Lean Mixtures

[Tom-D]

Any one care to guess when this engine manual was written? and is it still relevant to us ?

11. Mixture Control Operation

a. Definitions Of Settings Applicable To Manual Control

(1) "Full-Rich" is the setting of the mixture control lever in the position giving maximum fuel flow.

(2) "Best Power" or "Maximum Power" is the setting of the mixture control level which, with a given fixed throttle setting, results in the maximum engine R.P.M. at the leanest fuel flow; i.e., further leaning to the mixture would cause a decrease in engine R.P.M.

(3) "Rich Best Power" is the setting of the mixture control lever which, with a given fixed throttle setting, results in the maximum engine R.P.M. at the richest fuel flow.

(4) "Smooth Operation" is obtained by setting the mixture control lever for "Best Power" and then enriching the mixture until the engine speed drops 20 to 30 R.P.M.

(5) "Maximum Economy" is obtained by adjusting for "Best Power" and then leaning the mixture to obtain a decrease in engine speed of 40 to 50 R.P.M.

b. The mixture control should never be used without a full understanding of its effect on the engine and a knowledge of the general conditions under which the engine is operating.

c. It is possible to ruin an engine in a few minutes by improper use of the mixture control, as too lean a mixture will rapidly overheat the engine.

d. When using the mixture control, the cylinder temperatures must be carefully checked to prevent overheating.

e. The mixture control is set at a "full rich" position for take-off and landing.

f. Above 5000 feet the mixture control is adjusted for smooth operation.

 

[Ddayle]

I hope this is still valid. having only a single EGT probe Or no EGT, in the planes I fly, this is how I fly. But my guess is that Glen Curtis wrote it in 1922. Dave

 

[Tom-D]

I hope this is still valid. having only a single EGT probe Or no EGT, in the planes I fly, this is how I fly. But my guess is that Glen Curtis wrote it in 1922. Dave

It was not a Curtis, but it is in a MM written long before the current bunch jumped on the LOP band wagon.

 

[sba55]

It was not a Curtis, but it is in a MM written long before the current bunch jumped on the LOP band wagon.

When was it written? Seeing as though the vast majority of piston engines were being run mostly LOP starting more than 30 years ago, I assume this must have been before then?

 

[Ted]

Seeing as though the vast majority of piston engines were being run mostly LOP starting more than 30 years ago...

They were? The transport category piston aircraft were once they achieved injected engines that had good enough mixture distribution to do so, but to say that about the general aviation fleet of horizontally opposed engines (or even the big injected radials not in transport category aircraft) is inaccurate at best.

As to the manual, I'm guessing early aviation, but still bears resemblence to some modern manuals. Not much has changed in these engines.

 

[Palmpilot]

Was this ever true?

"too lean a mixture will rapidly overheat the engine."

 

[Tom-D]

this quote came from the 1932 Warner 125 manual. and carried thru all the models to include Warner 200.

and no, too lean mixtures (past the peak) will not cause over heating, but leaning to best power (rich of peak) may be in the red box and can harm the engine.

 

[Tom-D]

Was this ever true?

"too lean a mixture will rapidly overheat the engine."

Depends upon the amount of leaning, lean into the best power range, yes, it can cause overheating, but leaning into the lean of peak area, does not.

 

[Tom-D]

They were?

Yes they were.


The transport category piston aircraft were once they achieved injected engines that had good enough mixture distribution to do so, but to say that about the general aviation fleet of horizontally opposed engines (or even the big injected radials not in transport category aircraft) is inaccurate at best.

most of the big radials used a pressure injected carbs. and yes they can run full rich, auto rich, lean and auto lean. or any variation of each, plus the ability to retard/advance the timing in flight. the engines that were equipped with multi port fuel injection were not used by the airlines and very fer were built. jets came along and put an end to them.
As to the manual, I'm guessing early aviation, but still bears resemblance to some modern manuals. Not much has changed in these engines.

100LL burns pretty much the same in all engines, being able to vary the mixture is a function of design. some do some don't.

 

[airguy]

Was this ever true?

"too lean a mixture will rapidly overheat the engine."

Taken strictly by itself, with no other context - no. Too lean a mixture will silence the engine.

In reality, the answer is "It depends".

