The valve mechanism is generally classified by the method of valve arrangement as either side valve or over head valves. The cam shaft which is run through timing gears rotates at half of the number of rotations per minute of the crankshaft. The valves are opened and closed according to the valve timing diagram. The condition when both the inlet and exhaust valves are open simultaneously is called valve overlap. Similarly, the exhaust valve is opened before 46 0 of BDC and closed after 10 0 of TDC. Exhaust valve lead helps to increase overall efficiency by decreasing the work done required to forcing out the burned gas.The inlet valves is generally opened before 10 0 of TDC which is known as valve lead and closes almost 46 0 after BDC which refers to valve lag.Intake valve lag helps to intake more air/fresh charge by the help of inertia effect of flowing air/ air-fuel mixture.

Overlap is a secondary means of cooling of exhaust valve by fresh intake.Valve overlap/exhaust valve lag helps to scavenging of burned gas at region clearance volume.Valve overlap/ intake valve lead ensure the complete opening of intake valve when piston reached at TDC (and begins the intake stroke) so that it can suck large amount of air, fresh charge during suction stroke.Need or Advantages of valve lead, valve lag and overlap The overlap should not excessive so that it causes the expelling of fresh charge through the exhaust valve, or sucking of burned gas into the cylinder. The valve overlapping is a secondary means of cooling the exhaust valve by fresh air intake during the overlap. Ie the period of inlet valve lead and period of exhaust valve lag referred as overlap. The intake valve opens before piston reaches the TDC and exhaust didn’t close until piston passes the TDC. It happened at the end of exhaust stroke and at the beginning of intake. Valve overlap in valve timing diagramValve overlap is the short period of time in which both the inlet and the exhaust valve is open. 🔗 Variable valve timing VVT of internal combustion engine - Advantages Valve lag – Inlet valve close after piston passed the BDC (at beginning of compression stroke), the exhaust close after piston passed TDC (at the beginning of suction stroke). But in a real engine, the inlet valve is set to close some degree (time) after the piston reaches the BDC and exhaust valve close after piston passed the TDC. Valve lag in valve timingIn the case of closing of the valve, in theoretical valve timing, the inlet valve close when the piston reaches the BDC and exhaust valve close when piston reaches the TDC. Both situations are described as valve lead. Valve lead - Inlet open before piston reach TDC (at the end of exhaust stroke), Exhaust open before piston reach BDC (at the end of power stroke). Theoretically the inlet valve open at TDC and exhaust valve open at BDC, but in engine design, it is set to that the inlet valve open before the piston reaches the top dead center, and exhaust valve open before the piston reaches the bottom dead center.

The term “lead” and “lag” used to represent where the actual opening and closing of valve occurred. However, in a practical situation, opening and closing of engine valve occur at some degree (time) before or after TDC and BDC. Valve lead in engineTheoretically, the engine valve opens and closes at TDC (Top Dead center) and BDC (Bottom Dead Centre).

Valve timing diagram 4 stroke engine