The key lies in the CAM CAM shaft. As the camshaft rotates, the CAM as the piston games open/close the inlet valve and exhaust valve in a timely manner. Therefore, the shape of the CAM and the working condition of engine under different rotation speed has a direct link.
In order to understand why, let's imagine a is extremely slow speed running engine - 10 or 20 RPM - in this case, the piston needs a few seconds to complete a cycle. In reality, the normal operation of the engine can't so slowly, but we think also just as well. Under such a slow speed, we want to shape of the CAM is: the piston downward movement into the intake stroke (known as the check point on the TDC), open intake valve. The piston to the end, the inlet valve will shut down. Burning after the stroke, the exhaust valve in the piston (called the next check point of the business data catalog) to open, closed at the end of the piston of the exhaust stroke. As long as the engine running under this very slow speed, the good working condition will continue. But if, in the case of speed increase? Let us to study them carefully. In the speed of 10-20 RPM work well under the condition of the structure of the camshaft, under the condition of the speed increase is no longer suitable for. Engine speed of 4000 RPM, valve open and close to 2000 times per minute, namely 33 times per second. In this way, the piston movement speed is very fast, so the speed of the air/fuel mixture into the cylinder is also very fast.
As the inlet valve opens, the piston begins its intake stroke, air/fuel mixture began to accelerate in the circulation of air inlet pipe, preparation for entry into the cylinder. The piston is up in the intake stroke, air/fuel mixture will at high speed. If the inlet valve closed suddenly, all the air/fuel mixture will be inaccessible due to blocked cylinder. By extending into the valve open time, when the piston to compress stroke, the speed of air/fuel mixture momentum will continue to force the mixture into the cylinder. Therefore, the faster the engine operation, the air/fuel mixture is the faster movement, we hope the inlet valve open time is longer. In the case of high speed, we also hope that the valve opening more, this parameter is called the valve lift, it is determined by the CAM contour line.