The Importance of Bearings in Internal Combustion Engines
An internal combustion engine is a type of engine that converts the energy resulting from the ignition of a fuel/air mixture into mechanical energy to power a machine. The most common type of internal combustion engine is the reciprocating internal combustion engine. In engines of this type, the ignition and burning process takes place within a cylinder equipped with pistons driven by the pressure of the combustion gases. The force from the gas pressure is then transmitted to the crankshaft linked to the piston by a connecting rod.
Within internal combustion engines, bearings play a critical role. A bearing is a device used to support a mechanical element and allow movement relative to another element with minimal power loss. The rotating components of internal combustion engines are equipped with a type of sliding bearing known as sleeve bearings. Reciprocating engines are characterized by the cycling loading of their parts, including their bearings. This loading is the result of alternating pressure of the combustion gases within the cylinders. Rolling bearings, which transmit loads via balls to a relatively small area of the ring surface, cannot withstand the loading conditions of internal combustion engines. As such, only sliding bearings, which distribute the applied load over a relatively large area, can be used.
There are four types of sliding bearings used in internal combustion engines: main crankshaft bearings, connecting rod bearings, small end bushings, and camshaft bearings. Main crankshaft bearings support the crankshaft and provide its rotation under inertial forces generated by the shaft and oscillating forces transmitted by the connecting rods. Main crankshaft bearings are mounted in the crankcase and consist of two parts - an upper and lower part. The upper part typically features a hole for oil to pass through and feed the holes in the crankshaft. Some types of main crankshaft bearings, known as flange main bearings, feature thrust bearing elements that support axial loads and prevent movement along the crankshaft axis.
Connecting rod bearings provide rotational motion of the crank pin within the connecting rod, which transmits cycling loads applied to the piston. These bearings are mounted in the larger end of the connecting rod and typically consist of two commonly interchangeable parts. The third type, small end bushings, provide relative motion of the piston in conjunction with the connecting rod to which the piston is joined. As their name suggests, they are on the smaller end of the connecting rod. Small end bushes are cycle-loaded by the piston pushed by the alternating pressure of the combustion gases. Finally, camshaft bearings support the camshaft and provide its rotation.
It is pivotal that the bearings in an internal combustion engine be lubricated properly. The main purposes of engine bearing lubrication are the significant decrease in friction, which in turn minimizes heat, and the removal of foreign particles from the rubbing surfaces. Engine bearings typically work in hydrodynamic areas of friction, meaning there is a constant presence of lubricant between the bearing and journal surfaces. A continuous supply of sufficient amounts of lubricant is pivotal to the normal working of engine bearings and is carried out by the engine’s lubrication system.
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