Page 21 - Automotive Engineering

CH AP TER 1 .1 Piston-engine cycles of operation

Power phase (Fig. 1.1-9(c)) Shortly before the cylinder in a four-stroke engine, due to pumping and
piston reaches the innermost position to the cylinder friction losses at low speeds. At higher engine
head on its upward compression stroke, highly pressur- speeds the situation is reversed, and the two-
ised liquid fuel is sprayed into the dense intensely heated stroke’s Rootes blower will consume proportionally
air charge (Fig. 1.1-9(c)). Within a very short period of more engine power – this could be up to 15% of the
time, the injected fuel droplets will vaporise and ignite, developed power at maximum speed.
and rapid burning will be established by the time the h) With reduced engine load for a given speed, a two-
piston is at the top of its stroke. The heat liberated from stroke engine blower will consume proportionally
the charge will be converted mainly into gas-pressure more of the power developed by the engine.
energy which will expand the gas and so do useful work in
driving the piston outwards. i) A two-stroke engine runs smoother and relatively
An overall view of the various phases of operation in quietly, due to the absence of reversals of loading
a two-stroke-cycle three-cylinder diesel engine is shown on bearings as compared with a four-stroke
in Figs. 1.1-9(d), and Figs. 1.1-9(e) and (f) show the engine.
cycle of events in one crankshaft revolution expressed in
terms of piston displacement and cylinder pressure.
1.1.5 Comparison of S.I.
and C.I. engines
1.1.4.1 Comparison of two- and
four-stroke-cycle diesel engines The pros and cons of petrol and C.I. engines are now
considered.
A brief but critical comparison of the merits and limi-
Fuel economy The chief comparison to be made
tations of the two-stroke-cycle diesel engine compared
between the two types of engine is how effectively each
with the four-stroke power unit is made below.
engine can convert the liquid fuel into work energy.
a) Theoretically, almost twice the power can be devel- Different engines are compared by their thermal effi-
oped with a two-stroke engine compared with ciencies. Thermal efficiency is the ratio of the useful
a four-stroke engine. work produced to the total energy supplied. Petrol
engines can have thermal efficiencies ranging between
b) A comparison between a typical 12 litre four-stroke
engine and a 7 litre two-stroke engine having the 20% and 30%. The corresponding diesel engines generally
same speed range would show that they would de- have improved efficiencies, between 30% and 40%. Both
velop similar torque and power ratings. The ratio of sets of efficiency values are considerably influenced by
engine capacities for equivalent performance for the chosen compression-ratio and design.
these four-stroke and two-stroke engines would Power and torque The petrol engine is usually
be 1.7:1. designed with a shorter stroke and operates over a much
larger crankshaft-speed range than the diesel engine. This
c) In a four-stroke engine, the same parts generate enables more power to be developed towards the upper
power and empty and fill the cylinders. With the speed range in the petrol engine, which is necessary for
two-stroke engine, the emptying and filling can be high road speeds; however, a long-stroke diesel engine
carried out by light rotary components. has improved pulling torque over a relatively narrow
d) With a two-stroke engine, 40–50% more air con- speed range, this being essential for the haulage of heavy
sumption is necessary for the same power output; commercial vehicles.
therefore the air-pumping work done will be At the time of writing, there was a trend to in-
proportionally greater. corporate diesel engines into cars. This new generation of
engines has different design parameters and therefore
e) About 10–20% of the upward stroke of a two-stroke
does not conform to the above observations.
engine must be sacrificed to emptying and filling the
Reliability Due to their particular process of com-
cylinder.
bustion, diesel engines are built sturdier, tend to run
f) The time available for emptying and filling a cylinder
cooler, and have only half the speed range of most petrol
is considerably less in a two-stroke-cycle engine – engines. These factors make the diesel engine more re-
something like 33% of the completed cycle as liable and considerably extend engine life relative to the
compared to 50% in a four-stroke engine. Therefore petrol engine.
more power will be needed to force a greater mass Pollution Diesel engines tend to become noisy and
of air into the cylinder in a shorter time. to vibrate on their mountings as the operating load is
g) Compared with a two-stroke engine, more power is reduced. The combustion process is quieter in the petrol
needed by the piston for emptying and filling the engine and it runs smoother than the diesel engine. There

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