explain the terms:valve overlap,valvelead and valve lag.?

Answer 1

Overlap is where both Exhaust and Inlet valves on the same cylinder are open at the same time., this happens at around Top Dead Centre as the Inlet needs to be open as soon as the piston starts to go down the bore while the Exhaust needed to stay open until the piston had stopped rising.
A lot of overlap used to be a feature of racing engines but variable valve timing has changed things rather, and excessive overlap gives a lousy idle and can cause starting problems.
Large amounts of ovrlap are usually associated with poorly designed cylinder heads and high revs.

Valve Lead is the amount the valve opens before the piston arrives at Top Dead Centre (TDC) but is normally associated with steam engines which have front and back dead centres not top and Bottom. Some Steam Railway engines have variable valve events ( timing) and some have Negative lead in full gear, the valve does not open until after the piston has passed TDC.

Valve lag, (Not Lap) lag is where the valve cannot exactly follow the cam profile, whip in pushrods, rockers, pedestals etc cause this .

Valve Lap is a term used in steam locomtives not cars

Answer 2

Valve Overlap Definition

Answer 3

Valve overlap is where the exhaust valve stays open a little longer than normal and uses the exhaust gases to help suck in the new mix of fuel and air,This help cool the cyl.
Lag is the opposite.