Running a Piston Engine 'Lean of Peak EGT'

[NoPax]

Any thoughts on the issue from the professional mechanics out there.

Is this a practice that is bad for the engine, not bad for the engine, worth pursuing or not?

Anybody out there that has done this over a prolonged period of time?

Any research on the issue?

I heard a Malibu pilot one time say he did this to keep the temps in an acceptable range...I kinda thought maybe if he lowered the nose etc etc, he could have avoided having to do that.

 

[USMCmech]

The main problem with running LOP is that the engine will vibrate badly because each cylinder is hitting peak EGt at a different mixture setting. This is notheing other than piss poor quality controll by the engine manufacturers.

If you have GAMIjectors all cylinders will peak at the same time, allowing you to run LOP without shakeing the fillings out of your teeth. Any airplane I ever own will have these installed right away.

THe other problem is that to do this properly without damageing your engine you NEED to have a multicylinder engine monitor. Since HP drops off dramaticly after peak you need to be running your EGT just outside the dangerzone peak EGT if you don't want to fly too slow.

Is this a practice that is bad for the engine, not bad for the engine, worth pursuing or not?

Once you have the vibration problem and the insrumentation problem solved, it is completely harmless and may infact be better for your engine. Since there isn't any unburned fuel when LOP you don't have to worry about lead fouling.

You can deffinately save a lot on fuel.

Anybody out there that has done this over a prolonged period of time?

Yeah, all the old piston engine airliners and Bombers from WW2 ran this way all the time. They used diferent instrumentation to achive the same result.

Any research on the issue?

http://www.avweb.com/news/columns/182146-1.html

 

[NoPax]

Thank you so much

Great post. At first the practice seemed unusual to me, but it's starting to make more sense.

 

[cforst513]

i was always told to fly 50 rich of peak.

 

[PAPA FOX!]

i was always told to fly 50 rich of peak.

Unless below 60-65% power that actually is the WORST place to run an engine. The CHT's and peak pressure are the highest when run there. If you are operating at 50 ROP and rich OR lean it the CHT's will be cooler! I wish more pilots knew this. I only learned it on the link that USMC posted. Very useful to know!

 

[NoPax]

i was always told to fly 50 rich of peak.

Actually I was taught to fly 50º lower than peak - so work that one out! Yes you can go either way - rich or lean to get the same result.

In the airplane I'm currently flying - an Aero Commander 500B - the exhaust stacks (on top of the engine) leave a nice residue on the top cowlings.

If the residue is white - it has been run lean
If the residue is grey - it has been run at peak
If the residue is brown - it has been run too rich

At least that was what I was told, and it seems to make sense.

I've been flying them for about 1100hrs, and have only had one problem with the engine, probably unrelated to how I was running it (a push-rod valve seal gave in), and I do run them as lean as possible, but I'm without the GAMI injectors and sensors for each cylinder/exhaust stack. There is talk about getting them, but it would be expensive. My residue is always pure white.

When I swap out an airplane with the other pilot, usually his is greyish.

And what are you doing over here anyway...

 

[Lead Sled]

Unless below 60-65% power that actually is the WORST place to run an engine. The CHT's and peak pressure are the highest when run there. If you are operating at 50 ROP and rich OR lean it the CHT's will be cooler! I wish more pilots knew this. I only learned it on the link that USMC posted. Very useful to know!

John Deakins has some pretty good articles at www.avweb.com and you can also get more information on LOP operations at www.gami.com

'Sled

 

[banned username 1]

I spoke with Bruce Bohannon, pilot of the Exxon Tiger who does all of the time to climb and altitude records in his highly modified RV/Rocket/who-knows-what and he said you can do almost anything as far as leaning goes as long as it's below 75% power.

That was great advice about the vibration issue and makes a lot of sense. I know when I'm flying my RV-8 and watching the fuel flow, the thought of 100LL selling for over $4 a gallon takes a little joy out of the rest of the fun.

 

[avbug]

Unless below 60-65% power that actually is the WORST place to run an engine.

Ah, no. Stochiometric, often referred to as peak, is where the most complete combustion takes place, and also where temperatures are highest.

Pilots who run slightly rich of peak are risking running some cylinders at peak, because the cylinders don't run the same (due to a number of factors). At operations greater than 75% rated power, one stands a chance in some engines of detonation when operating close to peak, especially in the presence of superheated carbon or lead deposits in the cylinder.

Running Lean of Peak has been standard practice in some engines for a great many years, and yes, temperatures are cooler than at peak. Rich of peak, you can think of lower temperatures as being fuel cooled, and lean of peak you can think of lower temperatures being air cooled.

GAMI is nice, but not the be-all nor end-all of products.

Many pilots using EGT as a reference to establishing mixture settings think the markings on the alcor guages mean something, and they don't. The company removed the temperature markings a long time ago when they figured out all the gauges couldn't be calibrated the same, or consistantly, and went with hash marks to represent nothing more than general reference points. Each tick isn't really 25 degrees.

Pilots also fail to realize when trying to fine tune their engine to a gnat's breath of refinement, that gauges and instruments put in the cockpit are cheap idiot gauges which seldom give accurate information. That goes for temp, voltage, etc. That's why when working on the equipment, we use calibrated test equipment instead of cockpit instrumentation.

The various multipoint cht and egt systems are dandy, except that very seldom, if you take the time to pull and test each thermocouple for calibration, will you find them calibrated properly or delivering accurate information. The instrument only gets an electrical signal from the thermocouple probe, it needs to know how that signal equates to temperature. Grab five different probles and immerse them in the same hot liquid, and you'll get five different readings.

I've never seen an engine shake itself off the mounts or crack a mount if it's run lean of peak and everything is properly calibrated, and one doesn't need gami injectors to do that. If you're getting vibration as described before, then you've got far more wrong with your engine than slightly differing injector deliveries, and you're probably experiencing detonation. That's another matter completely.

In the airplane I'm currently flying - an Aero Commander 500B - the exhaust stacks (on top of the engine) leave a nice residue on the top cowlings.

If the residue is white - it has been run lean
If the residue is grey - it has been run at peak
If the residue is brown - it has been run too rich

At least that was what I was told, and it seems to make sense.

Sounds good, in theory. Those deposits...those white depoits...that's lead. Tetraethyl lead, burned and processed in your engine, and it's very toxic. Grey doesn't mean it's been run at peak, white doesn't mean it's been run lean, and brown could be several factors, but likely includes the burning of oil.

I've set mixture by the color of flame in the exhaust collector stacks, but setting it based on the color of your exhaust stains is asking for trouble.

 

[UA-RESURRECTED]

Stochiometric, often referred to as peak, is where the most complete combustion takes place, and also where temperatures are highest.

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?

At operations greater than 75% rated power, one stands a chance in some engines of detonation when operating close to peak...

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'.

lean of peak you can think of lower temperatures being air cooled.

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?

Also, what about engines with a tachometer only? Leaning for best power is easy, but does anyone have a procedure for leaning for best economy with a tachometer only?

 

[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.