The theory of1compressionignition enginesContents1.1 5Introduction51.1.1Historical51.1.2Classifications51.2Two-stroke and four-stroke engines61.2.1Two-strokeengines71.2.2Four-strokeengines1.2.3Evaluation of power output of two-stroke7andfour-strokeengines81.2.4Other operatingparameters1.3Airstandardcycles:constantpressure-constant9volume-dualcombustion1.3.1Theoretical expressions for air standard9cycles1.3.2Further comments on air standard cycles13141.4Basic thermodynamics of real gases141.4.1Gasproperties151.4.2Combustion171.4.3Dissociationandreactionkinetics171.5Real diesel engine cyclic processes171.5.1Closed period191.5.2Openperiod211.6Detailed cycle analysis methods211.6.1Closed period221.6.2Open period (gas exchangeprocess)251.6.3Completion ofcalculation sequence25ReferencesThispage has been reformatted by Knovel toprovide easier navigation
This page has been reformatted by Knovel to provide easier navigation. 1 The theory of compression ignition engines Contents 1.1 Introduction 5 1.1.1 Historical 5 1.1.2 Classifications 5 1.2 Two-stroke and four-stroke engines 5 1.2.1 Two-stroke engines 6 1.2.2 Four-stroke engines 7 1.2.3 Evaluation of power output of two-stroke and four-stroke engines 7 1.2.4 Other operating parameters 8 1.3 Air standard cycles: constant pressure—constant volume—dual combustion 9 1.3.1 Theoretical expressions for air standard cycles 9 1.3.2 Further comments on air standard cycles 13 1.4 Basic thermodynamics of real gases 14 1.4.1 Gas properties 14 1.4.2 Combustion 15 1.4.3 Dissociation and reaction kinetics 17 1.5 Real diesel engine cyclic processes 17 1.5.1 Closed period 17 1.5.2 Open period 19 1.6 Detailed cycle analysis methods 21 1.6.1 Closed period 21 1.6.2 Open period (gas exchange process) 22 1.6.3 Completion of calculation sequence 25 References 25
Thetheoryof compression ignition engines5air-fuel ratios well in excess of stoichiometric, is ensured. The1.1Introductionmixing process is crucial to the operation of the Diesel engineand as such has received a great deal of attention which is1.1.1Historicalreflected in a wide variety of combustion systems which mayconvenientlybegrouped intwobroad categories,viz.Although the history of the diesel engine extends back into theclosingyears of the19thcenturywhenDrRudolfDieselbegan(a)DirectInjection (D)Systems asusedinDIengines,inwhichhispioneeringworkonairblastinjectedstationaryengines,andthe fuel is injected directly into a combustion chamber formedinspiteof thedominant position itnowholds in manyapplications,in the clylinderitself,i.e.between a suitably shapednon-stationarye.g.marinepropulsion and landtransport,both road and rail,itpiston crown and a fixed cylinder head in which is mounted theistodaythesubjectof intensivedevelopmentand capableoffuel injector with its single or multiple sprayorifices or nozzles.improvements.These willguaranteethe diesel engineanassured(See Figures 1.I and 1.2.)place as the most efficient liquid fuel burning prime mover yetderivedBefore1914,building ontheworkof DrRudolf Diesel inGermanyandHubertAkroydStuart intheUK,the dieselengineInjectorwasusedprimarilyinstationaryandshippropulsionapplicationsin theformofrelativelylow speed four-stroke normally aspiratedenginesThe1914-18wargave considerableimpetus to thedevelopmentofthehigh speeddieselenginewithits much higher specificoutput,with a viewto extending its application to vehicles.Although the first generation of road transport engines wereundoubtedly of the spark ignition variety,the somewhat laterFigure1.1Quiescentcombustionsystem.Application-Four-strokedevelopment of diesel engines operating on the self or com-andtwo-strokeenginesmostlyabove150mmbore(Bensonandpression ignition principle followed soon after so thatby theWhitehouse)mid 193Os the high speed normally aspirated dieselengine wasfirmlyestablishedasthemostefficientprimemoverfortrucksand buses. At the same time with the increasing use ofturbocharging it began to displace the highly inefficient steamengine in railway locomotives while the impending 1939-45war gave a major impetus to the development of the highlysupercharged diesel engine as a newaero engine,particularlyinGermany.Since the 1939-45 war every major industrial country hasdeveloped its own range of diesel engines.Its greatestmarketpenetration has undoubtedly occurred in thefield of heavyroadtransport where,at any rate in Europe,it is now dominant.It isparticularly in this field where development, in the direction