The displacement or the displacement means for the cylinder of piston engines the enclosed volume , which consists of the working path of the single piston stroke results and the effective piston cross-sectional area. It thus defines the volume that a motor is displaced by the piston stroke of all total.


As effective piston cross-sectional area is generally the area enclosed by the piston ring and the inner cylinder wall circular area A display ( A = \ pi \ times r ^ 2 Wherein r the radius of the piston surface is).

Not to be set, however, is a larger piston surface which is composed of a curved instead of a schedule is piston end. In machines whose pistons a continuous piston rod has such as in steam locomotives customary to determine the stroke volume, the circular cross-sectional area of the piston rod from the piston cross-section to be withdrawn.

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

The stroke volume V results with words A_w as an effective piston area and h as stroke to
V = A_w \ cdot h ,

where the unit of measurement generally cubic centimeters or liters are given.

With the same parameters ( speed and mean effective pressure ), the displacement is an indicator for comparing the performance of different machines and one of the characteristics of an internal combustion engine . A distinction is made in addition to the single cylinder displacement and the total capacity of a machine, resulting from the multiplication of the number of cylinders with Zylinderhubräume.

Depending on whether the piston is greater than or less than the diameter of the cylinder, this is called a long-stroke or short-stroke . Bore and stroke are the same size, then one speaks of a square stroke ratio .

Important factors:

V_h → cylinder engine;
V_h → engine displacement;
V_c → compression space;
V → combustion chamber;
i → cylinder number;
d → cylinder diameter;
h (Or: s ) → stroke, stroke or short stroke.

cylinder displacement

The displacement is the volume between the bottom and top dead center of the piston of an engine cylinder, and is usually expressed in cm 3 or l. The following applies:

V_h = \ frac {d ^ 2 \ cdot \ pi} {4} \ cdot h .

Engine Displacement

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

V_h v_h = \ cdot i .

combustion chamber

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

V = + v_h V_c .

With dual-piston engines have two cylinders together a combustion chamber, the situation is more complicated.
Specifications in automotive internal combustion engines

Huber allow short because of the larger cylinder of larger cross-sectional area to accommodate valves. They have – with the same speed – compared to long-Hubern lower piston speed are so fixed speed and are therefore used mostly in sports cars and motorcycles.

Long Huber enable better combustion chamber design and improved combustion.

In general, the combustion chamber is similar shortly after ignition, ie, 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 length is, 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 through it.

Long Huber therefore have a slightly higher efficiency .

The trend in automotive construction is increasingly in smaller and therefore lighter engines, by virtue of the smaller heat transfer surfaces with the same power have lower thermal losses. However, this does not necessarily less efficient high-volume engines.
displacement and torque

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

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


Has a naturally aspirated engine has a displacement of 2000 cm 3, the maximum torque is usually at 170 to 210 Nm. It is much higher (about 230 nm), it can be assumed that the motor has a motor, such as charging of an exhaust gas turbocharger or a compressor .
Capacity and road tax

The engine displacement is often the tax applied, whereby the so-called tax formula is used (situation in Germany). Since taxes are usually staggered over again engines with capacities were built just below these thresholds, for example, 1998 cm ³ instead of 2000 cc. The type designation of a vehicle often has the capacity in the specific name.

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

old StVZO control formula: Total displacement = Cdot 0 {,} 78 \ D ^ 2 \ times H \ times Z (D = bore, H = stroke, Z = number of cylinders, bore and stroke H D Note prior to the calculation of half a millimeter, rounding the result to the nearest cubic centimeter. \ Pi / 4 here is rounded to 0.78, therefore the capacity differs according to the old tax formula right down significantly from the actual displacement from)
new EU tax formula: Total displacement = 0 {,} 7854 \ times D ^ 2 \ times H \ times Z (D = bore, H = stroke, Z = number of cylinders, bore and stroke H D are to full millimeters mathematically rounded up or down; \ Pi / 4 here is rounded up to 0.7854)

As part of the – amended several times – tax formula other rounding rules, common than in the technology, there were and are always variations in the capacity indicated in the technical data of the producer’s and the administrative data for taxation on the other. By grinding cylinders in an engine overhaul enlarged holes are not considered for taxation. On implementation of the displacement tax in Germany was intended by the legislator to tax the engine power; since this time was not easy to measure, however, was the volume – by different for two-stroke and four-stroke engines formulas – used as a measure of engine power, details see below taxable horsepower .

Numerous laws, such as the German car tax law , take the displacement as the basis, for example, driving licenses at mopeds . In new registrations of passenger cars in Germany in 2008 30.3% of the vehicles had an engine displacement from 1800 to 1999 cc.

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