Do You Agree With This?

[uwochris]

Hey guys,

I have another Q regarding mixture control, and I hope I can get some feedback.

In one book, I read the following:

" An aircraft's engine will run hotter with a lean mixture than with a rich mixture because the leaner mixture is slower to burn, exposing the cylinder walls to higher temperatures for longer periods of time. A richer mixture will burn faster, exposing the cylinders to high temps for a relatively shorter period of time."

What do you think about this?

I always thought that the leaner mixture is burning faster, and this speeds up the combustion cycle as more heat is produced, and more work is being done.

Also, I read that as you lean, the mixture begins to burn faster, and the Peak Pressure Point (PPP) will occur closer to Top Dead Center, resulting in higher pressure and higher temps- this is the direct cause of the high CHT with a leaner mixture (assuming you're not lean of peak). This seems to contradict what I read above.

What am I missing?

Thanks in advance.

 

[mar]

Take with grain of salt; call in morning

Hey man, lots of interesting questions lately. Is this just some light reading for you--or are you preparing for a big test?

Just curious.

I happen to hold an FE-Recip certificate but that doesn't mean much. From the gauge of your questions I get the feeling you could teach me something.

My understanding of fuel mixture vis-a-vis CHT is pretty simple. A rich mixture results in lower CHT because the excess fuel simply absorbs some of the heat from combustion.

It's that simple in my mind.

For example:

On the R-2800, we can develop greater manifold pressure if we use water injection.

In order to get the greater power (physicists and engineers, I'm sure, will have an issue with my word choice) the carburetor is automatically leaned out, BUT, the injection of water substitutes the extra fuel that would be present with a full rich mixture, HENCE, extra power, cooler CHTs....

Like I said, that's leaning for dummies. I don't know anything about the speed of the burning or PPPs.

I hope you get a better answer.

 

[uwochris]

No test or anything like that... I've just been doing some reading on this stuff lately.

Specifically, most of my questions lately have arised after I read John Deakin's article on www.avweb.com called "Mixture Magic." Here, he gets into the relationships between EGT, CHT, fuel flows, temps, etc and tries to dispell some "old wives tales", or common misconceptions about leaning.

I actually emailed him directly with the same Qs I asked here. I am hoping he responds. If and when he does, I will gladly share his responses here.

One bad tendencies of mine is that I simply cannot accept things without knowing WHY! I am sure I have annoyed many flight instructors with my questions, but I feel it is important to have a good understanding of this kind of stuff. If I read something and see something I don't understand, I'll ask and ask and ask until I get an answer.

Anyway, I hope I'm not ticking anybody off here with my Qs.

 

[uwochris]

Forgot to ask....

Mar, do you fly for Ken Borek Air? Your signature is the same as the company's motto!

 

[mar]

Great questions!

If Mr. Deakin does respond I'd be interested to read your questions and his answers.

It's good to scratch the head every now and then.

I fly for Northern Air Cargo. You'll probably see the same motto on the website.

Honestly, it probably deserves a little modification.

You know:

Anything (If we can fit it through the door or it won't take too long to take apart).

Anytime (If we're not too tired and cranky)

Anywhere (If there's enough wind and we're light enough to take off 'cause nobody wants to stay there).

 

[uwochris]

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. "

 

[A Squared]

Re: Anything Anywhere, Anytime.


Originally, that phrase was the motto of Air America, an organization for which the words were not merely an idle boast. Whether or not you agree with Air America's purposes and methods, you have to have a certain respect for what they did and the conditions under which they did it.

Recently, the motto has been co-opted by a number of operators attempting to cash in on that cachet. NAC, FS air, and Ken Borek Aviation, I'm sure are only a few of many. It's a testimony to the general sleazieness of marketing people everwhere.
(Nothing personal, mar, I'm sure that my company would have stolen it if yours hadn't thought of it first.)

 

[mar]

Sleazy marketing people

AA--No offense taken man.

I'm just glad that if NAC is going to co-opt another organization's motto they have the balls to do so unabashedly. I mean, what were the other options?

--NAC: When it absolutely, positively has to be there...some time this week.

--NAC: We're not happy, till you're not happy.

--NAC: We love to fly but this ain't no hobby, SHOW ME THE MONEY!!!!