How Do You Lean Your Mixtures?

chris

Well-Known Member
Hey guys,

I hope everyone had a good xmas and ate a lot of turkey.

I'm just wondering as to how you guys lean out your mixtures? Do you use Peak EGT or Best Power- why?

If you fly a constant speed prop, do you first set your MP and RPM and then lean, or do you do it differently? Is there a best method to use?

I always say the red knob as a way to control your fuel flow mainly; however, in an article I read, the author says he relies on it for controling his speed (go fast vs. better range). The author always cruises at WOT, then he leans it out, and then he sets his RPM. I thought this was interesting and am not sure why he does it this way.

BTW, the article is "Mixture Magic" by John Deakin. I found it on www.avweb.com.
 

E_Dawg

Moderator
Chris, remember that guy gets away with what he does for a few reasons:
He's got those GAMIjectors that fine tune the fuel injection for each cylinder, so no cylinder gets any more or less fuel than any other cylinder. And he's got a CHT probe on every one of his cylinders.
So he can pretty much see everything that's going on inside his engine.

For those of us who rent or own airplanes without all that great instrumentation, it's more back to the old lean-to roughness, then enrichen a bit. Even with a CHT gauge, yes it will give you CHT information for one cylinder, but how do you know that is the hottest cylinder? Same goes for EGT gauges.

I think the value in that article is more to disprove some of those old wives tales he talks about; i.e. you can just pull the prop back to 25 and leave the throttle at full on climbout in many airplanes without hurting the engine. In fact, that may be actually better for some engines because of a power enrichment valve which adds excess fuel at full throttle (that you obviously don't get when you pull it back to 25/25).

Also, you can keep the pressures up inside the cylinders to avoid shock cooling (if it even exists) during descents by pulling the prop way back to 1900 - 2000 and keeping the MP up enough to give a steady descent.

That is much better for the engine than keeping the RPMs up at cruise value and just pulling back the throttle for descent; if you do that the pressures and temperatures will drop much faster inside the cylinders.

Hope you had a good xmas too.
-Ed
 

bluelake

Well-Known Member
I am 100% with SkyGuyEd on this...

I printed out most if not all of John Deakins articles, and I read them over and over about a few months apart. I use them mostly to educate myself on the concepts and issues and ideas relating to engine management.

In every plane I fly, I got only one EGT. If that EGT does not work, I got nothing. As a CFI, I teach to the manufacturuers recommendations.
 

chris

Well-Known Member
Thanks for the responses.

Deakin does seem to encourage operating LOP a lot in his work, and I have always been taught differently. Then again, as you mentionned, none of the planes I fly have the GAMIjectors or any of the other high tech engine gauges he has on his Bonanza.

I tend to lean it to peak EGT more than Best Power, but have recently been using Best Power more. I guess it depends on the type of flight you are doing.

Also, he mentioned how a richer mixture on climbs provides cooling by slowing the combustion cycle and thus, causing the peak pressure pulse in the cylinders to occur farther from Top Dead Center, which reduces the risk of detonation... can anyone explain this in more detail? I don't quite understand it... I always figured that having excess fuel causes cooling due to the vaporization of the fuel, but Deakin says this is not the case.

Anyways, it's bedtime now.

Thanks again.
 

pkloop

New Member
turn out to rough turn back in about 3 turns. played with it today at 7,500 seems to work just as well as using egt
 

Minuteman

“Dongola”
The Warrior operating handbook uses the lean-with-WOT as the "economy" mixture setting. Basically, you go wide open, then back off the mixture until you get the desired RPM (that's opposed to leaning for peak EGT at a given RPM)
 

EatSleepFly

Well-Known Member
I'm spoiled. The Bonanza that I fly has a full digital engine monitor and GAMIjectors. At 75% power it should be no leaner than 50 Rich of Peak, and at 65% power I usually run 100-150 Rich of Peak for best power. I don't know what kind of Bonanza Deakin flies, but the Continental IO-520 in the one that I fly is not tolerant of operating Lean of Peak.
 

Alchemy

Partner, Ally, Friend
Most of the planes I fly have no working EGT gauges. The 182RG has a digital CHT gauge for each cylinder and a digital fuel flow indicator, I usually lean for proprer fuel flow per the POH and verify that I'm not running the CHT's too hot.

