The purpose of a turbocharger
is to improve upon the size-to-output efficiency
of an engine by solving one of its cardinal limitations.
A naturally aspirated automobile engine uses only
the downward stroke of a piston to create an area
of low pressure in order to draw air into the cylinder.
Because the number of air and fuel molecules determine
the potential energy available to force the piston
down on the combustion stroke, and because of the
relatively constant pressure of the atmosphere,
there ultimately will be a limit to the amount of
air and consequently fuel filling the combustion
chamber. This ability to fill the cylinder with
air is its volumetric efficiency. Because the turbocharger
increases the pressure at the point where air is
entering the cylinder, and the amount of air brought
into the cylinder is largely a function of time
and pressure, more air will be drawn in as pressure
increases. The additional air makes it possible
to add more fuel, increasing the power output of
the engine. Also, the intake pressure can be controlled
by a wastegate, which bleeds off excess boost from
the turbocharger.
The application of a compressor to increase pressure
at the point of cylinder air intake is often referred
to as forced induction. Centrifugal superchargers
operate in the same fashion as a turbo; however,
the energy to spin the compressor is taken from
the rotating output energy of the engine's crankshaft
as opposed to exhaust gas. For this reason turbochargers
are ideally more efficient, since their turbines
are actually heat engines, converting some of the
thermal energy from the exhaust gas that would otherwise
be wasted, into useful work. Contrary to popular
belief, this is not totally 'free energy', as it
always creates some amount of exhaust backpressure
which the engine must overcome. Superchargers use
output energy from an engine to achieve a net gain,
which must be provided from some of the engine's
total output; either directly or from a separate
smaller engine, perhaps electrically driven from
the main engine's generator.
Turbocharging is very common on diesel engines
in conventional automobiles, in trucks, locomotives,
for marine and heavy machinery applications. In
fact, for current automotive applications, non-turbocharged
diesel engines are becoming increasingly rare. Diesels
are particularly suitable for turbocharging for
several reasons:
• Naturally-aspirated diesels will develop
less power than a gasoline engine of the same size,
and will weigh significantly more because diesel
engines require heavier, stronger components. This
gives such engines a poor power-to-weight ratio;
turbocharging can dramatically improve this P:W
ratio, with large power gains for a very small (if
any) increase in weight.
• Diesel engines require more robust construction
because they already run at very high compression
ratio and at high temperatures so they generally
require little additional reinforcement to be able
to cope with the addition of the turbocharger. Gasoline
engines often require extensive modification for
turbocharging.
• Diesel engines have a narrower band of engine
speeds at which they operate, thus making the operating
characteristics of the turbocharger over that "rev
range" less of a compromise than on a gasoline-powered
engine.
• Diesel engines blow nothing but air into
the cylinders during cylinder charging, squirting
fuel into the cylinder only after the intake valve
has closed and compression has begun. Gasoline/petrol
engines differ from this in that both fuel and air
are introduced during the intake cycle and both
are compressed during the compression cycle. The
higher intake charge of temperature forced-induction
engines reduces the amount of compression that is
possible with a gasoline/petrol engine, whereas
diesel engines are far less sensitive to this.
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