Reducing the power output, for part-load operation, is
done by controlling the amount of (air+fuel) mixture per
cycle. This amount is regulated by using a throttle valve in
the intake, thus controlling the quantity of mixture
allowed into the cylinders (Bquantitative control[). Using
a throttle valve to accomplish this results in poor part-load
efficiency, as the partly closed valve represents a resistance
for the flow, and leads to pressure losses (Bpumping
losses[). Another important factor limiting the engine
efficiency, is the necessity of avoiding auto-ignition of the
mixture. Simply put, when the temperature in the
unburned mixture exceeds the mixture’s auto-ignition
temperature, it will auto-ignite, leading to a very rapid rate
of pressure rise and very high temperatures. In the best
case, this will result in pressure oscillations, and thus
vibrations, manifested as a knocking or pinging sound,
hence the term Bknocking[ combustion. In the worst case,
it will severely damage the engine. Auto-ignition must thus
be avoided. This effectively limits the compression ratio of
the engine. As the theoretical cycle efficiency increases
with compression ratio, this thus limits the efficiency.