Leaning in the Climb

[cytoxin]

uh huh huh

As stated above, when you go up in DA, the NA engine can no longer generate more than 75% anyway so...

I'm not sure why so many pilots struggle with this. Of course, 'Recommended leans', just rich of peak and vague FAA warnings about 'too lean' without any real explanation of in-reference-to-what and why, don't help.

-jjf

Edit: I also agree that the constant EGT method works, but I think that pilots should really understand what is going on. So I have question:

When we go up, the air gets 'thinner' - IE, fewer molecules per volume. So mixture must get leaner to maintain the same air/fuel ratio. If we are burning less fuel, then there is less energy being released.

A smaller release of energy over the same interval of time is the same as saying we have less 'power' available. We see this at the crankshaft at higher DAs.

But, if we accept that the total energy available is now smaller, why does constant EGT work? In other words, if the total pie we are measuring gets smaller, why does keeping the 'thermal slice' the same size work well?

flame front propagtion? you tell me

 

[zav6a]

Step right in

Probably going to step in it here, but perhaps a mixture of a particular composition burns at approximately the same temperature, regardless of how much of it is stuffed in the cylinder. There is just more of it at higher power (lower DA).

I use a campfire analogy to understand the constant EGT method. Too much fuel, not enough air, and the flame is dark red (cool). Remember your color-temperture spectrum from phsysics? Get the combination of fuel and air just right and it is orange-white (hotter). With the Constant EGT method, we feed the fire extra fuel to keep it on the cool side.

You can observe the same phenomenon campfires at high altitude observe these same rules.

Maybe some of those optical monitors that they installed in the old radials would help solve the debate. My guess is that the flame color (temperature) of rich mixtures in supercharged cylinders would be still be cooler than stoichiometric mixtures at 55%.

All of this is complicated by rate of combustion etc. but the general physics of combustion ought to dictate regardless of how fast it is occuring.

 

[Fitz]

No miss-step at all

Probably going to step in it here, but perhaps a mixture of a particular composition burns at approximately the same temperature, regardless of how much of it is stuffed in the cylinder. There is just more of it at higher power (lower DA).

You're not stepping in it at all. But think about what we are measuring. To use your campfire analogy. Imagine we act as a temperature probe just over the fire (we stick our hands out to warm them in the heated air rising from the fire). Are our hands going to feel the same temperature if the fire itself gets smaller?

The volume of each cyl is unchanged, as is the size of our EGT probe in the exhaust pipe. But the number of fuel and air molecules is going down. The probe needs excited molecules to bump into it to register temperature.

A really big clue is CHT. Look at CHT in the two different runs in the the AdvancedPilot presentation. The fire is smaller, but the fireplace is the same temperature!

I'm sorry if it seems I'm being coy. I'm not sure if they arrived at this starting with theory, on the basis of collected data, or even simple empiracle testing, but it is pretty brilliant. Since we can hear it straight from one of the sources, I'm loathe to 'step in it' myself.

-jjf

 

[zav6a]

Both Feet

Maybe the heat we feel over the fire should be equated to horsepower and the temperatrue of the flames to EGT.

I'm a monitoring nut and find reasons to plot cht, egt, fuel flow, and density altitude in various combinations. I end up flying fairly high most flights, up to 14K and higher, so I have a lot of sample points for various DAs. I've always been a bit puzzled by the fact that although I'm less than 50% at 14K, I can still generate EGTs of about 1340. I don't spend a lot of time looking for peak at WOT at sea level (for fear of the evil detonation) but at 8K, and 75%, I don't get much over 1400 degrees. So, in spite of burning almost 2 gph or 25% less fuel, my EGTs only go down about 3 - 4 percent. My CHTs at 14 K also manage to peak somewhat near (but not as close) to the 8K peak. I can understand that given the limited cooling effciency of thin air.

You can demostrate the same without making yourself dizzy by logging EGTs at low loads and high loads - simulating high altitude (once the intake valve closes and the piston starts up - the engine does not know the difference).

So, all that said, I end up using the target EGT approach sometimes and feel pretty comfortable with it. I more often use the target fuel flow for altitude approach though! Not as sensistive to partially fouled plugs, and I get to use the data from all that charting!

The other advantage of the fuel flow for altitude approach is you know which you are, LOP or ROP. Before I had fuel flow data, I've been surprised to find I was rich 1280 when I thought I was lean 1280 and visa versa.

Finally, just to throw more fuel on the fire - I always climb LOP once out of the pattern. The difference in power is not that significant at 10 - 20 degrees LOP, I am not in that big of hurry, and it runs a lot cooler. I seldom start at less than 5000 feet DA though.

If that is not clear - it is because I need to sniff some oxygen.

 

[fodrv7]

Heat & Temp

Maybe the heat we feel over the fire should be equated to horsepower and the temperatrue of the flames to EGT.

Good analogy. Taking it a little further;
The Volume (Mass) of Fuel/Air being burnt in a campfire or Combustion chamber or candle for that matter, is not going to alter the temperature of the combustion- assumming it is close to the correct stoiciometric mixture. Chemistry controls the temperature. Different fuels burn at different temperatures. Candle wax versus magnesium.

Heat (produced) on the other hand is determined by the mass of fuel/air AND the temperature.

Remember, digressing for a moment, Power = Torque by time (RPM). You can have low torque x high RPM (Subbie car engine) or large torque x low RPM (Aero Engine) and achieve the same power.

For combustion we have a similar equation.
Power (Heat) = Mass (Fuel/Air) x Temp. (Sorry Walter. Bad Maths, but it's just an analogy)
What we are doing with throttle/pitch/mixture is playing around with the two variables, but we don't really want to alter the temperature too far away from Peak. (except to protect the engine)

Pete.

 

[Fitz]

Maybe the heat we feel over the fire should be equated to horsepower and the temperatrue of the flames to EGT.

But that is not what we are measuring. EGT is very much like our hands, at a fixed distance from the fire (in a chimney over a fireplace would be more accurate). And CHT (at least measurement principle wise) is like a metal can enclosing a lightbulb. The light bulb heats the can to a relatively constant temperature. If the light bulb wattage changes, what happens to that temperature?

