.9 (charge 60% to90%) while, in supercharged engines, a volumetricefficiency of 1.2 to 1.6 (charge 120% to 160%) is pos-sible.

The charge can additionally be improved by a lowerflow resistance of the fresh gases and by lower inter-nal cylinder temperatures. This is achieved by:

● Optimally structured induction pipes

● Favourable combustion-chamber shapes

● Large inlet passages

● Several inlet valves per cylinder

● Good cooling

The charge deteriorates as a result of:

● The flow resistance of the throttle valve.

● Decreasing valve opening times at higher speeds.

● Lower air pressure, with an increase in altitude to100 m engine power drops by roughly 1 %.

Compression ratioCombustion chamber.This is the space enclosed bythe cylinder, the cylinder head and the piston crown.Its size changes continually during a stroke. The com-bustion chamber is at its largest when the piston is atBDC and at its smallest when the piston is at TDC. Thelargest combustion chamber is composed of theswept volume and the compression chamber.

Compression space Vc.This is the smallest combus-tion chamber.

Swept volume Vh.This is the space between the twopiston dead centres TDC and BDC.

Total swept volume VH.This is derived from the sumtotal of the swept volumes of the individual cylindersof an engine.

Both the inlet valve and the exhaust valve areopened in the transition phase from the exhauststroke to the induction stroke.

The volumetric efficiency is the ratio of fuel-airmixture actually drawn in in kg to the theoreticallypossible (complete) cylinder charge with fuel-airmixture in kg.

λL = �m

m

t

z

h

�


1100

188

In spite of the significantly increased compressionwork with ε = 9, the utilisation of the significantly in-creased pressure differential with the same fresh-gascharge results in a work gain or a power increase ofmore than 10% and a reduction in fuel consumptionof roughly 10%.

Reasons for power increase:

● Better removal of burned gases from the smallercompression space.

● Higher temperature during compression, betterand more complete carburation.

● On account of the high compression, the burnedgases can expand to a larger volume, the exhaust-gas temperature decreases and less thermal en-ergy is lost through the exhaust.

The final compression temperature rises as the com-pression ratio increases (Table 1). The compressionratio is therefore limited by the auto-ignition temper-ature of the fuel.

In supercharged engines the compression is lower asthe air is admitted in a highly compressed state intothe cylinder.

Boyle-Mariotte's LawThe upward and downward movement of the pistonin the cylinder causes the pressure and the tempera-ture also to be altered with the volume.

Back in the 17th century the physicists Boyle andMariotte had already discovered that volume andpressure in the cylinder change in inverse proportionwith a constant temperature.

If, for instance, the volume is reduced to an 8th, so thepressure increases by a factor of 8 (Fig. 2).

Comparing the space above the piston before com-pression (swept volume Vh + compression space Vc)with the space above the piston after compression(compression space Vc) produces the compressionratio ε (Fig. 1).

The higher the compression ratio of a spark-ignitionengine, the better the utilisation of fuel energy andthus the engine's efficiency.

Compression

= ratio

Swept volume + Compression space��

Compression spaceS

tro

ke s

BDC

TDC

Vh

Str

oke

s

BDC

TDC

Vc

Vh + Vc

Vcε =

Fig. 1: Compression ratio

12345678

TDC

BDC

12345678

TDC

BDC

12345678

TDC

BDCCompression heat nottaken into account

Compression heattaken into account

V1 = 400 cm3

p1 = 1 barT1 = 273 K

p ·V = const.

V2 = = 50 cm3

p2 = 1 bar · 8 = 8 barT2 = 273 K

400 cm3

8V3 = = 50 cm3

p3 = 8 bar · 2 = 16 barT3 = 273 K · 2 = 546 K

400 cm3

8

p 3 ·V3

T3

p1 ·V1

T1=

Compressionε = 8 : 1

21

Compressionε = 8 : 1

1 3

State 12State 3State

Fig. 2: Ratio of pressure, volume and temperature during compression

Compression ratio

Final compression pressure

Maximum compression pressure

Pressure during openingof exhaust valve

Final compression temperature

7

~10 bar

~30 bar

~ 4 bar

400 °C

9

~16 bar

~42 bar

~ 3 bar

500 °C

Table 1: Comparison of compression ratios

The product of pressure and volume is con-stant.


1100

189

Gay-Lussac's LawBy including the temperature in the ratio of volumeand pressure, the French physicist Gay-Lussac dis-covered the following regularity:

When the gas is heated by 273 K, it expands to twicethe volume.