If you are rich of peak and producing a large fraction of total engine power, then as you lean the mixture (and keep all other factors the same) the EGT will climb and you'll get a bit more CHT as well. Once you reach peak EGT (stoichiometric fuel/air ratio) and continue to lean, you are now operating on the lean side of peak and any further leaning leads to COOLER operation as well as a decrease in engine power, to the point where the engine quits operating.

The boogeyman in the closet that causes engine damage when leaning is detonation - and that typically only occurs under high power loads (wide open throttle, high manifold pressure, more than 75% power) and with a mixture running from slightly lean of peak to about 75 degrees rich of peak - that's what Deakin calls the "red box" that you need to avoid.

I took a flight yesterday in a C-172 at 7500', trued out at 119KTAS, running WOT LOP (wide open throttle, Lean of Peak) at 7.9gph, 2500 rpm on a FP prop and my CHT's in cruise were between 325 and 275. Once you get up high enough above the ground that your reduced manifold pressure does not allow the engine to exceed 75% total power at WOT, then you can leave the throttle open and control power with mixture without fear of detonation. Anytime you're making more than 75% power, keep it rich and keep it cool. I takeoff and climbout with a quite rich mixture to keep the CHT's down, and then I pull the mixture back as the RPM rises due to increasing airspeed as I level off (I normally cruise between 7500-10500, so I'm well below 75% power). From that point until descent I use mixture alone to control RPM at cruise power.

It should be noted that in order to do this, you need good balanced injectors (I have GAMI) and good multi-cylinder engine monitoring, and know how to use both of them. In this case ignorance may be bliss, but it's damned dangerous too.

 

[Ted]

100LL burns pretty much the same in all engines, being able to vary the mixture is a function of design. some do some don't.

Right, but that doesn't address the general aviation aircraft.

 

[Tom-D]

Right, but that doesn't address the general aviation aircraft.

Sure it does,,,,, can you vary the mixture on your aircraft?

 

[Ted]

Sure it does,,,,, can you vary the mixture on your aircraft?

Yes, but I was addressing the statement:

Seeing as though the vast majority of piston engines were being run mostly LOP starting more than 30 years ago

As it relates to GA.

 

[airguy]

Yes, but I was addressing the statement:



As it relates to GA.

Are you saying that the operating principles of the big-bore radials of yesteryears airlines were different? The physics don't change with the engine design.

 

[jesse]

Are you saying that the operating principles of the big-bore radials of yesteryears airlines were different? The physics don't change with the engine design.

He was addressing the statement that "the vast majority of piston engines were being run mostly LOP starting more than 30 years ago... ".

There were examples of people running LOP more than 30 years ago - but the vast majority of the GA fleet was not running LOP.

 

[Tom-D]

The boogeyman in the closet that causes engine damage when leaning is detonation - and that typically only occurs under high power loads (wide open throttle, high manifold pressure, more than 75% power) and with a mixture running from slightly lean of peak to about 75 degrees rich of peak - that's what Deakin calls the "red box" that you need to avoid.

"Lean mixtures do cause detonation". is an OWT that has been proven wrong many times, if it were true, your engine will detonate every time you pull the mixture to full lean to shut down.

Piston speed and high pressures caused by wide open throttle and low RPM cause detonation. not mixtures.

 

[Ted]

Are you saying that the operating principles of the big-bore radials of yesteryears airlines were different? The physics don't change with the engine design.

You're missing my point. I'm not questioning that they can or cannot be. My question is what were people actually doing with their mixture knobs?

 

[airguy]

"Lean mixtures do cause detonation". is an OWT that has been proven wrong many times, if it were true, your engine will detonate every time you pull the mixture to full lean to shut down.

Piston speed and high pressures caused by wide open throttle and low RPM cause detonation. not mixtures.

Detonation is caused by a mix of factors - primarily those you named, but also influenced (heavily) by mixture and combustion chamber temperature (preignition, if you care to lump that in with detonation, since it will rapidly lead to detonation).

 

[Tom-D]

I contend, that if you have been following the procedures in the POH that tells you to lean the mixture until the engine stutters then richen it to smooth operation, you have gone to LOP operations with out being aware of it.

and that has been the advice of engine operators/manufacturers since the 20s.

think about it, the engie will not stutter until you have gone past the peak and kept going until it ran too lean on the leanest cylinder which started to starve for fuel.

 

[Ted]

Piston speed and high pressures caused by wide open throttle and low RPM cause detonation. not mixtures.

