Page 12 - Automotive Engineering

Piston-engine cycles of operation CHAPTER 1.1

to the crankshaft rotating half a revolution or 180 .Itis proportions ranging from 10 to 17 parts of air to one
also equal to twice the crank-throw. part of petrol by weight.
i.e. L ¼ 2R An engine which induces fresh charge by means of
where L ¼ piston stroke a depression in the cylinder is said to be ‘normally aspi-
and R ¼ crank-throw rated’ or ‘naturally aspirated’.
Compression stroke (Fig. 1.1-3(b)) Both the inlet
Thus a long or short stroke will enable a large or small
turning-effort to be applied to the crankshaft and the exhaust valves are closed. The piston begins to
respectively. ascend towards the cylinder head (Fig. 1.1-3(b)). The
Cylinder bore The cylinder block is initially cast induced air-and-petrol charge is progressively com-
with sand cores occupying the cylinder spaces. After the pressed to something of the order of one-eighth to one-
tenth of the cylinder’s original volume at the piston’s
sand cores have been removed, the rough holes are ma- innermost position. This compression squeezes the air
chined with a single-point cutting tool attached radially and atomised-petrol molecules closer together and not
at the end of a rotating bar. The removal of the unwanted only increases the charge pressure in the cylinder but
metal in the hole is commonly known as boring the cyl- also raises the temperature. Typical maximum cylinder
inder to size. Thus the finished cylindrical hole is known compression pressures will range between 8 and 14 bar
as the cylinder bore, and its internal diameter simply as with the throttle open and the engine running under
the bore or bore size.
load.
Power stroke (Fig. 1.1-3(c)) Both the inlet and the
1.1.1.2 The four-stroke-cycle exhaust valves are closed and, just before the piston ap-
spark-ignition (petrol) engine proaches the top of its stroke during compression,
a spark-plug ignites the dense combustible charge
The first internal-combustion engine to operate suc- (Fig. 1.1-3(c)). By the time the piston reaches the in-
cessfully on the four-stroke cycle used gas as a fuel and nermost point of its stroke, the charge mixture begins to
was built in 1876 by Nicolaus August Otto, a self-taught burn, generates heat, and rapidly raises the pressure in
German engineer at the Gas-motoreufabrik Deutz the cylinder until the gas forces exceed the resisting load.
factory near Cologne, for many years the largest manu- The burning gases then expand and so change the piston’s
facturer of internal-combustion engines in the world. It direction of motion and push it to its outermost position.
was one of Otto’s associates – Gottlieb Daimler – who The cylinder pressure then drops from a peak value of
later developed an engine to run on petrol which was about 60 bar under full load down to maybe 4 bar near
described in patent number 4315 of 1885. He the outermost movement of the piston.
also pioneered its application to the motor vehicle Exhaust stroke (Fig. 1.1-3(d)) At the end of the
(Fig. 1.1-3). power stroke the inlet valve remains closed but the ex-
Petrol engines take in a flammable mixture of air and haust valve is opened. The piston changes its direction of
petrol which is ignited by a timed spark when the charge motion and now moves from the outermost to the in-
is compressed. These engines are therefore sometimes nermost position (Fig. 1.1-3(d)). Most of the burnt gases
called spark-ignition (S.I.) engines. will be expelled by the existing pressure energy of the
These engines require four piston strokes to complete gas, but the returning piston will push the last of the
one cycle: an air-and-fuel intake stroke moving outward spent gases out of the cylinder through the exhaust-valve
from the cylinder head, an inward movement towards port and to the atmosphere.
the cylinder head compressing the charge, an outward During the exhaust stroke, the gas pressure in the
power stroke, and an inward exhaust stroke. cylinder will fall from the exhaust-valve opening pressure
Induction stroke (Fig. 1.1-3(a)) The inlet valve is (which may vary from 2 to 5 bar, depending on the engine
opened and the exhaust valve is closed. The piston speed and the throttle-opening position) to atmospheric
descends, moving away from the cylinder head pressure or even less as the piston nears the innermost
(Fig. 1.1-3(a)). The speed of the piston moving along position towards the cylinder head.
the cylinder creates a pressure reduction or depression Cycle of events in a four-cylinder engine (Figs.
which reaches a maximum of about 0.3 bar below at- 1.1-3(e)–(g)) Fig. 1.1-3(e) illustrates how the cycle of
mospheric pressure at one-third from the beginning of events – induction, compression, power, and exhaust – is
the stroke. The depression actually generated will phased in a four-cylinder engine. The relationship
depend on the speed and load experienced by the between cylinder pressure and piston stroke position
engine, but a typical average value might be 0.12 bar over the four strokes is clearly shown in Figs. 1.1-3(f) and
below atmospheric pressure. This depression induces (g) and, by following the arrows, it can be seen that
(sucks in) a fresh charge of air and atomised petrol in a figures of eight is repeatedly being traced.

5