ofturbocharging in its various forms,has been rapid during thelast twenty years,and where much of the current research anddevelopmenteffortis concentratedHowever.a continuousprocess of uprating and refinement has been applied in all itsFigure 1.2High swirl system.Application to virtuallyall truck andfields of application,from the very largest low speed marinebus sized engines, but increasingly also to the high speed passengertwo-stroke engines,throughmedium speed stationary enginescarenginetosmallsinglecylinderenginesforoperationinremoteareaswith minimum attendance.There is littledoubt that it will continueto occupya leadingpositionin thespectrum of reciprocating(b)IndirectInjection(IDDSystemsasused inIDIenginesinprime movers, so long as fossil fuels continue to be availablewhichfuel is injected into aprechamber which communicatesand,provided it can be made less sensitiveto fuel qualitywellwiththe cylinder through a narrow passage.The rapid transferinto the era of synthetic or coal derived fuels.of air from the main cylinder into the prechamber towards topdead centre (TDC)of the firing stroke promotes a veryhighdegreeofairmotionintheprechamberwhichisparticularly1.1.2Classificationsconducive to rapid fuel-air mixing.(See Figure 1.3.The major distinguishing characteristic of the diesel engine is,Combustion systems aredescribed inmoredetail inChapter4 and generally in Chapters 22 to 29 describing engine types.of course,thecompression-ignition principle,i.e.theadoptionA furthermajor subdivision of diesel engines is into two-strokeof a special method of fuel preparation.Instead of relying onand four-stroke engines, according to themanner in which thethepassage of a spark atapredetermined pointtowards the endof thecompression process toigniteapre-mixedand whollygas exchange process is performed.gaseous fuel-airmixture in approximately stoichiometricproportions as inthe appropriatelynamedcategory of spark-1.2Two-strokeandfour-stroke enginesignition (SI)engines,the compression ignition (CI)engineoperateswithaheterogeneouschargeofpreviouslycompressedair and afinelydivided spray of liquid fuel.The latteris injectedAnevenmorefundamental classificationofdieselenginesthanthataccordingto combustion systemis intotwo-strokeor fourintotheengine cylindertowardstheendof compressionwhen,after a suitably intensive mixing process with the air already instrokeengines,althoughthis latterclassification appliesequallythe cylinder,the self ignitionproperties of the fuel causetosparkignitionenginesandcharacterizesthegasexchangcombustiontobe initiatedfrom small nuclei.These spread rapidlyprocesscommontoallairbreathingreciprocatingengines.Thefunctionofthegas exchageprocess,inbothcases,is to effectsothatcompletecombustionofallinjectedfuel,usuallywith
1.1 Introduction 1.1.1 Historical Although the history of the diesel engine extends back into the closing years of the 19th century when Dr Rudolf Diesel began his pioneering work on air blast injected stationary engines, and in spite of the dominant position it now holds in many applications, e.g. marine propulsion and land transport, both road and rail, it is today the subject of intensive development and capable of improvements. These will guarantee the diesel engine an assured place as the most efficient liquid fuel burning prime mover yet derived. Before 1914, building on the work of Dr Rudolf Diesel in Germany and Hubert Akroyd Stuart in the UK, the diesel engine was used primarily in stationary and ship propulsion applications in the form of relatively low speed four-stroke normally aspirated engines. The 1914-18 war gave considerable impetus to the development of the high speed diesel engine with its much higher specific output, with a view to extending its application to vehicles. Although the first generation of road transport engines were undoubtedly of the spark ignition variety, the somewhat later development of diesel engines operating on the self or compression ignition principle followed soon after so that by the mid 1930s the high speed normally aspirated diesel engine was firmly established as the most efficient prime mover for trucks and buses. At the same time with the increasing use of turbocharging it began to displace the highly inefficient steam engine in railway locomotives while the impending 1939-45 war gave a major impetus to the development of the highly supercharged diesel engine as a new aero engine, particularly in Germany. Since the 1939-45 war every