What is the main purpose of crankcase ventilation?

[LaPreston]

The main purpose of crankcase ventilation is to discharge the combustible mixture and exhaust gases that have leaked into the crankcase, thereby improving the vehicle's driving performance. During engine operation, the high-pressure combustible mixture in the combustion chamber and the burned gases will, to some extent, leak into the crankcase through the gap between the piston assembly and the cylinder, causing blow-by. The blow-by gases consist of unburned fuel vapor, water vapor, and exhaust gases, which dilute the engine oil, reduce its performance, and accelerate its oxidation and deterioration. The lower part of the cylinder block, where the crankshaft is installed, is called the crankcase. The crankcase is divided into the upper crankcase and the lower crankcase. The upper crankcase is cast integrally with the cylinder block, while the lower crankcase is used to store lubricating oil and seal the upper crankcase, hence it is also known as the oil pan.