[A]ssumming it is close to the correct stoiciometric mixture...

That is getting to the heart of the matter. The fixed EGT method (as described) most assuredly achieves the correct air fuel mixture for our purposes. But what does 'correct' mean? Are we just looking for the most complete combustion? In stoichiometric terms that would be peak EGT (lambda 1.0).

Don't get frustrated. I posed this question to engineers taking a seminar. They had literally just studied the science, and it still took a lot of big hints before several of them went 'Ahhh!' All agreed that it is a freakin' brilliant example of applied engineering.

-jjf

 

[zav6a]

hints

I'll just take the answer - hold the hints.

 

[Fitz]

OK, in brief

I'll just take the answer - hold the hints.

I really should let the right folks take the bow, but I know the feeling, so... To more clearly 'get' it, let's shift our focus.

First, ask yourself, why do we run so rich for full power takeoffs at low density altitudes?

The stock answer is cooling.

This leads many pilots to envision fuel acting as some sort of pour-on coolant. But that is largely a myth. Because of the relatively tiny quantities involved, 'charge cooling' (as it is called in automotive circles) is, at best, just a few degrees C. The primary reason that richer equates to cooler is that when we change the air fuel mixture, the speed of the flame front changes (why this equates to cooler is best left for an even 'longer' answer).

So, in stoichiometric terms (air fuel mixture), we have three points. Optimum combustion chemically speaking happens at peak EGT. An air fuel ration of about 14.7:1 with gasoline. We even call this point "stoich". But for technical reasons also best left for a 'long' answer, "Best Power" does not typically occur at stoich, but at a richer mix. Somewhere around 12.5:1 (engine specific). Then we have a third point, the point we are taking off at full rich and down low, which could be described as 'the closest to Best Power we can reasonably get without heading into detonation or otherwise hurting the engine...'

*That* is the point we want to run at. And, with a properly adjusted engine in good order, that point has been set for us by the engine maker. Essentially, full rich already represents 'As hard as the engine can safely work WOT where air and fuel are both pentiful'.

At high density altitude we don't need to use fuel for cooling. The amount of air available for combustion is limited so we can't generate the same levels of heat and power. Detonation requires heat, fuel, and pressure and we now have less of all three.

So where to run the engine? Here is the brilliant part - as hard as the engine makers say it safely can!

Some obviously very bright folks realized that a) the people who built the thing and tested it have set this point down low and b) EGT, under normal circumstances, is a good indicator of how 'hard' the engine is working.

The EGT stays constant because we aren't just getting learner in the absolute (less fuel) sense, but in the stoichiometric ratio sense as well. In other words, our 'third' point above is moving towards true Best Power as fast as it can.

We are not using EGT to keep our air/fuel ratio constant, we are using it to keep the heat and pressure that the engine is subjected to constant. As we get higher we dispense less fuel, but convert it to work more efficiently - all staying within limits and safety margins set by the manufactuer. Considering the operational simplicity it is very, very ingenious.

-jjf

 

[zav6a]

Hmmmm

My brain is holding on hard to the campfire analogy but can't find any holes in your logic - EGT as an effective work meter. I'll have to think about what happens when cross-over occurs - that is - when (at the altitude) the target EGT is actually higher than best power EGT. But then again, probably no need to complicate matters - the target EGT approach was intended as guide for ROP ascents to 75%.

This thread did open a new train of thought - how closely does EGT relate to actual combustion temperature? Clearly there are other variables, like timing and how much residual from combustion still therefore remains when the exhaust valve opens. I'll have to do some surfing.

 

[Fitz]

My brain is holding on hard to the campfire analogy but can't find any holes in your logic - EGT as an effective work meter. I'll have to think about what happens when cross-over occurs - that is - when (at the altitude) the target EGT is actually higher than best power EGT. But then again, probably no need to complicate matters - the target EGT approach was intended as guide for ROP ascents to 75%.

This thread did open a new train of thought - how closely does EGT relate to actual combustion temperature? Clearly there are other variables, like timing and how much residual from combustion still therefore remains when the exhaust valve opens. I'll have to do some surfing.

Don't read too much into the analogy. As you noted, the approach is for normally aspirated, ROP climbs to non-O2 type levels. I've actually tested this with a lambda meter in the exhaust of a 182 up to 10,000'. The ratio leans as the temperature remains constant. Also, at 10,000' the mixture was still a tad rich of Best Power in my test. Presumably target would become higher than Best Power at some point, but once you get into double digits you really need to fine tune most normally aspirated singles to get much climb at all.

Temperature is a fascinating subject. For example, the reason that CHT peaks a little rich of stoichiometric (where EGT peaks) isn't just that we are insuring that virtually all the O2 possible gets used. It also has to do with speed and concentration in flame front. In other words, it isn't just thr 'boom', it is also that timing and location gives a greater opportunity for that heat to be transfered to the top of the engine.

-jjf

 

[David-aviator]

My brain is holding on hard to the campfire analogy but can't find any holes in your logic - EGT as an effective work meter. I'll have to think about what happens when cross-over occurs - that is - when (at the altitude) the target EGT is actually higher than best power EGT. But then again, probably no need to complicate matters - the target EGT approach was intended as guide for ROP ascents to 75%.

This thread did open a new train of thought - how closely does EGT relate to actual combustion temperature? Clearly there are other variables, like timing and how much residual from combustion still therefore remains when the exhaust valve opens. I'll have to do some surfing.

Wow! 109 posts on this subject, never realized it was so complicated .

Back when I was with Lycoming I'd pull the mixture in climb to get an EGT rise and let it roll. In cruise it was pull it back until the engine rumbled a little and move it forward just a bit to smooth things out. And there I would sit for hours in the LEZ as the country moved by. Sometimes, flying into cold air it would cause a rumbled but a little more rich and all was well.

One thing I can say about this simple procedure, it did not harm the engine as far as I know and the plugs were always clean.

dd

 

[Fitz]

Wow! 109 posts on this subject, never realized it was so complicated .

