The purpose of a turbocharger is to improve the size-to-output efficiency of an engine by solving one of its serious 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. In view of the fact that 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 will ultimately be a limit to the amount of air and there after, fuel filling the combustion chamber. This ability to fill the cylinder with air is its volumetric efficiency. As the diesel 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, escalating the power output of the engine. The intake pressure can also be controlled by a wastegate that bleeds off excess boost from the turbocharger.
The use 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 diesel 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, at all times, 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 in conventional automobiles, trucks, locomotives, marine and heavy machinery applications that have diesel engines. 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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