major industrial country has developed its own range of diesel engines. Its greatest market penetration has undoubtedly occurred in the field of heavy road transport where, at any rate in Europe, it is now dominant. It is particularly in this field where development, in the direction of turbocharging in its various forms, has been rapid during the last twenty years, and where much of the current research and development effort is concentrated. However, a continuous process of uprating and refinement has been applied in all its fields of application, from the very largest low speed marine two-stroke engines, through medium speed stationary engines to small single cylinder engines for operation in remote areas with minimum attendance. There is little doubt that it will continue to occupy a leading position in the spectrum of reciprocating prime movers, so long as fossil fuels continue to be available and, provided it can be made less sensitive to fuel quality, well into the era of synthetic or coal derived fuels. 1.1.2 Classifications The major distinguishing characteristic of the diesel engine is, of course, the compression-ignition principle, i.e. the adoption of a special method of fuel preparation. Instead of relying on the passage of a spark at a predetermined point towards the end of the compression process to ignite a pre-mixed and wholly gaseous fuel-air mixture in approximately stoichiometric proportions as in the appropriately named category of sparkignition (SI) engines, the compression ignition (CI) engine operates with a heterogeneous charge of previously compressed air and a finely divided spray of liquid fuel. The latter is injected into the engine cylinder towards the end of compression when, after a suitably intensive mixing process with the air already in the cylinder, the self ignition properties of the fuel cause combustion to be initiated from small nuclei. These spread rapidly so that complete combustion of all injected fuel, usually with air-fuel ratios well in excess of stoichiometric, is ensured. The mixing process is crucial to the operation of the Diesel engine and as such has received a great deal of attention which is reflected in a wide variety of combustion systems which may conveniently be grouped in two broad categories, viz. (a) Direct Injection (DI) Systems as used in DI engines, in which the fuel is injected directly into a combustion chamber formed in the clylinder itself, i.e. between a suitably shaped non-stationary piston crown and a fixed cylinder head in which is mounted the fuel injector with its single or multiple spray orifices or nozzles. (See Figures 1.1 and 7.2.) Figure 1.1 Quiescent combustion system. Application-Four-stroke and two-stroke engines mostly above 150 mm bore (Benson and Whitehouse) Figure 1.2 High swirl system. Application to virtually all truck and bus sized engines, but increasingly also to the high speed passenger car engine (b) Indirect Injection (IDI) Systems as used in IDI engines in which fuel is injected into a prechamber which communicates with the cylinder through a narrow passage. The rapid transfer of air from the main cylinder into the prechamber towards top dead centre (TDC) of the firing stroke promotes a very high degree of air motion in the prechamber which is particularly conducive to rapid fuel-air mixing. (See Figure 1.3.) Combustion systems are described in more detail in Chapter 4 and generally in Chapters 22 to 29 describing engine types. A further major subdivision of diesel engines is into two-stroke and four-stroke engines, according to the manner in which the gas exchange process is performed. 1.2 Two-stroke and four-stroke engines An even more fundamental classification of diesel engines than that according to combustion system is into two-stroke or fourstroke engines, although this latter classification applies equally to spark ignition engines and characterizes the gas exchange process common to all air breathing reciprocating engines. The function of the gas exchage process, in both cases, is to effect Injector
