20-09-2008, 10:54 PM
Valve timing is the regulation of the points in the combustion cycle, at which the valves are set to open and close. Since the valves require a finite period of time in which to open or close without abruptness, a slight lead-time is always necessary for proper operation. The design of the valve-operating cam provides for the smooth transition from one position to the other, while the cam setting determines the timing of the valve.
In a typical four-stroke engine, the inlet valve is set to open before TDC (top dead centre), towards the end of the exhaust stroke and close after BDC (bottom dead centre), at the start of the compression stroke. While the intake valve should open, theoretically at TDC, most engines utilise an intake valve opening, which is timed to occur a few degrees prior to the arrival of the piston at TDC on the exhaust stroke. This is because by the time the valve becomes fully open, the piston would have travelled considerably down the bore, and since the valve would have to be fully closed before BDC, the actual time the valve would be fully open would be minimal.Additionally, the inertia of the incoming mixture plays a big role.
Keeping the inlet valve open after BDC forces more mixture to pack into the cylinder, in spite of the fact that the piston is moving upwards. The exhaust valve is set to open before BDC, towards the end of the power stroke and close after TDC, at the beginning of the intake stroke. The reason the exhaust valve is opened before BDC is to prevent the exhaust gases from forming a high-pressure cushion, which would impede the movement of the piston and rob the engine of power. This also ensures that the valve is fully open at the start of the exhaust stroke. Keeping the exhaust valve open after TDC ensures that the entire burnt mixture is thoroughly scavenged..
