Engine displacement

The displacement or the displacement referred to the cylinders of reciprocating engines the enclosed volume, which results from the working path of each piston stroke and the effective piston cross -sectional area. So as to define the volume as a whole is displaced in a motor by the stroke of all the pistons.

  • 3.1 Displacement and torque
  • 3.2 Displacement and vehicle tax


An effective cross sectional area of ​​the piston is usually to see the area enclosed by the piston ring and the inner cylinder wall, the circular surface ( with the radius of the piston face ).

On the other hand not to recognize a larger piston area, which results from a curved rather than a flat piston end. In machines whose pistons a continuous piston rod, such as in conventional steam engines, the circular cross-sectional area of the piston rod has to be removed from the piston cross-section for the determination of the stroke volume.

The piston is the path which is traveled between the two extreme positions of the working piston, usually called in internal combustion engines as the upper and lower dead center.

The stroke volume thus results with to be an effective piston area and stroke as

Where as a unit cubic centimeters or liters are usually given.

With the same parameters ( engine speed and brake mean effective pressure) of the cylinder capacity is an indicator to compare the performance of various characteristics of the machine and an internal combustion engine. It is also distinguished by the individual cylinder displacement and total displacement of a machine, which results from the multiplication of the Zylinderhubräume with the number of cylinders.

Depending on whether the stroke is larger or smaller than the diameter of the cylinder, one speaks of a long stroke or a short stroke. Are bore and stroke the same size, so it is called a square stroke ratio.


Important parameters:

Cylinder Displacement

The displacement is the volume between the upper and lower dead point of the piston engine of a cylinder and is usually expressed in cm3 or l. The following applies:

Engine capacity

For multi- cylinder engines with cylinders Z is the subsequent formula, provided that the individual cylinders have the same displacements:

Combustion chamber

The combustion chamber is the area enclosed by the cylinder, piston and cylinder head volume; it is dependent on the position of the piston, in operation, thus time-dependent. The piston is at bottom dead center, then:

With dual-piston engines, two cylinders have a combustion chamber together, the conditions are more complicated.

Specifications in automotive internal combustion engines

Short Huber allow for the larger cylinder cross-sectional area to accommodate larger valves. You have - at the same speed - compared to long- Hubern a lower piston speed, and are therefore fixed speed and are therefore mostly used in sports cars and motorcycles.

Langhuber enable better combustion chamber design and a better combustion.

In general, the combustion chamber is similar shortly after ignition, so then when the gas is at its hottest, a flat cylinder, so it has a large surface area relative to volume. The smaller the cylinder bore, and the longer the stroke, the less flat the shape of the combustion chamber at this time is, the smaller the combustion chamber and the lower surface of the heat conduction losses therethrough.

Therefore, long-stroke engine with a slightly higher efficiency.

The trend in automotive engineering is increasingly smaller and therefore lighter engines with the same power as these have lower thermal losses due to the smaller heat transfer surfaces. However, this does not necessarily mean a lower efficiency of large-volume engines.

Displacement and torque

For a given cylinder pressure means the engine size determines the torque. Displacement medium pressure and speed together affect performance.

Rule of thumb for normally aspirated engines: The value for the maximum torque in Nm measured here is approximately 100 Nm per liter of displacement. However, a deviation of the torque value of about 10 to 15 percent is in this case often present, generally downward but also increasingly upward.

Has a naturally aspirated engine had a displacement of 2,000 cm3, the maximum torque is usually at 170 to 210 Nm. Is it much higher ( about 230 Nm) is to assume that the motor via a motor charging features such as a turbocharger or a supercharger.

Displacement and road tax

The capacity is often also used for tax assessment, in which case the so-called tax formula is used ( situation in Germany ). Since taxes are usually staggered engines with displacements were always built just below these limits, for example, 1998 cm ³ ³ instead of 2000 cm. The type designation of a motor vehicle often has the Displacement in the more specific name.

The calculation of the displacement of reciprocating engines according to the control formula made ​​in Germany since 1989 by two different methods:

  • Old traffic regulations control formula: Total displacement = (d = bore, h = hub, Z = number of cylinders, bore D and stroke H are before the computation to the nearest half millimeter, rounding the result to the nearest cubic centimeter Note: is completed here on 0.78. thus giving way to the cylinder capacity in the old tax formula right down significantly from the actual engine capacity from )
  • New EU tax formula: Total displacement = (d = bore, h = hub, Z = number of cylinders, bore and stroke H D are on full millimeter mathematically rounded up or down, is here rounded to 0.7854 )

In part in the Da - amended several times - Tax formula other rounding rules apply, as customary in the art, there were and are always variations in the displacement information in the technical data of the producer's and the administrative data for the taxation on the other. By grinding cylinder at an engine overhaul enlarged holes will not be considered for taxation. Upon introduction, the displacement control in Germany was intended by the legislature to tax the engine power; For details, used as a measure for the engine performance, see fiscal horsepower - but since this was not easily measurable at that time, the engine capacity was - according to different for two-stroke and four-stroke engines formulas.

Numerous laws, such as the German motor vehicle tax law, take the engine capacity as the tax base, for example, categories of licenses for mopeds. For new registrations of passenger cars in Germany in 2008 30.3 % of the vehicles had a ³ engine capacity from 1800 to 1999 cm.

Related Topics

  • The compression ratio refers to the ratio before the combustion chamber volumes and after compression.