Responses From Mr. Deakin Himself!


Well-Known Member
Hey guys,

As you know, I have been obsessed lately with mixture control Qs. I actually emailed John Deakin, and he shared his wisdom with me, and I'd like to share his explanations with you.

Here's my Qs and Mr. Deakin's responses:

#1: You seemed to emphasize those charts showing the relationship of CHT, EGT, BSFC, and BHP to fuel flow and Temperature a lot; however, one question I have not not been to find an answer to is: why does CHT peak prior to EGT? I always thought they peaked together before I read your article... it just does not seem intuitive. Is the reason very complex, or is there a simple explanation, that even a pilot can understand?

"Excellent question! In a perfect world, with a perfectly homogeneous mixture of fuel and air (oxygen), they'd peak together, with power. But the mixture in the combustion chamber is not homegenous, there are varying little spots of "richer" and "leaner" spots. The result of this is that at peak EGT, there are still unburned molecules of fuel remaining that simply didn't light off for lack of "local oxygen" when the flame front passed. By adding just a little more fuel, more of THOSE molecules light off, producing more power, and a bit more CHT. Slightly richer yet, and the change in speed of combustion takes effect, the power pulse is very slightly later, and thus, more power. "

#2: This question has to do combustion. On one hand, you mentionned how detonation can occur at high MP and low RPM- by operating like this, you risk detonation because the Peak Pressure Point (PPP) occurs closer to Top Dead Center (TDC) than it does with faster RPM (smaller combustion chamber, higher pressure, higher temperature). I realize that ignition timing is fixed for us as well, and that if we could only advance the timing, this problem may not result.
On the other hand, you mentionned how with take off power, if you lean the mixture out, the combustion cycle speeds up, and this also puts the PPP closer to TDC, again risking detonation.

These 2 statements seem contradictory to me. In the first scenario (low RPM, high MP), it seems that the combustion cycle is slowed down... ie) here I am assuming that low RPM causes the combustion cycle to slow down (pistons move up and down slower), and that high RPM causes it to speed up- I am not sure if this is correct, but it is what I assumed.

"This one boggled my mind at first, too. Picture full takeoff power, full rich, and let's just say Peak Pressure (PP) occurs at, oh, 12 degrees after TDC, ok? Now, if you reduce RPM, it doesn't affect the speed of the FLAME FRONT, or the combustion event very much, but it does take longer in time to get from spark to 12 degrees after TDC, right? Since it takes longer to get to that same 12 degrees, the PP must occur a few degrees closer to TDC.

By moving the same combustion event (roughly) closer to TDC, you make it happen in a smaller space, with less mechanical advantage, so you get more heat for less power. Picture any given combustion event occuring AT TDC, then think of it occurring at 90 ATDC.

The second scenario changes the MIXTURE, with the RPM remaining constant. By leaning from full rich, you speed the flame front up, which makes it happen closer to TDC. "

In the second scenario, the combustion cycle speeds up (opposite to above), yet you risk detonation in both cases. How can this be?
I figured that since ignition timing is fixed, anything that speeds the combustion cycle should make the PPP occur farther than TDC, and anything that slows the cycle should put the PPP closer to TDC, yet I appear to be wrong. Maybe it has something to do with the speed at which the mixture burns??

"The first scenario has everything to do with how fast the piston gets past TDC, with the same combustion event.

The second has everything to do with the flame front speeding up. "

#3: Is it true that a leaner mixture is slower to burn than a richer one? In one aviation textbook, the author claims that engine temps are higher with leaner mixtures because the mixture is slower to burn than a richer one, exposing the cylinder walls to high temps for long periods of time. Since the rich mixture burns faster, it exposes the cylinder walls to high temps, but only for a short period of time, so the temps are lower. Do you agree with this?

"The fastest burning mixture occurs at about peak EGT. If you move away from peak EGT in EITHER direction, combustion slows. "