Quote:
Originally Posted by mort
Hi drmiller100,
The errors in this calculation are,
1. taking the displacement of the engine as the volume of pumped air. The volume of air at 1 atm will be less than the displacement by approximately the volumetric efficiency.
2. one displacement occurs for 2 revs of the engine in a 4 stroke.
If VE at idle is 20% and divide by 2 then pumping power = 0.65 hp.
Which really does pass the big picture bs test.
I'm OK with 1000 rpm and 5 psi, but I will note that many engines idle slower. The standard idle manifold vacuum is about 17 to 22 in hg, 1 atm is 29.9 in hg so a good idle manifold pressure is about 9.5 psi. The suction across the throttle is then 14.7 - 9.5 = 5.2, so excellent guess.
-mort
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So I thought about this some more. You are right about the 4 stroke, but the same calcs are also valid for cruising down the freeway.
VE at low rpms is pretty easy to get over 90 percent. and I am not aware of any variable displacement engines.
My premise is the displacement is the displacement. And it takes energy to maintain a pressure differential.