2 replies to this topic

[Indian Chief]

I have my Energy Lab exams in about a week's time. We have to test several types of IC engines and air compressors. Apparently, one of the most commonly asked questions during the exams is about pumping losses.

But I haven't been able to find a simple definition of it anywhere. So can anyone here help?

Thanks.

[Chui]

At part throttle the pressure drops below atmospheric as the piston descends on the intake stroke and is above atmospheric during the exhaust stroke. If you look at at a P-V diagram of a Realistic Otto Cycle and you'll notice the strange looping portion that runs parallel to the x-axis. If you greatly expand this for part throttle [see Engines: An Introduction by John Lumley] this 'looping' represents work that must be done on the gases. This is pumping work.

Typically, pumping work is included in the friction. However, if avg cylinder pressures are known or estimated during intake and exhaust strokes the pumping work can be estimated as the pressure difference times the swept volume of the engine.

Pumping losses become sizeable when the manifold vacuum is high [i.e., part throttle].

However cryptic, elusive and vague this is I hope it helps.

[Indian Chief]

Thanks, Chui.