Page 36 - Piston Engine-Based Power Plants

28 Piston Engine-Based Power Plants

Starting with the piston at the top of its chamber, and the chamber
empty, the first stroke in an intake stroke in which either air (diesel
cycle) or a fuel and air mixture (Otto cycle) is drawn into the piston
chamber by movement of the piston to expand the volume of the
enclosed space with the air or air fuel mixture valve open. This valve
closes at the end of the first stroke. The second stroke is a compression
stroke during which the gases in the cylinder are compressed by the
piston returning towards the top of its chamber. In the case of the
Otto cycle, a spark ignites the fuel air mixture at the top of this sec-
ond stroke, creating an explosive expansion of the compressed mixture
which forces the piston down again. This is the power cycle. In the die-
sel cycle fuel is introduced through a separate nozzle close to the top
of the compression stroke, igniting spontaneously in the hot gas with
the same effect. After the power stoke, the fourth stroke is the exhaust
stroke during which the exhaust gases are forced out of the piston
chamber though the second valve, now open. This closes at the end of
the fourth stroke and the cycle begins again. In both spark ignition
and diesel engines a large flywheel attached to the crankshaft stores
angular momentum generated by the power stroke and this provides
sufficient momentum to carry the crankshaft and piston through the
three other stokes required for each cycle.

The shaft of an engine that is fitted with a single piston and cylinder
will receive a power impulse once every two rotations, leading to a rela-
tively uneven transfer of power. However if the engine has multiple cylin-
ders, the cycle of each can be staggered relative to the others so that they
deliver their power sequentially, leading to a much more even rotational
motion. For a four-stroke engine it is normal for four (or a multiple of
four) pistons to be attached to the crankshaft, with one of each set of four
timed to produce a power stroke while the other three move through dif-
ferent stages of their cycles. In this case the power strokes of the four pis-
tons should take place at 180 intervals during the rotation of the shaft

for optimum delivery. The introduction of fuel and air and the removal
of the exhaust gases are then controlled by valves which are mechanically
timed to coincide with the various stages of the cycle on each cylinder.

Two-Stroke Engines
The two-stroke engine is much simpler than the four stroke. This is
both an advantage and a limitation. In a two-stroke engine, intake and
exhaust strokes are not separate and there are no valves. Instead the