crankcase ventilation
From Glossary of Meteorology
The emission of unburned fuel to the atmosphere from the crankcase of an internal combustion engine.
Formerly a large source of hydrocarbons to the atmosphere, the rate of escape has been greatly reduced by recycling of the escaped gases to the cylinders by means of a device called the positive crankcase ventilation (PCV) valve. Crankcase ventilation contributes around 5% of unburned hydrocarbon emissions to the atmosphere.
