Editing Forced induction (section)

== Operating principle ==
=== Overview ===
Forced induction is often used to increase the power output of an engine.<ref name="Dorries2004">{{cite book|author=Elisabeth H. Dorries|title=TechOne: Automotive Engine Repair|url=https://books.google.com/books?id=KsYFA0skuZ8C&pg=PA106|date=December 2004|publisher=Cengage Learning|isbn=1-4018-5941-0|page= 106}}</ref> 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 [[atmospheric pressure|surrounding atmosphere]]; however a forced induction engine produces "boost",<ref name="Magazines1982">{{cite journal |author= Don Fuller |title= Turbocharging - This time it's here to stay|journal=Popular Mechanics |url=https://books.google.com/books?id=-9kDAAAAMBAJ&pg=PA75|date=December 1982|publisher=Hearst Magazines|pages= Page 75,109|issn=0032-4558}}</ref> 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]].<ref name="Thermodynamics1">{{cite book |author1= Cengle, Y.A. |author2= Boles, M.A. |year=2008 |title=Thermodynamics: An Engineering Approach |edition= 6th |location= New York, N.Y. |publisher=McGraw-Hill |pages= 70, 590}}</ref> 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 engine]]s are well suited to forced induction, since the lack of fuel in the intake air means that higher [[compression ratio]]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 {{convert|18000|ft|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.<ref name="knuteson">{{cite journal |title=Boosting Your Knowledge of Turbocharging |last=Knuteson |first=Randy |issue=July 1999 |journal= Aircraft Maintenance Technology |access-date=18 April 2012 |url=http://www.kellyaerospace.com/articles/Turbocharging.pdf |archive-url=https://web.archive.org/web/20120617094358/http://www.kellyaerospace.com/articles/Turbocharging.pdf |archive-date=17 June 2012 |url-status=dead }}</ref> 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 {{cvt|29.5|inHg|kPa}}.<ref name="knuteson" />