Combustion chamber

from Wikipedia, the free encyclopedia
Recess in the piston as the combustion chamber of a diesel engine
Recess in the cylinder head as a combustion chamber

A combustion chamber is in an internal combustion engine during ignition to the piston bordering gas-filled space in which the combustion begins.

In the usual reciprocating piston engine , the piston passes through the displacement before and after .

In the simplest version as a two-stroke engine , the combustion chamber is surrounded by the cylinder cover with spark plug , the piston with the piston rings , and to a lesser extent by the cylinder . In the usual four-stroke engine , its valves are added in the cylinder head .

In the rotary piston engine , the combustion chamber is limited by the piston and the housing wall.

The combustion chamber must be gas-tight in order to start the engine. The sealing is demanding because high temperatures and pressures occur during operation.

Design options

The shape of the combustion chamber influences the mixing of the gas with fuel, the combustion process and the resulting pressure on the piston. The shape of the combustion chamber has a decisive influence on the fuel consumption , the performance and the tendency of an engine to knock .

The combustion chamber of two-stroke engines can be freely designed without valves, but they have other disadvantages. The shape of the combustion chamber is unfavorable in rotary piston engines, which is one of their fundamental disadvantages.

In four-stroke reciprocating engines, the valve arrangement and the shape of the combustion chamber are closely linked. The diversity is greatest in two-valve designs: Valves hanging in parallel can be arranged parallel to the cylinder's longitudinal axis, in which case the combustion chamber is usually a recess in the cylinder head around the valves. If they are inclined to the longitudinal axis of the cylinder, the combustion chamber is usually wedge-shaped or roof-shaped. The larger slope of the roof then carries the valves, the smaller the spark plug. This shape was developed by Harry Ricardo and allows relatively high compression ratios. Even in the early days of automobile construction at the beginning of the 20th century, high-performance engines were made with V-shaped valves and a hemispherical combustion chamber. This offers uniform flame paths in all directions and a very favorable ratio of surface to volume and thus low heat losses; the valves are then at 90 degrees to each other. With increasing compression ratios, however, it became apparent that, due to the large volume of the hemisphere, the piston crown had to be arched up to an unfavorable extent if high compression is desired. This gives the combustion chamber a sickle shape, which is very unfavorable for the spread of the combustion. For this reason, from the 1950s onwards, engines with smaller valve angles were developed, the combustion chamber of which was only a spherical cap and no longer a full hemisphere, which significantly increased the efficiency of these combustion chambers. In the case of four-valve constructions, the combustion chambers are usually designed in the shape of a roof, the pairs of valves then each stand in parallel on one of the roof surfaces.

In most practical applications, the combustion bowl does not cover the entire area of ​​the cylinder bore, but is significantly smaller in diameter. The piston is then brought as close as possible to the resulting flat surface of the cylinder head in the edge area of ​​the bore. During the compression process, the mixture located in this edge area is then squeezed out at high speed and ensures rapid swirling and thus good combustion of the mixture. This is why this construction is called a pinch edge.

With rotary piston engines there is

  • Circumferential inlets, circumferential outlets
  • Side inlets, side outlets

The side design enables lower consumption and lower emissions. The shape of the combustion chamber remains unaffected.

Special forms

Heron's head

The Heron head, in which the combustion chamber lies completely in the piston crown and the cylinder head is completely flat, is a special design of combustion chambers. This construction is easy to manufacture because the cylinder head only needs to be machined flat, but has disadvantages in terms of power development because of the necessarily small valves and also requires a large and heavy piston. The best-known representatives of this design are the medium-pressure engine of the Audis of the 1960s, the boxer engine of the Alfasud , the V-engines from Moto Morini from the 1970s and the V-engines of the small series from Moto Guzzi .

Three-ball vortex tub

Another special form is the three-ball vortex tub. In this design, long favored by BMW in the M10 and M30 engine series, small spherical caps are arranged around the two valves and the spark plug and the piston crown has an oval recess. This design was supposed to improve the smoothness, but limited the performance, which is why it was usually reworked into a hemispherical shape when increasing performance.

Fireball combustion chamber

The Fireball combustion chamber developed by the Swiss engineer May resembles a Heron head around the inlet valve and has a very small combustion bowl directly in front of the exhaust valve and a very high compression ratio. As a result, the engine is economical in the low to medium load range, but at the price of a loss in performance in the upper load range. The latter played no role in the only commercial application of this principle, the HE variant of Jaguar's twelve-cylinder engine.

G and M motors

A combustion chamber version for diesel engines is that of G- and M-firing process of the MAN . Here the cylinder head is completely flat and the combustion chamber is a deep, hollow spherical depression in the piston crown. The sucked-in air rotates very quickly in this recess and gradually releases the fuel injected into the combustion chamber.

Individual evidence

  2. ↑ : Jaguar XJ 12 L - if you want something more (luxury). Retrieved April 20, 2020 .


  • Richard van Basshuysen, Fred Schäfer (Hrsg.): Handbook internal combustion engine . 5th edition. Vieweg + Teubner, Wiesbaden 2010, ISBN 978-3-8348-0699-4 .
  • Rudolf Pischinger, Manfred Klell, Theodor Sams: Thermodynamics of the internal combustion engine . 3. Edition. Springer, Vienna 2009, ISBN 978-3-211-99276-0 .