The oil crankcase is a foundational structure in an internal combustion engine, serving as the main housing for the rotating assembly. It is the lower section of the engine block that surrounds the crankshaft. The crankcase is a sealed environment that shields the engine’s moving parts from external contaminants like dirt and moisture. Its primary functions are to provide structural support for the engine’s core components and to manage the engine’s lubricating oil supply.
The Crankcase: Location, Structure, and Primary Role
The crankcase is located at the bottom of the engine block, situated above the oil pan, which bolts to it to form a complete enclosure. In most modern four-stroke engines, the crankcase is integrated into the engine block casting, making it a single, rigid unit. It is typically made from durable, lightweight materials like aluminum alloy to dissipate heat and reduce overall engine weight. The crankcase houses and supports the crankshaft, which converts the pistons’ up-and-down motion into rotational energy. It provides mounting points for the main bearings, which hold the crankshaft in precise alignment.
The crankcase also functions as the reservoir for the engine’s lubricating oil, especially in wet sump systems. The oil pump draws oil from this reservoir and circulates it under pressure to lubricate the bearings, connecting rod bearings, and other moving parts. This circulation reduces friction, prevents wear, and helps carry heat away from the engine.
Crankcase Ventilation and the PCV System
During engine operation, combustion gases inevitably leak past the piston rings and into the crankcase. This phenomenon is known as “blow-by,” consisting of unburnt fuel, air, and exhaust by-products. If left unmanaged, these gases create excessive pressure within the sealed crankcase. This pressure buildup can force oil past the engine’s seals and gaskets, leading to external oil leaks and potential engine damage.
Blow-by gases also contain moisture and contaminants that mix with the lubricating oil, accelerating its degradation and forming sludge. Sludge formation can clog oil passages and lead to lubrication failure. The Positive Crankcase Ventilation (PCV) system manages this internal pressure and contamination. The system uses a one-way PCV valve and engine vacuum to draw the blow-by gases out of the crankcase. These gases are routed back into the engine’s intake manifold to be re-burned in the combustion chambers.
This process prevents the release of harmful pollutants and maintains a balanced, slightly negative pressure within the crankcase. The PCV valve regulates the flow of gases, restricting it at high vacuum (like idling) and opening wider at lower vacuum (higher engine speeds) to ensure efficient ventilation under all operating conditions.