6Diesel EngineReferenceBook(a)(D(c)Fiqure1.4Two-strokeengines:(a)Loopscavenqedenqine:(b)Exhaust valve-in-head engine; (c) Opposed piston engine (BensonandWhitehouse)the exhaust ports. As a result the degree of charge purity (i.e. theproportion of trapped air)attheend ofthescavengingprocesstendstobe low.Asecondadversefeatureresultingfromsymmetrical timingFigure1.3Prechambersystem-compressionswirl.Applicationis loss of trapped chargebetween inletandexhaust port closuretraditionallytohigh speedpassengercarenginesbutnow increasinglyand susceptibilityto furtherpollution of the trapped chargereplaced by direct injection enginewith exhaust gas returned to the cylinderby exhaust manifoldpressure wave effects.The great advantage of the system is itsexpulsionoftheproducts of combustion fromtheenginecylinderoutstanding simplicity.and their replacement by a fresh air charge in readiness for thenext working cycle.1.2.1.2Uniflowscavengesinglepistonengines(Figure1.4b)1.2.1Two-strokeengines(Figures1.4a,b,c)In engines of this type admission of air to the cylinder is usuallyeffected bypiston controlledports while theproducts ofIn two-stroke engines combustion occurs in the region of topcombustion are exhausted through a camshaft operatedexhaustdead centre(TDC)of everyrevolution.Consequentlygasvalve.Such systemsare preferable from the standpoint ofexchange also has to be effected once per revolution in theScavenging in thatthe'uniflow'motion ofthe airfromtheinletregionofbottomdeadcentre(BCD)andwithminimumlossofportsupwardsthroughthecylindertendstoleadtophysicalexpansion work of thecylindergasesfollowing combustion.displacement of,rather than mixing with,the products ofThis implies that escape of gas from the cylinder to exhaustcombustion thus giving improved charge purityat the end of theand charging with fresh airfrom the inlet manifold must occurscavengingprocess.Atthesametimeitisnowpossibletoadopunderthemostfavourablepossibleflowconditionsovertheasvmmetricaltimingoftheexhaustandinletprocessesrelativeshortest possible period. In practice the gas exhange orto bottom dead centre (BDC) so that, with exhaust closureSCAVENGINGprocess intwo-strokeengines occupiesbetweenpreceding inletclosure thedanger of escape of fresh charge into100°and 150°ofcrank angle(CA)disposed approximatelytheexhaustmanifoldpresentintheloopscavengesvstemissymmetricallyaboutBDC.completelyeliminated.ThissystemhasbeenadoptedinanumberTwo-stroke engines may be subdivided according to theof stationary and marine two-stroke engines.particular scavenging systemused into thefollowing sub-groups1.2.1.3Uniflow scavengeopposed piston engines1.2.1.1 Loop scavenged engines (Figure 1.4a)(Figure 1.4c)This is the simplest type of two-stroke engine in which bothInenginesofthistypeadmissionofairiseffectedby'airpistoninletand exhaustare controlled byports in conjunction withacontrolledinletports,andrejectionofproductsofcombustionsinglepiston.Inevitablythis arrangementresults in symmetricalby'exhaustpistoncontrolledexhaustports.Themotionofthetiming which from the standpoint of scavenging is not ideal.Intwo sets of pistons is controlled by either two crankshaftsthe first instance the'loop'air motion in the cylinder is apt toconnected through gearing. or by a signle crankshaft with theproduce a high degree of mixing of the incoming air with the'top'bank ofpistonstransmitting their motion tothe singleproductsofcombustion,insteadofphysicaldisplacementthrough
Figure 1.3 Prechamber system-compression swirl. Applicationtraditionally to high speed passenger car engines but now increasingly replaced by direct injection engine expulsion of the products of combustion from the engine cylinder and their replacement by a fresh air charge in readiness for the next working cycle. 1.2.1 Two-stroke engines (Figures IAa, b, c) In two-stroke engines combustion occurs in the region of top dead centre (TDC) of every revolution. Consequently gas exchange also has to be effected once per revolution in the region of bottom dead centre (BCD) and with minimum loss of expansion work of the cylinder gases following combustion. This implies that escape of gas from the cylinder to exhaust and charging with fresh air from the inlet manifold must occur under the most favourable possible flow conditions over the shortest possible period. In practice the gas exhange or SC AVENGING process in two-stroke engines occupies between 100° and 150° of crank angle (CA) disposed approximately symmetrically about BDC. Two-stroke engines may be subdivided according to the particular scavenging system used into the following sub-groups. 