Overhead camshaft, referred to as OHC. General camshaft installation position of the engine under the home, in the home, the top three forms. The overhead camshaft is placed in the cylinder head, inside the combustion chamber, directly driving the rocker arm, the valve, without having to pass the longer putter. The number of components that require reciprocating movement in the overhead camshaft structure is much smaller compared to the same number of valves (ie, the overhead valve structure), thus greatly simplifying the gas distribution structure, significantly reducing the engine weight, while But also improve the transmission efficiency, reducing the working noise. Although the overhead camshaft makes the structure of the engine more complex, it brings superior engine performance (especially a significant improvement in ride comfort) and a more compact engine structure, enabling engine manufacturers to quickly become widely used in the product This design. The overhead camshaft is not necessarily different from the way the overhead valve is driven. Power can be transmitted through the timing belt, chain or even gear set to the overhead camshaft.

The overhead camshaft can be divided into the following two forms according to the number of camshafts contained in the gas distribution structure:

The single overhead camshaft is a design that only provides one camshaft in the cylinder head. The in-line cylinder engine with this design requires only one camshaft mounted above the cylinder head, while the V-cylinder engine requires two camshafts, which are placed on one side of the cylinder group.

In a single overhead cam design, the components that require reciprocating motion and the total mass of the push rod type engine are significantly reduced under the same conditions. So the single overhead camshaft can increase the engine speed, so as to increase the output power of the engine with the same output torque. In this design, the camshaft can be opened or closed by the rocker arm directly or through the rocker arm without the need for a push rod type engine like an overhead valve, which needs to be moved by china valve lifter, a long putter and a rocker arm The movement of the cam on the camshaft is transmitted to the valve in the cylinder head.

Compared to the push rod structure, single overhead camshaft design makes the engine structure (mainly gas distribution structure) more compact. This advantage is particularly pronounced when multiple valve designs are used (ie, one cylinder has two or more valves). But sometimes for the regulation of a single overhead camshaft engine typical. The Hillman urchin is a small two-door sedan designed and manufactured in the early 1960s. The rear alloy engine is improved by the Coventry Climax series of racing engines. In this engine, the intake and exhaust valves are placed on the same side of the engine body, eliminating the need for a cross-flow cylinder head design that facilitates the operation of the spark plug.

But the single overhead camshaft also has its shortcomings. As the intake valve and exhaust valve in the inlet position in different, the valve opening and closing time accuracy will be affected. In the early 1980s, Toyota and Volkswagen used a direct-drive, parallel-valve structure in a single-cylinder two-valve engine with a single camshaft to make the volume further compact. Toyota uses a hydraulic china valve lifter, while Volkswagen uses a barrel tappet, and is also equipped with a valve clearance adjustment gasket to precisely control the valve opening and closing time. This design may be the most compact and simple in all valve arrangement of single overhead camshaft engines.

Double overhead camshaft

The double overhead camshaft is a valve arrangement in which two camshafts are provided in the cylinder head. Two camshafts control the intake and exhaust valves, respectively. Depending on the structure of the engine (mainly in the form of cylinders), a typical double overhead camshaft car engine can have up to two to four different camshafts.

The twin overhead camshaft engine does not necessarily have more than two intake and exhaust valves, but if the valve of the multi-valve engine needs to be driven directly (although generally driven by the china valve lifter), the double overhead cam The shaft is indispensable. Not all of the double overhead camshafts are multi-valve - two valve shafts are often equipped with two camshafts before the multi-valve technology is popular. But today, double overhead camshafts have been equated with multi-valve technology, because in almost all of the double overhead camshaft engines, each cylinder has three to five valves.

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