Piston Engines
A piston engine, is a heat
engine that uses one or more pistons to convert pressure into a rotating
motion. There may be one or more pistons. Each piston is inside a cylinder,
into which a gas is introduced, either already hot and under pressure
(steam engine), or heated inside the cylinder either by ignition of
a fuel air mixture (internal combustion engine) or by contact with a
hot heat exchanger in the cylinder (stirling engine).
The hot gases expand, pushing the piston to the bottom of the cylinder.
The piston is returned to the cylinder top (Top Dead Centre) either
by a flywheel or the power from other pistons connected to the same
shaft. In most types the expanded or "exhausted" gases are removed from
the cylinder by this stroke. The exception is the Stirling engine, which
repeatedly heats and cools the same sealed quantity of gas.
In some designs the piston
may be powered in both directions in the cylinder in which case it is
said to be double acting.
In all types the linear movement of the piston is converted to a rotating
movement via a connecting rod and a crankshaft or by a swashplate. A
flywheel is often used to ensure smooth rotation. The more cylinders
a reciprocating engine has, the more vibration-free (smoothly) it can
run also the higher the combined piston displacement volume it has the
more power it is capable of producing.
It is common for such engines
to be classified by the number and alignment of cylinders and the total
volume of displacement of gas by the pistons moving in the cylinders
usually measured in cubic centimeters (cc). For example for internal
combustion engines, single and two-cylinder designs are common in smaller
vehicles such as motorcycles, while automobiles typically have between
four and eight, and locomotives, and ships may have a dozen cylinders
or more. Cylinder capacities may range from 10cc or less in model engines
up to several thousand cc in ships' engines.
The compression ratio is
a measure of the performance in an internal-combustion engine or a Stirling
Engine. It is the ratio between the volume of the cylinder, when the
piston is at the bottom of its stroke, and the volume when the piston
is at the top of its stroke.
Cylinders may be aligned
in line, in a V configuration, horizontally opposite each other , or
radially around the crankshaft. Opposed piston engines put 2 pistons
working at opposite ends of the same cylinder and this has been extended
into triangular arrangements such as the Napier Deltic. Some designs
have set the cylinders in motion around the shaft, see the Rotary engine.
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