OK let's try this:
We all can agree that the pressure above the piston on the power stroke is greater than the pressure above the piston on the compression stroke and is a net positive. This is what we're after, positive work.
Now think about the intake and exhaust strokes. The pressure above the piston on the exhaust stroke is greater than the pressure above the piston on the intake stroke. This is what we have to pay to transfer gasses out and in the cylinder, negative work. The term used is pumping work. Pumping work is quite large when the pressure above the piston on the intake stroke is low in a throttled condition.
The power loop of fig2 is compression and power strokes while the pumping loop is the exhaust and intake strokes. Areas inside loops are work. Power loop is positive work, pumping loop is negative.
Crankcase pressure cancels out of all four strokes because it is assumed constant.
Help any?
Last edited by strokersix; 04/13/11 07:24 PM.
