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Old 02-28-2012, 09:30 PM   #39 (permalink)
drmiller100
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Quote:
Originally Posted by mort View Post
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
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.
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