Running a Piston Engine 'Lean of Peak EGT'

[erj-145mech]

The temperatures are on a parabolic curve in relation to the fuel flow. Thats how you can run rich of peak and lean of peak, with peak at the tip of the parabola.

 

[avbug]

Would that be strictly exhaust temperatures, or general engine temp??? I always thought that running an engine too lean resulted in higher engine temperatures. Were you only referring to exhaust temps?

UA, Think about the term, "peak." Forget about engine temperature or exhaust temperature...peak is where you'll find the peak temperatures. Lean of peak you may think of the mixture as being air cooled, and rich of peak you may think of it as fuel cooled. What's the hottest part? Considering it's the fuel that's burning, you might correctly guess it's the fuel air mixture combustion temperature...what you measure as exhaust gas temperature.

Everything else is largely fluff. It's the fuel air mixture as it burns that determines CHT, that determines valve temperature, that is a primary determining factor in detonation or a good clean burn. The only issue you need to worry about with CHT is exceeding temperature limitations. But it's burning gasses that give you the temperature itself, give you the most immediate and accurate feedback as to proper mixture, and what really make the engine go.

If you were taught that running lean of peak gives higher temperatures, you were taught wrong. Your temperatures peak at...peak. Some will tell you that temps peak rich of peak, though this isn't true. You'll see a slight disparity between true peak and indicated peak because of where you measure the exhaust gas temperature, but fuel burns most completely and hottest at it's most ideal mixture setting and at the ideal timing (compression being a determining factor in the final temperature)...which occurs at stochiometric (peak), and at a variable timing (something we don't usually get in most aircraft piston engines).

So would you agree with the practice of using a "full-rich" setting whenever operating above 75% maximum power? This is the basic premise of what I was taught, unless the density altitude is higher than 5,000'.

I would most definitely not agree with that premise, except where a manufacturer might specifically dictate it as the appropriate operating practice. Lean appropriately.

Here again, I always though that too lean meant too hot. You seem to state the opposite, that temperatures are cooler with leaner mixtures. This doesn't seem likely. Can you clarify?

Think of it in simple terms. Oxygen serves to support the burning or combustion process. Not enough oxygen in the mixture, you are getting an incomplete burn. Spray fuel into the chamber and enrichen it, and you'll see the temperatures drop. That makes sense to you, because you know that enrichening the mixture will cause a temperature drop.

Take away that fuel, and eventually you'll reach a point where the mixture is such, at about a 15:1 ratio, that fuel burns most completely. As you lean to that point, temperatures continue to increase. When you reach this point, "peak," you're getting the most out of your fuel...it's burning most completely, and not surprisingly, hottest. As you continue to lean the mixture however, now there's not enough fuel for the amount of oxygen or air available. Too much air, not enough fuel.

Imagine a coal burning stove. It's going great ganders, the room is heating up, people are starting to peel off layers like onions. You get a metal bucket, and scoop out about 3/4 of the burning coal, and toss it in the snow outside. It sizzles, it hisses, and dies.

The stove, having a lot less fuel to burn now, is cooler. This isn't a perfect example, because in truth it's more like closing your throttle than adjusting the mixture. But less fuel for a given quantity of air in a closed environment such as a cylinder combustion chamber means that the temperature gets progressively cooler as the mixture gets lean of peak.

Too much fuel when rich, too much air when lean. The temperature doesn't climb, it drops, lean of peak.

You don't run it there without having a very good idea of what you're doing, because the mixture tends to burn less and less evenly. When the slug of air that enters the combustion chamber is saturated with fuel, getting a fairly even burn isn't hard. But when it's lean, getting the same even homogenous mixture isn't so easy. The fuel burns unevenly, even explosively. An explosion really isn't anything more than a really fast burn, and detonation in your engine is nothing more than a rapid, uneven burning.

Where detonation becomes a problem is when the timing is off, when the mixture fires early. This is most likely close to peak, though uneven burning occurs more and more as you go leaner and leaner (and you get a rougher and rougher engine). Close to peak at high power settings, the mixture tends to heat up quicker at high power settings, when cylinder pressures are high. Add something to cause an early ignition to that fuel air mixture, while the piston is still on the upstroke compressing the mixture in the cylinder, and it fires while it's still going up. Too soon before it gets to the top, or top dead center (TDC).

Imagine climbing up the ladder of a bunk bed while someone at the top kicks you in the face. You fall back down. Same for the piston, only it can't go back down, it just takes that kick in the face and keeps on coming up...and if it happens enough, it's "legs" (connecting rod) breaks, or the temperature just gets hot enough to burn a valve or melt a hole in the piston.

Staying rich all the time isn't going to help...it must means the engine isn't running efficiently, and you may even be doing more harm than good. Too much fuel leads to inadequate cylinder lubrication, and potentially greater wear.