One complete cycle of a four cylinder, four stroke engine. The volume displaced is marked in orange.

Engine displacement is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre (TDC) to bottom dead centre (BDC). It is commonly specified in cubic centimeters (cc), litres (l), or (mainly in North America) cubic inches. Motive power output of a combustion engine is directly proportional to the engine displacement. Engine displacement does not include the volume of the combustion chamber.

[edit] Definition

Engine displacement is determined from the bore and stroke of an engine's cylinders. The bore is the diameter of the circular chambers cut into the cylinder block.

Example: The 427 Chevy bore is 4.312 in, and the stroke is 3.65 in, therefore the displacement for this 8-cylinder engine is

 (3.141 / 4) * 4.312 in * 4.312 in * 3.65 in * 8 = 426.3 in3. 

[edit] Units of measure

The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. Please improve this article and discuss the issue on the talk page. (July 2009)

This section needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (July 2009)

In the United States, the cubic inch was the commonly used unit of measurement until the 1980s by the manufacturers to express the displacement of engines for cars, trucks, etc. (e.g., the "426" in 426 HEMI refers to 426 cubic inches displaced). It is still used for this purpose in the context of the classic-car hobby, auto racing, and so forth.^{[citation needed]}

The automotive industry nowadays uses the International System of Units / le Système international d'unités (SI), a modern metric system for this purpose worldwide (e.g. 6.1 litre HEMI). However, on markets accustomed to cubic inches, the actual displacement measurements of an engine are still given by many manufacturers in these units, usually along with metric value; e.g. the 6.1 litre HEMI's published displacement is 370.0 CID/6,059 cc.^[1][2][3][4] 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.).

[edit] Governmental regulations

Taxation of automobiles is sometimes based on engine displacement, rather than the actual power output. Displacement is a basic fundamental of engine design, whereas power output depends a great deal on other factors, particularly on how the car manufacturer has tuned the engine from new. 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) litre, and another at the level of about 1.6 litres. 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.^[5]

The Japanese method is similar to the European taxation by classes of displacement, plus a vehicle weight tax.

In the American system, which also 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 registered in 2008, or later, are taxed based on carbon dioxide emissions.^{[citation needed]}

Displacement is also used to distinguish categories of (heavier) motorbikes with respect to drving licence and insurance requirements. In France and some other EU countries, mopeds of less than 50 cm³ displacement (and usually with a two-stroke engine), 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 litres becomes effective 2.0, 2.0 becomes effective 3.0), although actual power outputs are far greater (the 1.3 litre 13B can produce power comparable to a 3.0 V6 engine, and the 2.0 litre 20B can produce power comparable to a 4.0 V8 engine).^{[citation needed]} As such, racing regulations actually use a much higher conversion factor.

[edit] Automotive model names

In the automotive industry, engine displacement is frequently encoded in the auto manufacturer's model names. For instance, the BMW 528 automobile is a model in the 5-series with a 2.8 litre engine, and Nissan's Teana 350JM is a car with a 3498 cc (213.5 cubic inch displacement (CID)) engine). Motorcycles are often labeled similarly. However, this can be misleading. For instance, the BMW 335i only has a 3.0 litre (turbocharged) engine.

[edit] See also

• Active Fuel Management
• compression ratio
• engine tuning
• variable displacement

[edit] References