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Unformatted text preview: P8250 — Cussons Technology — Automotive 1 Engine Test Set Page 16 6 THEORY 6.1 Introduction. The Internal Combustion (1C) engine is still today the predominant prime mover for a host of
applications such as vehicle prepulsion, marine propulsion, generator sets etc. Two principal
classes of IC engines exist. The ﬁrst, Where the combustion is initiated by a spark is termed
‘Spark Ignition’ (SI) and embodies Petrol and Gas engines. The second, Where combustion is
initiated spontaneously by virtue of the rise in temperature during the compression process is
termed ‘Compression Ignition’ (CI), and embodies Diesel engines. The following sections
describe the theory associated with IC engines and standard experiments designed to demonstrate these principals. 6.2 Criteria for engine performance. The overall efﬁciency of a power plant is usually expressed as
_ W Q2. 44 000 lLI/[aj ‘ 0) _ q
Tl — Qnetp in“) 13",. [qt:4] rnfj E‘h I/(ipOlr IE.) r'mr]
' Ml €919;ng are ,
Where W is the net power output per unit mass of fuel and amp is e not calonﬁc value of the ﬁiel at constant pressure. The net output of an IC engine is called the brake power (b.p.), and for
this reason the overall efﬁciency is termed the brake thermal eﬁ‘iciency , nb. If the brake power is expressed in kW, the ﬁiel ﬂow mf in kg/s, and om in kJ/kg, then
r b. . .7
m = __p_ L2,) The Speciﬁcﬁle! conszmzption (s.f.c) is frequently used as an alternative criterion of performance
and is deﬁned as the rate of fuel consumption per kW of brake power and is expressed as follows = C3) As can be seen, for any given fuel the s.f.c. is inversely proportional to 11b. Most common hydrocarbon fuels have very similar caloriﬁc values, and the s.f.c. can therefore be used when
comparing efﬁciency of engines using different ﬁiels. ‘ In IC engines, an appreciable part of the losses is due to mechanical friction, and it is informative
if the thermodynamic and friction losses are separated. The overall efﬁciency is therefore
analysed as the product of the indicated themzal eﬁcz‘ency n, and the mechanical eﬁi‘ciency nm, as follows “b = mm s) m x Qnetlp i. p The indicated power (i.p.) is the actual rate of work done by the working ﬂuid on the piston, and
the difference between the i.p. and the hp. is the power absorbed by mechanical ﬁiction (piston,
bearings etc.). As the name implies, in can be determined ﬁom the indicator diagram which for
a four stroke engine has the form as shown in the ﬁgure below The negative work of the small
anticlockwise loop is termed the pumpng power, and is the result of viscous friction in the
induction and exhaust strokes. The difference between a four stroke IC engine and other CUSSOHS 102, Great Clowes Street. Manchester M7 1RH. U.K.
——TECHNOLOGY Ltd— Phone +44 161 833 0036 email [email protected] P8250 —— Cussons Technology — Automotive 1 Engine Test Set Page 17 reciprocating machines therefore, is that the area of the small anticlockwise loop must be
subtracted from the area of the main diagram to determine Indicated mean eﬁ'ective pressure
(i.m.e.p.), which would then be the height of a rectangle of the same area as this resultant area
and with the same length as the pv diagram (L). The i.p. can then be expressed as follows. i.p.=100xpmxSxAxanc kW where S is the stroke in In; A is the piston area in nu2 , as the number of cylinders, and pm the i.m.e.p. in bar. C is the number of machine cycles per second and for a four stroke engine, this is
equal to half the engine speed (rev/s) whilst for a two stroke engine it equals the engine speed. Engine Indicator diagram Whereas i.m.e.p. is considered a useful guide to engine per’formanCe, in practice it is usual to use
the concept of brake mean eﬁ'ectz'Ve pressure (b.m.e.p.) as an indicator. The b.m.e.p., denoted by
pm ,7 is deﬁned in a similar way to pm above. b.p.=100Xpmb xSxAxanc kW ' (i) where pm is again in bar. The b.rn.e.p. may be regarded as that part of the i.m.e.p. which is
imagined to contribute to brake power, the remainder being used to overcome ﬁiction. An increase in b.m.e.p. therefore implies that the cylinder volume (S x A), can be smaller for a given
output. In the Automotive 1 engine test set the parameters monitored relating to the engine brake poWer are the engine speed and the electrical output of the Alternator (Volts and Amps) driven by the
engine. This output can be converted to Brake Power (b.p.) as follows. Vxlx‘lOO , 1.;
b.Pl =—“——— _ Watts Where V is the Alternator output volts, I is the Alternator output Current and u is the Alternator
efficiency. u varies with speed and output current and values can be obtained from ﬁgure 2. The
brake power can then be expressed in terms of Torque and engine speed as follows. CUSSOT‘IS 102. Great Clowes Street. Manchester M7 1RH. U.K.
