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I've been working on some formulas related to fuel economy in cars. One of them is for pumping loss. Here it is.
HP=CID*RPM*P/792000
CID: cubic inch displacement
HP: horsepower
RPM: engine revolutions per minute
P: pressure difference between exhaust and intake in psi
The 792000 comes from these numbers:
2 rev/cycle * 60 sec/min * 12 in/ft * 550 (ft*lb/sec)/hp
A Mercury with a 429 cubic inch engine goes 60 MPH at 2200 RPM with the exhaust pressure 9 psi higher than the intake manifold pressure. What is the pumping loss?
HP=429 * 2200 * 9 / 792000
HP=10.725 horsepower (pumping loss)
How about an Accord with a 2.0 L engine running at 2500 RPM with an intake pressure of 7 psi and an exhaust pressure of 15 psi?
HP=CID*RPM*P/792000
CID=61*2.0=122 cubic inches
RPM=2500 rev/min
P=15-7=8 psi
HP=122*2500*8/792000
HP=3.08 horsepower wasted to pump air
One of the most effective ways to reduce pumping loss is to deactivate some cylinders when the load is low. This is done in some Chevrolet, Chrysler and Honda vehicles. This reduces both the displacement and the intake vacuum. I tried that in my Accord for a while by removing half of the rocker arms. The engine braking was much less. If I could turn off all the cylinders (by closing all the valves) when decellerating, the engine would turn very easily and use no gasoline while descending hills.
I put back the rocker arms after a few hundred miles because the extra vibration was destroying my exhaust system.
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