Crankcase

The crankcase is the lower portion of an internal-combustion engine that houses the crankshaft and forms the structural foundation of the engine block. It exists to provide a rigid, sealed enclosure in which the crankshaft can rotate, to carry the loads generated as the pistons drive the crank, and to contain and manage the engine's lubricating oil. In effect it is the engine's main case: the part to which the cylinder block above and the sump below are referenced.

In the great majority of modern engines the crankcase is cast in one piece with the cylinder block, the two together forming the cylinder-block-and-crankcase assembly, with the cylinders rising above and the crankshaft chamber below. The crankshaft is supported along its length by main bearings, whose lower halves are held by main-bearing caps or, in stiffer designs, by a one-piece ladder frame or bedplate bolted into the bottom of the crankcase. These bearings locate the crankshaft precisely and react the enormous, fluctuating forces transmitted down the connecting rods from the pistons, so the crankcase must be strong and dimensionally stable.

The crankcase also seals and contains the lower end of the engine. Oil thrown from the bearings and draining from the cylinder head collects in the sump beneath it, and oil seals at the front and rear of the crankshaft prevent leakage where the shaft passes out to drive the timing system and the gearbox. The chamber's shape and internal windage trays influence how the rotating assembly interacts with the oil, affecting both lubrication and frictional drag.

A defining practical feature is crankcase ventilation. During combustion a small amount of gas inevitably escapes past the piston rings into the crankcase, a phenomenon known as blow-by, which raises pressure and contaminates the oil with fuel, water and combustion by-products. A positive crankcase ventilation, or PCV, system draws these gases out and routes them back into the intake to be burned, relieving pressure, protecting seals and reducing emissions, replacing the older practice of simply venting them to the atmosphere.

From a maintenance perspective the health of the crankcase ventilation matters more than the casting itself, which rarely fails. A blocked PCV valve or breather can cause pressure to build, forcing oil past seals and gaskets and leading to leaks, while excessive blow-by is often a symptom of worn rings or bores. The oil it contains must be changed at the prescribed intervals because the crankcase is where contaminants accumulate.

The crankcase is intimately related to the components it serves: it houses the crankshaft, which is driven through the connecting rods by the pistons, and as the lower structure of the block it is a fundamental element of the internal-combustion engine.