Back when I was with Lycoming I'd pull the mixture in climb to get an EGT rise and let it roll.

But if you left it there the mixture became richer as you climbed. The engine didn't mind running cooler and generating less energy and it is actually pretty hard to foul and engine with 100 LL once the points are hot enough to activate the additives. The idea behind the constant EGT approach is that you climb faster, use less $4+/gallon fuel, and don't run the engine outside spec.

Pocket protector types everywhere, including at Lycoming usually shudder at the rough engine method of cruise leaning. The problem is that where roughness occurs is pretty much arbitrary. In other words, the engine is rough because the fuel distribution is uneven - not because you are at a particular air/fuel ratio.

So you are leaning to the limits of the fuel distribution system, which varies widely, then enrichening slightly. You could have been running at peak, LOP, ROP, whereever. At cruise power settings, catastrophic failure is unlikely regardless of what you do with the mixture, but fuel and engine efficiency are definately effected, and if you happen to be landing just rich of peak, where CHTs and pressures are highest, you are needlessly putting extra wear and tear on the engine as well.

EGT peak at least puts you at a known starting point - 14.7:1. Relative temperature offsets aren't perfect, but it certainly puts you closer to points the engine was designed to run.

-jjf

 

[zav6a]

Convinced

"Don't read too much into the analogy. As you noted, the approach is for normally aspirated, ROP climbs to non-O2 type levels. I've actually tested this with a lambda meter in the exhaust of a 182 up to 10,000'. The ratio leans as the temperature remains constant. Also, at 10,000' the mixture was still a tad rich of Best Power in my test. Presumably target would become higher than Best Power at some point, but once you get into double digits you really need to fine tune most normally aspirated singles to get much climb at all."


Simple is best. More enrichment and detonation buffer where needed at high loads and low altitude and linear decrease in that buffer to the point that it is no longer necessary. Hard to argue with that.

So, given the differences between specific engines and installations, how would one go about establishing a safe but efficient(non-fuel wasting) target EGT? I'll go back up and see if folks already addressed this but it seems that I ought to be able to find the EGT corresponding to about 150 ROP at anywhere between 6 and 8k and thats my number for climb up to and beyond that point (until the "crossover" point mentioned earlier).

 

[gmcjetpilot]

But if you left it there the mixture became richer as you climbed. The engine didn't mind running cooler and generating less energy and it is actually pretty hard to foul and engine with 100 LL once the points are hot enough to activate the additives. The idea behind the constant EGT approach is that you climb faster, use less $4+/gallon fuel, and don't run the engine outside spec.

Pocket protector types everywhere, including at Lycoming usually shudder at the rough engine method of cruise leaning. The problem is that where roughness occurs is pretty much arbitrary. In other words, the engine is rough because the fuel distribution is uneven - not because you are at a particular air/fuel ratio.

So you are leaning to the limits of the fuel distribution system, which varies widely, then enrichening slightly. You could have been running at peak, LOP, ROP, where ever. At cruise power settings, catastrophic failure is unlikely regardless of what you do with the mixture, but fuel and engine efficiency are definitely effected, and if you happen to be landing just rich of peak, where CHTs and pressures are highest, you are needlessly putting extra wear and tear on the engine as well.

EGT peak at least puts you at a known starting point - 14.7:1. Relative temperature offsets aren't perfect, but it certainly puts you closer to points the engine was designed to run.

-jjf

Fitz you post is laser spot on, I agree but a few comments.

Lycoming recommends (allows) the roughness method, lean till it shakes and than move the red knob in a little to get smooth operation. It's "approved". However Lyc does not approve hunting for peak to get your "bench mark" at higher power settings, as you also pointed out rightly (ed-corrected).

Yes, the roughness in the roughness method is from uneven fuel distribution and one cylinder power dropping first, but that is a different issue, not much you can do about that in the AIR or with a carb. It's a function of the engine not pilot technique. Clearly the roughness method is only good at low power where you can't do harm.

I think you said this as well, at some point you can run ROP, Peak, LOP or what ever and you will do no harm, with the caveat all temps are within acceptable or desirable limits. Lyc does allow or suggest you can run peak EGT at or less than 75% pwr. I don't think they recommend it, but it is allowed.

That is why I suggest don't get too rambunctious leaning at higher power, and do no leaning until at least at 5,000 feet. Even if you mess up, environ or engine conditions are poor or critical, you're not going to do harm. In a RV, a climb to 5,000 feet is not a long period of time anyway. After that lean away.

Leaning in climb near sea level at high power may work with target EGT, O2 sensors and so on, but than again we may be outsmarting our self into unseen consequences that the paper or test stand analysis did not indicate. Lyc has found that time and again. What works on the test stand does not work in the plane. Every engine, engine installation, pilot and instrumentation is differnt.

 

[Mike S]

What works on the test stand does not work in the plane. Every engine, engine installation, pilot and instrumentation is differnt.

AMEN.

The same engine/prop/airframe, (basicly everything identical) wont yield identical results in two different planes, no matter how much of a "clone" factor there is.

Not to discount Walters efforts/data-------Test stand info in a critical and SAFE way to gain knowledge, but that is only accurate to a point, even if it is 99%. The final nth degree must be gained by individual testing of each a/c.

The truly amazing thing about all this discussion is in the knowledge floating arround out there.

George, Walter, Fod7, ------all of you, thanks for the info.

Ironflight thanks for starting this one. GREAT READING.

Mike

 

[Ted Johns]

Don't read too much into the analogy. As you noted, the approach is for normally aspirated, ROP climbs to non-O2 type levels. I've actually tested this with a lambda meter in the exhaust of a 182 up to 10,000'. The ratio leans as the temperature remains constant. Also, at 10,000' the mixture was still a tad rich of Best Power in my test. Presumably target would become higher than Best Power at some point, but once you get into double digits you really need to fine tune most normally aspirated singles to get much climb at all.

Temperature is a fascinating subject. For example, the reason that CHT peaks a little rich of stoichiometric (where EGT peaks) isn't just that we are insuring that virtually all the O2 possible gets used. It also has to do with speed and concentration in flame front. In other words, it isn't just thr 'boom', it is also that timing and location gives a greater opportunity for that heat to be transfered to the top of the engine.

