Editing Engine displacement

{{Short description|Volume swept by all of the pistons}}
{{redirect|Swept volume|the 3D display technology|Swept-volume display}}
{{Use dmy dates|date=December 2019}}
{{Use British English|date=January 2020}}
[[File:Displacement.gif|thumb|upright=1.35|One complete cycle of a four-cylinder, [[four-stroke engine]]. The volume displaced is marked in orange.]]
'''Engine displacement''' is the measure of the [[cylinder (engine)|cylinder]] [[volume]] swept by all of the pistons of a [[piston engine]], excluding the [[combustion chamber]]s.<ref>{{cite web |title=Piston Engine Displacement |url=https://www.engineeringtoolbox.com/piston-engine-displacement-d_811.html |website=The Engineering Toolbox |access-date=18 August 2021}}</ref> It is commonly used as an expression of an engine's size, and by extension as an indicator of the [[engine power|power]] (through [[mean effective pressure]] and [[rotational speed]]) an engine might be capable of producing and the amount of fuel it should be expected to consume. For this reason displacement is one of the measures often used in advertising, as well as regulating, motor vehicles.

It is usually expressed using the [[metric system|metric]] units of [[cubic centimetre]]s (cc or cm<sup>3</sup>, equivalent to [[Litre#SI prefixes applied to the litre|millilitres]]) or [[litre]]s (l or L), or{{snd}}particularly in the United States {{snd}}[[cubic inch]]es (CID, cu in, or in<sup>3</sup>).

==Definition==
The overall displacement for a typical reciprocating piston engine is calculated by multiplying together three values; the distance travelled by the piston (the [[Stroke (engine)#Stroke length|stroke length]]), the [[Area of a circle|circular area]] of the cylinder, and the number of cylinders in the whole engine.<ref>{{cite web |title=Math for Automotives - Displacement of a Piston |url=https://www.avc.edu/sites/default/files/studentservices/lc/math/piston_displacement.pdf |website=arc.edu |publisher=Antelope Valley College |access-date=18 August 2021}}</ref>

The formula is:

:<math> \text{Displacement} = \text{stroke length} \times \pi \left(\frac{\text{bore}}{2}\right)^2 \times \text{number of cylinders}</math>

Using this formula for non-typical types of engine, such as the [[Wankel engine|Wankel design]] and the oval-piston type used in [[Honda NR]] motorcycles, can sometimes yield misleading results when attempting to compare engines. Manufacturers and regulators may develop and use specialised formulae to determine a comparative nominal displacement for variant engine types.

==Governmental regulations==
{{main|Road tax}}
In several countries fees and taxes levied on road vehicles by transport authorities are scaled in proportion to engine displacement. In countries where this is practised, vehicle manufacturers often seek to increase power output through higher-revving engines or [[Turbocharger|turbocharging]], instead of increasing the displacement.

Examples of countries where the road taxes are based upon engine displacement:
* In some European countries, and which predates the EU, there is one charge for engines over 1.0 litre, and another at the level of about 1.6&nbsp;litres.
* In the United Kingdom, cars registered after 1 March 2001 are taxed based on the exhaust emissions. However, cars registered before this date are taxed based on engine displacement. Cars under 1549&nbsp;cm<sup>3</sup> qualify for a lower tax rate.<ref>[http://www.direct.gov.uk/Motoring/OwningAVehicle/HowToTaxYourVehicle/HowToTaxYourVehicleArticles/fs/en?CONTENT_ID=10012524&chk=X2gG9G Direct.gov.uk] {{webarchive |url=https://web.archive.org/web/20060616173240/http://www.direct.gov.uk/Motoring/OwningAVehicle/HowToTaxYourVehicle/HowToTaxYourVehicleArticles/fs/en?CONTENT_ID=10012524&chk=X2gG9G |date=16 June 2006 }}: The Cost of Vehicle Tax for Cars, Motorcycles, Light Goods Vehicles and Trade Licences.</ref>
* In Japan, the engine displacement is one of the factors (along with overall vehicle size and power output) used to determine the vehicle size class and therefore the cost of road tax for the vehicle.
* In France and some other EU countries, [[moped]]s with a displacement of less than {{convert|50|cc|cuin|1|abbr=on}} can be driven with minimum qualifications. This led to all light motorbikes having a displacement of about 49.9&nbsp;cm<sup>3</sup>.
* In many areas of the United States, Canada (except Quebec<ref>{{Cite web|url=https://saaq.gouv.qc.ca/en/saaq/rates-fines/vehicle-registration/additional-registration-fee-large-cylinder-capacity-vehicles/|title=Additional Registration Fee for Large Cylinder Capacity Vehicles|last=SAAQ|website=SAAQ|language=en|access-date=12 March 2018}}</ref>), Australia and New Zealand, the road taxes are not based on engine displacement. However, the engine displacement is often used in low-powered scooters or mopeds to determine whether a licence is required to operate the vehicle. A common threshold is {{convert|50|cc|cuin|1|abbr=on}}.

Wankel engines are able to produce higher power levels for a given displacement. Therefore, they are generally taxed as 1.5 times{{citation needed|date=November 2019}} their stated physical displacement (1.3&nbsp;litres becomes effectively 2.0, 2.0 becomes effectively 3.0), although actual power outputs can be higher than suggested by this conversion factor. The nominal displacement of a Wankel engine is 3 times smaller than the physical displacement, but this is compensated by the fact that the shaft has 3 times the rotational speed of the rotor. The nominal displacement is the swept volume of a single chamber.

== Automotive model names ==
Historically, many car model names have included their engine displacement. Examples include the 1923–1930 [[Cadillac Type V-63|Cadillac Series 353]] (powered by a 353 [[Cubic inch]]/5.8-litre engine), and the 1963–1968 [[BMW New Class#1800|BMW 1800]] (a 1.8-litre engine) and [[Lexus LS|Lexus LS 400]] with a 3,968 cc engine.  This was especially common in US [[muscle car]]s, like the [[Ford Mustang]] Boss 302 and 429, and later GT 5.0L, The [[Plymouth Roadrunner]] 383, and the [[Chevrolet Chevelle]] SS 396 and 454.

However, trends towards [[engine downsizing|downsizing]] and hybrid/electric drivetrains since 2010 have resulted in far fewer model names being based on the engine displacement.

==See also==
* [[Active Fuel Management]]
* [[Bore (engine)]]
* [[Compression ratio]]
* [[Stroke (engine)]]
* [[Variable displacement]]

== References ==
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{{Automotive engine}}
{{Authority control}}

{{DEFAULTSORT:Engine Displacement}}
[[Category:Engine technology|Displacement]]