In the 172's I lean until the engine gets rough then enrichen a few turns. Taking off at density altitudes above 3000ft I do a full throttle static runup and lean the mixtures till the engine gets rough, then enrichen a few turns. Set the mixture again passing 6000, 9000, (if needed) and cruise.
 

ananoman

New Member
[ QUOTE ]
Thanks for the responses.

Deakin does seem to encourage operating LOP a lot in his work, and I have always been taught differently. Then again, as you mentionned, none of the planes I fly have the GAMIjectors or any of the other high tech engine gauges he has on his Bonanza.

I tend to lean it to peak EGT more than Best Power, but have recently been using Best Power more. I guess it depends on the type of flight you are doing.


[/ QUOTE ]

The reason you have been taught differently is because most engines will not run LOP. The induction systems on our engines are poorly designed and do not do a good job evenly distributing fuel and air to the various cylinders in the engine. The old radial engines had a supercharger on the back of the engine and equal length intake pipes that did a good job of mixing the fuel/air and delivering it evenly to the cylinders, making LOP easy and safe. The only modern engines made for LOP operation are found in the Piper Malibu/Mirage.

GAMI injectors make it possible to run other engines LOP, by evening out the amount of fuel delivered to each cylinder.

Since time is usually money, I will lean for best power. Unless you are holding or have a good tail wind, I do not see much benefit in leaning for best economy.



[ QUOTE ]
Also, he mentioned how a richer mixture on climbs provides cooling by slowing the combustion cycle and thus, causing the peak pressure pulse in the cylinders to occur farther from Top Dead Center, which reduces the risk of detonation... can anyone explain this in more detail? I don't quite understand it... I always figured that having excess fuel causes cooling due to the vaporization of the fuel, but Deakin says this is not the case.

[/ QUOTE ]

In order for combustion (fire) to occur, we need three things. Fuel, air and a source of ignition. We also need the fuel and air to be in the correct proportions. You have probably heard of the trick of throwing a cup of gas onto a lit match and putting it out (legal disclaimer...don't try this at home). It works, because there is too much gas, and not enough air. It is really no different than lighting a fire and blowing it out (too much air).

If the fuel and air are in the correct proportion to each other, combustion is very rapid. If we increase the amount of either fuel or air over what is optimum, the speed of combustion is decreased.

In our engines, the ignition system is very primitive. The spark always occurs at about 22-24 degrees before TDC. This is a compromise. In automobiles and other more advanced systems, ignition timing varies depending on operating conditions. In general the higher the rpm, the more advanced (before TDC) you want the spark. This is because it takes awhile for the flame front to travel through out the cylinder. At higher rpm you want the ignition earlier so that pressure will reach a suitabily high level before the piston gets too far down on the power stroke. At lower rpm, you want to retard the timing (closer to TDC), so the pressure will not rise too much before the start of the power stroke.

Since we cannot vary our igniton timing, the only control we have is the mixture. By running too rich (rich of peak) or lean (lean of peak), we cause the fuel air mix to burn slower, delaying peak pressure in the cylinder. It does not really have anything to do with 'extra fuel' evaporating and cooling the engine. The FAA implies this in their materials, because they want to make things easy to understand.

The reason we run rich of peak to cool the engine, is because we cannot usually run most engines LOP, and because the most important time to keep temperatures and pressures under control is during high power operations. At take off, engine power is limited by the amount of air available in the combustion chamber. Since air is the limiting factor, and we want to make as much power as possible, it makes sense to use extra fuel to keep temperature and pressure under control. This ensures that every available air molecule is available for combustion.
 

RiddlePilot

New Member
In the Seminole in cruise, 'bout 125 rich-of-peak works great, just as the POH says.

The C172Ss over here have fuel flow gauges, and if I'm on an XC, I'll generally lean to the appropriate fuel flow. If on a local training flight, generally 75-100 rich-of-peak. In the event the EGT doesn't work, lean to rough, then back to peak.

I'm pretty spoiled in the Turbo Seminole, as it has a digital readout on all sorts of engine parameters. For cruise, I'll lean to approximately 1450-1550F • (turbine inlet temperature) and call it good.
 

SteveC

Really?
Staff member
[ QUOTE ]
Since time is usually money, I will lean for best power. Unless you are holding or have a good tail wind, I do not see much benefit in leaning for best economy.

