What is the purpose of a crankcase ventilation system in an internal combustion engine?

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The main idea is to control and recover the gases that escape from the combustion chamber into the crankcase, rather than letting them build up or vent freely. During operation, some combustion gases blow past the piston rings into the crankcase—this is blow-by. If those gases aren’t managed, they raise crankcase pressure, force oil seals to leak, dilute the oil, and create sludge. The crankcase ventilation system provides a controlled path for those gases to leave the crankcase and return to the engine—usually into the intake manifold or a separate collector—so they can be burned in the next cycle or safely captured. This reduces emissions, keeps the oil cleaner, and protects seals and gaskets.

In contrast, increasing crankcase pressure would worsen leaks, injecting fuel into the crankcase isn’t how engines are designed to operate, and recycling exhaust gases back to the intake manifold describes the exhaust gas recirculation system, not crankcase ventilation.