-jjf

I don't buy the light bulb analogy. It seems that this issue revolves around temperature, btu/hour/, and thermal resistance of the EGT probe (or your hands, in case of the light bulb!). Temp. probes are designed to be low thermal mass at the sense point so as to avoid requiring a lot of BTUs/hour to read correctly. An ideal sensor would be mass-less, and would require 0 btu/hour to read temperature correctly. Of course, EGT probes cannot be ideal, but are likely fairly insensitive to btu/hour issues at the range of engine output we are discussing here.

BTW, thanks for the commentary on this thread Fitz, the O2 sensor info has been very entertaining.

 

[David-aviator]

EGT peak at least puts you at a known starting point - 14.7:1. Relative temperature offsets aren't perfect, but it certainly puts you closer to points the engine was designed to run.

-jjf[/QUOTE]

I have not measured A/F ratio on a Lycoming but have done it with a borrowed O2 sensor on the Subaru H6. We have reduced fuel consumption considerably by reducing fuel pressure but this is something you don't want to do without making sure the ratio is at least 12:1 for take off. In cruise it runs 14-15:1. Robert Paisly of California was the first pilot to do this. We've reduced fuel pressure from 38-40 to 30-32 with considerable savings and no harm to the engine. Some are running at 28.

Some guys are installing permanently an O2 sensor to have a constant A/F reading. This is great info with any engine for all modes of flight. I am convinced my Lycoming was running at about 8:1 on warm days based on fuel flow alone. No 0360 needs 14-16 gph, especially on a hot day, so I leaned it slightly on take off on those type days.

By the way, I did lean during climb, not once but gradually all the way up to usually 11-12000' on long cross countries. It had to be done or the engine would choke on the rich mixture fuel.

dd

 

[Fitz]

Lycoming recommends the roughness method, lean till it shakes and than move the red knob in a little to get smooth operation. It is "approved". However Lyc does not approve hunting for peak to get your "bench mark" at higher power settings, as you suggest.

Regarding roughness. Rather it is approved our not, engineers always hate it because the engine is running at a relatively unknown point.

Regarding 'hunting for peak at high powers', I'd have to say that is the absolute opposite of what I have been endorsing. That is the whole reason I am so fond of O2 sensing - I can fine tune to the engine makers recommended mix for the situation with better precision and no dependance on peak. I'm sorry if I have not been clear enough.

-jjf

 

[gmcjetpilot]

stand corrected

Regarding roughness. Rather it is approved our not, engineers always hate it because the engine is running at a relatively unknown point.

Regarding 'hunting for peak at high powers', I'd have to say that is the absolute opposite of what I have been endorsing. -jjf

That was poorly worded on my part, sorry, I understand what you meant and just wrote it wrong. In agree with you 100%. Thanks I corrected it hope.

As far as the roughness method, heck with the engineers I don't like it. Ever since I was a private pilot I never liked the idea. I got the why and where of the rough part, but the part about "move the mixture back towards rich to get smooth operations" is the part I have an issue with. You really don't know where you are. This method is in thousands of AFM's. Of course with a single EGT there's some risk, since you have 100% sure idea if that cyl was first to peek. You hope it's on the correct cylinder. I think many RV's with a single EGT out it on the same jug that's also usually has the hottest CHT, #4. Right or wrong a single probe has its limitation, but it's better than nothing.

In the day where rental planes (still the case no doubt) did not even ONE EGT gauge. So the roughness method was the only thing available. When CFI'ing long ago I can't remember seeing an engine monitor on the single engine fleet with a few exceptions. It's a method of necessity not prefrence.

If building a RV on the cheap (nothing wrong with that) try to install at least one CHT and one EGT. Most put it on the #4 jug. Of course 4 channels of CHT and 4 of EGT is best. The FAR's don't require any EGT or CHT gauge nor does Lycoming. Of course Lycoming recommends not going over 400F CHT continuous for longevity. How do you know that with out a CHT gauge.

With the advent of all the fancy engine instruments we know way more, may be too much.

 

[gmcjetpilot]

Excellent, but question

No 0360 needs 14-16 gph, especially on a hot day, so I leaned it slightly on take off on those type days.

By the way, I did lean during climb, not once but gradually all the way up to usually 11-12000' on long cross countries. It had to be done or the engine would choke on the rich mixture fuel. dd

Interesting comments on O2 sensor. I am going to guess you have a innate sense of your engine, some got the touch. I mean that as a complement and with all do respect. Clearly you have a deep understanding of how engines work and mixture ratios, no doubt more than I do or may be the average pilot.

However what do you mean by "lean a little" and "hot day". I think I know what you mean, I think, but in engineering terms and explaining a technique I or others could use, something may be lost in translation. By hot day I guess high density altitude. How high or hot? You say "lean a slightly". Do you move the red knob back to a preset known position you estimate from prior knowledge, or do some calibration during run up or FF? I am not challenging you in anyway and they way you operate is your prerogative, just trying to understand how may be I could use this technique or if I would want to.

 

[Fitz]

I don't buy the light bulb analogy. It seems that this issue revolves around temperature, btu/hour/, and thermal resistance of the EGT probe (or your hands, in case of the light bulb!). Temp. probes are designed to be low thermal mass at the sense point so as to avoid requiring a lot of BTUs/hour to read correctly. An ideal sensor would be mass-less, and would require 0 btu/hour to read temperature correctly. Of course, EGT probes cannot be ideal, but are likely fairly insensitive to btu/hour issues at the range of engine output we are discussing here.

BTW, thanks for the commentary on this thread Fitz, the O2 sensor info has been very entertaining.

I'm sorry, I'm not following you. The issue isn't rather the sensor itself consumes BTUs, but what it is measuring. Let's assume the EGT measurement is perfect. Now, instead of providing the heated air in the exhaust pipe from an engine, let's say we use a fan and a can of sterno.

Are you saying that even if I cover 1/2 the opening on the sterno the EGT measurement will not change? I'm saying that EGT measures the results of combustion (how hot we heated things too), not the temperature of combustion iteself.

