|Temperature||Up to 500°C (petrol engine)|
Up to 900°C (Diesel engine)
|Pressure||12 - 20 bar (petrol engine)|
Up to 50 bar (Diesel engine)
BDC = bottom dead-center
|Piston travels from BDC to TDC.|
Valves are closed.
Smallest volume at TDC.
TDC = top dead-center
In the combustion stroke, the flame length in the following combustion, should be kept as short as possible through high compression and swirling, thus increasing the economic efficiency and the performance. Through a low exhaust gas recirculation during certain operating conditions, the nitrogen oxide proportion is reduced, and a high level of efficiency is still maintained.
The valves are closed during almost the entire stroke. Only at the beginning is the intake-valve still briefly open. The petrol-air fuel-mixture in the Otto engine, is compressed to between 9:1 and approx. 13:1 or in the Diesel engine (see picture) the pure air is compressed to between 14:1 and 20:1, if the piston travels from BDC to TDC and the combustion occurs before TDC. Immediately preceding the combustion, the fuels must be converted into a gas. For this to take place, a sufficient amount of temperature is necessary.
A high compression ratio has a positive influence on the torque, the performance and the fuel consumption, this is because the carbon- and the oxygen atoms are brought closer together, thus providing for short reaction paths. The result is, a quicker and stronger combustion, with a low pollutant content.
The upper limit of the compression ratio in the petrol engine is exceeded if the mixture uncontrollably self-ignites (knocking). Thereby, the combustion temperature rises above the stress-limit which leads to noises which are similar to those of a mechanical defect, and the strain on the crank mechanism is higher.
The Diesel engine can have as much compression as the smooth running, the engine service life, the material complexity or the amount of nitrogen oxide allows.
To determine the actual condition of the engine, without dismantling the cylinder head, the compression can be measured. In the petrol engine the spark plugs - in the Diesel engine the nozzles - are removed. The sealings of the valves, the cylinder head gasket and the cylinder sleeves are examined. The measured compression lies somewhat lower than the actual running compression. 03/12