1.2.Ll Loop scavenged engines (Figure L4a) This is the simplest type of two-stroke engine in which both inlet and exhaust are controlled by ports in conjunction with a single piston. Inevitably this arrangement results in symmetrical timing which from the standpoint of scavenging is not ideal. In the first instance the 'loop' air motion in the cylinder is apt to produce a high degree of mixing of the incoming air with the products of combustion, instead of physical displacement through Figure 1.4 Two-stroke engines: (a) Loop scavenged engine; (b) Exhaust valve-in-head engine; (c) Opposed piston engine (Benson and Whitehouse) the exhaust ports. As a result the degree of charge purity (i.e. the proportion of trapped air) at the end of the scavenging process tends to be low. A second adverse feature resulting from symmetrical timing is loss of trapped charge between inlet and exhaust port closure and susceptibility to further pollution of the trapped charge with exhaust gas returned to the cylinder by exhaust manifold pressure wave effects. The great advantage of the system is its outstanding simplicity. 7.2.7.2 Uniflow scavenge single piston engines (Figure IAb) In engines of this type admission of air to the cylinder is usually effected by piston controlled ports while the products of combustion are exhausted through a camshaft operated exhaust valve. Such systems are preferable from the standpoint of scavenging in that the 'uniflow' motion of the air from the inlet ports upwards through the cylinder tends to lead to physical displacement of, rather than mixing with, the products of combustion thus giving improved charge purity at the end of the scavenging process. At the same time it is now possible to adopt asymmetrical timing of the exhaust and inlet processes relative to bottom dead centre (BDC) so that, with exhaust closure preceding inlet closure the danger of escape of fresh charge into the exhaust manifold present in the loop scavenge system is completely eliminated. This system has been adopted in a number of stationary and marine two-stroke engines. 7.2.7.5 Uniflow scavenge opposed piston engines (Figure IAc) In engines of this type admission of air is effected by 'air piston' controlled inlet ports, and rejection of products of combustion by 'exhaust piston' controlled exhaust ports. The motion of the two sets of pistons is controlled by either two crankshafts connected through gearing, or by a signle crankshaft with the 'top' bank of pistons transmitting their motion to the single
Thetheory of compression ignition engines7crankshaft through a crosshead-siderod mechanism.By suitable(a) the longer period available for thegas exhange processoffsetting of thecranks controllingtheairand exhaustpistonsand the separation of the exhaust and inlet periods-asymmetrical timing can be achieved.apartfromthecomparativelyshortoverlap-resultingIt is evident that this system displays the same favourablein a purer trapped charge.characteristics as the exhaust valve in head system, but at the(b) the lower thermal loading associated with engines inexpenseofevengreatermechanical complications.Its outstandingwhich pistons, cylinder heads and liners are exposed toadvantage is the high specific output per cylinder associatedthe most severe pressures and temperatures associatedwithtwopistons.However,thesystemisnowretainedonlyinwithcombustiononlyeveryotherrevolution.large low speed marine,and smaller medium speed stationary(c)Easier lubrication conditionsforpistons,ringsand linersandmarineengines,Inhighspeedformitisstillemployedfordue to the absence of ports,and the idie stroke renewingnaval purposes suchas in somefastpatrol vesselsandmineliner lubrication and giving inertia lift off to rings andsearchers, although its use in road vehicles and locomotives issmall and large end bearings.discontinued.These factors make it possible for the four-stroke engine toachieveoutputlevelsof theorderof 75%of equivalenttwo1.2.2Four-stroke engines (Figure 1.5)strokeengines.Inrecentyearsattentionhasfocusedparticularlyon three-cylinder high speed passenger car two-stroke enginesThe vast majority of current diesel engines operate on the four-asapossiblereplacementforconventional four-cylinder,four-stroke principle in which combustion occurs only every otherstroke engines with considerable potential savings in space andrevolution,again in theregion of top dead centre(TDC),andweight.withtheintermediaterevolutionand itsassociatedpistonstrokesgiven over to thegas exchangeprocess, In practice the exhaust1.2.3Evaluation of power output of two-stroke andvalve(s)openwell beforebottomdeadcentre(BDC)followingthe expansion stroke and only close well after the following topfour-strokeengines (Figures 