—TECHNOLOGY Ltd— Phcne +44 161 833 0036 email [email protected] _ P8250 — Cuesons Technology — Automotive 1 Engine Test Set Page 13
= 2 x II x N x T W a 00
l 000 where the engine speed N is in rev/sec. It can be seen from equations (i) & (ii) that Torque and
b.m.e.p. are directly proportional and can be related by the following expression. b.p. 105  (‘1'
T= 4 H x pm, ,xSxAxanc Nm forafour stroke engine, and .
x
105 ' .
and T = x pm x S x Axanc Nm for atwo stroke engine. 2 x 1'1
6.3 Comparisons between SI and CI Engines. 6.3.1 SI Engines. The air standard cycle employed in SI engines is the Otto cycle and analysis shows that the cycle
efficiency improves along with the brake thermal efficiency, with increase in compression ratio.
There is an upper limit to which the compression ratio can be increased however, which is related
to the fact that ﬁle ﬂuid compressed in the cylinder is a mixture of fuel vapour and air. The
temperature of the mixture increases during compression and too high a compression ratio could
result in pre—z'gnitt'on before spark—ignition. Even if the compression ratio is not high enough to
cause preignin'on, detonation may result. Combustion begins near the spark plug a short time
after the formation of the spark and sets of a relatively slow ﬂameless reaction occupying a delay
period until the ﬂame front develops. This delay period although short (0.002 seconds)can
translate in high speed engines to appreciable crank movement and as a result the point of
ignition is usually well in adVance of top dead centre. As the ﬂame front spreads in a uniform
manner across the combustion space, it compresses the unburnt mixture before it. The
temperature of the unburnt portion is raised therefore by both the radiation ﬁom the ﬂame as well
as by the compression and so if the original compression and temperature were to high self
ignition will occur. The resulting violent pressure rise and associated shock wave as the ﬂame
front accelerates rapidly are allied to the detonation affect. Modern engines and fuels tend to
permit compression ratio's of upto 10:1 Having ﬁxed the compression ratio the next factor governing performance, is the rise in pressure
obtainable during combustion, as an increase in peak pressure will result in an increase in
b.m.e.p. and consequently b.p.. The peak pressure is ﬁxed by the amount of fuel that can be
burnt, and hence the amount of oxygen available. Theoretically it will be a maximum if a
stoichiometric mixture is used i.e. just sufﬁcient oxygen to burn all the fuel. Stoichiomenic air. to
fuel ratio's (AFR) by weight are almost identical for all liquid fuels, i.e. approximately 14.5:1. In
practice, maximum brake thermal efﬁciency is achieved at a slightly weak mixture, giving
complete combustion of the fuel, and maximum power with a slightly rich mixture where
complete combustion of the oxygen occurs. CUSSODS 102, Great Clowes Street. Manchester M7 1RH. U.K.
