Page 46 - Piston Engine-Based Power Plants

38 Piston Engine-Based Power Plants

valve is open so and the gas in the cylinder is drawn in or expelled at con-
stant pressure and it is assumed that during these stokes no energy is
either produced or used. Only the volume within the cylinder changes.
These strokes are represented between points 1 and 2 in Fig. 4.2.

The two remaining strokes define the engine. The first is a compression
stroke, 2 3 in the diagram, in which the fuel air mixture is compressed.
This reduces its volume, increases its pressure and increases the tempera-
ture because the temperature of a gas rises when it is compressed. At the
end of the compression stroke the fuel is ignited and the chemical reaction
that takes place releases large amounts of heat. This is considered, in the
idealised cycle, to take place instantaneously while the volume within the
cylinder does not change. However the pressure of the gases rises dramati-
cally (3 4). The hot, high-pressure gases then force the piston away,
expanding the volume within the cylinder as the pressure drops (4 5). At
the end of this stroke the exhaust valve is opened and any excess heat and
pressure is released (5 6). This is considered in the ideal case to be
another instantaneous process. Finally, the exhaust stroke takes place
(2 1) while the combustion gases are expelled at constant pressure.

In this ideal form, there are two phases in which the both volume
and pressure change. The compression stroke is the first and during this
work is done on the gas to compress it and so energy is expanded. The
second phase is the power stroke in which the expanding gases force the
piston to move. This generates power. Mathematically, the net amount
of useful work that the engine provides is that generated in the power
stroke minus that used during the compression stroke. It is represented
graphically by the area within the cycle diagram in Fig. 4.2.

FUEL PREPARATION AND COMBUSTION

In order for a spark ignition engine to operate smoothly, the composi-
tion of the air fuel mixture must be carefully controlled and the rate
at which it is delivered to the engine cylinders must be controlled too.
In virtually all engines until the 1980s these processes were controlled
mechanically using a device called a carburettor. Since then most
engines have converted to fuel injection because this allows closer con-
trol of the fuel mixture and therefore closer control of the emissions
from the engine. Modern engines use electronically controlled injection
systems.