The Red Knob....

[E6BAV8R]

I was reading http://www.avweb.com/news/pelican/182084-1.html "Mixture Magic" over on Avweb and he does some very interesting tests with mixtures and provides some very good information. However, one thing I came across shocked me:

"...where I mention how much difference mixture makes in the speed of combustion. At takeoff power, if we bring the mixture control back a bit from full rich, the rate of combustion speeds up, and puts that pressure pulse closer to TDC (Top Dead Center). Again, a very bad thing. (Combustion speed reaches maximum around 50° F to 75° F rich of peak EGT, and further leaning causes it to slow down again.)

"Our POHs instruct us to use full-rich for takeoff. The extraordinarily rich mixture is required to assure that detonation does not occur. The conventional wisdom is that the purpose of the "excess" fuel is to cool the engine, but in fact its primary purpose is to slow the combustion rate and delay the PPP, which eliminates the risk of detonation by reducing the pressure peak. This does, in fact, result in cooler operation, but that's actually a second-order effect of the delayed PPP. (If we could just retard the ignition timing for takeoff, we wouldn't need to throw all that extra fuel at the problem.)"

PPP = Point of Peak Pressure

Now, what I bolded is the part that made me think - Does the mixture really increase the rate of combustion of the spark plug? I could see that the leaner the mixture, the less fuel there would be to ignite and would need to delay the spark slightly to achieve its 'normal' peak combustion at TDC.

Nonetheless, he does provide some very good information and graphs to the points he makes and does provide some interesting views towards what GA pilots have always been taught to do with leaning and setting standard MP and RPM settings during flight.

[SteveC]

Originally Posted by E6BAV8R

Now, what I bolded is the part that made me think - Does the mixture really increase the rate of combustion of the spark plug? I could see that the leaner the mixture, the less fuel there would be to ignite and would need to delay the spark slightly to achieve its 'normal' peak combustion at TDC.

Don't think about when or how fast the spark plug fires, think about how fast the flame front propogates through the fuel/air mixture in the cylinder. How quickly the "explosion" happens, in other words.

[E6BAV8R]

I see it now - thanks for the help.

[SteveC]

If it is over-rich I would think that it would tend to dampen the flame propogation when compared to a stoichiometric (ideal fuel/air ratio) mixture. In other words, the extra fuel kind of gets in the way. There isn't enough air available to burn all the fuel, so the extra fuel does not contribute to the explosion, but rather slows it down as it propogates through the cylinder.

Be careful about quoting me on this stuff, though, since I'm just talking from a layman's understanding. I keep hoping that one of the true technical gurus will jump in and give you the straight scoop rather than my hodge-podge description.

edit to add: hey, wait! You changed your answer while I was typing my reply!

[Berkut]

Yep, rich mixtures burn more slowly than lean ones*, and that's why we use a very rich mixture on takeoff.

The ideal point for pressure to peak inside the cylinder is about 15 degrees after top dead center. That point is where we have the best mechanical advantage between the connecting rod and the crankshaft. The problem with an aircraft engine is keeping the point of peak pressure (PPP) happening at 15 degrees after TDC, because it's affected by several different things:

1. High manifold pressure tends to advance the PPP, moving it closer to top dead center.

2. Low RPM tends to advance the PPP, moving it closer to TDC.

3. Rich mixtures tend to retard the PPP, moving it farther from TDC.*

4. Advancing or retarding the ignition timing also advances or retards the PPP. Since most aircraft engines have magnetos with fixed timing, we can't really do anything about this one.

When you are taking off, you are operating at high power, high manifold pressure, and low airflow (less cooling). Higher power requires higher mean pressure inside the cylinder. High MP is pushing the PPP closer to TDC, which isn't good. To counter this, your carburetor or fuel injection system will enrichen the mixture at full throttle, slowing down the combustion and retarding the PPP so that it moves back to around 15 degrees after TDC where it belongs. The extra fuel is there for timing purposes, to retard the PPP. This is also cooler, which is nice since we have less airflow at takeoff speeds and the higher mean pressure drives up CHTs.

So why doesn't your car have to go through all this crap? Because your car has variable ignition timing, and it knows how to use it. Your car also strives to maintain that ~15 degree after TDC PPP, but it has tools that the aircraft engine doesn't. Ideally, your car could sense pressure inside the cylinder, see where it peaks, and adjust the ignition timing as required to advance or retard the PPP. But it can't do that, or at least none that I know of can. What it can do is look at all the factors that affect PPP. It knows the manifold pressure from the MAP sensor. It knows the RPM and crank position from the crank angle sensor. The fuel injection maintains an ideal mixture using the mass airflow and oxygen sensors. The computer can use all of that information to calculate the PPP and retard or advance the ignition timing accordingly.

Want to know where the old "don't operate oversquare" myth has its roots? Consider the effect on the PPP when you have low RPM and a high manifold pressure.

*There's more to these statements, but that's another post.

[TFaudree_ERAU]

Berkut, that was a very intuitive post. Thanks for that information.

[aloft]

Originally Posted by SteveC

...it would tend to dampen the flame propogation when compared to a stoichiometric (ideal fuel/air ratio) mixture.

Oww...that word hurt my brain....