When the gas is prevented from expanding, e.g. dur-ing compression (Fig. 2, Page 188), the pressure isdoubled. However, the final pressure is lower due tothe dissipation of heat at the cylinder walls.

10.3.3 Combustion sequence of spark-ignition engine

Because only a very short time period is available forcombustion of the fuel-air mixture (combustion is al-ready completed shortly after TDC), fuel and oxygenmolecules must be close to each other in the com-pressed mixture. The oxygen required for combus-tion is removed from the drawn-in air. Because the aircontains only roughly 20 % oxygen, a proportionateamount of air must be admixed with the fuel. Theminimum air quantity required for complete com-bustion, the theoretical air requirement, is roughly14.8 kg air for 1 kg petrol (~ 12 m3 at a density of ρ = 1.29 kg/m3).

The carbon contained in the fuel burns with the oxy-gen to form carbon dioxide (CO2) and the hydrogencontained combines with the oxygen to form watervapour (H2O). The nitrogen contained in the air doesnot assume a predominant role in the combustion.However, toxic nitrogen oxides (NOx) are created athigh pressures and combustion temperatures.

Complete combustion:

The chemical energy of the fuel is converted into ther-mal energy.

If, for example, only 13 kg air are available for 1 kgpetrol, the fuel-air mixture will be too rich (1 :13). Be-cause there is not enough oxygen available, part ofthe carbon burns only incompletely to form carbonmonoxide CO, which is toxic.

Incomplete combustion:

If, for example, 16 kg air is available for 1 kg petrol,the fuel-air mixture will be too lean (1 : 16). Com-plete combustion can indeed occur, but because ofthe lower amount of available fuel which can evap-orate, the interior cylinder chamber is cooled to alesser extent with the result that the engine mayoverheat.

Knocking combustionA spark-ignition engine will knock if the fuel-air mix-ture, instead of the combustion initiated by the ig-nition spark, ignites by itself (Fig. 1).

This auto-ignition, which simultaneously initiatesinflammation in several spark cores, results in apremature, sudden combustion, during which theglobular flame fronts collide with each other. Thisgives rise to combustion velocities of 300 m/s to500 m/s, which in turn result in excessively highpressures (Fig. 2).

The knocking or often also pinging noise in the en-gine is caused by shock waves which are triggeredby the different spark cores and result in individualengine components vibrating. Knocking results inincreased mechanical and thermal load on thecrankshaft drive and reduced power.

If a gas is heated at constant pressure by 1 K (1 °C), it expands by a 273rd part of its volume.

C + O2 ➞ CO2 + thermal energy

2 H2 + O2 ➞ 2 H2O + thermal energy

2 C + O2 ➞ 2 CO + heat

Unburnedmixture

Spark core

Flame front byauto-ignition

Flame front byignition sparks

Burnedmixture

Undesirableauto-ignition

Fig. 1: Knocking combustion

Knockingcombustion

Normalcombustion

TDC

p in

bar

Crankshaft angle in ° CA

Fig. 2: Pressure characteristic during combustion

Causes of knockingApart from the use of unsuitable fuels, knocking canalso be caused by:

● Excessively advanced ignition.

● Uneven mixture distribution in the cylinder.

● Poor heat dissipation due to carbon-residue de-posits or faults in the cooling system.

● An excessively high compression ratio, e.g. whena thinner cylinder-head gasket is used.

Acceleration knockingThis occurs primarily when accelerating under fullload from low engine speeds. It is usually caused byfuel with an insufficient octane number (RON) and in-correct spark adjustment.

High-speed knockingThis is knocking which usually occurs in the upperspeed range at full load. It is often caused by fuel witha MON that is too low or fuel in which the differencebetween RON and MON (= sensitivity) is great. It fre-quently cannot be detected in good time because ofthe louder noises inside the vehicle. Overheating ofthe engine can cause damage such as burned pistoncrowns and cylinder heads as well as piston seizures.

Uncontrolled ignitionThis is triggered by glowing parts in the engine com-bustion chamber already at a stage before the onsetof normal ignition of the fuel-air mixture by the igni-tion spark (uncontrolled advanced ignition).

10.3.4 Combustion sequence of diesel engine

Ignition lag in a diesel engineThe period of time required for internal mixture for-mation up to the initiation of combustion is called theignition lag.

When the engine is a normal operating temperature,the ignition lag is normally approx. 0.001 s (1/1,000 s).It is substantially dependent on …

● … the structure of the fuel molecules (ignitionquality, cetane number).