-jjf

 

[Fitz]

[M]aking sure the ratio is at least 12:1 for take off.

It had to be done or the engine would choke on the rich mixture fuel.
[/QUOTE]

FWIW, with the fuel flow set per the placard on a C206H Lambda measured about 0.75 (about 11:1 with 100LL) so *at least* 12:1 is probably pretty sound advice.

I am actually interested in experimenting with precise O2 measurements LOP. But I'll have to probably spring for GAMI injectors first...

As for choking on rich - I hear you. I used to do a lot of flying of woefully underpowerd planes out of Flagstaff.

-jjf

 

[Fitz]

So, given the differences between specific engines and installations, how would one go about establishing a safe but efficient(non-fuel wasting) target EGT? I'll go back up and see if folks already addressed this but it seems that I ought to be able to find the EGT corresponding to about 150 ROP at anywhere between 6 and 8k and thats my number for climb up to and beyond that point (until the "crossover" point mentioned earlier).

Well, in the AdvancedPilot example, they were starting with a manufacturer's specified fuel flow for specific conditions. I suspect that the racing folks, who have spent some serial experimentation time might have some good suggestions on refining this number.

-jjf

 

[zav6a]

Target

I'm sure I saw Walter provide an opinion on how to come by a target on one of those earlier 109 messages.

 

[Ted Johns]

I'm sorry, I'm not following you. The issue isn't rather the sensor itself consumes BTUs, but what it is measuring. Let's assume the EGT measurement is perfect. Now, instead of providing the heated air in the exhaust pipe from an engine, let's say we use a fan and a can of sterno.

Are you saying that even if I cover 1/2 the opening on the sterno the EGT measurement will not change? I'm saying that EGT measures the results of combustion (how hot we heated things too), not the temperature of combustion iteself.

-jjf

Sure, the sensor can "consume" BTUs. Back to that in a bit. What I am saying is that I think combustion gas temp. remains roughly constant for a given A/F ratio throughout a reasonable range of air densities. The air density affects cylinder filling, and thus hp, but not combustion temp.

Now, if the sensor has high thermal mass because (for example) it has too much thermal conductivity to some heat sink like the exhaust system, BTUs per hour would affect the equilibrium temperature of the sensor. The sensor would "consume" BTUs.

It is my supposition that a good sensor doesn't have that problem. How good is the average aircraft EGT sensor? I don't know.

So, Yes, I am saying that if you cover half the sterno can heater, a good temp sensor will not change it's reading. The output air is just as hot. There is just less air to heat up a thermally lossy system (i.e. a bad temp sensor).

Interestingly, simple thermal resistance / thermal mass (or capacitance) systems model just like resistance / capacitance in electronics.

 

[Fitz]

Sure, the sensor can "consume" BTUs. Back to that in a bit. What I am saying is that I think combustion gas temp. remains roughly constant for a given A/F ratio throughout a reasonable range of air densities. The air density affects cylinder filling, and thus hp, but not combustion temp.

If I am understanding you, you are saying that there is a direct relationship between absolute EGT and a specific air/fuel ratio.

My understanding has always been different. In fact, I always believed that the reason we typically use peak EGT, then adjust relative to it is because absolute EGT values cannot be safely equated to specific AFRs.

If you are correct and there is an absolute relationship, shouldn't that mean that the EGT reading I get ground leaning at 1200 RPM should match the EGT reading I get when I peak and lean at cruise?

Similiarly, if I am leaned for cruise and throttle way back, wouldn't EGT remain constant under your theory as long as can keep trading airspeed to hold altitude?

-jjf

 

[David-aviator]

Interesting comments on O2 sensor. I am going to guess you have a innate sense of your engine, some got the touch. I mean that as a complement and with all do respect. Clearly you have a deep understanding of how engines work and mixture ratios, no doubt more than I do or may be the average pilot.

Deep understanding? Not really. Most of what I know comes from reading what guys like you and others write, I am no engineer.

I sense there is a certain frustration with Lycoming among the engineers who are tuned in here. The engine is not much different than the Farmall tractor I learned to operate over 50 years ago, it was quite reliable and straight forward, not at all complicated. However, to operate it required a bit of common sense. If the impulse coupling were not engaged on start, it could break your arm cranking it. That kind of stuff.

I had an EGT guage hooked up to one cylinder on the 0235 and 0360. How much meaninful information does that provide? Not much really. Generally, I leaned the engine off the vibration mode - too lean, for whatever reason, I tend to believe it was lack of fuel not flow that caused it. Why else would it happen when flyinging into colder air - to make it run smooth. It is not very scientific but then, neither is the engine.

If you guys really want to get wound up on the science of an engine, get into some the deep secrets of todays auto engines. They are incredible.

dd

 

[Tandem46]

DD,
I sent you a pm.
Tobin

 

[fodrv7]

Refined engines

DD
So true.
Compared with the Lycoming in my RV-7, the V6 2.7 litre engine in my Audi has 5 valves per cylinder, Variable inlet manifold length, variable exhaust valve lift and timing, one turbo per bank, one ignition coil per spark plug, is smooth as silk and approaches the 6600rpm redline with such enthusiasm that if the tipronic gearbox did not intervine and select the next gear it looks like it would spin happily to 8000rpm. It has a dead flat torque curve from 1800 to 5000rpm and is wisper quite; unless you boot it, when it develops a fabulous menacing growl.

Pete.

 

[Fitz]

I sense there is a certain frustration with Lycoming among the engineers who are tuned in here.

Why not spell out exactly what some of those frustrations are? Reading Lycoming's document SSP700A I got the 'sense' that Lycoming thinks the average pilot is probably too slow and clueless to operate a pointy stick, let alone a big red knob. I also got the 'sense' that the author kicks his dog and doesn't eat enough fiber, but so what?

This isn't an EST class, we are simply talking about Engine 101. Yes, I'm an engineer but this is all stuff you can get from a good engine tuner. Furthermore, when we put aside all the touchy feely human emotion nonsense, there is pretty much nothing I am going to say about an aircraft engine that you can't see in Lycoming's own charts and data.