—TECHNOLOGY Lta— Phone +44 161 833 0036 email [email protected] pm as new farmstqu
Wu w.. Hammer; “1‘ 'nmv if"??? r”?  rim WMT P8250 — Cussons Technology — Automotive 1 Engine Test Set Page 19 Effect of mixture strength on performance i brake thermal eff i May hrzlln: thnrmnl nff I
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I mixture AFR, however reliable ignition by spark is only achievable over a narrow band of AFR
values. Instead approximately stoichiometric ratio's are maintained at all loads with regulation
being achieved by varying the mass of the mixture induced in each working cycle. This method is
referred to as Throttle or Quantity governing. Throttling results in subatmospheric pressures in
the cylinder during the induction stroke, thus increasing the pumping loss. Part load efﬁciency of
SI engines is consequently poor. 6.3.2 CI Engines. Since the fuel is introduced into the cylinder of a CI engine only when combustion is required,
is. towards the end of the compression stroke, proignition cannot occur. Moreover, since the
fuel is injected at a controlled rate, the simultaneous combustion of the whole mixture cannot
happen, thus the problem of detonation associated with SI engines does not arise. It follows that
compression ratio's in CI engines can be much higher, and in fact there is a lower limit of about
12:1 below which compression ignition of common diesel fuels is not possible. In a CI engine most of the combustion takes place at relatively constant pressure, such that the
maximum pressure reached in the cylinder is largely determined by the compression ratio
employed. The upper limit of compression ratio is therefore ﬁxed by the strength of the cylinder,
con rod, bearings etc, whose stresses are determined by the peak pressure forces. Usually
compromises between high efﬁciency and low weight/cost result in maximum limit on
compression ratio's in the order of 20:1 A comparison of air—standard efﬁciencies of the Diesel
and Otto cycles suggests that the CI engine will be more efﬁcient, a fact home out in practice. The factor limiting i.m.e.p. (and consequently b.m.e.p.) of a CI engine, is the change in volume
which occurs during the combustion at constant pressure. This is a maximum when all the ﬁle]
that can be burnt in the oxygen available is injected into the cylinder, suggesting again,
stoichiometric AFR. In practice, because the fuel is injected as ﬁne liquid droplets, mixing has to
take place at the point of injection to ensure full combustion. Despite careful attention to inlet
port, fuel injector, and combustion chamber design, it is not possible to burn all the fuel
completely if stoichiometric ratio's are used, and the minimum AFR corresponding to full load
usually lies between 18:1 and 25:1. Consequently ﬂJB engine must be larger than would be the 011880118 102, Great Clowes Street. Manchester M7 1 RH. U.K. ‘ —TECHNOLOGY Ltd— Phone +44 161 833 0036 email [email protected] i"? P8250 — Cussons Technology — Automotive ‘1 Engine Test Set Page 20 case if it were capable of burning stoichiometric ratio's. This results in CI engines being generally
larger and heavier than SI engines although the former has a higher thermal efﬁciency. It is found that very weak mixtures can be ignited and burnt in a CI engine so that it is possible to
govern power output by Varying fuel supply. Although this results in a increase in indicated
thermal efﬁciency at part load, the fall in mechanical efﬁciency more than outweighs this eﬁect,
and the brake thermal efﬁciency always falls off. Nevertheless the reduction in efﬁciency with
decrease in load is not so marked as in SI engines. Governing the power output by varying the
mixture is usually referred to as Quality governing. 6.4 Engine performance characteristic. Up to new engine performance has been considered in terms of work done per complete cycle,
however the actual power output will depend on the rotational speed. It would seem that, other
factors being equal, the power output of an engine can be raised by increasing its speed upto its
mechanical limit. In practice it is found that the maximum indicated work per cycle varies
considerably with speed, and beyond a certain speed the i.p. will fall with further increase in
speed."I'l1e reduction is chieﬂy due to decrease in the mass of the charge induced in each cycle.
Theoretically a naturally aspirated engine should induce a mass equivalent to its swept volume at
ambient pressure and temperature. As a result of ﬂuid friction and charge expansion, signiﬁcantly
less is taken in at high engine speeds when gas velocities are high and the manifold is hot. This "breathing" capacity of an engine is expressed in terms of volumetric eﬁ‘z‘ciency deﬁned as m =39 00) Vs where V, is the volume of induced charge per induction stroke , and V, is the engine swept
volume. The volume of the induced charge is the combination of the volume of air and fuel
induced, and at typical AFR values, due to the relative densities of the two ﬂuids the fuel volume
flow is very small in comparison to the air. When comparing engine performance, it is oﬂen
standard practice to correct power readings to a reference pressure and temperature, thus
compensating for differing ambient conditions and there effect on mass charge. There are various
standards in existence for this corrective procedure, some merely accounting for variations in
pressure and temperature, whilst others include corrections for humidity and consequently
moisture content in the air. A typical correction factor, ct, for SI engines, to multiplied by the
observed power for correction purposes, is given as 1.2 CLE '
_ T .
at, = [4'] (taken from the British standard as 5514) (I I)
pr _ gypsy T ‘ 1’ where p is absolute barometric pressure in kPa, p S is the saturated water vapour pressure in kPa at the applicable temperature (at Q = 100), $25 is the relative humidity, and T is the absolute air
temperature in deg K, Subscript r corresponds to the value under standard reference conditions,
and these are given in the standard as p = lOOkPa, T = 298 K, and Z = 0.3. Subscript y
corresponds to the values under test ambient conditions. The indicated work per cycle also falls at low speeds. This is due partly to increased leakage of
charge past valves and piston for which more time is available at low speeds. It is also due to
reduced turbulence and increased cooling time available, thus making combustion less efﬁcient.