● … the temperature of the compressed air beforethe start of injection.

● … the degree of atomisation during injection (ex-tent of the injection pressure, size of the fueldroplets).

The higher the pressure and the temperature, theshorter the ignition lag.

Injection takes place in the diesel engine in such away that the main fuel quantity is only admitted in-to the combustion chamber when the initial parts ofthe fuel have ignited in the chamber with the resultthat further-injected fuel is continuously burned.

Diesel knockingWhen the engine and intake-air temperatures arelow, for example when the engine is started fromcold, the time needed to form the internal mixtureis prolonged. The ignition lag becomes too great(over 0.002 s) and the collected fuel burns sudden-ly with a loud noise, resulting in the diesel engineknocking. The sudden combustion is triggered byseveral spark cores which originate from accumu-lated fuel in the combustion chamber. The highpressure peaks thereby generated can result indamage to the crankshaft drive. The knocking canbe reduced by preinjecting a small amount of fueland by using diesel fuel with a higher cetane num-ber in the winter months.

10.4 Pressure-volume diagram (p -V diagram)

Spark-ignition engineThe relationships between pressure, volume andtemperature of gases can be carried over for thepower cycle of a four-stroke spark-ignition engineinto a pressure-volume diagram (p-V diagram). Ac-cording to Boyle-Mariotte and Gay-Lussac, this pro-duces an ideal diagram in which the volume doesnot change, i.e. remains constant, at the respectivepiston reversal points at BDC and TDC during thecombustion process and the exhaust process.

Ideal constant-volume combustion, as depicted inFig. 1, Page 191, calls for the following precondi-tions:

● The cylinder contains only fresh gases and noresidual exhaust gases.

● Complete combustion of the fuel-air mixture.

● Loss-free charge cycle.

● No heat transfer at the cylinder.

● Constant volume during combustion and cool-ing.

● The combustion chamber must be gastight (pis-ton rings).


1100

190

The ignition lag is the period of time between thestart of injection and the initiation of combustion.

Constant-volume combustion: Sudden combus-tion takes place with a constant volume.


1100

191

Process sequence1 ➠ 2 Compression of the fuel-air mixture, pres-

sure increase, no addition of heat.

2 ➠ 3 Combustion of the fuel-air mixture, pres-sure increase with constant volume, i.e. thepiston remains for the brief period of com-bustion at TDC, addition of heat.

3 ➠ 4 Expansion.The gas under high pressure ex-pands and moves the piston to BDC, the start-ing volume is reached. No dissipation of heat.

4 ➠ 1 Cooling. The process takes place with a con-stant volume. The pressure drops as a resultof heat dissipation until the starting pres-sure at point 1 is reached again.

Energy gain, energy lossThe area created in the diagram (Fig. 1) with the cor-ners 1-2-3-4 reflects the work gained during a powercycle (surface +).

The work gained could be greater if the exhaust valvewere not to open already at point 4 but only after thegases have expanded down to the starting pressureat point 5.

However, this is not possible in practice because ex-tending the expansion is associated with increasingthe stroke (long-stroke engine). Thus the area 1-4-5reflects the work lost.The work gained can be increased by increasing thecompression ratio.

Diesel engineIn contrast to a spark-ignition engine, the pressuretheoretically does not change during the combustionprocess; this phenomenon is referred to therefore asconstant-pressure combustion. In reality, neither theconstant-volume nor the constant-pressure processtakes place in ideal circumstances because the con-ditions cannot be maintained.

Actual p -V diagramThe pressure characteristic during the four strokes ofa power cycle can be recorded with a piezoelectricindicator on the running engine and displayed as a

curve on the screen. Here the differences from theideal p -V diagram can be clearly seen. In practice,the curve shapes of a spark-ignition engine and adiesel engine still differ only in the extent of the pres-sures (Fig. 2).Because of the significantly higher combustion pres-sure in a diesel engine and the subsequent expan-sion of the burned gases to 4 bar…6 bar, the exhaustgases cool more markedly than in a spark-ignitionengine. This results in a reduction of the exhaust-gaslosses, which in turn increases the work gained andwith it the efficiency. The thermal load on the valvesis lower. However, modern diesel engines are nolonger able to deliver sufficient heat to heat the vehi-cles with the result that auxiliary heaters are neces-sary.

Error detection in the p -V diagramLarger deviations from the normal pressure charac-teristic enable errors in the engine settings (mixtureformation, spark adjustment, compression) andabove all errors arising from knocking phenomena tobe detected (Fig. 1, Page 192).