Frankly, I find even the idea of technical folks at Lycoming being digusted with their users galling. Remember, it is 2006 and the number one cause of premature failure on a normally aspirated Lycoming engine is manufacturing flaws. Forget crankshaft problems, were talking stuff like chambers out of round, valve guides mounted off center... *Manufacturing* 101 type stuff, at least today.

I'm not upset with you. I just wish that folks at Lycoming would accept that some of us want to fully understand what we are going in the cockpit. Telling us not to worry our pointy little heads instead of giving sound technical explanations can grate on those of us who fall in this camp.

However, to operate it required a bit of common sense.

Are we talking about 'common sense' or 'tribal lore'? Newly minted pilots learn leaning from newly minted CFIs, who were themselves once student pilots... At some point far back in the chain the ju-ju magic of some alpha male (or female) was mimiced, but any caveats or meaningful explanations are always long forgotten.

Think about it, the engineers at Lycoming set a fuel flow for the engine's full rich setting. And you'd be hard pressed to find official recommendations to overide that for climbs (Service Instruction 1094D repeatedly warns against it). But you meantioned leaning until EGT comes alive for climbing.

Is that 'sense', as in Age of Reason? Did you put thought into detonation margins or the expected variations in air cooling? Or is it 'lore'? As in, someone told you that you will use less fuel and climb faster, but it won't hurt the engine. There is nothing wrong with using tried and true procedures even without complete understanding. But we need to be careful about equating the procedures to 'good sense' when underlying conditions change.

I had an EGT guage hooked up to one cylinder on the 0235 and 0360. How much meaninful information does that provide? Not much really.

Well, EGT was enough to get Lindy across the Atlantic. It also cut the number of P-38 pilots dying in post mission fuel starvation accidents over the Pacific down to nearly zero in WW-II. It also was the foundation of emissions control science for a long time and the foundation for automative tuning until just a few years ago.

Because of my engineer's enthusiasm for wideband sensing people might get the idea that I pooh pooh EGT. Not so. It is a simple way to get a real glimpse into what is occuring in the engine. It has its limits and can even be misleading, but just looking at EGT, throttle position, and RPM can give you amazing insight into an engine's health and performance. In other words, the information is there, but you have to learn the language.

With a rudimentary float carb, information specifically useful for leaning might be another story (another place Lycoming and I agree, but I'll get to that below).

I tend to believe it was lack of fuel not flow that caused it. Why else would it happen when flyinging into colder air - to make it run smooth.

What we believe isn't always what is so. I once used Venturi the Flatulent Fairy to explain the odd thermal behavior of a particular updraft float carb. All and all, a pretty convincing spiel, but he wasn't there when we finally took the thing apart.

However, in this case both Lycoming Service Instruction 1094D and I agree with you. The engine is rough because of misfires and the misfires occur because some cyls are too lean. The problem is not general air/fuel ratio, but terrible mix distribution, especially as fuel flow decreases. In 1094D, Lycoming goes so far as to contend that with their float type carbs distribution is so bad that EGT leaning may not be practical because the probe cannot be placed in the leanest cyl (each cyl varies widely).

With Lycoming fuel injected engines we know that fuel distribution with smaller flows is still a problem because many stock engines cannot be run LOP without roughness, but the same engine can be run at stoichiometric ratios above 1.0 when injectors are replaced or the distribution otherwise leveled.

Here is a case were 'lore' can break down. A carb driven Skyhawk is going to always run rough ROP. A 172R, with fuel injection - you really don't know. Just rich of 'rough' may well put the engine exactly where you don't want to be. Hence EGT and fuel flow gauges are standard on R models.

It is not very scientific but then, neither is the engine.

Actually, that is unfair. A huge amount of R&D went into aircraft engines in the first half of the 20th century. That research isn't just the foundation of the modern understanding of combustion, we're talking about core research in cooling, ignition, detonation, the first significant study of mixture... You name it.

You cannot pick up a research paper at a conference on engine design *today* without still finding citations to this work, some of it now 60 years old. Remember, aviation represented a significant challenge in power, weight, and efficiency. All modern auto engines stem from this work and it is a real testament to the scientific approach and enormous investment that some 50 year old engine designs remain difficult to top for their application today.

Just because, as an applied science, we are talking about something that Gomer can fly and Goober can service with a big hammer right there in Mayberry doesn't mean that the engines are not major technological achievements.

If you guys really want to get wound up on the science of an engine, get into some the deep secrets of todays auto engines. They are incredible.

Tell me about it, as I recently commented in a PM the oil cooling in some Porche engines, engines which are achieving Best Power at mixes as lean as .9 Lambda, are amazing. But they are also complex. I'm not all that sure I would want to be running a GA single behind one. Especially if it was a rental and I didn't know whose 'ju-ju' had been last used on it!

-jjf

 

[gmcjetpilot]

But do we need it in a plane

DD
So true.
Compared with the Lycoming in my RV-7, the V6 2.7 litre engine in my Audi has 5 valves per cylinder, Variable inlet manifold length, variable exhaust valve lift and timing, one turbo per bank, one ignition coil per spark plug, is smooth as silk and approaches the 6600rpm redline with such enthusiasm that if the tipronic gearbox did not intervene and select the next gear it looks like it would spin happily to 8000rpm. It has a dead flat torque curve from 1800 to 5000rpm and is whisper quite; unless you boot it, when it develops a fabulous menacing growl.

Pete.

Pete we don't need 30 valves on an engine going 2,500 rpm, nor do we need the weight. Fat or flat torque curves are great, but when you operate 100% of you time in cruise at one power setting, who cares. I concur cars are great but the paradigm, the reality of the demands of each application are completely different. What if the jet engine was not invented and billions where put into piston engine development over the last 5 decades? Who knows.

Take that Audi engine, all the hoses, wires, pumps and computers and do a weight to HP comparison (don't forget to add a gear reduction for the prop if needed, which is likely) the Lyc will come out way a head. Not the be a cheer leader for Textron, but the comment Fitz made about the technology of the lowly air-cooled engine is really amazing and true.

