Page 49 - Piston Engine-Based Power Plants

Spark Ignition Engines 41

called the throttle. This controls the absolute amount of air that can
enter the engine, whatever the fuel air ratio. If more fuel and air are
allowed in, the engine runs faster or accelerates.

An important feature of the spark ignition engine is that its speed is
self-limiting because the speed or power of the engine is determined
by the amount of fuel that is fed to it. Since the engine is fed with a
mixture of fuel and air, controlling the quantity using the throttle will
control the engine speed.

More modern engines use an electronically controlled fuel injection
system to ensure that the proportion of fuel in the air is exactly correct
during each moment of the engine’s operation. This enables tight con-
trol of emissions. For a conventional automobile engine the most power
is obtained with a relatively rich fuel mixture while the best economy is
obtained with a leaner mixture. However the efficiency will also depend
upon how well the fuel and air are mixed. The better the mixing, the fas-
ter and more completely combustion of the fuel can take place. The
amount of time available for this to occur is very short. For engine run-
ning at 4000 rpm, there will be 66 revolutions and 33 firings of each cyl-
inder each second, allowing 8 ms for each power stoke. The actual
combustion probably takes place in less than one-tenth of this time.
Combustion of the mixture in the cylinder is normally carried out
using a spark plug. This has two electrodes which are inside the com-
bustion chamber at the top of the cylinder. At a pre-determined point
during each cycle a spark is generated across these electrodes to ignite
the mixture. The combustion must then spread almost instantaneously
through the compressed fuel air mixture. More technically complex
engines can use a pre-ignition chamber in which a small amount of an
air fuel mixture rich in fuel is admitted and ignited. This pre-ignition
then spreads into the main cylinder where an air fuel mixture contain-
ing a much greater proportion of air is ignited.
The point at which ignition takes place, the ‘timing’ of the engine,
is important for a number of reasons. Ideally it should take place when
the piston is at TDC but this may not allow sufficient time for com-
bustion to take place fully. It is common, therefore to ‘advance’ the
timing so that ignition takes place slightly before the piston reaches
TDC. Timing will also be affected by another important engine param-
eter, the compression ratio.