Ignition point too advanced:The highest possible pressure is already reached be-fore the piston has arrived at top dead centre. Thehigh pressures and temperatures created result inknocking combustion, poorer exhaust-gas valuesand loss of power, which can be identified from thesmaller area in the diagram.

Addition of heat

Dissipation of heat

TDC BDCV

1

4

3

2

5pamb

p

bar

cm3

Fig. 1: Ideal constant-volume process (p -V diagram)

The p -V diagram can be used to calculate the ef-fective work Weff of an engine by subtracting thelost work (area –) from the produced work (sur-face +) (Fig. 1, Page 192).

60

V in cm3

E = Start of injection= Ignition point

Diesel engineSpark-ignition engine

TDC BDCVh

E

50

40

30

20

10

0

–1

bar

Pre

ssu

re p

in

cyli

nd

er

Induction

Ex-haust

Combustion

Compression

Fig. 2: Actual p -V diagram


1100

192

Ignition point too retarded:Normal rise of the compression line up to top deadcentre. After a short drop in the pressure after TDC, itrises again but can no longer reach the maximumcombustion pressure because due to the retarded ig-nition point the piston has moved too far in the di-rection of BDC before the fuel-air mixture has burnedfully.The consequences are loss of power, higher fuelconsumption and risk of overheating.

Leaking valves or piston rings:Normal pressure build-up not possible, the rise of thecompression line is flatter. The maximum combus-tion pressure cannot be reached even when the igni-tion point is correct. The consequences are loss ofpower and poorer exhaust-gas values.

10.5 Timing diagram

The opening and closing angles of the inlet and ex-haust valves are entered in degrees of crankshaft rev-olutions (Fig. 2).The opening angles of the valves andthe shape of the timing cams are determined by wayof tests for each type in such a way that the enginedelivers the best power possible. Because this is notpossible over the entire speed range, engines areequipped with adjustable inlet camshafts. The open-ing and closing angles of the inlet valves can bechanged by a specific adjustment angle (variable tim-ing).The timing angles of the individual engines deviatefrom each other to the extent that each engine has itsown timing diagram, e.g. Fig. 2.As a rule, the angles from the opening through to theclosing of the valves are greater the higher the nor-mal running speed of the engine is.

Symmetrical timing diagram. The angles Io beforeTDC and Ec after TDC are identical in size, as are theangles Eo before BDC and Ic after BDC.

Asymmetrical timing diagram. One of the two anglepairs is unequal.

10.6 Cylinder numbering, firingorders

Cylinder numbering.The designation of the individ-ual cylinders of an engine is standardised. The count-ing of the cylinders starts from the end opposite theoutput end. In the case of V-, VR- and opposed-cylin-der engines, the counting starts on the left cylinderbank and then each bank is counted through (Fig. 3).

Valves, pistonrings leaking

p i

n b

ar

Vh

Ignition pointretarded

VhVh

Ignition pointadvanced

Fig. 1: p -V diagrams of faulty engines

This provides an overview of the timing angles ofthe valves and the valve overlap.

Io: IV opens 15° before TDCIc: IV closes 40° after BDC

Eo: EV opens 44° before BDCEc: EV closes 22° after TDC

Valveoverlap

TDC

0°…

40° beforeTDC

BDC

Eo

Ic

Io

Ec

Co

mp

ress

ion

Comb

ustio

n

Exh

aust

Ind

uct

ion

Fig. 2:Timing diagram of a four-stroke spark-ignition engine

V-VR-engine

W-engine

VR-engineOpposed-cylinder engine

In-line engine V-engine

Output1

43

2

5

87

6

9

1211

10

Output Output

132

465

Output

12 3

45

6

Left side Output

Right side

12

34

15°1

23

45

6

60°- 90°

12

34

5

6

7

8

Output

Fig. 3: Cylinder numbering


1100

193

Firing order and ignition interval in multiple-cylinder engines (Fig. 1).Firing order. This indicates the order in which thepower strokes of the individual cylinders of an en-gine follow each other.

Ignition interval.This indicates the interval in crank-angle degrees in which the power strokes or the fir-ing operations of the individual cylinders follow eachother. The greater the number of cylinders, the small-er the ignition interval. Engine operation becomessmoother and the torque output is more regular.

