Page 58 - Piston Engine-Based Power Plants

50 Piston Engine-Based Power Plants

exchange involved in the other two strokes.) At position 1 in the dia-
gram the cylinder is assumed full of air and this air is compressed by
the piston as it moves to position 2. This compression stroke reduces
the volume, increases the pressure and increases the temperature of the
air. Fuel is injected at position 2 and ignites, dramatically increasing
the temperature and pressure further as the piston begins to move
away from TDC and the volume in the cylinder expands. This is fol-
lowed by the power stroke, 3 4, as the volume inside the cylinder
increases and the pressure falls. Finally at the end of the power stroke,
4, the exhaust valve is opened and the excess pressure is released, again
instantaneously in this ideal version. There then follow the exhaust
stroke and the intake stroke, both of which take place at position 1.

If Fig. 5.2 is compared to Fig. 4.2, which shows the cycle for a
spark ignition engine, the only difference is in the change that occurs
when combustion takes place. In the spark ignition engine this is
assumed to take place instantaneously within the cylinder, at constant
volume because the piston does not have time to move during the
explosive combustion. In the diesel cycle the combustion takes longer
and is assumed to take place at constant pressure as the piston moves
away from TDC.
The compression stroke, 1 2, requires energy to be used to com-
press the gas in the cylinder. On the other hand, the power stroke,
from 2 to 4, generates power. The net amount of power available for
useful work is the difference between the two. This is represented
mathematically by the area within the cycle in the diagram.

DIESEL ENGINE COMBUSTION AND TURBOCHARGING

The primary difference between a spark ignition engine and a diesel
engine is the manner in which ignition occurs. The diesel engine does
away with the ignition system and instead relies on the temperature of
the air in the cylinder being high enough to ignite the fuel spontane-
ously. This fuel is added at the end of the compression stroke whereas
in a spark ignition engine the cylinder already contains an air fuel
mixture.

In order to achieve ignition, the air in the cylinder is compressed
much more than in a spark ignition engine and it is the compression
that raises the temperature. The compression ratio of a diesel engine is