Quote:
Originally Posted by Old Mechanic
Consider this a hypothetical situation.
What would be the difference in power produced by the same engine in these two scenarios.
Throttle plate positioned to maintain half of atmospheric pressure.
Throttle plate wide open at an altitude, like 18,000 feet where atmospheric pressure is 50% of sea level.
The difference in power produced between these two scenarios would define the loss due to throttle restriction.
Aircraft engines have had this scenario since planes were capable of reaching 18,000 feet altitude (I did not try to figure out the exact altitude where atmospheric pressure is precisely 50% of sea level).
My point is the principle cause of inefficiency is due to lower in cylinder compression. This is well defined by the different altitude scenario.
Best BSFC is a combination of the ideal RPM combined with the lowest manifold vacuum.
Other than that the atmospheric pressure available is another factor.
Losses are due to throttle restriction (the example above), which I think is fairly minor, heat loss, friction, and the energy required to overcome the changes in velocity inherent in all every reciprocating engine.
Grind two opposing lobes on the camshaft and you have a pump, suction, discharge.
Old timers used to do this with many engines to make them a high capacity pump. Take half the cylinders and grind two lobes on the cam to make half the engine an air pump.
Pumping losses have been debated years ago on this site. In my opinion the term "pumping losses" is not very precise as far as what is actually a pumping loss and what is other losses. I guess the debate will continue, without resolution.
regards
mech
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I had never thought of it this way, and you make a good point. Thank you for bringing it up.
If I'm not mistaken though, airplanes have better BSFC at higher altitudes...I am not sure of this though. But the colder intake charge, and greater pressure ratio should improve the thermodynamic efficiency right?
I take back what I said about the "compression ratio" changing. It makes no sense if you think about it in the airplane context. I guess there is a difference, that at high altitudes the air is colder because the throttle effectively heats up the air slightly.
However I know that compression and combustion temperature are important factors in combustion efficiency...idle engines do produce the most emissions afterall.
mort, I think you are right that the combustion efficiency is affected by part loads, but I don't know if it's the main cause for the drop in efficiency...perhaps sometime someone should try to see what BSFC maps would look like if it were just friction/throttling causing the problems. Friction alone seems to consume a very large proportion of power when operating at low loads, so I feel like most of the effects should be attributable to friction. Maybe when I have some time I'll compute a few points using thermodynamic cycle efficiency, friction, and pumping together. If the efficiency drop off isn't as much as in real engines, then it would be reasonable to conclude that combustion efficiency has some significant effect.