A typical power curve against speed therefore is as shown below. It is evident that the power developed in an IC engine, is dependant on the mass of charge
induced, and it would seem that only the ambient conditions prevalent may effect this. A much CUSSOHS 102, Great Clowes Street. Manchester M7 1RH. U.K.
—TECHNOLOGY Ltd— Phone +44 161 333 0036 email [email protected] P8250 ~— Cusso'ns Technology — Automotive 1 Engine Test Set Page 21 F,
ii a
r
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r. greater increase in power can be obtained by compressing the charge prior to induction with some
type of Supercharger. A Roots blower or centrifugal compressor is normally used for this
purpose, the former gear driven from the crankshaft, whilst the latter obtaining its power from
turbine driven from the exhaust gases, and is termed a Turbocharger. Although the net increase
__I in power obtained ﬁom supercharging or turbocharging can be quite considerable, it has little ‘ effect on brake thermal efﬁciency, as the ﬁrel must be increased in proportion to the air charge to
"4 maintain the required AFR. _.l “were;
i ' warm] E ‘ CUSSODS 102, Great Clowes Street. Manchester M7 1RH. U.K.
g —TECHNOLOGY Ltd— Phone +44 161 833 0036 email [email protected] P8250 — Cuesons Technology — Automotive 1 Engine Test Set Page 28  '1
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"_ 3 .4 a 12 1a 20 24 23 32 35 4o 44 T Alternator Current  Amps :2 .1 .3 CUSSOHS 102. Great Clowes Street. Manchester M7 1RH. U.K.
—TECHNGLOGY Ltd— Phone +44 16‘! B33 0036 emaii [email protected] P8250  Cusslons Technology — Automotive 1 Engine Test Set Page 31 12 APPENDIX 2  AIR FLOW MEASUREMENT BY ORIFICE
PLATES 12.1 Introduction. Both the inlet and discharge air ﬂows are measured by square edged oriﬁce plates with corner tappings designed in accordance with section 7 of British Standard BS 1042: Patti: 1992. The
working equations for the oriﬁce plates are: 0.5
Volume Flow Rate, Q = c e a; a2 [2 39—]
. p Q = Volume ﬂow rate, m3 I see
d = Oriﬁce diameter, m
p = Air density at upstream conditions, kg 1' n13 Ap = Pressure difference across oriﬁce, Pa
C = Coefﬁcient of discharge a = Expansibility factor. B = Velocity of approach factor. The constants C, e, and E are expressed as follows 6 0.75
Cuefﬁcient of Discharge, C = 05959 + 0.0312132»! _ 0.134;?! + 0.00295z5[;0 J
. en 2
_ Area Ratio, B = where D= Upstream diameter, 111 4Q
Hvd Rep = Reynolds number referred to diameter D = where v = Kinematic viscosity, mzlsec Expansibilityfactcr. a =1a (o.41+o.35a4)ﬁg_
. K 1 p1 = Absolute upstream static pressure, Pa K = Isentropic exponent
1 It can be seen that volumetric ﬂow rate and Renolds number Re are dependant on one another
and to satisfy the equation therefore reiteraﬁon is necessary. Velocity of Approach Factor, E = CUSSOI‘IS 102, Great Clowes Street. Manchester M7 1RH. U.K.
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A W I
t l .T P8250 — Cussons Technology — Automotive 1 Engine Test Set Page 32 12.2 Application To Inlet Air Flow Oriﬁce And Surge Tank. The oriﬁce diameter ﬁtted to the inlet surge tank used to measure the inlet air ﬂow is sized to suit
the engine, (the diameter is stamped on the plate), Whilst the air is drawn from a ﬂee space which
is effectively of inﬁnite diameter, thus: D=co so thatthe area ratio, B=O
therefore, (3 = 0,5959.
And, by calculation the velocity'of approach factor, B = 1,0
Substituting into
0.5
Volume Flow Rate, Q: Ce EE 0'2 [29—3] e __ 
. 4 p 2 _ I
_ . 2 AP “'5 Ci 7: i 3 m m
.erlds: . Q = 0.66188 5' a’ and assuming a perfect gas the air density p '= P l R ' T where R can he assumed to be 237.1 .T/kg
K. Note: The oriﬁce diameter is stamped on the oriﬁce plate on top of the surge chamber CUSSOI’IS 102' Great CloWes Street. Manchester M7 1RH. U.K.
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 Internal combustion engine, U.K, Test Set Page, Engine Test Set, great clowes street

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