hevansrv7a
Well Known Member
Thanks, Milt
As you point out, BMEP can either be measured or calculated. I did not realize (sorry) that you had actual measurements. In either case, MEP or BMEP is an average or "mean". But you are right that not all curves are equal and that is what I was asking. So, your #1 means that the curve is flatter (more area under it) right? For the HP to be better while the pressure peak is lower can only happen if the curve is flatter. Now, correct me (anyone) if I'm wrong, but doesn't that implicitly mean a nice, even, long-lasting burn? That's exactly what I would expect from LOP, too. Because..
If you run LOP in a hypothetical perfect engine, you are assured that all the fuel will be consumed. (BTW - diesel engines use WOT and control power only with control of the amount of fuel injected.) That is clearly not true of ROP. When a given amount of fuel is burned, it requires a given amount of oxygen, which is present in air in a constant percentage. When you have that you have stoichiometric conditions which are fixed by chemistry and physics. In ROP there is wasted fuel. I will take a guess that best power and peak EGT will coincide closely, since it takes temperature to expand the gasses that drive the piston down and greater power thus requires greater temperatures if all else is equal.
LOP is not maximum power, since it must mean that for a given cylinder's volume and volumetric efficiency, extra air displaces some fuel, so you can't get maximum power from that cylinder while LOP. I believe, however, that for a given HP less than maximum, LOP is going to be more efficient in most cases.
BTW - some experts say that the rich mixture we use for operations >75% is not really cooling the engine, but retarding the timing. The math they present suggests that they are right in that there isn't enough wasted fuel to account for the cooling effect. I'm convinced, anyhow. That does lead to some interesting questions for the guys with electronic ignitions, though.
Yours for helping each other learn,
h
Thanks, Milt.N395V said:Thank God civil discourse backed up by fact and logic.
Your math and logic are correct and impeccable however unless I am mistaken the formula yields a straight or nearly straight line. If the actual pressure curves were pyramidal or trapezoidal mean effective pressure would have a direct relationship to peak. However these curves are more parabolic and require (I think) a derivative to relate peak to mean. Now I am no mathemetician and may be wrong. I think you are right - h.
The curves I reference are actual pressure, temp measurements taken from an engine on a test stand, the bsfc is a calculated wave form derived from measured fuel flow and massaged with measured BHP (If I remember the Church of ADA seminar correctly) So I think my difference between theory #1 and your calculation rests in the precision of relating peak to mean using derivatives as opposed to what appears to me to be a linear formula. With the added effects of actual measurement as opposed to calculation.
As you point out, BMEP can either be measured or calculated. I did not realize (sorry) that you had actual measurements. In either case, MEP or BMEP is an average or "mean". But you are right that not all curves are equal and that is what I was asking. So, your #1 means that the curve is flatter (more area under it) right? For the HP to be better while the pressure peak is lower can only happen if the curve is flatter. Now, correct me (anyone) if I'm wrong, but doesn't that implicitly mean a nice, even, long-lasting burn? That's exactly what I would expect from LOP, too. Because..
If you run LOP in a hypothetical perfect engine, you are assured that all the fuel will be consumed. (BTW - diesel engines use WOT and control power only with control of the amount of fuel injected.) That is clearly not true of ROP. When a given amount of fuel is burned, it requires a given amount of oxygen, which is present in air in a constant percentage. When you have that you have stoichiometric conditions which are fixed by chemistry and physics. In ROP there is wasted fuel. I will take a guess that best power and peak EGT will coincide closely, since it takes temperature to expand the gasses that drive the piston down and greater power thus requires greater temperatures if all else is equal.
LOP is not maximum power, since it must mean that for a given cylinder's volume and volumetric efficiency, extra air displaces some fuel, so you can't get maximum power from that cylinder while LOP. I believe, however, that for a given HP less than maximum, LOP is going to be more efficient in most cases.
BTW - some experts say that the rich mixture we use for operations >75% is not really cooling the engine, but retarding the timing. The math they present suggests that they are right in that there isn't enough wasted fuel to account for the cooling effect. I'm convinced, anyhow. That does lead to some interesting questions for the guys with electronic ignitions, though.
Yours for helping each other learn,
h
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