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Camshaft The Speed Is Higher, The Running Smoothness Is Better

Oct 20, 2017

The camshaft is a part of the piston engine. Its function is to control the opening and closing action of the valve. Although in a four-stroke engine the camshaft's rotational speed is half that of the crankshaft (the camshaft has the same rotational speed as the crankshaft in a two-stroke engine), however, usually its speed is still high, and the need to withstand a lot of torque, so the design of the camshaft in the strength and support requirements are high, the material is generally special cast iron, Occasional forgings are also used. Because the valve motion is related to the power and running characteristics of an engine, the camshaft design plays an important role in the engine design process.

The main body of the camshaft is a cylindrical bar with the same length as the cylinder group. A plurality of cams are set on the top to drive the valve. One end of the camshaft is the bearing support point, and the other end is connected with the driving wheel.

The side of the cam is egg-shaped. The purpose of the design is to ensure that the cylinder full intake and exhaust, specifically in the shortest possible time to complete the valve opening and closing action. In addition, considering the durability of the engine and the smoothness of the operation, the valve can not be caused by the increase and deceleration process in the opening and closing actions too much impact, otherwise it will cause serious valve wear, noise increase or other serious consequences. Therefore, the cam and engine power, torque output and the smoothness of the operation has a very direct relationship.

Generally speaking, in a straight-line engine, a cam corresponds to a valve, a V-type engine, or a horizontal-pair engine that shares a cam per two valves. The rotor engine and the valveless valve engine have no need of Cam because of their special structure.

For a long time, the bottom-mounted camshaft is the most common in internal combustion engines. Typically, the valve is located at the top of the engine, the so-called OHV (overheadvalve, top-valve) engine. At this point, the camshaft is usually located on the side of the crankcase, and the valve is controlled by a gas distribution mechanism (e.g. tappet, push rod, rocker arm, etc.). The bottom cam shaft is called therefore also generally lateral cam shaft. Because the camshaft is far away from the valve in such an engine, and each cylinder usually has only two valves, the speed is usually slower, the smoothness is poor and the output power is low. However, this structure of the engine output torque and low-speed performance is relatively good, the structure is relatively simple, easy to repair.

Most of the cars are now equipped with a top-mounted camshaft. The overhead camshaft structure makes the camshaft closer to the valve, reducing the waste of the back-and-forth kinetic energy caused by the large distance between the camshaft and the valve. The engine of the overhead camshaft is more rapid because of the valve opening and closing, thus the speed is higher and the running smoothness is better. An earlier engine with a top-mounted camshaft structure is SOHC (Singleoverheadcam, overhead single camshaft) engine. The engine has only one camshaft installed at the top, so that there are usually only two to three valves per cylinder (one to two, one for exhaust), and the high-speed performance is limited. Some of the technical updates are DOHC (DOUBLEOVERHEADCAM, overhead double camshaft) engine, which is equipped with two camshaft, each cylinder can be installed four to five valves (intake of two to three, exhaust two), high-speed performance has been significantly improved, But at the same time Low-speed performance will be affected, the structure will become complex, difficult to repair.

According to the number of camshaft, it can be divided into single top camshaft (SOHC) and double overhead camshaft (DOHC). The single overhead camshaft is only one camshaft, and the double overhead camshaft is a two-piece, which is too straightforward to explain.

A single overhead camshaft with a camshaft on the cylinder cover directly drives the inlet and exhaust door, which is simple in structure and suitable for high-speed engines. In the past, the side-mounted camshaft, that is, the camshaft in the cylinder side, is directly driven by the timing gear. In order to transform the camshaft rotation into the reciprocating movement of the valve, the valve tappet must be used to transmit power. In this way, the reciprocating movement of the parts more, inertia mass, not conducive to high-speed engine movement. Moreover, the slender tappet has certain elasticity, which can cause vibration, accelerate the wear of parts and even lose control of the valve.

The overhead double camshaft is fitted with two camshaft on the cylinder head, one for driving the intake valve and the other for driving the exhaust valve. The design requirements of camshaft and valve spring are not high with double overhead camshaft, especially suitable for the hemispherical combustor with valve v configuration, and can be used in conjunction with four valve air distribution mechanism.