[/ QUOTE ]
The one situation that I would consider a more economical setting is when you are aiming for a destination farther out in your plane's range profile (based on current load and fuel), and want to make the destination without a fuel stop on the way. Then it will often save both fuel and time (i.e. money) to run with both a lower power setting and leaning for best economy. (Time saved by not making the fuel stop even though your true airspeed is lower).
 

chris

Well-Known Member
Thanks again for all the responses. I did a little more research on engines and ignition timing (howstuffworks.com has some pretty good info, not relating to planes though). However, 2 questions still remain, and I hope I can still get some feedback:

1. In regards to ignition timing, I read (and anonman mentionned too) how if you increase RPM, ideally the spark would ignite earlier as the pistons are travelling faster; likewise, if you decrease RPM the pistons travel slower and you want the spark to occur a little later to ensure that the peak pressure point does not occur too close to TDC... however, Deakin says that as you lean your mixture, the combustion cycle speeds up, and this increases the risk of detonation because he says the peak pressure point is now closer to TDC.... this all seems contradictory to what I mentionned above about RPM (i.e. high RPM is associated with faster combustion cycles but not with detonation... operating at high MP and low RPM risks detonation, not the other way around).

2. Can someone explain why the CHT peaks before EGT as you lean? Why don't they peak together?

Thanks again for any responses.
 

ananoman

New Member
[ QUOTE ]
However, 2 questions still remain, and I hope I can still get some feedback:

1. In regards to ignition timing, I read (and anonman mentionned too) how if you increase RPM, ideally the spark would ignite earlier as the pistons are travelling faster; likewise, if you decrease RPM the pistons travel slower and you want the spark to occur a little later to ensure that the peak pressure point does not occur too close to TDC... however, Deakin says that as you lean your mixture, the combustion cycle speeds up, and this increases the risk of detonation because he says the peak pressure point is now closer to TDC.... this all seems contradictory to what I mentionned above about RPM (i.e. high RPM is associated with faster combustion cycles but not with detonation... operating at high MP and low RPM risks detonation, not the other way around).

[/ QUOTE ]

It is not correct to say that high RPM is associated with faster combustion cycles. The speed of combustion at a certain mixture setting will remain fairly constant. This is the reason we would like to advance the timing at high rpm. Since it takes awhile for the flame front to travel throughout the cylinder and pressure to rise, we 'start the fire' sooner. If we do not do this, then our engine will lose power.

The problem with almost all recip aviation engines is that, unlike modern automobile and motorcycle engines, ignition timing is fixed. In other words, in your car, your ignition timing is advanced at high rpm operations and retarded during low rpm cruising. This delivers optimum performance and economy. Most modern car engines also have 'knock sensors' mounted on the engine block that can sense detonation and retard the timing. Combined with the oxygen sensors in the exhaust system, these engines are continually adjusting the mixture and the spark timing for optimum performance.

In aviation engines, spark timing is fixed. It never changes. Ignition timing is optimized for full throttle, high rpm operations (high RPM is a relative term, since our engines have a low redline compared to car engines that can rev to 6-9,000rpm and motorcycles that redline at 16,000). This gives the most power during takeoff and helps avoid detonation during full power operation. When we cruise at a lower rpm, it would be nice to be able to retard the timing a little, but this is not possible.

Even though spark timing is fixed, we can still control the speed of combustion with the mixture. If we run very rich, the fuel/air mixture will burn fairly slowly. This has the same effect of retarding the spark timing. If we lean, the fuel/air mixture will burn faster. This is similar to the effect of advancing the timing. If we lean beyond a certain point, the fuel/air mixture burns slower again, effectively retarding the timing.

If we go out and take off at full power with a lean mixture (but not lean of peak EGT), the speed at which the fuel/air mix in the cylinders burns will increase. This can cause peak pressure to occur too early, when the piston is not sufficiently past TDC. Detonation or engine damage can result. This is why we run rich at takeoff.

If we try to cruise at an excessively high MAP, and low RPM we can again cause peak pressure to occur while the piston is too close to TDC, leading to engine killing detonation. If we run at high MAP, low RPM, with a lean mixture, we are even more likely to damage the engine, since the peak pressure will occur even closer to TDC. This is why we use a power chart to determine the correct MAP/RPM combination and carefully lean in cruise. (It is important to remember that below about 65% power, cylinder pressures will be low enough, that it becomes very difficult to cause engine damage with improper leaning.)