If engineers where given a blank sheet and wanted to make a light aircraft engine, it would look a lot like an air-cooled, horizontally opposed, direct drive Lycoiming and not a 30 valve Audi, IMHO. I know you did not say the Audi would make a great aircraft engine, just admiring the technology, but how would that smooth acceleration and power out the corner, relate to cruising in a light plane.

I get your desire and others wish that this smooth turn key sewing machine power could translate to a plane. It can in a way; we have the Mazda or Subaru conversion efforts. However the reality is they are "adaptations" and a series of workarounds to get car engines to work in a plane. However even though I'm not a fan of alternative power, my hat is off to them. They have come a long way and at least get acceptable performance out of them. However the weight issue (and drag issue) is still a big issue and think it always will be. Weight is an aircraft's enemy. Fly a light simple RV than a heavy one. It'll be worth 1000 words. In a "Sport" plane handling is key and that is a weight issue. In a straight and level cruiser weight is not so much a factor. So each to his or her own. Radiator drag takes a different approach than just coping an air-cooled installation.

After 50 years the Lyc is still the bench mark the alternative engine guys try to match. Honda had a prototype aircraft engine? Where did that go. No doubt financial issues, like number of units built to cost and profit are in the way. As well, no doubt the performance edge was still better with the lighter, simpler Lyc. For all the critics of Lyc, give them credit where it's due. It is made to only look crude on purpose.

Water cooling a piston aircraft engine opens a whole world of issues, not the least of which is what to do with the radiator (drag) and weight. In an airframe made for an air cooled engine, fitting a water cooled engine is going to be a compromise. The water cooled engine needs an airframe designed around it, just for it. The ill fated Pond racer comes to mind as a dedicated water cooled design. Of course the P-51 is another example, but it's a 400 MPH plane and the aerodynamics (transonic) of that radiator installation is quite different. Listen to a P-51 pilot stuck waiting to takeoff on the radio, they get real excited, because they have cooling problems, as do some Subaru installations in RV's. Big air cooled raidial not so much problem on taxi.

It's great to talk technology and gee whiz stuff, but you have to talk about the negative. Part of the Audi's smooth quite power is from the water cooling. Cost, liability and other issues keeps truly "NEW" honest to goodness piston aircraft engines coming to the market, especially with such a small market. However if I would not be surprised if that NEW aircraft engine looked just like todays Lyc or TCM.

 

[Ted Johns]

If I am understanding you, you are saying that there is a direct relationship between absolute EGT and a specific air/fuel ratio.

My understanding has always been different. In fact, I always believed that the reason we typically use peak EGT, then adjust relative to it is because absolute EGT values cannot be safely equated to specific AFRs.

If you are correct and there is an absolute relationship, shouldn't that mean that the EGT reading I get ground leaning at 1200 RPM should match the EGT reading I get when I peak and lean at cruise?

Similiarly, if I am leaned for cruise and throttle way back, wouldn't EGT remain constant under your theory as long as can keep trading airspeed to hold altitude?

-jjf

Well, let me think about the physics of the experiment.
I believe the energy of the chemical reaction to be a constant. A given number of Kcals per mole of reactant will be liberated. The resultant combustion gas temperature will depend on the starting temperature of the reactants, and the presence of diluents, i.e. lean => excess oxygen, rich => excess 100LL. These diluents will not contribute to the reaction, but will add thermal mass. This will result in a different temperature equilibrium for the given thermal mass of combustion gas. BTW, one can see that reduced air density does not affect the diluent ratio.

So, starting temperature of the reactants is indeed an issue. Additionally,
real world EGT sensors may have large offset errors, hopefully much smaller gain errors. So using absolute measurements from an EGT sensor is not reliable. The (peak EGT - xxx delta) scheme that pilots use is simply a way to "calibrate" the sensor at time of use.

With calibrated EGT sensors, and sensors to measure the temperature of the reactants, a computer could use EGT as an accurate A/F ratio meter. No reason this couldn't be accurate on the ground at idle, or at cruise at altitude.

The better absolute accuracy, and lack of dependency on starting reactant temperature, is what is nice about the oxygen sensor.

 

[fodrv7]

Too sophisticated

George,
Can't disagree with one word of your reply.
After all, the three things I want from an aircraft engine are reliability, reliability and reliability. After that power to weight is worth chasing.
And in the interest of reliability, simple is better. Aslo helps the weight as you say.
But, I do love the modern technology.
Pete.

 

[Fitz]

I believe the energy of the chemical reaction to be a constant.

Actually, it isn't. The long molecules in the fuel break down into different compounds under different conditions. New cars don't knock when we step on the gas because idle is too lean. Idle is actually at 14.7:1, the stoichiometric optimum. They knock because the conditions are right for the fuel break down into more auto-igniting compounds.

The latent energy in the fuel remains pretty constant, but the amount we actually release for a given mass changes. But let's ignore this variable.

With calibrated EGT sensors, and sensors to measure the temperature of the reactants, a computer could use EGT as an accurate A/F ratio meter. No reason this couldn't be accurate on the ground at idle, or at cruise at altitude.

Unfortunately, no. The parts of the equation that I have been trying to hint at are volume and time. We are not measuring the temperature of the flame front, we are measuring the temperature of the gas pushed out of the cyl during the exhaust stroke.

The volume of the cyl remains unchanged, but the mass of reactant and the speed at which it is consumed changes. We can actually see a case of this pretty easily.

When we switch from both mags to L or R alone, RPM (assuming fixed pitch prop) drops. This is primarily because the mix in the cyl is being less completely consumed (as the piston moves down pressure drops - with just one spark, pressure drops below a critical threshold before the flame front can rach all the fuel). We've released less energy, but what does EGT do? It goes up (yes, enough to see on a typical EGT gauge)! Conversely, there are examples we definately don't want to see. When there is pre-ignition or detonation, EGT goes down.

