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1.1 Introduction1.1.1 HistoricalAlthough the his tory of the
diesel eng ine extends back into theclosing years of the 19th
century when Dr Rudo lf Diesel beganhis pioneering work o n air
blas t injected s tationary engines, andin spite of the dom inant
position it now holds in ma ny applications,e.g. marine propulsion
and land transport, both road and rail, i ti s today the subject of
intensive development and capable ofimprovements . These wil l
guaran tee the diesel engine an assuredplace as the most efficient
l iquid fuel burning prime mover yetderived.Before 1914, bui lding
on the work of Dr Rudolf Diesel inGerman y and Hubert Akroyd Stu
art in the UK, the diesel eng inewas used primarily in stationary
and ship propulsion a pplicationsin th e form of relatively low
speed four-stroke norm ally aspiratedengines.The 1914-18 w ar gave
considerable impetus to the develop-ment of the high speed diesel
engine with its much higher specificou tpu t , with a view to
extending i ts application to vehicles .Al t h o u g h th e first
generation of road transport engines wereundoubtedly of the spark
igni tion variety, the somewhat laterdevelopment of diesel engines
operating on the self or com-pression igni tion principle followed
soon after so that by themid 1930s the high speed norm ally
aspirated diesel engine wasfirmly established as the most efficient
prime mover for trucksand buses . At the sa me t i me w i th t he i
n c r e as i n g u s e o fturbocharging it began to displace th e
highly inefficient s teamengine in rai lway locomotives whi le the
impending 1939-45war gave a major impetus to the development of the
highlysupercharged diesel engine as a new aero engine, particularly
inGermany .Since th e 1939-45 w ar every major industrial country
hasdeveloped its own range of diesel engines. Its greatest
marketpenetration has undou btedly occurred in the f ield of heavy
roadtransport where, at any rate in Europe, i t is now dominant. It
isparticularly in this field where development, in the direction
ofturbocharging in i ts various forms, has been rapid during
thelast twenty years , and where much of the current research
anddevelopment effort i s concentrated. However, a
continuousprocess of uprating and refinement has been applied in
all i tsfields of application, from the very largest low speed
marinetwo-s t roke engines , through m edium speed s tationary
enginesto small single cylinder engines for operation in remote
areaswith minimu m attendance. There is little doubt that it will
continueto occupy a leading posi t ion in the spectrum of
reciprocatingprime movers , so long as fossi l fuels continue to be
avai lableand, provided it can be made less sensitive to fuel quali
ty , wellinto the era of synthetic or coal derived fuels .1.1.2
ClassificationsTh e major dis tingu ishing characteris tic of the
diesel engine is ,of course, th e compression-ignition principle,
i.e. th e adoptionof a special method of fuel preparation. Instead
of relying onthe passage of a spark at a predetermined point tow
ards the endof the compression process to igni te a pre-mixed and
whollygaseous fuel-air mi x tu re in ap p rox i ma te ly
stoichiometricproportions as in the appropriately named category of
spark-ignition (SI) engines , th e compression ignition (CI)
engineoperates w ith a heterogeneous charge of previou sly com
pressedair and a finely divided spray of liquid fuel . The latter
is injectedinto the engine cylinder towards the end of compression
wh en,after a sui tably intensive m ixing process with the air
already inthe cylinder, the self igni tion properties of the fuel
causecombu stion to be initiated from small nuclei. These spread
rapidlyso that complete combustion of all injected fuel, usual ly
wi th
air-fuel ratios well in excess of s toichiometric, is ensured .
Th em ix ing process is crucial to the operation o f the Diesel en
gineand as such has received a great deal of attention which
isreflected in a wide variety of combustion systems which
mayconveniently be grouped in two broad categories , viz.(a )
Direct Injection (DI) Systems as used in DI engines, in whichth e
fuel i s injected directly into a combustion chamber formedin th e
clylinder itself, i.e. between a suitably shaped non-stationarypis
ton crown and a fixed cylinder head in which is mounted thefuel
injector with i ts s ingle or multiple spray ori f ices or no zzles
.(See Figures 1.1 and 7.2.)
Figure 1.1 Quiescent combustion system.
