Engine displacement is the measure of thecylindervolume swept by all of the pistons of apiston engine, excluding thecombustion chambers.[1] It is commonly used as an expression of an engine's size, and by extension as an indicator of thepower (throughmean effective pressure androtational 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 themetric units ofcubic centimetres (cc or cm3, equivalent tomillilitres) orlitres (l or L), or – particularly in the United States – cubic inches (CID, c.i.d., cu in, or in3).
The overall displacement for a typical reciprocating piston engine is calculated by multiplying together three values; the distance travelled by the piston (thestroke length), thecircular area of the cylinder, and the number of cylinders in the whole engine.[2]
The formula is:
Using this formula for non-typical types of engine, such as theWankel design and the oval-piston type used inHonda 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.
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 orturbocharging, instead of increasing the displacement.
Examples of countries where the road taxes are based upon engine displacement:
Wankel engines are able to produce higher power levels for a given displacement. Therefore, they are generally taxed as 1.5 times[citation needed] their stated physical displacement (1.3 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.
Historically, many car model names have included their engine displacement. Examples include the 1923–1930Cadillac Series 353 (powered by a 353Cubic inch/5.8-litre engine), and the 1963–1968BMW 1800 (a 1.8-litre engine) andLexus LS 400 with a 3,968 cc engine. This was especially common in USmuscle cars, like theFord Mustang Boss 302 and 429, and later GT 5.0L, ThePlymouth Roadrunner 383, and theChevrolet Chevelle SS 396 and 454.
However, trends towardsdownsizing and hybrid/electric drivetrains since 2010 have resulted in far fewer model names being based on the engine displacement.
