Engine displacement

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 inches, cubic centimeters, or litres. In a piston engine, this is the volume that is swept as the pistons are moved from top dead center to bottom dead center.

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Standard engines

In a standard piston engine (an Otto or Diesel engine), displacement is calculated by multiplying the number of cylinders in the engine with the area of a piston and the length of the stroke. With circular pistons, displacement can be calculated from the bore diameter and stroke using the following formula:

[itex]{\pi\over 4} \times \mbox{bore}^2 \times \mbox{stroke} \times \mbox{number of cylinders}[itex]

Displacement in other engine types (especially for a Wankel engine) is much more complicated.

Displacement is equal to the volume of combustible air/fuel mixture ingested during one cycle of all the cylinders at 100% volumetric efficiency. Thus, a four-stroke engine ingests its displacement in combustible mixture in two engine revolutions, while a two-stroke engine needs only one engine revolution to do so.

Engine power is thus dependent on the quantity of air/fuel mixture ingested and the efficiency of its combustion and conversion into power. To increase the quantity of mixture combusted, the engine displacement can be increased, the speed of operation of the engine can be increased, or the mixture can be delivered at a higher pressure, which is the function of such devices as turbochargers and superchargers.

All other factors being equal, a larger displacement engine is therefore more powerful than a smaller one. It is the easiest method of adding power since it neither requires higher rotational speeds nor complicated auxiliaries; however, engine weight and bulk increase proportionally. The ease of adding power this way (along with the lack of performance effects such as turbocharger lag caused by the time needed to spin up the turbine of the turbocharger) led to the sayings There's no substitute for cubic inches or, alternatively, There's no replacement for displacement commonly quoted by devotees of large-engined cars.

The added mass and size reduce a vehicle's maneuverability however, and in applications where that is important, alternative methods for increasing power are commonly employed. Additionally, because the efficiency of the engine is not improved, fuel consumption rises dramatically.

In cars, engines over 8 litres displacement are extremely rare in the last half-century and most modern cars utilise engines much smaller than that; 1 to 2 litres for smaller cars, 3 to 5 litres for larger and faster cars.

Five to 10 litre engines are used in many single and twin engine propeller-driven aircraft. Much larger engines tend to be diesel engines fitted to trucks, ships, railroad locomotives and those used to drive stationary generators. The displacement of each cylinder in such an engine may be much larger than that of a whole car engine.

Governmental regulations

In many nations levels of taxation on automobiles have been based on engine displacement, rather than on power output or vehicle weight. 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 common to 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 100 cubic inches, which is approximated to 1.6 litres. The British system is different than the European one even though it is in the Eu. It is very similar, except that the peculiar Royal Automobile Club formula for approximating the power of primitive engines was maintained over many decades instead of displacement (this calculation does not include the stroke of the piston). The Japanese is similar to the European, except that automobiles are inspected after three years very harshly. It is only in the American system, which includes Canada, Australia and New Zealand, that there is not this sort of taxation per engine displacement. Also, in The Netherlands, road tax is not based on engine displacement but on vehicle weight.

Example regulations

• Ireland: under 1.4 L; 1.4-1.9 L; over 1.9 L
• Korea: under 0.8 L; 0.8-2.0 L; over 2.0 L
• Spain: under 1.6 L; over 1.6 L
• Taiwan: under 2.0 L; over 2.0 L
• Philippines (proposal from 2000): under 1.6 L; 1.6-2.0 L; 2.0-2.8 L; over 2.8 L
• Bulgaria: a special tax on non-European cars over 2.8 L, later amended to over 3.0 L
• Belgium and Portugal have a proportional tax including reference to displacement

Increase and decrease of typical engine displacement in the US

Once V8 engines became expected on large American cars in the late 1950s, and continuing to the oil crisis in the 1970s, there was an engine displacement race in the industry. Firms would put badges on the fenders of cars giving the displacement in cubic inches. This was also a sort of trademark as well. There's a famous Beach Boys song, "409", which refers to any full-size Chevrolet which had an engine of that size in it, regardless of trim level. This number was not the model number of the car.

In the mid-1960s, Chrysler offered a V8 engine of 426 cubic inches (6981 cm³) on its muscle cars and pony cars. Soon Ford came out with one of similar size, but it couldn't use the same label, so the engine was made and labelled as 427 cubic inches (6997 cm³). When Ford improved its engine by changing ancillary equipment, to indicate the change they put badges labelled "428" on such cars, and subsequently did the same to get "429".

With the oil shocks of the 1970s, American firms started selling cars with smaller engines. The Chevrolet Vega was initially touted as having an engine of 1998 "cc" (cubic centimetres), given in metric because it equates to 122 cubic inches, which would have been considered laughable to declare on the American market. This also differs from the European convention of two significant figures, which was in the U.S. European car models usually have a number of three digits. In this instance, the numbers are considered trademarks. These two factors in the world marketplace contributed to American cars now getting labelled in the European manner. Engines like that of the Vega would now be called 2.0 (being litres).

Conversions

• 1 L ~ 61 inch³
• 1 inch³ ~ 16 cm³

The big engines listed above are mostly 7.0 litres. The 3.5 litre engines listed on American cars today as being large are much smaller than the 350 cubic inch (5.7 L) engines that once were considered medium size.

The 3.5 litre engine is 213 cubic inches. The 1964 Mustang's smallest Ford V8 engine of 289 cubic inches is 4.7 litres.

However, modern electronically-controlled engines these days are much more efficient, and the cars they are fitted in are lighter, so the difference in performance is not as great as might otherwise be supposed.

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