The crankcase is the housing for the crankshaft within a reciprocating internal combustion engine. In most modern engines, the crankcase is integrated into the engine block. Two-stroke engines use a crankcase compression design which results in the fuel/air mixture passing through the crankcase before entering the cylinders. In this configuration, the crankcase does not include an oil sump. In four-stroke engines, the crankcase features an oil sump at the bottom, with most of the engine’s oil being held in the crankcase. The fuel/air mixture does not pass through the crankcase in a four-stroke engine, though a modicum of exhaust gases commonly enter as residual blow-by from the combustion chamber.
Most two-stroke engines use a crankcase compression design, wherein a vacuum draws the fuel/air mixture into the engine as the piston moves upwards. Then, as the piston moves downward, the inlet port is uncovered and the now-compressed mixture of fuel and air is pushed from the crankcase through to the combustion chamber. Crankcase component designs such as this one are commonly used in small gasoline engines in motorcycles, generator sets, and powered garden equipment. Crankcase compression design is also found in small diesel engines, though it is far less common.
Another type of two-stroke engine crankcase design is the lubricating crankcase. This is used in large two-stroke engines that do not utilize a crankcase compression, but rather a separate supercharger that draws the fuel/air mixture into the compression chamber. Lubricating crankcases are similar to a four-stroke engine in that they are solely used for lubrication purposes. Four-stroke engine crankcases contain the engine’s lubricant oil, either in the form of a wet sump system or dry sump system. In a dry-sump system, there are two or more oil pumps and an independent oil reservoir, while a wet sump system utilizes the main oil pan below the engine and a single pump.
There are two types of crankcases used in four-stroke engines: oil circulation and combustion gas ventilation. In an oil circulation configuration, engine oil is circulated around a four stroke-engine, with the majority of the circulation taking place in the crankcase. Oil is stored either at the bottom of the crankcase or in a separate reservoir. From here, the oil is pressurized by an oil pump and then sent into the crankshaft and connecting rod bearings and onto the cylinder walls. From here, the oil slowly drips off into the bottom of the crankcase.
In the second type, combustion gas ventilation, piston rings are intended to seal the combustion chamber from the crankcase. Despite this, it is normal for some combustion gases to escape around the piston rings and make their way into the crankcase. This is known as blow-by. If too much of this gas accumulates in the crankcase, it would cause unwanted pressurization of the crankcase, contamination of the oil, and rust from condensation. In order to prevent this, modern engines use a ventilation system to remove the combustion gases from the crankcase. Generally, the gases are fed through to the intake manifold.
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