The complete explanation for both these is fairly complicated, but a big factor is pretty easy. We don't 'explode' all the fuel in the chamber (in fact, we put a lot of effort into stopping that from happening). We do a 'controlled burn' over an interval of time. We use the energy released several ways (rather we'd like to or not). One of the ways that we use it is to heat the exaust gas behind the flame front itself. When we detonate, all the mix is consumed much more quickly than normal, so we spend less time doing this (EGT goes down). When we ignite using a single mag the pressure curve changes and more of the mix is physically farther from the ignition source - so we spend more time heating gas behind the flame front (EGT goes up).

The better absolute accuracy, and lack of dependency on starting reactant temperature, is what is nice about the oxygen sensor.

No arguments there! O2 sensing does not even care what the fuel is (you only need the stoichiometric multiplier for the fuel if you want to convert Lamba into AFR). But that is is also how I confirmed my own hypothesis that the constant EGT reflects leaning air/fuel ratio.

-jjf

 

[Walter Atkinson]

Fitz:

**When we go up, the air gets 'thinner' - IE, fewer molecules per volume. So mixture must get leaner to maintain the same air/fuel ratio. If we are burning less fuel, then there is less energy being released.**

Sorta, but not really. When ROP, Hp is not signiificantly related to FF except at the exceptionally rich ranges. HP when ROP is related almost directly to mass airflow. Altering the FF has very little effect across the broad range of mixtures ROP.

**A smaller release of energy over the same interval of time is the same as saying we have less 'power' available. We see this at the crankshaft at higher DAs.**

BECAUSE the mass airflow is down.

**But, if we accept that the total energy available is now smaller, why does constant EGT work?**

Because if we hold the EGT constant at the same as at T.O., early in the climb we are rich enough to keep things cool. As the climb continues, that same EGT is getting closer to Best Power--which is what we want as we climb, the power is reduced as the mass airflow is reduced and we need the power to continue the climb. By the time we are at about 13k feet or so, the Target EGT is now almost exactly Best Power. Basically as we climb, by using raget EGT, we are maximizing power while maintaining deotnation margin, cool CHTs and optimum power for the altitude.

Kinda works nicely, huh?

BTW, your explanation of why CHTs stay cool with additional FF was elegant and well done. It's not the heat of vaporization. It's the lower ICP which results in few BTUs being transfered across the thermal boundary layer.

Also, someone suggested that these things need to be tested beyond the lab and on every installation. That is in violation of Sir Isaac Newton's Principia. For one to believe that, one must first believe that the F:A charge knows what kind of vessel it is in. "The physics is everyhwere the same," so this is true in all spark ignited, piston gasoline engines. There are no exceptions IF, IF and only IF the enigne we are talking about is a conforming engine. If it's not conforming, all bets are off. It won't run right no matter what. <g>

Regards,

Walter

 

[Fitz]

Because if we hold the EGT constant at the same as at T.O., early in the climb we are rich enough to keep things cool. As the climb continues, that same EGT is getting closer to Best Power--which is what we want as we climb, the power is reduced as the mass airflow is reduced and we need the power to continue the climb. By the time we are at about 13k feet or so, the Target EGT is now almost exactly Best Power. Basically as we climb, by using raget EGT, we are maximizing power while maintaining deotnation margin, cool CHTs and optimum power for the altitude.

Kinda works nicely, huh?

Actually, I still think it is a brilliant example of applied science (see my later post where I described it as three points, stoich (peak), Best Power, and 'where we don't break the engine' - then explained how constant EGT moves with the last towards Best Power).

I had planned on just opening the discussion, then letting you explain it (it was your presentation after all!) - but I got some pretty frustrated PM's, etc. so I explained as best I could.

-jjf

 

[fodrv7]

Keep it coming

Walter, Fitz,
Fascinating and all explained with brilliant clarity.

Keep it coming.

Thanks,
Pete.

 

[kentb]

What was it....

WOW this is the longest thread that I have ever read. Very interesting and made me think some things over.

I would like an explaination of something that happend to me a number of years ago. I thought that I understood it before I read these many posts, but now I would like some of you experts to explain it to me.

The situation was:
C172 8000 ft O320-E2D (150hp). I had lean until my single EGT peaked and then richened 50 degrees. I had be at this setting for about 20 minitues and then the engine got real rough. I quickly richened the mixture, but the roughness continued. I started looking for an airport and reduced power and landed. By the time I had landed 5 minutes later everthing was fine and I could not find anything wrong with the engine and ground runs were OK.

Later my CFI told me that it was detination.

What say you?

Kent

 

[dan]

What say you?

Carb ice. Just a WAG.

 

[gmcjetpilot]

Ditto

Carb ice. Just a WAG.

SKYHAWK info

Page 4 and 5 Carb ice

 

[David-aviator]

Later my CFI told me that it was detination.

What say you?

Kent

Vote #3 - carb ice.

I had it happen same symptoms in a C150 at 9500' in July with a surface temp of about 90. On descent, it ran fine after 5000' but landed anyhow and called a favorite mech. He said carb ice (rest his soul, the old coot has gone West). Took off and flew home without further event. The C150 with Cont 0200 was ice prone anytime....really dumb design with carb up front behind the prop.

dd

 

[Fitz]

Vote 4...

Above even 5000' it is very hard to generate the heat and pressure nec. to get detonation. It sounds like carb ice to me.

-jjf

 

[J-42]

Six years later has anyone put an ICP in and flown it?

... May be I'll buy a data logger to measure my own ICP http://www.tfxengine.com/hardware3.html, here is a plot:

[/I]





G: I don't know what data you've seen and you are not talking. I am not trying to sell you on the idea of EI but consider this. You agree 25 degrees is OK and is designed for 100% power. Now refrence the ElectroAir charts; you have to get down to 83% percent power to see the timing move 1 degree. Seems safe to me? From the charts, at 75% power the advance is about 7-8 degrees. This is not radical stuff. You need to be at 50% power to see the max advance, about +18 degrees over 25. The main benifit Walter is the hotter and longer duration spark. The small advance is just frosting on the cake. BTW all the manufactures will BURN in a new timing curve at your request. If you want to maintain a fixed 25 degrees you can do that as well.

Clearly you feel that without ICP there is NO safe way to advance the timing.

IT was a pleasure. Take care. G

Six years later has anyone put an ICP in and flown it?