Propeller rpm and effect on engine cooling

When breaking an engine in we use full throttle and high RPMs in order to maintain high cylinder pressures to assist with ring seating. Coincidentally the full throttle fuel flow also aids in cooling providing there's adequate fuel flow. Is your friend certain that his full throttle fuel flow is adequate? If he's a teeny bit low on flow at full power it'll show up as high temps during climb out.
 
I'll check the full throttle fuel flow, among other things, next time we go up.

Additional data point: his full throttle rpm indication is only 2,500 rpm. He says they checked with a test tach and his actual is 2,600 rpm, which is still low and he's aware of the need for adjustment. This was partially what led me to think that low rpm was part of his cooling issue.
 
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FastEddieB said:
I'll check the full throttle fuel flow, among other things, next time we go up.
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It should be in the neighborhood of 18-19gph

Speed is really key. 85-90 knots will run pretty hot. I usually cruise climb about 100 knots in a J.
 
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I never saw temps that high on the M20J....in fact I don't think I ever saw temps above 380. Sounds like something is wrong to me
 
Ted DuPuis said:
Note I said "these engines." Not your Ford's engine.

Yes, low RPM and high manifold pressure is a common detonation point. But on these engines, your detonation propensity goes down with lower RPM because of the loss of power. Ultimately they spin slow at any of their RPMs. It's not like going from 4,000 RPM to 2,000 RPM.
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He might be running a early J with 25btdc timing and in that case he is better off with as much rpm as he can get, given timing is fixed. RPMS increase detonation margin, reduce throttle or LOP IF NECESSARY.


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FastEddieB said:
I think my thesis depends not on the fact that the propeller provides additional cooling at a higher rpm. It's that the engine generates less combustion chamber pressures at higher rpm, therefore reducing CHT's.
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From my vague recollection of high school physics: Power = Work/Time = Force*Distance/Time

If you start with say 24"/2500RPM and then switch to 23"/2700RPM and the POH says both are 73% (making up numbers for the sake of example here), it would seem to me that the amount of combustion chamber pressure per unit of time is the same since force=combustion chamber pressure and distance/time=RPM. However, higher RPM = more friction = more heat.

Disclaimer: "it would seem to me"
 
asicer said:
From my vague recollection of high school physics: Power = Work/Time = Force*Distance/Time

If you start with say 24"/2500RPM and then switch to 23"/2700RPM and the POH says both are 73% (making up numbers for the sake of example here), it would seem to me that the amount of combustion chamber pressure per unit of time is the same since force=combustion chamber pressure and distance/time=RPM. However, higher RPM = more friction = more heat.

Disclaimer: "it would seem to me"
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I like that "per unit of time" proviso. That may be the key.

Taking your example further, how about 26"/2200 rpm? "Oversquare" prohibition is overstated, but stems from the fact that eventually too high mp combined with too low rpm sets the stage for much higher combustion chamber pressures and the nasty detonation/preignition scenario it can set up.

In any case, I queried an engine "guru", who opined that more rpm = more heat, so for now I stand corrected and have to think back to where my misconception came from.
 
I got permission from my "guru" - Mike Busch - to quote him:

"CHTs are NOT lower at higher RPM. Generally they are higher.

420F is an acceptable CHT for a Lycoming during climb, though 400F would be even better."

So, I stand corrected. I just need to figure out how that wrong idea got into my mind in the first place.
 
Aaronk25 said:
He might be running a early J with 25btdc timing and in that case he is better off with as much rpm as he can get, given timing is fixed. RPMS increase detonation margin, reduce throttle or LOP IF NECESSARY.
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Why do I bother...
 
here's a vid I took in cruise if it helps
see description for conditions
(go ahead, tell me how I am toasting my engine too!)
 
Just a quick fact: Thermal events per second (that is: rpm) is a big factor on your CHTs. It actually has a bigger effect on your piston temp gradient (that is: heat at the top of piston vs. heat at the skirt) but we don't measure them. Less time to dissipate heat == higher temp, so higher RPM generally means higher CHTs but often lower(only slightly though) EGTs due to lower peak cylinder pressures.

(and there's no detonation, ever, in a piston engine. sorry. can't help myself).
 
If you are parked, you probably have your best cooling at high rpm. If you are flying and have Ram effect, you probably have better cooling at lower rpm. Higher rpm has more combustion events per given time frame.
 
mtuomi said:
Less time to dissipate heat == higher temp, so higher RPM generally means higher CHTs but often lower(only slightly though) EGTs due to lower peak cylinder pressures.
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GlennAB1 said:
Higher rpm has more combustion events per given time frame.
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This line of reasoning doesn't jibe with the equations. Higher RPM may have less time to dissipate heat and higher RPM means more combustion events per unit time, but these are offset by generating less heat per combustion event. For a given HP output, you only need to dissipate a fixed number of joules/second regardless of RPM. I suspect the higher heat from higher RPM is due to other factors.
 
asicer said:
This line of reasoning doesn't jibe with the equations. Higher RPM may have less time to dissipate heat and higher RPM means more combustion events per unit time, but these are offset by generating less heat per combustion event. For a given HP output, you only need to dissipate a fixed number of joules/second regardless of RPM. I suspect the higher heat from higher RPM is due to other factors.
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You're oversimplifying it. The piston and cylinder head work as a heat sink, and higher pressure tends to leave more residual heat to the exhaust gases, since the piston is moving away from the heat source after combustion (and slower flame propagation also means more residual heat when exhaust valve opens etc) Total thermal load stays the same in joules/second, but piston takes less of that load and more of it gets transferred into the cylinder head and exhaust gases (and then piston moves the heat to the cylinder head etc). This is also what GT Power modeling suggest, and real life testing has confirmed it. It's a very complicated process (we also have to remember that lower pressures at higher frequencies tend to break the boundary layer between the cylinder barrel and combustion chamber etc).
 
mtuomi said:
You're oversimplifying it. The piston and cylinder head work as a heat sink, and higher pressure tends to leave more residual heat to the exhaust gases, since the piston is moving away from the heat source after combustion (and slower flame propagation also means more residual heat when exhaust valve opens etc) Total thermal load stays the same in joules/second, but piston takes less of that load and more of it gets transferred into the cylinder head and exhaust gases (and then piston moves the heat to the cylinder head etc). This is also what GT Power modeling suggest, and real life testing has confirmed it. It's a very complicated process (we also have to remember that lower pressures at higher frequencies tend to break the boundary layer between the cylinder barrel and combustion chamber etc).
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So what you're saying is that the total thermal load is the same but the load distribution is different? Hm, interesting. Yeah, I can see that.
 
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