Engine displacement

Engine displacement is the volume swept by the piston(s) in a single movement. In the very familiar four-stroke piston engine (but also in the two stroke engine) this is the volume that is swept as the piston(s) move from top dead center to bottom dead center. It can be specified in cubic centimeters, liters, or cubic inches. An engine's displacement is a basic mechanical feature such that, all other factors being equal, more engine displacement means more horsepower.

Alternatively, displacement must sometimes be defined as the total volume of air/fuel mixture an engine draws in during one complete engine cycle, howsoever defined and subject to further interpretation by taxation and racing authorities.

The engine's displacement is often used in the manufacturers nomenclature. For instance, the BMW 528 is a 5-series car with a 2.8 litre engine and Nissan's Teana 350JM is a Teana with a 3498cc (213.5 CID) engine. Motorcycles are often labelled this way.

Units of measure
The cubic inch was often formerly used (until the 1980s) to express the displacement of engines for new cars, trucks, etc. (e.g., the "426" in 426 HEMI refers to 426 cubic inches displaced). It is therefore still used for this purpose in the context of the classic-car hobby, auto racing, and so forth. The auto industry nowadays uses SI for this purpose (e.g. 6.1 L HEMI). However, the actual displacement measurements of an engine are still given by many manufacturers in cubic inch displacement (usually along with cc; e.g. the 6.1 L HEMI's published displacement is 370.0 CID/6,059 cc). Some examples of common CID-to-litre conversions are given below. Note that nominal sizes are not always precisely equal to actual sizes. This principle is frequently seen in engineering, tool standardization, etc. (for ease of use) and in marketing (when a big round number sounds more impressive, is more memorable, etc.).

Governmental regulations
Taxation of automobiles is commonly based on engine displacement, rather than power output. Displacement is basic to an engine design whereas power output depends a great deal on other factors, including wear and even the weather. This has encouraged the development of other methods to increase engine power, such as variable valve timing and turbochargers.

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.

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. However, Swedish cars which was registered in 2008 or later, have it's tax based on its carbon dioxide emissions.

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.