(Automotive engineering: Vehicle components, Engine, transmission, and exhaust) Blow-by is unwanted leakage of gas under pressure, such as that from a piston or its sealing rings.
⇒ The blow-by that occurs when too much pressure from the combustion chamber leaks into the crankcase
needs to escape somewhere. ⇒ All piston engines experience blow-by of compressed gas past the piston rings. ⇒ If the crankcase were completely closed, the blow-by would raise its internal pressure to unsafe levels.
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