There are a lot of factors that play into detonation, but your mixture setting is one of them. So are intake air temperature, cylinder head temperature, fuel, manifold pressure, RPM, ignition timing, and (big one) how your engine is designed. I'm sure I'm missing several factors, those are just ones that come off the top of my head.

What's important to remember is that where the "red box" is located depends on all of those factors. The "slightly LOP to 75 ROP" only applies to certain engines under certain conditions.

 

[Tom-D]

Detonation is caused by a mix of factors - primarily those you named, but also influenced (heavily) by mixture and combustion chamber temperature (preignition, if you care to lump that in with detonation, since it will rapidly lead to detonation).

you are splitting hairs. you can cause detonation at any mixture and any MAP.

If you don't believe me, try pulling the prop all the way out at any RPM./mixture, and watch the cylinders leave.

 

[Ted]

Detonation is caused by a mix of factors - primarily those you named, but also influenced (heavily) by mixture and combustion chamber temperature (preignition, if you care to lump that in with detonation, since it will rapidly lead to detonation).

You have that backwards. Detonation comes before preignition.

Detonation is effectively having additional ignition points after the spark plug has fired.

Preignition comes after detonation. Hot spots, created by detonation, then ignite the mixture prior to the spark plug firing.

 

[Tom-D]

There are a lot of factors that play into detonation, but your mixture setting is one of them. So are intake air temperature, cylinder head temperature, fuel, manifold pressure, RPM, ignition timing, and (big one) how your engine is designed. I'm sure I'm missing several factors, those are just ones that come off the top of my head.

What's important to remember is that where the "red box" is located depends on all of those factors. The "slightly LOP to 75 ROP" only applies to certain engines under certain conditions.

Mixture is a factor of how fast it will happen, that's all.

Cylinder pressures get too high for any reason it will happen. it's just a matter of when. you can run any mixture you like and never detonate, but run the RPM too low and it will happen at any mixture.

 

[Ted]

you are splitting hairs. you can cause detonation at any mixture and any MAP.

While true, the mixture and MAP are the primary means that a pilot has to get out of a detonation spot and change operating point something more favorable.

If you don't believe me, try pulling the prop all the way out at any RPM./mixture, and watch the cylinders leave.

It won't be quite that dramatic. More likely you'll get into some bad vibration areas with the prop that could cause one of the blades to fall off, at which point the whole engine will come off with it. Once found, I doubt the cylinders will still be attached.

 

[Tom-D]

You have that backwards. Detonation comes before preignition.

Detonation is effectively having additional ignition points after the spark plug has fired.

Preignition comes after detonation. Hot spots, created by detonation, then ignite the mixture prior to the spark plug firing.

Detonation can best explained by thinking it is spontaneous combustion of the fuel air ratio. with in a confined space. it is best known for the noise it makes in your car's engine, the "Knock" you hear when you forgot to shift down going up a hill.

 

[Ted]

Mixture is a factor of how fast it will happen, that's all.

Cylinder pressures get too high for any reason it will happen. it's just a matter of when. you can run any mixture you like and never detonate, but run the RPM too low and it will happen at any mixture.

In my experience performing detonation testing on the engines most of us fly (horizontally opposed), the mixture will have a bigger influence than the RPM as far as detonation goes for the reasonable operating range (we don't do tests at full throttle and 1500 RPM because you don't run there).

The most likely problem at high manifold pressures and low RPMs can be crankshaft related or propeller related. Of course, there are a whole host of problems that could ruin your day very quickly.

 

[Ted]

Detonation can best explained by thinking it is spontaneous combustion of the fuel air ratio. with in a confined space. it is best known for the noise it makes in your car's engine, the "Knock" you hear when you forgot to shift down going up a hill.

Which my car doesn't do now, but my '68 Cadillac did.

A visual understanding of what the pressure trace looks like helps to understand the sound you hear in your car:

 

[Tom-D]

While true, the mixture and MAP are the primary means that a pilot has to get out of a detonation spot and change operating point something more favorable.

I disagree with that, your easiest method is the big handle marked prop.

It won't be quite that dramatic. More likely you'll get into some bad vibration areas with the prop that could cause one of the blades to fall off, at which point the whole engine will come off with it. Once found, I doubt the cylinders will still be attached.

I think You are agreeing with me.

 

[Ted]

I disagree with that, your easiest method is the big handle marked prop.

Well, on certain engines that will have more of an impact than others, but regardless I would definitely push the miture forward.

I think You are agreeing with me.

