Valve Timing Diagram

Valve Timing Diagram Points : valve timing diagram, problems of camshaft and its drive system, problems in internal combustion engines, ic, engine, auto diesel To become conversant with this subject, it is necessary to understand the meaning of top dead centre, bottom dead centre, lead, lag and overlap. Top Dead-Centre (T.D.C) is when a piston is at the top of its stroke; that is to say it is on the point of changing from upward to downward motion and is at the highest point reached.

Bottom Dead Centre (B.D.C) is when a piston is at the bottom of its stroke. It will be on the joint of changing from downward to upward motion and is at the lowest point reached.

Lead: A valve is said to be given lead when it opens before the piston has reached top or bottom centre.

Lag: A valve is said to have lag when it Closes after (he piston has reached bottom centre. and closes or in some cases opens after the piston is at T.D.C. Overlap: On most engines the inlet valve begins to open before the exhaust valve is completely closed. Therefore, there is a short period during which both valves are open: this is referred to as overlap.

Exhaust Valve Period: In order to get rid of the burnt gases from which the power stroke has been derived and to acquire a better scavenging effect, the exhaust valve is opened before B.D.C. This lead given to the exhaust valve may be between 40 degree and 65 degree according to design. Consequently, the valve will open before the power stroke is finished, hut this is done in order to give the gases, which have greatly increased in volume owing to expansion, plenty of time to be expelled from the cylinder. The slight loss of power is more than compensated for be the more complete scavenging action and avoidance of back pressure which is effected. Even after the piston has reached T.D.C. there is still a certain amount of exhaust gas in the combustion chamber, and as much of this as is possible must be allowed to escape under its own momentum, or it will impede and contaminate the new explosive mixture drawn into the cylinder.