Introduction of Camshaft
The camshaft is a component in a piston engine. Its function is to control the opening and closing action of the valves. Although the speed of the camshaft in a four-stroke engine is half that of the crankshaft (in a two-stroke engine the speed of the camshaft is the same as that of the crankshaft), it is still usually very high and has to withstand a large torque, so the camshaft is designed with high requirements for strength and support, and is generally made of special cast iron or occasionally forgings. Since the valve movement law is related to the power and running characteristics of an engine, camshaft design occupies a very important position in the engine design process.
Constructions of Camshaft
The body of the 6sa1 camshaft is a cylindrical bar the same length as the cylinder block. There are a number of cams set on it for driving the valves. One end of the camshaft is a bearing support point and the other end is connected to the drive wheel.
The sides of the cams are egg-shaped. It is designed to ensure adequate intake and exhaust of the cylinder, specifically to complete the valve opening and closing action in the shortest possible time. In addition, considering the durability and smoothness of the engine, the valves should not have too much impact due to the acceleration and deceleration process in the opening and closing action, otherwise it will cause serious wear of the valves, increase the noise or other serious consequences. Therefore, the cam and s4d102e engine power, torque output and smoothness of operation have a very direct relationship.
Generally speaking, in an inline engine, one cam corresponds to one valve, while a V-engine or a horizontally opposed engine shares one cam for every two valves. Rotor engines and valveless engines do not require cams due to their special construction.
Location of Camshaft
For a long time before, bottom-mounted camshafts were most common in internal combustion engines. Usually in such an engine, the valves are located at the top of the engine, the so-called OHV (OverHeadValve) type engine. In this case, the camshaft is usually located on the side of the crankcase and the valves are controlled by the valve distribution mechanism (e.g. tappet, pushrod, rocker arm, etc.). Therefore, bottom-mounted camshafts are also generally called side-mounted camshafts. Since the camshaft is farther away from the valves in such an engine, and there are usually only two valves per cylinder, the speed is usually slower, the smoothness is poor, and the output power is lower. However, the engine output torque and low speed performance of this structure is better, and the structure is simpler and easier to maintain.
Most production car engines today are equipped with overhead camshafts. The overhead camshaft structure brings the camshaft closer to the valves, reducing the waste of round-trip kinetic energy caused by the large distance between the camshaft and the valves in the bottom-mounted camshaft. The overhead camshaft engine has a higher speed and better running smoothness because of the quicker valve opening and closing action. An early engine with overhead camshaft structure is the SOHC (SingleOverHeadCam, overhead single camshaft) type engine. This engine has only one camshaft installed at the top, so there are generally only two or three valves per cylinder (one or two intake, one exhaust), limiting high-speed performance. The newer technology is the DOHC (DoubleOverHeadCam, overhead double camshaft) engine, which is equipped with two camshafts, each cylinder can be installed four to five valves (intake two to three, exhaust two), high-speed performance has been significantly improved, but at the same time the low-speed performance will be affected, the structure will also become complex, not easy to repair. Not easy to repair.
Classification of Camshaft
According to the number of camshafts, there are two types of camshafts: single overhead camshaft (SOHC) and double overhead camshaft (DOHC). Single overhead camshaft is only one camshaft, double overhead camshaft is two, which is too straightforward explanation.
Single overhead camshaft uses one camshaft in the cylinder head to directly drive the intake and exhaust valves, which has a simple structure and is suitable for high-speed engines. In the past, side mounted camshafts were generally used, that is, the camshaft was on the side of the cylinder and directly driven by the timing gear. In order to transform the rotation of the camshaft into the reciprocating motion of the valves, valve tappets must be used to transmit the power. Thus, the reciprocating motion has more parts and a large inertial mass, which is not conducive to high-speed engine motion. Moreover, the slender tappet has a certain elasticity, which tends to cause vibration, accelerate the wear of parts and even make the valves lose control.
The overhead double camshaft is equipped with two camshafts on the cylinder head, one for driving the intake valve and the other for driving the exhaust valve. The use of dual overhead camshafts does not require high design requirements for camshafts and valve springs, and is particularly suitable for hemispherical combustion chambers with a V-shaped valve configuration, and is also convenient for use with four-valve valve distribution mechanisms.
Transmission of Camshaft
Bottom-mounted camshafts usually use a star gear set (the so-called “control wheel”) with a roller chain or rack connected to the crankshaft. For noise control, the larger diameter camshaft end drive wheels are usually made of plastic or light metal, while the relatively small diameter crankshaft end drive wheels are mostly made of steel. Chain connections are also more common. This can be seen on both bottom-mounted and top-mounted camshafts. In order to reduce the noise (generally the “oscillation noise” caused by the chain movement), a hydraulic compression device and plastic guides are usually included.
The overhead camshaft construction is more commonly connected by a plastic rack and pinion chain. This rack and pinion chain is located outside the engine oil chamber and has a steel insert that is tensioned by means of an adjustable roller.
There is another structure that is now less common due to excessive power loss and complexity in the transmission process. This construction connects the overhead camshaft to the crankshaft by means of an eccentric linkage, a star gear set or a bevel gear set with an intermediate shaft.
Common transmission methods between the camshaft and crankshaft include gear drives, chain drives, and toothed tape drives. Most of the transmissions between the lower and middle camshafts and the crankshaft use cylindrical timing gear transmission, and generally only one pair of gear transmission is required from the crankshaft to the camshaft, and if the transmission gear diameter is too large, one more intermediate idler can be added. In order to mesh smoothly and reduce the working noise, the timing gears mostly use helical gears.
Chain drive is commonly used between the overhead camshaft and crankshaft, but its working reliability and durability are not as good as gear drive. In recent years, toothed tape has been widely used in high speed engines instead of drive chains, but chain drive is still used in some high power engines. Toothed tape has the characteristics of low working noise, reliable work and low cost. For dual overhead camshafts, generally the exhaust camshaft is driven by the crankshaft through timing toothed tape or chain, and the intake camshaft is driven by the exhaust camshaft through a metal chain, or both the intake and exhaust camshafts are driven by the crankshaft through toothed tape or chain.
When installing the camshaft, be sure to pay attention to the timing marks on the camshaft pulley or sprocket. Some engines do not have obvious timing marks, and the service person can mark the exact position of the crankshaft and camshaft before removing the camshaft, while some engines require special tools to perform timing adjustments.
Quenching of Camshaft
Quenching Quenching is one of the basic means of making steel stronger. Quenching steel to martensite and subsequently tempering it to improve toughness is the traditional method of making steel to obtain high comprehensive mechanical properties.
The working ability of a camshaft generally depends on strength and stiffness, and the camshaft is quenched so that the stiffness and strength of the camshaft are increased, resulting in more durability and higher quality.