Crankcase

A crankcase is the housing for the crankshaft in a reciprocating internal combustion engine. The enclosure forms the largest cavity in the engine and is located below the cylinder(s), which in a multicylinder engine is usually integrated into one or several cylinder blocks. Crankcases have often been discrete parts, but more often they are integral with the cylinder bank(s), forming an engine block. Nevertheless, the area around the crankshaft is still usually called the crankcase. Crankcases and other basic engine structural components (e.g., cylinders, cylinder blocks, cylinder heads, and integrated combinations thereof) are typically made of cast iron or cast aluminium via sand casting. Today the foundry processes are usually highly automated, with a few skilled workers to manage the casting of thousands of parts. A crankcase is the housing for the crankshaft in a reciprocating internal combustion engine. The enclosure forms the largest cavity in the engine and is located below the cylinder(s), which in a multicylinder engine is usually integrated into one or several cylinder blocks. Crankcases have often been discrete parts, but more often they are integral with the cylinder bank(s), forming an engine block. Nevertheless, the area around the crankshaft is still usually called the crankcase. Crankcases and other basic engine structural components (e.g., cylinders, cylinder blocks, cylinder heads, and integrated combinations thereof) are typically made of cast iron or cast aluminium via sand casting. Today the foundry processes are usually highly automated, with a few skilled workers to manage the casting of thousands of parts. A crankcase often has an opening in the bottom to which an oil pan is attached with a gasketed bolted joint. Some crankcase designs fully surround the crank's main bearing journals, whereas many others form only one half, with a bearing cap forming the other. Some crankcase areas require no structural strength from the oil pan itself (in which case the oil pan is typically stamped from sheet steel), whereas other crankcase designs do (in which case the oil pan is a casting in its own right). Both the crankcase and any rigid cast oil pan often have reinforcing ribs cast into them, as well as bosses which are drilled and tapped to receive mounting screws/bolts for various other engine parts. Besides protecting the crankshaft and connecting rods from foreign objects, the crankcase serves other functions, depending on engine type. These include keeping the motor oil contained, usually hermetically or nearly hermetically (and in the hermetic variety, allowing the oil to be pressurized); providing the rigid structure with which to join the engine to the transmission; and in some cases, even constituting part of the frame of the vehicle (such as in many farm tractors). A large number of small two-stroke engines use a sealed crankcase as a compression chamber for their mixture. These are very common as petrol or gasoline small engines for motorcycles, generator sets and garden equipment. Both sides of the piston are used as working surfaces: the upper side is the power piston, the lower side acts as a scavenging pump. As the piston rises, it pushes out exhaust gases and produces a partial vacuum in the crankcase, which draws in fuel and air. As the piston travels downward, the compressed fuel/air charge is pushed from the crankcase into the cylinder. Unlike larger engines, the crankcase does not contain engine oil because it handles the fuel/air mixture. Instead, oil is mixed in with the fuel supply as petroil, and this mixture provides lubrication for the cylinder walls, crankshaft and connecting rod bearings. Using ball or roller bearings for the bearings in such engines has always been more common than the oil-fed plain bearings used for other engines, as these are more amenable to using this oil mist lubrication. These engines have been used in larger sizes for small cars. They were considered in the 1980s as more fuel efficient engines for larger cars, but the increased concern of engine emissions prevented their use: mixing lubricating oil into the engine's intake mixture gave high HC emissions and this problem was not overcome. Small diesel engines may also use this type of crankcase compression. Such engines are still uncommon, compared to petrol engines, but they are used for generators and industrial equipment and are becoming available for diesel motorcycles. Large two stroke engines do not use crankcase compression, but instead a separate scavenge blower or supercharger. Their crankcases are reserved for lubrication purposes and are comparable to that of a four-stroke engine. Very large slow-speed two-stroke diesel engines, such as those in ships, may have their crankcase as a separate space from the cylinders, or as an open crank. The piston rod is supported by a separate crosshead. The spaces between the piston rod and the crosshead, or the crosshead to the crankshaft, may be largely open for maintenance access. As they are large and their movement is slow, the bearings may be made individually well enough sealed to catch and return the oil from the bearings. In a four-stroke engine, the crankcase is filled mainly with air and oil, and is largely sealed off from the fuel/air mixture by the pistons.