What is Crankcase In an Engine?
A crankcase is the housing for the crankshaft in a reciprocating internal combustion engine. In most modern engines, the crankcase is integrated into the engine block. It also functions as housing and protects the engine parts against dust, water, and splashing mud.
The crankcase stores lubricating oil required for lubricating the engine parts. The size of a crankcase is sufficiently large as it accommodates the revolving crankshaft with the connecting rod.
The crankcase not only gives support to the engine parts and engine mountings, but also withstands the loads caused by piston thrust, gas pressure, primary and secondary forces, couples, etc.
Therefore, the crankcase must be strong to withstand these loads and pressures. When the cylinder block and the crankcase are cast together in one unit, grey cast iron is used because it has rigidity, low cost, and high wear resistance.
Two-stroke engines typically use a crankcase-compression design, resulting in the fuel/air mixture passing through the crankcase before entering the cylinder. This design of the engine does not include an oil sump in the crankcase.
Four-stroke engines typically have an oil sump at the bottom of the crankcase and the majority of the engine’s oil is held within the crankcase. The fuel/air mixture does not pass through the crankcase in a four-stroke engine; however, a small amount of exhaust gasses often enters as “blow-by” from the combustion chamber.
An open-crank engine has no crankcase. This design was used in early engines and remains in use in some large diesel engines, such as used in ships.
Types of Crankcases
The cylinder block and the upper part of the crankcase are from an integral cast. Thus, a crankcase is usually divided into an upper and a lower section. The lower section is known as the ‘oil pan’ and acts as a reservoir for the storage of lubricating oil.
The lubricating oil is splashed due to the rotation of the crank and is also pumped to the engine bearings, thus lubricating the various engine parts. For cooling the lubricating oil, fins or ribs are provided on the outside of the oil pan.
These fins also increase the strength of the oil pan.
The joints between the upper section of the crankcase and the oil pan may be either on the level of the crankshaft axis or below this axis. The assembly of the upper section of the crankcase with the oil pan has been shown.
The main forces acting on a cylinder block are due to:
- Gas pressure includes the force of the explosion, and
- Inertial forces due to reciprocating masses.
Both these forces act along the connecting rod, i.e., the line of stroke. These forces tend to lift the cylinder blocks from the crankcase.
Therefore, in the case of a single-cylinder engine having a crankcase joint on the axis of the crankshaft, resisting forces are induced in the threads of the retaining bolts used at the joints. Note that the angularity of the connecting rod results in the horizontal forces on the cylinder walls and the crankshaft bearing.
Decreasing the length of the connecting rod increases the side forces. In the case of a multi-cylinder engine, the resulting stresses are divided between a greater number of bolts. In the case of 90° V-type engines, the component of stresses is equally divided in vertical and horizontal directions.
Therefore, the crankcase is split through the crankshaft axis. Such assembly makes the crankcase lighter because the oil pan size is increased which is usually made of aluminum alloy.
To minimize the resisting forces in the bolts used in the crankcase joint, the upper section of the crankcase is further extended below the axis of the crankshaft. The extension is from 50 mm to 75 mm below the crankshaft.
This decreases the size of the oil pan, but the crankcase rigidity is increased in the vertical direction. The upper section of the crankcase takes up the force of the explosion, whereas the oil pan bolts take only inertial forces.
A four-stroke cycle engine needs a heavier flywheel than a two-stroke cycle engine. Therefore, the crankcase of the four-stroke cycle engine is more robust than the two-stroke cycle engine. An engine following a mixed cycle has a high compression ratio and a large force of the explosion and therefore needs a stronger crankcase.