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The cam should open the valves at the right time by an exact distance and close them again after a precisely predefined time span in a certain amount of time.

How it works

Cams guide one or several valves. The course, stroke and the speed of the cams guide the opening times of the valves precisely in degree crank angle and thereby determine the operating performance of the engine in the single speed ranges. Except for the direct control via bucket tappets, the valve lift can be altered by changing the transmission ratio at the rocker arm or towing lever. Generally, the valve spring causes the closing of the valve if the cam admits it. Only mechanical desmodromic valve controls are an exception to this rule. Steep cams (see 2nd picture) are subject to more wear and allow a faster, perhaps quicker opening and subsequently later, perhaps quicker closing.

The cam can also be formed unsymmetrical. Figure 3 displays a normal cam whose leading track is different than the trailing one. If we assume a clockwise rotation, the diagram in picture 4 displays its valve lift. This diagram also provides an answer to the question of the purpose of degree numbers often mentioned in combination with sports camshafts. Thus, a 300° cam leaves the valve open during 300°-crank angle. If it opens as an intake valve, e.g., 50° before TDC, it closes 70° after BDC, so the degree number always refers to the crankshaft. You may anticipate already that such an engine does not operate smoothly in the lower speed range... 06/08

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