Quote:
Originally Posted by drmiller100
I use the terms pumping losses and throttling losses interchangably.
what is the difference?
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Hello drmiller100,
I do too, sometimes. But in context, mwebb's graph shows that at idle less air gets into the cylinder. That is due to the throttle being closed, and can be called throttling loss. But how much power is required to suck the air in, that is called pumping. It is power from the engine that cannot be used to move the car.
In a text book or even a piece of automotive journalism, pumping refers to the power used to suck air through the throttle. It is quite a slug of power at cruise and if you could eliminate it you'd notice the better FE. But it isn't anything like the power hog some believe. In the amazing graphs posted by Old Tele man, the FE for 1/4 throttle at 4000 rpm is half the FE of full throttle. At that rpm 1/4 throttle makes about 12 or 13 hp but is burning enough gas to make 25. The pumping power might be about 0.6 hp and the other 12(or so) hp is lost thermodynamic efficiency. But many writers seem to believe the opposite and that is one of my pet peeves. For lots of throttled operation the pumping power is a fraction of the power lost to thermodynamic inefficiency. (conventional Otto cycle, CR around 8 or 9, blah blah blah)
There are pumping losses that can be improved and some are really hard to reduce. All the air goes through the little annular space around the intake valve. Not much can be done about that. All the exhaust goes through the exhaust port, and then there is the whole exhaust piping and muffler. But you do your best. The air cleaner is a place where there is room for improvement, but a measurable improvement is going to be big and expensive. Better intake piping? In any case these are mostly important at high power out, and won't make any difference at cruise. And except for that exhaust port, all are an order of magnitude smaller than trying to suck air through the closed throttle. And the piping arangements are designed to improve performance under certain conditions so alterations to reduce pumping may not be any improvement at all.
In the case of an Atkinson cycle, as seen these days (Prius like), under cruise conditions the throttle will be open and the charge is limited by allowing some of the intake mixture to get pushed out the intake valve after the compression stroke has started, after BDC. But if the engine could idle, and you used the same pressure measuring setup in mwebb's graph, then the cylinder pressure at TDC would be the same. The graph would look almost the same. The same lower amount of intake air, but no throttle and no pumping.
-mort