dumb question on compressor displacement cfm

Nevertheless that's how electric motor HP is given, and the time dependence of power output that electric motors have but gas ones don't is part of the reason for the non-equivalence. Another may be because small gas engines have been found to rarely meet spec. If you want to compare gas HP to Sears HP they may be equal. Watt's original definition of the 24/7 horse power rating of his steam engines was extremely indefinite; at any instant several of the horses might be sleeping.

I'm 62 and found out last weekend that I can't quite manage 1 HP any more, while unloading a pallet of cement bags. Lifting an 80 Lb bag 4 feet in 1/2 second is a bit over 1 HP.

jsw

Of course. My point is simply that the unit of horsepower is a specific and well-defined physical quantity, and that the word "horsepower" on a gasoline engine is not that unit, and there is no conversion between the two because the latter usage can mean anything the manufacturer whimsically chooses. Contrast this to an automobile they are honest in specifying "brake horsepower" to reflect useful rpm x torque delivered to the wheels.

In the case of gasoline-powered air compressors, the situation is difficult to characterize in terms of true horsepower units, because the crankshaft power delivered by the engine is a complicated function of many factors, and the crankshaft sink at the piston compressor is likewise varying through factors like the cycle hysteresis.

correct

and that the word "horsepower" on a

Blather

and there is no conversion between the

As long as they choose it but that does not change the definition

Contrast this to an automobile they are honest in specifying

REAL blather. There is NO vehicle manufacturer that talks about "brake hp to the rear wheels.

torque at engine RPM

a complicated function of many factors,

Complete blather.

The real point is that manufacturer's either publish a spec sheet and meet those specs. If they don't they are guilty of misrepresentation or outright fraud.

But you have to realize that a gasoline engine has NO OVERLOAD capacity, unlike an electric motor.

At 900 rpm when the piston is at bottom dead center, the pressure inside the cylinder is not 1 atmosphere. It is somewhat less because of the drop in pressure due to the filter and intake manifold.

When the piston is at top dead center, there is still a little volume inside the cylinder. The compression ratio on compressors is quite high because they use reed valves. But it is still not infinite. Otherwise with just a slight change due to manufacturing tolerances or temperature changes, the piston would be hitting the head.

And then those reed valves on both the intake and the exhaust, need some pressure across them to cause them to open.

And then there is some leakage by the piston rings.

None of these are biggies but they all affect how much air actually gets pumped.

So you calculated the swept volume correctly, but the efficiency can only be determined by an actual test.

=20 Dan

Just to throw another (largely irrelevant) definition of HP into this discussion,