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The crankcase of a combustion engine accumulates gases and oil mist—called blowby—that can leak from several sources. The most important source of blowby is the combustion chamber, Figure 1 . Most of the combustion blowby occurs when the combustion chamber pressure reaches a maximum, during the compression and the expansion strokes. At high pressures, the gases leak to the crankcase around the piston rings and through the piston ring gap.
Figure 1. Combustion Blowby
Other important sources of blowby include the turbocharger shaft, air compressors and in some cases the valve stems. In total, these components can be responsible for as much as 40% of the crankcase blowby . Turbochargers and air compressors are often lubricated with oil supplied by the engine’s oil pump and drained back into the engine’s crankcase. The oil drain line from these components ensures that gas leaking past the turbocharger shaft and the piston rings of an air compressor will pass into the engine crankcase contributing to blowby.
Blowby amounts vary greatly depending on engine design, temperature operating conditions and engine wear. While a number of “rules of thumb” exist for estimating maximum engine blowby, they should be used with caution. Some of these estimates are outlined in Table 1.
|New engine||Blowby [dm3/s] = rated power [kW]/180|
Blowby [ft3/min] = rated power [hp]/120
|Worn engine||Blowby [dm3/s] = rated power [kW]/90|
Blowby [ft3/min] = rated power [hp]/60
|Blowby [dm3/s] = rated power [kW]/60|
Blowby [ft3/min] = rated power [hp]/40