Example: In the case of a 5-cylinder engine, the igni-tion interval is calculated from 720° CA : 5 = 144° CA.A star diagram is drawn as a substitute for the crank-shaft. Starting from the uppermost cylinder, which isdesignated as 1, the remaining cylinders are enteredaccording to the firing order 1-2-4-5-3 against the di-rection of rotation at an interval of 144° CA. In thisway, the firing order can be read off from each star di-agram.

Ignition interval 360°

2

Six-cylinder

in-line engine

7 x bearings

Eight-cylinder

V-engine - 90°5 x bearings

Five-cylinder

in-line engine

6 x bearings

Four-cylinderopposed-cylinderengine3 x bearings

Four-cylinder

in-line engine

5 x bearings

Three-cylinder

in-line engine

4 x bearings

Two-cylinder

in-line engine

2 x bearings

Two-cylinderopposed-cylinderengine2 x bearings

One-cylinder

vertical

2 x bearings

1Cylinders Strokes

1 Combustion Exhaust Induction Compression

11

22

1

12

2

1…3

12

3

3

1…4

2

1

4

3

1…5 1

5 4

32

1…6

2.5 3.4

1…4 5…890°

1234

1234

12345

123456

123456

87

Firing order1 – 3 – 2

Firing order1 – 3 – 4 – 21 – 2 – 4 – 3

Firing order1 – 4 – 3 – 2

Firing order1 – 2 – 4 – 5 – 3


Firing order1 – 5 – 3 – 6 – 2 – 4(or 1 – 2 – 4 – 6 – 5 – 3)(or 1 – 4 – 2 – 6 – 3 – 5)

Most commonfiring order1 – 5 – 4 – 8 – 6 – 3 – 7 – 2(1 – 8 – 2 – 7 – 4 – 5 – 3 – 6)(1 – 6 – 3 – 5 – 4 – 7 – 3 – 8)








1

32

4 5

1

23

4

1

32

4

1

32

21

1

2

1

32 4 5

6

1

1.5 6.2

3.7 4.8

13

24

5

7

68

90°

Fig. 1: Crankshaft designs, firing and power-stroke orders

Ignition interval = 720° CA

Cylinder number

10.7 Engine-performance curves

When these measured values are plotted in a chartagainst the speeds, the curves located by the corre-sponding measuring points produce the engine'sperformance curves (Fig. 1).

There are full-load and part-load curves.

Full-load curves. The engine at normal operatingtemperature is braked on a test bench with thethrottle valve fully open.

Full load refers to the load which an engine canovercome at the respective engine speed. The val-ues determined over the entire speed range underidentical load are the basis for the curve shapes fortorque, power and specific fuel consumption. It ispossible to determine from these curve shapes themaximum torque, the maximum power and theminimum fuel consumption for an allocated speed.

Part-load curves. Measurements at part load are al-so important in view of the fact that an engine israrely subjected to full load in everyday drivingconditions. Various series of tests are carried out atdifferent speeds for this purpose. The throttle valveis only partially opened in these tests.

Theoretically, both the fuel consumption and thetorque should be constant in the entire speed rangewith the throttle valve in the same position since inactual fact always the same amount of energy ofone cylinder charge should deliver the same rota-tory force to the crankshaft. Accordingly the powershould rise uniformly with the speed.

However, the power curve drops after the maxi-mum power is reached because as the speed in-creases the loss of torque can no longer be com-pensated.

Causes of deviation from the ideal state:

● Fluctuating charge in the lower and upper speedranges

● Air deficiency and poor swirling of the fuel-airmixture due to low flow velocity, thus slowerand incomplete combustion

● Heat losses

● Friction losses

Elastic range. This lies between the maximumtorque and the maximum power (Fig. 1). As speedreduces, the decreasing power is compensated byan increasing torque. The maximum torque shouldwhere possible be situated before the middle speedrange whereas the maximum power should be sit-uated well into the upper speed range. This pro-duces a broad elastic range, which has a favourableeffect on gearbox tuning because the torque bandincreases in size.

Fuel-consumption map. Torque is plotted againstspeed at different specific fuel consumptions in thediagram (Fig. 2). This results in curves with a con-stant specific fuel consumption which are some-times closed in on themselves.

Because the curves resemble shells, they are alsocalled conchoids. The diagram also contains furthercurves with constant effective power, from which itcan be seen that the engine can deliver the same ef-fective power with a completely different specificfuel consumption. Thus the engine in the diagramcan deliver power of 60 kW both with a specific fuel consumption of 320 g/kWh and with one of280 g/kWh together with increasing torque.


