E.

The gas-engine, the oil-engine, and the motor-car engine are similar in
general principles. The cylinder has, instead of a slide-valve, two, or
sometimes three, "mushroom" valves, which may be described as small and
thick round plates, with bevelled edges, mounted on the ends of short
rods, called stems. These valves open into the cylinder, upwards,
downwards, or horizontally, as the case may be; being pushed in by cams
projecting from a shaft rotated by the engine. For the present we will
confine our attention to the series of operations which causes the
engine to work. This series is called the Beau de Rochas, or Otto,
cycle, and includes four movements of the piston. Reference to Fig. 39
will show exactly what happens in a gas-engine--(1) The piston moves
from left to right, and just as the movement commences valves G (gas)
and A (air) open to admit the explosive mixture. By the time that P has
reached the end of its travel these valves have closed again. (2) The
piston returns to the left, compressing the mixture, which has no way of
escape open to it. At the end of the stroke the charge is ignited by an
incandescent tube I (in motor car and some stationary engines by an
electric spark), and (3) the piston flies out again on the "explosion"
stroke. Before it reaches the limit position, valve E (exhaust) opens,
and (4) the piston flies back under the momentum of the fly-wheel,
driving out the burnt gases through the still open E. The "cycle" is now
complete. There has been suction, compression (including ignition),
combustion, and exhaustion. It is evident that a heavy fly-wheel must be
attached to the crank shaft, because the energy of one stroke (the
explosion) has to serve for the whole cycle; in other words, for two
complete revolutions of the crank. A single-cylinder steam-engine
develops an impulse every half-turn--that is, four times as often. In
order to get a more constant turning effect, motor cars have two, three,
four, six, and even eight cylinders. Four-cylinder engines are at
present the most popular type for powerful cars.

THE MOTOR CAR.

[Illustration: FIG. 40.--Plan of the chassis of a motor car.]

We will now proceed to an examination of the motor car, which, in
addition to mechanical apparatus for the transmission of motion to the
driving-wheels, includes all the fundamental adjuncts of the
internal-combustion engine.[8] Fig. 40 is a bird's-eye view of the
_chassis_ (or "works" and wheel