"Charles Vincent" wrote in message
t...
RST Engineering wrote:
I could be very, very wrong, but my understanding of efficiency is that
25% efficiency means that a quarter of the energy goes to torque and 3/4
to heat. That would mean you throw away THREE horsepower's-worth instead
of four, no?

Jim



since such engines are no more than 25% efficient when it comes to
converting the heat of combustion into torque at the crankshaft. That
means that for every horsepower measured at the crank you must
generate at least four horsepower's-worth of heat in combustion.


You are correct, but that is also exactly what he said ---. i.e. generate
four hp in heat, only one available at the shaft.

Charles

Well, with only slight rephrasing, that is what was said at the beginning of
this thread. The rephrasing would be that we "throw away 3 horespower's
worth of heat out of every four".

Actually, that is slightly worse than I was taught; but I really did not
regard it as the primary issue under discussion--so I let it be. The number
that I was taught a little more than 40 years ago was 30% when running at a
moderately high percentage of power. I understand that there has been
considerable improvement since then, which could have brought the efficiency
up to around 33%, and I have read recently (from a forgotten source) that
the theoretical maximum is around 37% efficience for spark ignition engines.

So, for the moment, it is unclear (at least to me) whether the 25% figure is
slightly low, pertains to rich mixture opperation, or is related to the low
compression and the intake and exhaust plumbing of the engines in question.
In any case, even the 25% figure is really much better than we can obtain
from our supposedly more efficient automobile engines--especially when you
consider the waste in time spent idling or simply "spun up" to avoid lugging
(because the cost of not doing so is far greater).

However, the basic point of Bob's original post is very real; even on the
highway, cars and trucks normally run at a very low percentage of power--and
the few exceptions, such as pulling trailers far about the rated limit, seem
to result in increased failure rates and short TBOs.

I admit that I still believe a 1600, and especially an 1800, is fully
capable of producing 60 hp on takeoff and 30 hp in cruise with a 52 inch
diameter prop--and that the addition of thermal barrier and dispersant
coatings might allow a climb power approaching 40 hp. But none of that
detracts from the basic points--that cars and trucks normally opperate at
surprisingly low ower levels, and that 100 maximum continuous hp from a
Continental O-200 really does mean continuous. Taken in that context, tha
1600 might well be 40 hp maximum and 30 hp cruise; and the thermal coatings
might raise that to 45 hp maximum and 33 hp cruise.

Peter
(Former engineering student and shade-tree mechanic)