Answer for What is Lead-Lag and Overlap of Valve

Lead— The opening of the engine-valve before time is called “lead”.
Lag—The opening of the engine-valve after its proper time is “lag”.
Overlap-The duration for which the inlet and the exhaust valve remain open together is called overlap of valve. According to the principle the inlet valve should open at T.D.C. and close at B.D.C. Automobile Engineering Similarly, the exhaust valve should open at B.D.C. and close at T.D.C.