Overall yes, just adding more details.

 

[Tom-D]

That is exactly what we saw on the "O" scope in the Connie when the Newbee pilot pulled props first and then jammed throttles. BMEP goes out the bottom and CHT goes off the scale.

BTDT many times, that is why the newbi pilots got to set right seat for about 10,000 hours before getting to move over and be PIC in the Connie.

 

[Tom-D]

[/COLOR][/B]

Well, on certain engines that will have more of an impact than others, but regardless I would definitely push the mixture forward.

ever wonder why the TCDS tells us what prop we must use on fixed pitched prop installations? too high of a pitch will do what at full throttle at any mixture?

 

[Ted]

ever wonder why the TCDS tells us what prop we must use on fixed pitched prop installations? too high of a pitch will do what at full throttle at any mixture?

...and your engine rated at 2700 RPM won't get you off the ground very well if all you get out of it on the ground is 2000 RPM.

That would be a bigger factor, especially on any engine that has a fixed pitch prop that I've tested.

 

[dmccormack]

...and your engine rated at 2700 RPM won't get you off the ground very well if all you get out of it on the ground is 2000 RPM.

That would be a bigger factor, especially on any engine that has a fixed pitch prop that I've tested.

Well...

When I line up on the runway and add full power, I see 2000 RPM. As the airplane accelerates, the RPM increases -- slightly.

Once airborne on initial T/O climb RPM is usually 2300-2350.

I don't see 2550 (Max RPM) until straight and level.

 

[Ted]

Well...

When I line up on the runway and add full power, I see 2000 RPM. As the airplane accelerates, the RPM increases -- slightly.

Once airborne on initial T/O climb RPM is usually 2300-2350.

I don't see 2550 (Max RPM) until straight and level.

Perhaps I should've used 1500 RPM. Sorry for getting the specifics off, but you get my point.

Interesting data point on your plane, though! And apparently it hasn't detonated the cylinders off.

 

[dmccormack]

Perhaps I should've used 1500 RPM. Sorry for getting the specifics off, but you get my point.

Interesting data point on your plane, though! And apparently it hasn't detonated the cylinders off.

Nope! At 6-1 compression (maybe), I don't think detonation is a big problem.

 

[flyer]

One thing i have learned recently about lean mixtures is the accessories shops that are overhauling carburetors now a days wont set them rich enough. I have went through two rebuilt carbs in the last 2 years at a cost of 1800 bucks and both of them run way too lean. I just pulled my carb for the third time and sent it back to the factory with a note telling them to please set it richer so my engine will operate properly. We shall see.

 

[flyingcheesehead]

In my experience performing detonation testing on the engines most of us fly (horizontally opposed), the mixture will have a bigger influence than the RPM as far as detonation goes for the reasonable operating range (we don't do tests at full throttle and 1500 RPM because you don't run there).

So, how will detonation show up to the pilot? Will the CHT rise significantly? If I lean an engine and start hearing a slight "tick" but the temperatures all remain the same, am I OK and what is going on?

 

[Ted]

So, how will detonation show up to the pilot? Will the CHT rise significantly? If I lean an engine and start hearing a slight "tick" but the temperatures all remain the same, am I OK and what is going on?

You won't hear detonation in your plane. You'll see the CHTs go up on your engine monitor (and the EGTs should go down, but the CHTs going up is the big thing to notice). If you don't have an engine monitor, you won't notice the detonation until it causes major engine damage and things start going bad typically. If you hear an abnormal tick or miss, that'll be something else.

 

[flyingcheesehead]

You won't hear detonation in your plane. You'll see the CHTs go up on your engine monitor (and the EGTs should go down, but the CHTs going up is the big thing to notice). If you don't have an engine monitor, you won't notice the detonation until it causes major engine damage and things start going bad typically. If you hear an abnormal tick or miss, that'll be something else.

OK.

It's mixture-related - And may just be slightly unbalanced fuel flows. But it doesn't run "rough" by my definition. It's noticeable only if you're really paying attention.

 

[dmccormack]

You won't hear detonation in your plane. You'll see the CHTs go up on your engine monitor (and the EGTs should go down, but the CHTs going up is the big thing to notice). If you don't have an engine monitor, you won't notice the detonation until it causes major engine damage and things start going bad typically. If you hear an abnormal tick or miss, that'll be something else.

It's disconcerting that a phenomenon that can cause such damage is not clearly indicated...