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On page 1.8 I read that the "early opening of the exhaust valve during the power stroke is called valve lead, while the late closing of the inlet valve during the compression stroke is called valve lag".
Also, "The period either side of TDC where the inlet valve is opening and the exhaust valve is closing is called valve overlap."
However, another source, my old ATC book, defines it this way (ed.2001, page 147):
"The inlet valve opening just prior to the piston reaching TDC, and not closing until the piston has passed BDC, following the induction stroke, allows maximum time for the intake of the fuel-air mixture to occur. This is called valve lead and valve lag".
Is this contradictory and can anyone offer an explanation for this varying definition?
bobtait replied the topic: Re: Lead, lag and overlap
The early opening of a valve such as the inlet valve opening before TDC and the exhaust valve opening before BDC could both be described as valve lead.
The late closing of a valve such as the closing of the exhaust valve after TDC and the closing of the inlet valve after BDC could both be described as valve lag. However the period of inlet lead and exhaust lag either side of TDC is usually referred to as overlap because during this part of the cycle both valves are open [the inlet in the process of opening and the exhaust in the process of closing].
