Lead, Lag, and Overlap In the Valve Timing Diagram and Their Advantages

lag, lead and overlap Valve timing diagram four stroke SI engine

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Valve lead in engine

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

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. This is known 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). Both situations are described as valve lead.

Valve lag in valve timing

In 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. 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. This is known as valve lag.

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). Both were described as valve lag.

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Valve overlap in valve timing diagram

Valve overlap is the short period of time in which both the inlet and the exhaust valve is open. It happened at the end of exhaust stroke and at the beginning of intake. The intake valve opens before piston reaches the TDC and exhaust didn’t close until piston passes the TDC. Ie the period of inlet valve lead and period of exhaust valve lag referred as overlap. The valve overlapping is a secondary means of cooling the exhaust valve by fresh air intake during the 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.

Need or Advantages of valve lead, valve lag and overlap

  1. 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.
  2. Valve overlap/exhaust valve lag helps to scavenging of burned gas at region clearance volume.
  3. Overlap is a secondary means of cooling of exhaust valve by fresh intake.
  4. Intake valve lag helps to intake more air/fresh charge by the help of inertia effect of flowing air/ air-fuel mixture.
  5. Exhaust valve lead helps to increase overall efficiency by decreasing the work done required to forcing out the burned gas.


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