Editing Naturally aspirated engine

{{Short description|Type of internal combustion engine}}
[[File:4-Stroke-Engine-with-airflows_numbers.gif|thumb|upright|Typical airflow in a four-stroke engine:<br /> In stroke #1, the pistons suck in (aspirate) air to the [[combustion chamber]] through the opened [[inlet valve]].]]

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]].<ref>{{cite web |title=What is a normally aspirated engine? |url=http://ask.cars.com/2008/09/what-is-a-norma.html |date=2008-09-02 |access-date=2015-10-18 |work=ask.cars.com |archive-url=https://web.archive.org/web/20130622020433/http://ask.cars.com/2008/09/what-is-a-norma.html |archive-date=2013-06-22 |url-status=dead }}</ref>

== Description ==
In a naturally aspirated engine, air for combustion ([[Diesel cycle]] in a [[diesel engine]] or specific types of [[Otto cycle]] in petrol engines, namely [[Gasoline direct injection|petrol direct injection]]) or an air/fuel mixture (traditional Otto cycle petrol engines), is drawn into the engine's [[cylinder (engine)|cylinders]] by atmospheric pressure acting against a [[Vacuum|partial vacuum]] that occurs as the [[piston]] travels downwards toward [[Dead centre (engineering)|bottom dead centre]] during the intake [[stroke (engine)|stroke]]. Owing to innate restriction in the engine's inlet tract, which includes the [[Inlet manifold|intake manifold]], a small pressure drop occurs as air is drawn in, resulting in a [[volumetric efficiency]] of less than 100&nbsp;percent—and a less than complete air charge in the cylinder. The density of the air charge, and therefore the engine's maximum theoretical [[power (physics)|power output]], in addition to being influenced by induction system restriction, is also affected by engine speed and atmospheric pressure, the latter of which decreases as the operating [[altitude]] increases.

This is in contrast to a [[forced-induction]] engine, in which a mechanically driven [[supercharger]] or an exhaust-driven [[turbocharger]] is employed to facilitate increasing the mass of intake air beyond what could be produced by atmospheric pressure alone. [[Nitrous oxide]] can also be used to artificially increase the mass of oxygen present in the intake air. This is accomplished by injecting liquid nitrous oxide into the intake, which supplies significantly more oxygen in a given volume than is possible with atmospheric air. Nitrous oxide is 36.3% available oxygen by mass after it decomposes as compared with atmospheric air at 20.95%. Nitrous oxide also boils at {{convert|-127.3|F}} at atmospheric pressures and offers significant cooling from the latent heat of vaporization, which also aids in increasing the overall air charge density significantly compared to natural aspiration.

== Applications ==
Most [[automobile]] petrol engines, as well as many small engines used for non-automotive purposes, are naturally aspirated.<ref>{{cite web |url=http://www.privatefleet.com.au/glossary/what-is-a-naturally-aspirated-engine/ |title=What is a Naturally Aspirated Engine ? |website=Private Fleet |access-date=2017-02-17 |quote=Most motor vehicle engines are naturally-aspirated engines; however, turbocharging and supercharging are currently a very popular way of boosting power output for a number car marques.}}</ref> Most modern diesel engines powering highway vehicles are [[Turbocharger|turbocharged]] to produce a more favourable [[power-to-weight ratio]], a higher [[torque curve]], as well as better [[fuel efficiency]] and lower [[Vehicle emissions control|exhaust emissions]]. Turbocharging is nearly universal on diesel engines that are used in [[Rail transport|railroad]], [[Marine propulsion|marine engine]]s, and commercial [[:wikt:stationary|stationary]] applications ([[Electricity generation|electrical power generation]], for example). Forced induction is also used with reciprocating [[aircraft engine]]s to negate some of the power loss that occurs as the [[aircraft]] climbs to higher altitudes.

== Advantages and disadvantages ==
The advantages and disadvantages of a naturally aspirated engine in relation to a same-sized engine relying on [[forced induction]] include:

===Advantages===
* Easier to maintain and repair 
* Lower development and production costs
* Increased reliability, partly due to fewer separate, moving parts
* More direct throttle response than a turbo system due to the lack of [[turbo lag]] (an advantage also shared with superchargers)
* Less potential for overheating and or uncontrolled combustion (pinging/ knocking)

===Disadvantages===
* Decreased efficiency 
* Decreased [[power-to-weight ratio]]
* Decreased potential for [[Engine tuning|tuning]]
* Increased power loss at higher elevation (due to lower air pressure) compared to forced induction engines

== See also ==
*[[Carburetor]]
*[[Fuel injection]]
*[[Manifold vacuum]]

== References ==
{{reflist}}

{{Automotive engine |collapsed}}

[[Category:Engine technology]]
[[Category:Internal combustion engine]]