I used the Scangage to determine the HP used by the A/C. With the A/C off, the Scangage read 0.4 gph. With it on, the Scangage oscillated between 0.5 and 0.6. Call it 0.6.
So assuming 20% efficiency for the diesel at idle:
Fuel rate at idle, A/C off
0.4gphFuel rate at idle, A/C on0.6gphA/C fuel usage0.2gph23000BTU/hrFuel conversion efficiency20%Combustion energy to HP4600BTU/hrEnergy conversion factor2545BTU/hr/hpMechanical HP1.8
This is at idle. I don't know if it varies much with RPM.
Capitalism: The cream rises. Socialism: The scum rises.
I would assume that it would be pretty much a constant for when the compressor was running. Movement (and subsequent airflow over the condensor) would cause it not to run as much, but while it's on, I see no reason why it would vary.
I guess I was thinking more of average loss rather than instantaneous. If the pump is spinning faster, it won't be on as long to reach required pressure. Therefore over time the average would be the same, or at least close to it (as we all know that spinning faster will loose some efficiency to friction).
And I LOVE the "turbo button" analogy - any of you guys remember Knight Rider? "Turbo Boost KITT!!"
What gets me is going down hill with no gas - darn cpu adds fuel when I kick in the AC and it doesn't need to. Next trick will be the blower fan kill switch so the AC runs at highway speed without the fan running - just using the natural air flow - should save some juice that way - can't recirculate however.