Forced induction

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A turbocharger for a car engine Turbo Mini GT (cropped).jpg
A turbocharger for a car engine
A supercharger (on top of a dark-grey inlet manifold) for a car engine Supercharger mustang.jpg
A supercharger (on top of a dark-grey inlet manifold) for a car engine

In an internal combustion engine, 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. [1]


Operating principle


Forced induction is often used to increase the power output of an engine. [2] This is achieved by compressing the intake air, to increase the mass of the air-fuel mixture present within the combustion chamber. A naturally aspirated engine is limited to a maximum intake air pressure equal to its surrounding atmosphere; however a forced induction engine produces "boost", [3] 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. [4] However, considerations (such as cooling the combustion chamber, preventing engine knock 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 engines are well suited to forced induction, since the lack of fuel in the intake air means that higher compression ratios 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-strokes 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 18,000 feet (5,500 m), 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. [5] 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 29.5 inHg (100 kPa). [5]

Types of compressors

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

Related Research Articles

<span class="mw-page-title-main">Compression ratio</span> Ratio of the volume of a combustion chamber from its largest capacity to its smallest capacity

The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values.

<span class="mw-page-title-main">Diesel engine</span> Type of internal combustion engine

The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-called compression-ignition engine. This contrasts with engines using spark plug-ignition of the air-fuel mixture, such as a petrol engine or a gas engine.

<span class="mw-page-title-main">Turbocharger</span> Exhaust-powered forced-induction device for engines

In an internal combustion engine, a turbocharger is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to produce more power for a given displacement.

<span class="mw-page-title-main">Miller cycle</span> Thermodynamic cycle

In engineering, the Miller cycle is a thermodynamic cycle used in a type of internal combustion engine. The Miller cycle was patented by Ralph Miller, an American engineer, U.S. Patent 2,817,322 dated Dec 24, 1957. The engine may be two- or four-stroke and may be run on diesel fuel, gases, or dual fuel.

A stratified charge engine describes a certain type of internal combustion engine, usually spark ignition (SI) engine that can be used in trucks, automobiles, portable and stationary equipment. The term "stratified charge" refers to the working fluids and fuel vapors entering the cylinder. Usually the fuel is injected into the cylinder or enters as a fuel rich vapor where a spark or other means are used to initiate ignition where the fuel rich zone interacts with the air to promote complete combustion. A stratified charge can allow for slightly higher compression ratios without "knock," and leaner air/fuel ratio than in conventional internal combustion engines.

<span class="mw-page-title-main">Four-stroke engine</span> Internal combustion engine type

A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

  1. Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing vacuum pressure into the cylinder through its downward motion. The piston is moving down as air is being sucked in by the downward motion against the piston.
  2. Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
  3. Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. the compressed air-fuel mixture is ignited by a spark plug or by heat generated by high compression, forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
  4. Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust valve.
<span class="mw-page-title-main">Intercooler</span> Heat exchanger used to cool a gas after compression

An intercooler is a heat exchanger used to cool a gas after compression. Often found in turbocharged engines, intercoolers are also used in air compressors, air conditioners, refrigeration and gas turbines.

<span class="mw-page-title-main">Roots-type supercharger</span> A positive displacement lobe pump

The Roots-type blower is a positive displacement lobe pump which operates by pumping a fluid with a pair of meshing lobes resembling a set of stretched gears. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust. The most common application of the Roots-type blower has been the induction device on two-stroke diesel engines, such as those produced by Detroit Diesel and Electro-Motive Diesel. Roots-type blowers are also used to supercharge four-stroke Otto cycle engines, with the blower being driven from the engine's crankshaft via a toothed or V-belt, a roller chain or a gear train.

Volumetric efficiency (VE) in internal combustion engine engineering is defined as the ratio of the mass density of the air-fuel mixture drawn into the cylinder at atmospheric pressure to the mass density of the same volume of air in the intake manifold. The term is also used in other engineering contexts, such as hydraulic pumps and electronic components.

<span class="mw-page-title-main">Naturally aspirated engine</span> Type of internal combustion engine

A naturally aspirated engine, also known as a normally aspirated engine, and abbreviated to N/A or NA, is an internal combustion engine in which air intake depends solely on atmospheric pressure and does not have forced induction through a turbocharger or a supercharger. Many sports cars specifically use naturally aspirated engines to avoid turbo lag.

Indirect injection in an internal combustion engine is fuel injection where fuel is not directly injected into the combustion chamber.

Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.

A twincharger refers to a compound forced induction system used on some internal combustion engines. It is a combination of an exhaust-driven turbocharger and a mechanically driven supercharger, each mitigating the weaknesses of the other.

Twin-turbo refers to an engine in which two turbochargers work in tandem to compress the intake fuel/air mixture. The most common layout features two identical or mirrored turbochargers in parallel, each processing half of a V engine's produced exhaust through independent piping. The two turbochargers can either be matching or different sizes.

The MA09ERT is a mass-produced SOHC internal combustion engine manufactured by Nissan Motors. It is a series-charged design, seldom seen in Japanese vehicles.

<span class="mw-page-title-main">Turbo-diesel</span> Diesel engine with a turbocharger

The term turbo-diesel, also written as turbodiesel and turbo diesel, refers to any diesel engine equipped with a turbocharger. As with other engine types, turbocharging a diesel engine can significantly increase its efficiency and power output, especially when used in combination with an intercooler.

<span class="mw-page-title-main">Supercharger</span> Air compressor for an internal combustion engine

In an internal combustion engine, a supercharger compresses the intake gas, forcing more air into the engine in order to produce more power for a given displacement.

<span class="mw-page-title-main">Two-stroke diesel engine</span> Engine type

A two-stroke diesel engine is an internal combustion engine that uses compression ignition, with a two-stroke combustion cycle. It was invented by Hugo Güldner in 1899.

Internal combustion engines come in a wide variety of types, but have certain family resemblances, and thus share many common types of components.

<span class="mw-page-title-main">Internal combustion engine</span> Engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber

An internal combustion engine is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is typically applied to pistons, turbine blades, a rotor, or a nozzle. This force moves the component over a distance, transforming chemical energy into kinetic energy which is used to propel, move or power whatever the engine is attached to. This replaced the external combustion engine for applications where the weight or size of an engine was more important.


  1. "How Forced Induction Works". 23 September 2013.
  2. Elisabeth H. Dorries (December 2004). TechOne: Automotive Engine Repair. Cengage Learning. p. 106. ISBN   1-4018-5941-0.
  3. Don Fuller (December 1982). "Turbocharging - This time it's here to stay". Popular Mechanics. Hearst Magazines: Page 75, 109. ISSN   0032-4558.
  4. Cengle, Y.A.; Boles, M.A. (2008). Thermodynamics: An Engineering Approach (6th ed.). New York, N.Y.: McGraw-Hill. pp. 70, 590.
  5. 1 2 Knuteson, Randy. "Boosting Your Knowledge of Turbocharging" (PDF). Aircraft Maintenance Technology (July 1999). Archived from the original (PDF) on 17 June 2012. Retrieved 18 April 2012.