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Combustion in the combustion chamber acts on the piston with a pressure (force when multiplied by the piston head area) which generates a linear movement of the piston. This movement is then transformed into a rotating one since the piston is mounted on a reciprocating mechanism consisting of the crankshaft and conrod. The crankshaft rotates with the support of spherical bearings mounted in the crankcase referred to as the main bearings.
The force acting on the piston is transformed and transmitted through the conrod and crankshaft and must then be absorbed by the crankcase. The crankcase not only has to absorb these forces, it also has many other functions that make it an essential element of the engine. A fundamental role is to act as a pre-compression chamber. When the piston moves downwards in the cylinder, mixture is compressed and pushed into the transfer ducts, through the transfer ports and into the combustion chamber. In addition to this, the crankcase also has the function of being the bond between the engine’s different components such as the cylinder, crankshaft, carburettor, engine-chassis mount, ignition system and coil.
The material used to build the crankcases of modern 2-stroke competition engines, such as those used for karting purposes, is aluminium alloy, which reduces considerably the weight of the engine. Aluminium though has some weaknesses linked to the fact that it heats up more easily than cast iron and consequently deforms more. Its heating up generates a rise in temperature of the crankcase walls which consequently heats up the mixture in the crankcase. A fluid (mixture) that heats up increases in pressure and expands, reducing its density and the volumetric efficiency of the crankcase pump.
Temperature increases also generate expansion of the aluminium alloy and therefore the walls of the crankcase (they reach around 100°C). This expansion will be different in different areas of the crankcase since the component’s walls vary in thickness and also fresh mixture can act as more of a coolant in some areas than others. Consequently, the crankcase will deform and generate negative effects. For example, alignment between the two main bearings that carry the crankshaft can be lost. Gas sealing between the crankcase and cylinder can also become critical. Another important effect is that the perpendicularity of the cylinder’s axis to the crankshaft is at risk. Finally, main bearings can change the tightness of assembly of the roller balls inside the cages which can produce seizures and breaking of the cages.
The limited mechanical strength of light aluminium alloys from which crankcases are constructed also needs to be taken into account with regard to the threads of the holes for the holding down bolts and the cylinder head studs. To avoid rapid damage to the threads it is advisable for the effective threaded length to be not less than 2.5 times the diameter. Assistance can also be given by the insertion of steel threads that are much more resistant, especially when studs and bolts are frequently screwed and unscrewed. Since the crankcase working as a pump needs reduced internal volume to be effi cient, it is constructed of two halves symmetrical with respect to a vertical plane perpendicular to the axis of the crankshaft. When joined, the two halves must be completely gas tight and to do this a gasket is also positioned between the parts. Mixture loss would reduce the efficiency and performance of the engine.
To have good matching of the two symmetrical parts, production must be very precise. Cylindrical dowel pins also help couple the two elements. Sealing must also be obtained where the crankcase opens to permit the exit of the ends of the crankshaft. Oil seals are used on both sides and are rubber rings that have one or two ‘lips’ that seal the area around the crankshaft. The aim is to have good sealing but low friction loss generated by the ‘lips’ and the crankshaft that are in contact with one another. As already mentioned, the crankcase also has the role of absorbing all the forces transmitted by the crankshaft. To limit deformation of the crankcase it is built with ribbing all over its external surface. These ribs help both to strengthen the structure and to cool down the surfaces