Application-Four-strokeand two-stroke engines mostly above 150 mm
bore (Benson andWhitehouse)
Figure 1.2 High swirl system. Application to virtually all truck
andbus sized engines, but increasingly also to the high speed
passengercar engine
(b ) Indirect Injection (IDI) Systems as used in IDI engines
inwhich fuel i s injected into a prechamber which communicateswith
the cylinder through a narrow passage. The rapid transferof air
from the main cylinder into the prechamber towards topdead centre
(TDC) of the f i r ing stroke promotes a very highdegree of ai r
motion in the prechamber wh ich is particula rlyconducive to rapid
fuel-air mixing. (See Figure 1.3.)Com bustion system s are
described in more detail in Chap ter4 and generally in Chapters 22
to 29 describing engine types.A fur ther ma jor subdivis ion of
diesel eng ines is into two-strokean d four-stroke engines,
according to the manner in which th egas exchange process is
performed .1.2 Two-stroke and four-stroke enginesAn even more fund
am ental classif ication of diesel engines thanthat according to
combustion system is into two-stroke or four-stroke engines,
although this latter classification applies equallyto spark igni
tion engines and characterizes the gas exchangeprocess common to
all air breathing reciprocating engines. Thefunct ion of the gas
exchage process, in both cases, is to effect
Injector
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Figure 1.3 Prechamber system-compression swir l .
Application-traditionally to high speed passenger car engines but
now increasinglyreplaced by direct injection engineexpulsion of the
produc ts of com bustion from the engine cylinderand their
replacement by a fresh air charge in readiness for thenext working
cycle.1.2.1 Two-stroke engines (Figures IAa, b, c)In two-stroke
engines combustion occurs in the region of topdead centre (TDC) o f
e v e ry revolution. Con s e q ue n t ly gasexchange also has to be
effected once per revolution in theregion of bottom dead centre
(BCD) and with minimum loss ofexpansion work of the cylinder gases
following combustion.This implies that escape of gas from th e
cylinder to exhaus tand charging with fresh air from th e in le t
mani fo ld m us t occurunder the most favourable possible flow
condi tions over theshor tes t poss ib le per iod . In pract i ce
the gas e x h a n g e o rSC AV ENG ING process in two-stroke
engines occupies between100 and 150 of crank angle (CA) disposed
approximatelysymmetr i ca l ly about BD C.Two-stroke engines may be
subdivided according to theparticular scavenging system u sed into
the following sub-groups.1.2.Ll Loop scavenged engines (Figure
L4a)This is the s implest type of two-stroke engine in which
bothinlet and exhaust are controlled by ports in conjunction with
asingle pis ton. Inevi tably this arrangement results in
symmetricalt iming which from the standpoint of scavenging is not
ideal. Inth e first ins tance the ' l oop ' air motion in the
cylinder is apt toproduce a high degree of mixing of the incoming
air wi th theproducts of combustion, instead of physical
displacement thro ugh
Figure 1.4 Two-stroke engines: (a) Loop scavenged engine;
(b)Exhaust valve-in-head engine; (c) Opposed piston engine
(Bensonan d Whitehouse)th e exhau st ports. A s a result th e
degree of charge purity (i .e. th eproportion of trapped air) at
the end of the scavenging processtends to be low.A second adverse
featu re resulting from symmetr i ca l t imingis loss of trapped
charge between inlet and exhaust port closureand susceptibility to
fur ther pollution of the trapped chargewith exhaust gas returned
to the cylinder by exhaust manifo ldpressure wave effects. The
great ad vantage of the system is i tsouts tanding s implici ty
.7.2.7.2 Uniflow scavenge single piston engines (Figure IAb)In
engines of this type adm ission of ai r to the cylinde r is usu
allyeffected by pis ton con tro l led por ts whi le th e p rod uc t
s o fcombustion are exhausted through a camshaft operated exhaustva
lve . Such sys tems are p referab le f rom the s tandpoin t
ofscavenging in that the 'un i f low ' motion of the air from th e
inletports upwards through the cylinder tends to lead to physicald
i sp lacement of , ra ther than mix ing wi th , the p roducts ofcom
bustion thu s giving improved charge p uri ty at the end of
thescavenging process . At the same time it is now possible to
adoptasymm etrical t iming of the exhaust and inlet processes
relativeto bottom dead centre (BDC) so that, wi th exhaust
closurepreceding inlet closure the danger of escape of fresh charge
intothe exhaust manifold present in the loop scavenge system iscom
pletely eliminated. This system has been adopted in a num berof s
tationary and marine two-stroke engines.7.2.7.5 Uniflow scavenge
opposed piston engines(Figure IAc)In engines o f this type
admission of air is effected by 'a i r piston'controlled inlet
ports , and rejection of produc ts of com bustionby 'exhaust
piston' controlled exha ust ports . The motion of thetwo sets of
pis tons is controlled by ei ther two crankshaftsconnected through