1.6a and b)dead centre (TDC)position is reached.The inlet valve(s) openIn order to determine the power developed within the enginebeforethis latterTDC,giving aperiodof overlapbetween inletcylinderasaresultofgasforcesactingonthepistonasopposedvalve opening (IVO) and exhaust valve closing (EVC) duringto shaft power from the output shaft, it is necessary to have awhich the comparatively small clearance volume is scavengedrecordof thevariationofgas pressure (p)withstrokeorcylinderofmostoftheremainingproductsofcombustion.Followingvolume (V)referredtoasanIndicatorDiagram(orp-VDiagram)completion of the inlet stroke, the inlet valve(s) close well afterThis used to be obtained by mechanical means, but such crudethefollowingbottomdeadcentre(BDC).afterwhichthe‘closedinstrumentationhasnowbeencompletelyreplacedbyelectronicportionofthe cycle,i.e.thesequencecompression, combustion,instrumentsknown as pressure transducers.It is also generallyexpansionleadstothenextcycle,commencingagain withexhaustmore convenient to combine the pressure measurement with avalveopening (EVO)crank angle (CA)measurement,using aposition transducer inThe main advantages of the four-stroke cycle over its two-conjunction with a suitable crank angle marker disc,and subse-stroke counterpart are:quently convert crank angle to stroke values by a simple geometricInletExhausttransformation.valvevalveThe sequenceofevents for the twocycles maybe summarizedas follows:(a)Two-stroke cycle (asymmetrical timing)1-2compression2-3Closed Periodheat release associatedwithcombustionPiston34360°CAexpansion4-5blowdown5-6Open Periodscavenging6-1 supercharge(b)Four-stroke cycle1-2compressionClosed Period2-3heat release associatedwith combustion3-4 expansion720°CATDC4-5blowdown5-6exhaustIVOEVCOpen Period6-7 overlap7-8 induction8-1recompressionIn both cases the cycle divides itself into the closed periodduringwhich poweris beingproduced,and the open or gasexchangeperiodwhichmaymakeasmallpositivecontributionto power production or,inthe case of the four-stroke engine,EVOIVCunder conditions of adverse pressure differences between inletandexhaustmanifold,anegativecontribution.InthecaseoftheBDCfour-stroke engine the area enclosed by the p-V diagram for theFour-stroke engine (turbocharged)Figure1.5gas exchangeprocess,i.e.5-6-7-8,isknown asthepumping
crankshaft through a crosshead-siderod mechanism. By suitable offsetting of the cranks controlling the air and exhaust pistons asymmetrical timing can be achieved. It is evident that this system displays the same favourable characteristics as the exhaust valve in head system, but at the expense of even greater mechanical complications. Its outstanding advantage is the high specific output per cylinder associated with two pistons. However, the system is now retained only in large low speed marine, and smaller medium speed stationary and marine engines. In high speed form it is still employed for naval purposes such as in some fast patrol vessels and mine searchers, although its use in road vehicles and locomotives is discontinued. 1.2.2 Four-stroke engines (Figure 1.5) The vast majority of current diesel engines operate on the fourstroke principle in which combustion occurs only every other revolution, again in the region of top dead centre (TDC), and with the intermediate revolution and its associated piston strokes given over to the gas exchange process. In practice the exhaust valve(s) open well before bottom dead centre (BDC) following the expansion stroke and only close well after the following top dead centre (TDC) position is reached. The inlet valve(s) open before this latter TDC, giving a period of overlap between inlet valve opening (IVO) and exhaust valve closing (EVC) during which the comparatively small clearance volume is scavenged of most of the remaining products of combustion. Following completion of the inlet stroke, the inlet valve(s) close well after the following bottom dead centre (BDC), after which the 'closed' portion of the cycle, i.e. the sequence compression, combustion, expansion, leads to the next cycle, commencing again with exhaust valve opening (EVO). The main advantages of the four-stroke cycle over its twostroke counterpart are: Figure 1.5 Four-stroke engine (turbocharged) (a) the longer period available for the gas exhange process and the separation of the exhaust and inlet periods— apart from the comparatively short overlap—resulting in a purer trapped charge. (b) the lower thermal loading