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Camshaft position in the engine
For a long time in the past, bottom-mounted camshafts were most common in internal combustion engines. In a typical such engine, the valve is located at the top of the engine, the so-called OHV (Overhead Valve) engine. At this time, the camshaft is usually located on the side of the crankcase, and the valve is controlled by a gas distribution mechanism (such as a tappet, a push rod, a rocker, etc.). Therefore, the bottom-mounted camshaft is generally also called a side-mounted camshaft. Due to the fact that the camshaft is far from the valve in such an engine, and each cylinder usually has only two valves, the rotational speed is usually slow, the ride comfort is poor, and the output power is relatively low. However, this structure has excellent engine output torque and low speed performance, and its structure is relatively simple and easy to maintain.
Most production cars now have an overhead camshaft. The overhead camshaft structure makes the camshaft closer to the valve, reducing the wasted kinetic energy of the bottom-mounted camshaft due to the large distance between the camshaft and the valve. The engine of the overhead camshaft is more rapid in opening and closing of the valve, so the rotation speed is higher and the running smoothness is also better. The engine of the overhead camshaft structure that appeared earlier is a SOHC (Single Over Head Cam) engine. This type of engine has only one camshaft mounted on the top, so there are usually only two or three valves per cylinder (one or two intakes, one exhaust), limiting high-speed performance. Some of the newer technologies are DOHC-type (DoubleOverHeadCam, overhead dual camshaft) engines, which are equipped with two camshafts, each cylinder can install four to five valves (two to three intakes, row Two of the gas), high-speed performance has been significantly improved, but at the same time the low-speed performance will be affected, the structure will become complex and difficult to maintain.