CRANKCASE :
The crankcase supports the cylinders and the crankshaft and is an important structure in the internal combustion engine. It also functions like a 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. Various accessories like carburettor, fuel pump, generator, water pump, air cleaner, starting motor, fan, oil filter, oil body of cooler, etc. are also mounted on the crankcase. 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 and 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 resistance.
Types of Crankcases:
The cylinder block and the upper part of the crankcase form an integral cast. Thus a crankcase is usually di- vided 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 crank- case and the oil pan may be either on the level of the crankshaft axis or below this axis. In Fig. 3.5, 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:
1. Gas pressure including force of explosion, and
2. Inertial forces due to reciprocating masses.
Both these forces act along the connecting rod, i.e. line of stroke. These forces tend to lift the cylinder blocks from the crankcase.
Therefore in the case of a single cylinder engine having 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 case of a multi-cylinder engine, the resulting stresses are divided between more number of bolts. In case of 90° V-type engines, the component of stresses are equally divided in vertical and horizontal directions. Therefore the crankcase is split through the crankshaft axis, Such assem- bly makes the crankcase lighter because aluminium alloy.
To minimise 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 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 mixed cycle has a high compression ratio and large force of explosion and therefore needs a stronger crankcase.
Both these forces act along the connecting rod, i.e. line of stroke. These forces tend to lift the cylinder blocks from the crankcase.
Therefore in the case of a single cylinder engine having 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 case of a multi-cylinder engine, the resulting stresses are divided between more number of bolts. In case of 90° V-type engines, the component of stresses are equally divided in vertical and horizontal directions. Therefore the crankcase is split through the crankshaft axis, Such assem- bly makes the crankcase lighter because aluminium alloy.
To minimise 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 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 mixed cycle has a high compression ratio and large force of explosion and therefore needs a stronger crankcase.
