The connecting rod is an important component of the engine and serves for three purposes:
? it connects the piston with the crankshaft;
? it changes reciprocal motion of the piston into rotary motion of the crankshaft:
? it transfers forces generated by ignition of mixture in the combustion chamber, onto the crankshaft, causing torque on the latter one.
The connecting rod must meet very rigorous requirements – let’s think about transfer of huge longitudinal forces, created by ignition in the combustion chamber above the piston. In addition, there are forces of acceleration and deceleration, created by the constantly changing speed of the piston. In addition to all this, there are bending forces, caused by pendulum motion. For these reasons, the connecting rod must have enormous mechanical resistance, but at the same time should also be as light as possible.
Connecting rod structure and shape
The connecting rod is made of the best possible materials. For example, refined steel and refined cast iron are suitable for this purpose. At the top end there is the head, in which is located the connecting rod sleeve, including the piston pin and connecting the piston with the connecting rod. In some constructions, there is no sleeve and then the piston pin is fixed in the piston head hole mechanically through compression or thermally through heating of the piston (so that it expands) or through cooling of the pin with the use of carbon dioxide.
From the head down to the connecting rod, runs its stem which connects the head with the foot. In order to achieve maximum resistance of the stem, it is made as a cast or in a similar manner. Due to the lightness of such form and its bending resistance, such shape is a current standard.
The connecting rod foot, together with the foot cover, forms the bottom end of the connecting rod. This element is attached to the crankshaft in the part which leans sideways from the axis of the shaft. The use of plain bearing is justified by the great bearing capacity of this type of bearing and its ability to suppress noise and durability. In addition, such bearings require less space and are lighter than other types of bearings.
Another type of bearing is used in single-, double-cylinder and two-stroke engines. Here, due to a different type of lubrication (no oil from oil pan, only oil-air mixture) and use of the crankcase for pre-compression, the crankshaft is connected with the connecting rod by means of cylindrical bearings or their needle variants. In both cases, the reasons for their use are less rigorous lubrication requirements. From the constructional point of view, it is important that, unlike bushings, such bearing cannot be disassembled ? their assembly and disassembly requires the use of crankshaft which consists of multiple components.
Connection of rod with piston
In high-performance engines the most popular solution are titanium connecting rods. This modern material is much lighter than steel and satisfies the highest requirements. Currently, carbon fiber rods are tested – the advantage of such solution is high resistance at low weight, as well as quiet operation and virtually no wear. However, at this moment such connecting rods are too expensive to be used in serial production.
Lubrication of connecting rods is carried out by means of filling the oil pan with engine oil. Pressure generated by the pump feeds oil into the bearings through the lubrication holes in the shaft. A hole is drilled through the connecting rod’s foot, through which oil under pressure is sprayed onto the bearing of the connecting rod head and onto the piston bottom – apart from lubricating the bearings, it cools the piston itself effectively.