Valve timing

 The time at which valves open and close (valve timing) and the duration of the valveopening is stated in degrees of crankshaft rotation. For example, the intake valve normallybegins to open just before the piston has reached the top dead center. The valve remainsopen as the piston travels down to TDC and even past BDC. This is intake valve duration.Generally，the intake valve will open earlier and close later. Opening the intake valvebefore TDC and closing it after BDC increase the fill of air-fuel mixture in the cylinder.Opening the intake valve early helps overcome the static inertia of the air-fuel mixture atthe beginning of the intake stroke, while leaving the intake valve open after BDC takesadvantage of the kentia of the moving air-fuel mixture.  This increase volumetricefficiency.

    As the piston moves down on the power stroke past the 90 after TDC position,pressure in the cylinder has dropped, and the leverage to the crankshaft has decreased dueto connecting rod angle and crankshaft position. This ends the effective length of thepower stroke, and the exhaust valve can now be opened to begin expelling the burnedgases.The exhaust valve remains open until the piston has moved up past the TDCposition. This helps to remove as much of the burned gases as possible and increasevolumetric efficiency.

    The exhaust valve stays open for a short period of time during which the intake valveis also open. In other words, the end of the exhaust stroke and the beginning of the intakestroke overlap for a short period of time. This is called valve overlap. Valve timing andvalve overlap vary on different engines.