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Under blow (of English. Blow and by so blown over , sometimes leakage called) refers to the in compression in a combustion engine or piston compressor to the piston rings from the working space in the machine room over grazing gas. Even with optimal sealing, it amounts to approx. 0.5% to 2% of the total gas volume.

When compressing, the compression is lower and when burning the gas mixture, the maximum combustion chamber pressure . During the work cycle, hot gas, which is partially contaminated with aggressive pollutants, enters the crankcase and pollutes the engine oil. The entrainment of unburned fuel, especially when starting from cold, leads to oil dilution .

In order to prevent leaks in the crankcase due to the overpressure, it is vented. The crankcase ventilation usually leads into the air filter box or the airbox . The escaped gas is therefore sucked in again on the next intake stroke and any oil droplets and unburned fuel that have been carried along are burned afterwards. The introduction of combustion gases into the intake system can contaminate the air duct, throttle valve, turbocharger, valves, etc. The turbocharger and intercooler in particular can lead to performance losses and malfunctions.

The loss due to leakage (leakage loss) can be determined in an engine process calculation and is less than 1% for engines that are not worn and that are well maintained.

Leakage loss
Internal efficiency with real combustion process , without leakage
Internal efficiency with real combustion process , with leakage
Internal work of the high pressure process with wall heat transfer , without leakage, SI unit : Joule
Internal work of the high pressure process with wall heat transfer , with leakage, SI unit : Joule

See also


  • Richard van Basshuysen, Fred Schäfer (Hrsg.): Handbook internal combustion engine 1st edition. Friedr. Vieweg & Sohn, Braunschweig / Wiesbaden 2002, ISBN 3-528-03933-7
  • Rudolf Pischinger, Manfred Klell, Theodor Sams: Thermodynamics of the internal combustion engine . 2nd revised edition. The vehicle drive. Springer, Vienna / New York 2002, ISBN 3-211-83679-9