1100

194

The characteristic of an engine is derived fromthe measured values for power, torque and spe-cific fuel consumption determined on the testbench at different speeds.

Max. torque

150S

pe

c. f

ue

l co

nsu

mp

tio

n b

eff

Engine speed n

Torque M

beff

PowerPeff

Elasticrange

Max.power

Min. fuelconsumption

100

50

0

kW

90

0

180

270

Nm

0 2,000 6,0004,000 rpm

350

300

250

g/kWh

Torq

ue

M

Po

we

r P

eff

Fig. 1: Full-load curves of a four-stroke spark-ignition engine

beff = constant

En

gin

e t

orq

ue M

in

Nm

260 g/kWh 280 g/kWh

Engine speed n in rpm,

Peff = constant

300 g/kWh

320 g/kWh

80 kW

60 kW

40 kW

90 kW

Fig. 2: Fuel-consumption map, conchoids


1100

195

The terms power output per litre and weight-to-pow-er ratio have been introduced so that the individualengines can be compared with each other (Table 1).

Power output Weight-to-power ratio of

Engine type per litre engine vehicle

kW/l kg/kW kg/kW

Spark-ignition engines

Motorcycles30…100 0.5…3 2…9

Passenger cars 35…130 1.3…5 4…22

Racing cars …400 1…0.2 1.5…7

Diesel engines

Passenger cars20…50 1.8…5 12…25

Diesel engines

Commercial vehicles10…45 2.5…8 60…230

Supercharged engines

Diesel passenger cars30…70 1…4 9…20

Supercharged engines

Diesel commercial

vehicles18…55 2…7 50…210

Table 1: Power output per litre, weight-to-power ratio

10.8 Stroke-to-bore ratio, poweroutput per litre, weight-to-power ratio

Stroke-to-bore ratio

If the stroke is smaller than the bore, then the stroke-to-bore ratio is less than 1. It is more than 1 if thestroke is larger than the bore.

Short-stroke engines. In order for production en-gines to enjoy long service lives, it is important forthem not to exceed a mean piston speed of 20 m/s.Nevertheless, short-stroke engines are built in orderto achieve high engine speeds. These engines have astroke-to-bore ratio of less than 1 (0.9…0.7).

Long-stroke engines. The stroke-to-bore ratio isgreater than 1 (1.1…1.3). These engines are usedmainly to power commercial vehicles and buses.High mileages and greater torques are achievedthanks to the lower engine speeds and the greatercrank throw respectively.

Power output per litre

Fast-running engines are more suitable for vehiclepropulsion the greater their power is in proportion tothe swept volume and the lower their structuralweight is in proportion to the power.

This indicates the ratio of stroke to bore.

The power output per litre indicates the greatesteffective power of the engine per litre of sweptvolume.

The weight-to-power ratio of the engine indicatesthe structural weight of the engine per 1 kW ofgreatest effective power.

The weight-to-power ratio of the vehicle indicatesthe weight of the vehicle per 1 kW of greatest ef-fective power.

REVIEW QUESTIONS

1 Who built the first four-stroke engine and thefirst two-stroke engine respectively?

2 In what order do the strokes of a four-stroke en-gine take place?

3 What is the compression ratio of four-strokespark-ignition engines?

4 Within which limits do the compression andcombustion pressures of four-stroke spark-igni-tion engines lie?

5 What is the process of internal mixture forma-tion in a diesel engine?

6 What is stated in Gay-Lussac's Law?

7 Which substances are formed during the com-bustion of the fuel-air mixture?

8 What are the causes of knocking in spark-igni-tion engines?

9 What do you understand by ignition lag in adiesel engine?

10 What do you understand by knocking in a dieselengine?

11 What special features apply to a diesel engine?

12 What are the four assemblies of a spark-ignitionengine and of a diesel engine?

13 Which errors can be read off from a pressure-volume diagram?

14 What do you understand by a symmetrical tim-ing diagram?

15 What are the differences between short-strokeand long-stroke engines?

16 What is the firing order of a 6-cylinder in-lineengine?

17 How are the cylinders numbered in compliancewith standards?

18 What does the power output per litre indicate?

19 What does the weight-to-power ratio of the en-gine indicate?

20 What do you understand by the greatest effec-tive power of an engine?

21 What do you understand by full-load curves ofan engine?

22 Why is the torque of a spark-ignition engine notof equal magnitude over the entire speedrange?

23 What do you understand by the elastic range?

knjiga183_195

Documents