Engine displacement

Engine displacement, in a piston engine, defines the volume (normally stated in cubic inches, cubic centimeters, or litres) that is displaced as the pistons are moved from top dead center to bottom dead center.

In a standard piston engine, displacement is calculated from the bore and stroke with the following formula:

<math>{\pi\over 4} \cdot \mbox{bore}^2 \cdot \mbox{stroke} \cdot (\mbox{number of cylinders})<math>

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

Displacement is more-or-less equal to the volume of combustible air/fuel mixture ingested during one cycle of all the cylinders. 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.

It can be easily seen that 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 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/or faster cars.

Much larger engines tend to be diesel engines, fitted to trucks, ships railroad locomotives and 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.

In many nations, levels of taxation on automobiles have been based on engine displacement, rather than on power output, since displacement is easier to calculate. This has encouraged the development of other methods to increase engine power.

See also:

compression ratio, engine tuning