associated with engines in which pistons, cylinder heads and liners are exposed to the most severe pressures and temperatures associated with combustion only every other revolution. (c) Easier lubrication conditions for pistons, rings and liners due to the absence of ports, and the idle stroke renewing liner lubrication and giving inertia lift off to rings and small and large end bearings. These factors make it possible for the four-stroke engine to achieve output levels of the order of 75% of equivalent twostroke engines. In recent years attention has focused particularly on three-cylinder high speed passenger car two-stroke engines as a possible replacement for conventional four-cylinder, fourstroke engines with considerable potential savings in space and weight. 1.2.3 Evaluation of power output of two-stroke and four-stroke engines (Figures 1.6a and b} In order to determine the power developed within the engine cylinder as a result of gas forces acting on the piston as opposed to shaft power from the output shaft, it is necessary to have a record of the variation of gas pressure (/?) with stroke or cylinder volume (V) referred to as an Indicator Diagram (or p'-VDiagram). This used to be obtained by mechanical means, but such crude instrumentation has now been completely replaced by electronic instruments known as pressure transducers. It is also generally more convenient to combine the pressure measurement with a crank angle (CA) measurement, using a position transducer in conjunction with a suitable crank angle marker disc, and subsequently convert crank angle to stroke values by a simple geometric transformation. The sequence of events for the two cycles may be summarized as follows: (a) Two-stroke cycle (asymmetrical timing) 1-2 compression 1 2-3 heat release associated > Closed Period with combustion J 3-4 expansion 36O0CA 4-5 blowdown 1 5-6 scavenging f Open Period 6-1 supercharge J (b) Four-stroke cycle 1-2 compression 1 2-3 heat release associated > Closed Period with combustion J 3-4 expansion 72O0CA 4-5 blowdown 5-6 exhaust 6-7 overlap Open Period 7-8 induction 8-1 recompression In both cases the cycle divides itself into the closed period during which power is being produced, and the open or gas exchange period which may make a small positive contribution to power production or, in the case of the four-stroke engine, under conditions of adverse pressure differences between inlet and exhaust manifold, a negative contribution. In the case of the four-stroke engine the area enclosed by the p-V diagram for the gas exchange process, i.e. 5—6—7-8, is known as the pumping Inlet valve Exhaust valve Piston
8Diesel Engine Reference Bookgeometric swept volume Vwepr.In the caseoftwo-strokeengine,43Pwiththegas exchangeperiodoccupyingupto150°CA,(Vswep)em/Vswep may be considerably less than unity whileforfour-strokeengines itvaries between closeto unity and 0.8 (approx.)Similarlythe volumetric compression ratio (CR),whichagainis crucial in air standard cycle calculations,is usually based onEOthe effective swept volume4SO(Vawepc )efr+ Velearmncei.e.(CR)eff=(1.3a)SCVelearanceEC52-rather than the geometric value+BDC116Vswep +Vekewaince(1.3b)(CR)geom2Velearance+V(a)Finally, indicated thermal efficiency n or indicated specificfuel consumption i.s.f.c.areevaluated fromtheexpression4w(eqn (1.2a) or (1.2b)P(1.4a)ni=mr(kg/sec)CV(kJ/kg)where m, is the rate of fuel flow to the engine and CV is thelowercalorificvalueof thefuel andEOAm×3600i.s.f.c. =kg/kWhr(1.4b)W1042IC1.2.4 Other operating parameters8EC(a) Air-fuel ratioInstrokeThe combustion process is governed in large measure by the airfuel ratio in the cylinder, expressed either in actual termsPOut strokema, (kg/sec)(A/F)=i.e.(1.5a)(mr)kg/secV(b)where ma,is the rate of trapped airflow to the engine or relativeFigure 1.6Gas exchange period. p-V diagrams: (a) Two-stroketo the chemically correct or stoichiometric air fuel ratio for the(asymmetrical timing): (b)Four-stroke.