Page 16 - Automotive Engineering Powertrain Chassis System and Vehicle Body

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

iii) direct expulsion of fresh charge through the the ratio of the cross-sectional areas of the cylinder and
exhaust port. the inlet port and on the speed at which the piston is
moving. The pressure difference established between
These undesirable conditions may occur under dif-
ferent speed and load situations, which greatly the inside and outside of the cylinder will induce air at
influences both power and fuel consumption. atmospheric pressure to enter and fill up the cylinder.
Unlike the petrol engine, which requires a charge of air-
g) Far less maintenance is expected with the two- and-petrol mixture to be drawn past a throttle valve, in
stroke engine compared with the four-stroke the diesel-engine inlet system no restriction is necessary
engine, but there can be a problem with the prod-
and only pure air is induced into the cylinder. A maxi-
ucts of combustion carburising at the inlet, transfer, mum depression of maybe 0.15 bar below atmospheric
and exhaust ports. pressure will occur at about one-third of the distance
h) Lubrication of the two-stroke engine is achieved by along the piston’s outward stroke, while the overall
mixing small quantities of oil with petrol in pro- average pressure in the cylinder might be 0.1 bar or
portions anywhere between 1:16 and 1:24 so that, even less.
when crankcase induction takes place, the various Compression stroke (Fig. 1.1-8(b)) With both the
rotating and reciprocating components will be inlet and the exhaust valves closed, the piston moves
lubricated by a petroil-mixture mist. Clearly a towards the cylinder head (Fig. 1.1-8(b)).
continuous proportion of oil will be burnt in the The air enclosed in the cylinder will be compressed
cylinder and expelled into the atmosphere to add to into a much smaller space of anything from 1/12 to 1/24
unwanted exhaust emission. of its original volume. A typical ratio of maximum to
i) There are fewer working parts in a two-stroke minimum air-charge volume in the cylinder would be
engine than in a four-stroke engine, so two-stroke 16:1, but this largely depends on engine size and designed
engines are generally cheaper to manufacture. speed range.
During the compression stroke, the air charge initially
at atmospheric pressure and temperature is reduced in
volume until the cylinder pressure is raised to between
1.1.3 Four-stroke-cycle
30 and 50 bar. This compression of the air generates heat
compression-ignition (diesel) engine which will increase the charge temperature to at least

600 C under normal running conditions.
Compression-ignition (C.I.) engines burn fuel oil which Power stroke (Fig. 1.1-8(c)) With both the inlet
is injected into the combustion chamber when the air and the exhaust valves closed and the piston almost at
charge is fully compressed. Burning occurs when the the end of the compression stroke (Fig. 1.1-8(c)), diesel
compression temperature of the air is high enough to fuel oil is injected into the dense and heated air as
spontaneously ignite the finely atomised liquid fuel. In a high-pressure spray of fine particles. Provided that
other words, burning is initiated by the self-generated they are properly atomised and distributed throughout
heat of compression (Fig. 1.1-8). the air charge, the heat of compression will then quickly
Engines adopting this method of introducing and vaporise and ignite the tiny droplets of liquid fuel.
mixing the liquid fuel followed by self-ignition are also Within a very short time, the piston will have reached
referred to as ‘oil engines’, due to the class of fuel burnt, its innermost position and extensive burning then
or as ‘diesel engines’ after Rudolf Diesel, one of the many releases heat energy which is rapidly converted into
inventors and pioneers of the early C.I. engine. Note: in pressure energy. Expansion then follows, pushing the
the United Kingdom fuel oil is known as ‘DERV’, which piston away from the cylinder head, and the linear
is the abbreviation of ‘diesel-engine road vehicle’. thrust acting on the piston end of the connecting-rod
Just like the four-stroke-cycle petrol engine, the C.I. will then be changed to rotary movement of the
engine completes one cycle of events in two crankshaft crankshaft.
revolutions or four piston strokes. The four phases of Exhaust stroke When the burning of the charge is
these strokes are (i) induction of fresh air, (ii) com- near completion and the piston has reached the out-
pression and heating of this air, (iii) injection of fuel and ermost position, the exhaust valve is opened. The piston
its burning and expansion, and (iv) expulsion of the then reverses its direction of motion and moves towards
products of combustion. the cylinder head (Fig. 1.1-8(d)).
Induction stroke (Fig. 1.1-8(a)) With the inlet The sudden opening of the exhaust valve towards the
valve open and the exhaust valve closed, the piston end of the power stroke will release the still burning
moves away from the cylinder head (Fig. 1.1-8(a)). products of combustion to the atmosphere. The pressure
The outward movement of the piston will establish energy of the gases at this point will accelerate their
a depression in the cylinder, its magnitude depending on expulsion from the cylinder, and only towards the end of

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