Forced
Induction
Forced
induction, also known as pressure charging, is the forcing
of air into the cylinders of an engine in order to increase
the density of oxygen available for combustion, thereby increasing
the power and torque that the engine produces. Forced induction
is in contrast to natural, or unforced, aspiration. In modern
cars, the usual means of achieving forced induction is by
the use of a turbocharger or supercharger.
Forced induction can be applied to both gasoline and diesel
engines. In the case of gasoline engines its important that
forced induction does not lead to excessive cylinder pressures
and temperatures which could cause damage because of destructive
detonation. Fortunately, a small decrease in the compression
ratio of a gasoline engine allows considerably more pressure
charging without raising the cylinder pressure unduly. However,
lowering the compression ratio does reduce thermal efficiency
and therefore increase fuel consumption. Once confined to
use in high-performance sports cars and racing cars, forced
induction is now used widely in a variety of production cars.
In the case of the diesel engine there is actually more justification
for the use of forced induction because of the shorter time
available for mixing the fuel and air. Forced induction can
make up for the shortage of oxygen available from natural
aspiration due to this brief mixing period in diesel engines
and so increase engine power.
Car manufacturers today are motivated by a variety of reasons
to use forced induction (primarily in the form of turbocharging).
These include uprating a naturally aspirated engine to give
performance similar to that of an existing naturally aspirated
engine of larger swept volume, so that engine space, vehicle
weight, and fuel consumption can be kept down; and compensating
for power losses due to stricter emission controls.
In addition to the turbocharger (of which there are a number
of different kinds), forced induction can also be achieved
through the use of inertia ramcharging and a supercharger.
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