gearing, or by a s ignle crankshaft wi th the'top' bank of pis tons
transmitting their motion to the single
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crankshaft through a crosshead-siderod mec han ism. By
suitableoffsetting of the cranks control l ing the air and exhaust
pistonsasymmetr ica l t iming can be ach ieved.I t is evident that
this system displays the same favourablecharacter ist ics as the
exhaust valve in head system, but a t theexpense of even greater
mechanical com plications. Its outstandingadvantage is the high
specif ic output per cylinder associatedwith two pistons. However ,
the system is now retained only inlarge low speed marine, and
smaller medium speed stat ionaryand mar ine eng ines . In high
speed form it is st il l employ ed fo rnaval purposes such as in so
m e fast patrol vessels and m inesearchers, a l thou gh i ts use in
road veh icles and locom otives isd iscont inued.1.2.2 Four-stroke
engines (Figure 1.5)The vas t major i ty of current diesel engines
operate o n the four-stroke pr inciple in which combustion occurs
only every otherrevolution, again in the region of top dead centre
(TDC), andwith the intermediate revolution and i ts associated
piston strokesgiven over to the gas exchang e process. In practice
the exha ustvalve(s) open well before bottom dead centre (BDC) fol
lowingthe expansion stroke and on ly close w ell after the fo l
lowing topdead centre (TDC) posit ion is reached. Th e inle t
valve(s) openbefore this la tter TDC, g iving a period of overlap
between inle tva lve opening ( IVO) and exhaus t va lve c los ing
(EVC) dur ingw hi ch the comparatively small c learance volume is
scavengedof most o f the remaining produc ts o f combust ion . Fo l
lowingcompletion of the inle t stroke, the inle t valve(s) close
wel l afterth e fol lowing bottom dead centre (BDC), after which
the 'closed'port ion of the cycle , i .e. the sequence com
pression, com bustion ,expansion, leads to the next cycle, comm
encing again with exhaustva lve opening (EVO) .The main advantages
of the four-stroke cycle over i ts two-stroke counterpart are:
Figure 1.5 Four-stroke engine (turbocharged)
(a ) th e longer period ava i lab le for the gas exhange
processand the separat ion of the exhaust and inle t periodsapar t
f rom the compara t ive ly shor t ove r lapresu l t ingin a purer t
rapped charge.(b) the lower thermal loading associated with eng
ines inwhich pistons, cylinder heads and liners are exposed toth e
most severe pressures and temperatures associatedwith combustion
only every other revolution.(c) Easier lubricat ion condit ions for
pistons, r ings and l inersdue to the absence of ports, and the
idle stroke renewingl iner lubr ica t ion and giving ine r t ia l
if t off to r ings andsmall and large end bear ings .These factors
make it possible for the four-stroke engine toachieve output levels
of the order of 75% of equ iva lent two-stroke engines. In recent
years a t tention has focused part icular lyon three-cylinder high
speed passenger car two-s t roke eng inesas a possible replacement
for conventional four-cyl inder , four-stroke engines with
considerable po tentia l savings in space andweigh t .1.2.3
Evaluation of power output of two-stroke andfour-stroke engines
(Figures 1.6a and b}In order to determine th e power developed
within th e eng inecylinde r as a result of gas forces act ing on
the piston as opposedto shaft power from the ou tpu t shaft , i t
is necessary to have arecord of the variat ion of gas pressure (/?)
with stroke or cylin dervo lume (V) referred to as an Indicator D
iagram (or p'-VDiagram).This used to be obtained by mechanical
means, but such crudeins t rumenta t ion has now been com plete ly
replaced by electronicinstruments known as pressure transducers. I
t is a lso general lymore conven ient to combine the pressure
measurem ent wi th ac rank ang le (CA ) measurement , us ing a pos
i tion t ransducer incon junct ion with a suitable crank angle
marker disc , and subse-quent ly convert crank angle to stroke
values by a simp le geom etrict ransfo rmat ion .
The sequence of events for the two cycles m ay be sum marizedas
fo l lows:(a ) Two-stroke cycle ( asymmetr ica l t iming)1-2
compress ion 12- 3 heat release associated > Closed Periodwith
combust ion J3-4 expansion 36O 0C A4-5 b l o w do w n 15-6 scaveng
ing f Open Period6- 1 supercharge J(b ) Four-stroke cycle1-2
compress ion 1
2-3 heat release associated > Closed Periodwith combust ion
J3-4 expansion 72O 0C A4-5 b l o w do w n5-6 exhaus t6-7 overlap
Open Period7-8 i ndu c t io n8-1 recompressionIn both cases th e
cycle divides i tse lf into th e closed perioddur ing which power
is be ing produced, and the open or gasexchange per iod which m ay
make a smal l pos i t ive cont r ibu t ionto power production or ,
in the case of the four-stroke engine,under condit ions of adverse
pressure differences between inle tand exhau st manifold , a
negative co ntr ibutio n. In the case of thefour-stroke engine the
area enclosed b y the p-V diagram for thegas exchange process, i .e
. 567-8, i s known as the pumping
Inletvalve
Exhaustvalve
Piston
