In an

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

, forced induction is where turbocharging or supercharging is used to increase the density of the intake air. Engines without forced induction are classified as naturally aspirated.

Operating principle

Overview

Forced induction is often used to increase the power output of an engine. This is achieved by compressing the intake air, to increase the mass of the air-fuel mixture present within the

combustion chamber A combustion chamber is part of an internal combustion engine in which the air–fuel ratio, fuel/air mix is burned. For steam engines, the term has also been used for an extension of the Firebox (steam engine), firebox which is used to allow a mo ...

. A naturally aspirated engine is limited to a maximum intake air pressure equal to its surrounding atmosphere; however a forced induction engine produces "boost", whereby the air pressure is higher than the surrounding atmosphere. Since the density of air increases with pressure, this allows a greater mass of air to enter the combustion chamber. Theoretically, the vapour power cycle analysis of the second law of thermodynamics would suggest that increasing the mean effective pressure within the combustion chamber would also increase the engine's

thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For ...

. However, considerations (such as cooling the combustion chamber, preventing

engine knock In Spark-ignition engine, spark-ignition internal combustion engines, knocking (also knock, detonation, spark knock, pinging or pinking) occurs when combustion of some of the air-fuel ratio, air/fuel mixture in the cylinder does not result from ...

and limiting NOx exhaust emissions) can mean that forced induction engines are not always more fuel efficient, particularly in the case of high-performance engines.

Diesel engines

Four-stroke

diesel engine The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...

s are well suited to forced induction, since the lack of fuel in the intake air means that higher

compression ratio The compression ratio is the ratio between the maximum and minimum volume during the compression stage of the power cycle in a piston or Wankel engine. A fundamental specification for such engines, it can be measured in two different ways. Th ...

s can be used without a risk of pre-ignition. Therefore, the use of turbochargers on diesel engines is relatively commonplace. Two-stroke diesel engines have a significantly different operating principle to two-stroke petrol engines, and require some form of forced induction - generally a supercharger - in order to function.

High altitude uses

A reduced density of intake air is caused by the loss of atmospheric density seen with elevated altitudes. Therefore, an early use of forced induction was in aircraft engines. At , the air is at half the pressure of sea level, which means that an engine without forced induction would produce less than half the power at this altitude. Forced induction is used to artificially increase the density of the intake air, in order to reduce the loss of power at higher altitudes. Systems that use a turbocharger to maintain an engine's sea-level power output are called "turbo-normalized" systems. Generally, a turbo-normalized system attempts to maintain a manifold pressure of .

Types of compressors

The most commonly used forced-induction devices are turbochargers and superchargers. A turbocharger drives its compressor using a

turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced can be used for generating electrical ...

powered by the flow of exhaust gases, whereas a supercharger is mechanically powered by the engine (usually using a belt from the engine's crankshaft).

Associated technologies

Intercooling is often used to reduce the temperature of the intake air after it is compressed. Another less commonly used method is water injection (or methanol injection).

References

{{Authority control Superchargers Turbochargers