(Bensonand Whitehouse)particular fuel, i.e. excess air factor(A/F)actualloopwhichmaycontributepositiveornegativeloopworkto(1.5b)=the work associated with the power loop.Figures .6a and b are(A/F)stoichiometrictypical p-V diagrams of the open or gas exchange period forIn practice,for most hydrocarbon fuelstwo-stroke and four-stroke engines.In both types of engine the cyclic integral expression leads(A/F)soichiomeric = 14.9(1.5c)to the so-called indicated mean effective pressureand, depending on the combustion system used, the limitingJ pdvJawrelative air fuel ratio for smokefree combustion at full load is in(1.1)Pind =VsweptVsweptthe rangewherefdWrepresentsthe cyclic work withthe distinction that1.2<8<1.6the cycleoccupies360°for two-stroke and 720°forfour-strokebeing lower for IDI than for DI engines.engines.The power may then be evaluated from the following(b)Gas exchange parametersexpressions:Fortwo-stroke engines,in particular, it is vitallyimportantmake a distinction berween the trapped rate of airflow mat andWiaoaoke (W)= Pm (bar) eg(m)N (revs)the total rate of airflow supplied to the engine mg.This arises(1.2a)10-2from the fact that the scavenging process in two-stroke enginesis accompaniedby substantial lossofairtoexhaust,partly throughormixing with products of combustion and partly through short-circuiting (see section 1.3.1)and leads to the definition of trappingPinu (bar) Vswept (m)N。(rev/s)nWourstroke (kW)= efficiencyas(1.2b)2 ×10-2(ma)tnu =Forpurposesofcomparison with airstandard cycles (seesection(1.6a)(ma)1.3).itisappropriatetousetheeffectivesweptvolume(Vswepi)effi.e.that associated with the closed period only rather than theor its reciprocal, the scavenge ratio
Figure 1.6 Gas exchange period. p-V diagrams: (a) Two-stroke (asymmetrical timing); (b) Four-stroke. (Benson and Whitehouse) loop which may contribute positive or negative loop work to the work associated with the power loop. Figures 1.6a and b are typical p-V diagrams of the open or gas exchange period for two-stroke and four-stroke engines. In both types of engine the cyclic integral expression leads to the so-called indicated mean effective pressure \pdV JdW Pind= y = y (LI) v swept v swept where J dWrepresents the cyclic work with the distinction that the cycle occupies 360° for two-stroke and 720° for four-stroke engines. The power may then be evaluated from the following expressions: Pmd (bar) K^nt (m3 )7Ve (rev/s)n , rts\\T\ swept v / e v / /1O \ ^two-stroke (kW) = — (1.2a) or Pind (bar) ^ent (m3 )7Ve (rev/s)« TI/ /VYXA ^W CP L v ' c v / OU\ Wfour-stroke (kW) = ~ TTT^ (1.2b) Z, XlU For purposes of comparison with air standard cycles (see section 1.3), it is appropriate to use the effective swept volume (Vswept)eff, i.e. that associated with the closed period only rather than the geometric swept volume Vswept. In the case of two-stroke engine, with the gas exchange period occupying up to 15O0CA, (Vswept)eff/ Vswept may be considerably less than unity while for four-stroke engines it varies between close to unity and 0.8 (approx.). Similarly the volumetric compression ratio (CR), which again is crucial in air standard cycle calculations, is usually based on the effective swept volume /^r> , (^swept )eff + ^clearance ,IO N i.e. (CK)eff = r? (1.3a) ^clearance rather than the geometric value ^swept + ^clearance (^)geom - v (1.3D) ^clearance Finally, indicated thermal efficiency T]1 or indicated specific fuel consumption i.s.f.c. are evaluated from the expression W(eqn(1.2a)or(1.2b)) Tt. — f i 4a) 11 rhf (kg/sec) CV(kJ/kg) v ' ' where mf is the rate of fuel flow to the engine and CV is the lower calorific value of the fuel and i.s.f.c.= ^ X . 360°kg/kWhr (1.4b) W 1.2A Other operating parameters (a) Air-fuel ratio The combustion process is governed in large measure by the air fuel ratio in the cylinder, expressed either in actual terms ma, (kg/sec) - (A/F)=(i^ <'•*» where ma is the rate of trapped airflow to the engine or relative to the chemically correct or stoichiometric air fuel ratio for the particular fuel, i.e. excess air factor £ =rJA/F)aC""" <'- 5 b> (A/r ) stoichiometric In practice, for most hydrocarbon fuels (A/F)stoichlometnc =± 14.9 (1.5c) and, depending on the combustion system used, the limiting relative air fuel ratio for smokefree combustion at full load is in the range 1.2 < £ < 1.6 being lower for IDI than for DI engines. (b) Gas exchange parameters For two-stroke engines, in particular, it is vitally important to make a distinction between the trapped rate of airflow mat and the total rate of airflow supplied to the engine ma. This arises from the fact that the scavenging process in two-stroke engines is accompanied by substantial loss of air to exhaust, partly through mixing with products of combustion and partly through shortcircuiting (see section 1.3.1) and leads to the definition of trapping efficiency as (mn )f ^=W c-6a) or its reciprocal, the scavenge ratio In stroke Out stroke