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Engine displacement is defined as the total volume of air/fuel mixture an engine can draw in during one complete engine cycle; it is normally stated in cubic centimeters, liters, or cubic inches. In a piston engine, this is the volume that is swept as the pistons are moved from top dead center to bottom dead center.
An engine's displacement is one of the basic mechanical features that determines maximum possible power output. All other factors being constant, increasing engine displacement will cause some increase in horsepower.
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Calculation
Engine displacement is calculated using the bore, stroke, and number of cylinders:
or
Governmental regulations
In many nations levels of taxation on automobiles have been based on engine displacement, rather than on power output. Displacement is easy to identify and difficult to modify whereas power output must be tested. This has encouraged the development of other methods to increase engine power.
There are four major regulatory constraints for automobiles: the European, the British, the Japanese, and the American. The method used in some European countries, and which predates the EU, has a level of taxation for engines over one (1.0) liter and another at the level of about 100 cubic inches, which is approximated to 1.6 liters. The British system of taxation depends upon vehicle emissions for cars registered after 1 March 2001 but for cars registered before this date it depends on engine size. Cars under 1549 cc qualify for a cheaper rate of tax.[1]
The Japanese is similar to the European taxation by classes of displacement, plus a vehicle weight tax. In the American system, which includes Canada, Australia and New Zealand, there is not this sort of taxation per engine displacement. In The Netherlands and Sweden, road tax is based on vehicle weight.
Displacement is also used to distinguish categories of (heavier) motorbikes with respect to license requirements. In France and some other EU countries, mopeds, usually with a two-stroke engine and less than 50 cm3 displacement can be driven with minimum qualifications (previously, they could be driven by any person over 14). This led to all light motorbikes having a displacement of about 49.9 cm3. Some people tuned the engine by increasing the cylinder bore, increasing displacement; such mopeds cannot be driven legally on public roads since they do no longer conform to the original specifications and may go faster than 45 km/h.
Wankel engines, due to the amount of power and emissions they create for their displacement, are generally taxed as 1.5 times their actual physical displacement (1.3 liters becomes 2.0, 2.0 becomes 3.0), although actual power outputs are far greater (the 1.3 liter 13B can produce power comparable to a 3.0 V6, and the 2.0 liter 20B can produce power comparable to a 4.0L V8). As such, racing regulations actually use a much higher conversion factor.
See also
References
- ^ The Cost of Vehicle Tax for Cars, Motorcycles, Light Goods Vehicles and Trade Licences." Direct.gov.uk