Confused yet? REMEMBER, NO MATTER WHAT RPM WE ARE OPERATING AT, WE ALWAYS WANT PEAK CYLINDER PRESSURE TO OCCUR AT ABOUT THE SAME NUMBER OF DEGREES AFTER TDC. Since our ignition timing is fixed, changing RPM or adjusting the mixture will both affect the rate this pressure rise occurs.
 

Alchemy

Partner, Ally, Friend
See the space between the top of the cylinder and the roof of the cylinder chamber? This is the "combustion chamber". Its size varies depending on the position of the cylinder in its cycle of rotation. The spark plug always goes off at the beginning of the cycle. So, in high rpm operation, the spark plug fires, then the cylinder retreats at peak pressure.




When the engine is at a slow rpm, it takes longer for the "combustion chamber" to "get big". The spark plug fires at the same time, but the cylinder doesn't have enough time to go down and create a large combustion chamber.



This is why operating at very high manifold pressures and low rpm settings can cause detonation. You have a lot of force from the burning fuel air mixture, and a smaller combustion chamber over which you are distributing that force. Think about exploding a hand grenade in a port-a-potty vs exploding that same hand grenade in the superdome. Safe to say, there's gonna be a lot more structural damage to the port-a-potty. High manifold pressure is that hand grenade. Running the engine at low rpm makes your combustion chamber that port-a-potty. (at least in direct drive engines)
 

agcatman

New Member
[ QUOTE ]
In aviation engines, spark timing is fixed. It never changes.

[/ QUOTE ]

I understand your point, but if I'm not mistaken, don't "shower of sparks" ignitions advance timing for engine startup? I'm pretty sure that's correct and that advancement remains in effect as long as the starter switch is engaged.

[ QUOTE ]
The old radial engines had a supercharger on the back of the engine and equal length intake pipes that did a good job of mixing the fuel/air and delivering it evenly to the cylinders, making LOP easy and safe.

[/ QUOTE ]

Yes, but this still is not recommended procedure for all radial engines. And even then it is generally accepted that LOP operations will result in having to overhaul sooner. Which isn't a big deal for an airline, but when I operated round motors it was generally regarded as a not real good idea.
 

Ralgha

Well-Known Member
The LASAR ignition system adjusts the timing to optimize it for all power settings. It makes a noticable difference too, one of our planes has it, quite nice.
 

RiddlePilot

New Member
[ QUOTE ]
I understand your point, but if I'm not mistaken, don't "shower of sparks" ignitions advance timing for engine startup? I'm pretty sure that's correct and that advancement remains in effect as long as the starter switch is engaged.


[/ QUOTE ]

The "shower of sparks" method of start provides a later spark at roughly 5 degrees ATDC. And as I recall, this only happens while the starter motor itself is engaged, as it is an electrically-driven system.

As a side note, the impulse coupling start method continues to provide a later spark to roughly 300 RPM, and since it's a mechanical system (not electrical), it has nothing to do with the starter motor.
 

agcatman

New Member
[ QUOTE ]
[ QUOTE ]
I understand your point, but if I'm not mistaken, don't "shower of sparks" ignitions advance timing for engine startup? I'm pretty sure that's correct and that advancement remains in effect as long as the starter switch is engaged.


[/ QUOTE ]

The "shower of sparks" method of start provides a later spark at roughly 5 degrees ATDC. And as I recall, this only happens while the starter motor itself is engaged, as it is an electrically-driven system.

As a side note, the impulse coupling start method continues to provide a later spark to roughly 300 RPM, and since it's a mechanical system (not electrical), it has nothing to do with the starter motor.

[/ QUOTE ]

That's what I was thinking. I've always been more impressed with SOS as opposed to impulse coupling, seems like a much easier start.
 

chris

Well-Known Member
Once again, thanks for the responses.

I feel as if I may be beating this dead horse with a stick, and I really appreciate everyone who continues to respond to my inquiries.

Anonoman, in response to your post I have another question... at the beginning of your response, you mention that RPM does not necessarily affect the combustion cycle... however, the best time to ignite the mixture (proper timing) depends on 3 things: mixture, MP, and RPM.

If you have high RPM, are not the pistons moving up/down faster than at a lower setting? Does this not imply that the combustion cycle (intake, compression, power, exhaust) occurs faster than at low RPM? At high RPM, assuming the ignition timing is fixed and all else is equal (mixture, MP), the point where the peak pressure pulse (PPP) will occur should be later, correct? (like retarding the timing) This will lead to cooling.


Thanks in advance.
 
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