[RBD]

My father-in-law recently purchased a Diamond DA40, which is equipped with the Garmin G1000 avionics package. He has owned several planes over the years, but none have had such detailed engine monitoring. I know that it is becoming more commonplace to run LOP and I've even heard that it is better for the engine. I'm looking for a good explanation of the logic behind LOP, as well as the pros and cons. Thanks for your help and I hope to see all of your beautiful RVs at Sun'n'Fun....please don't get mad if I drool on your canopy.

Cliff notes: Looking for info on LOP procedures, pros/cons.

 

[Bryan Wood]

Go to www.avweb.com and search John Deacon's articles called the Pelicans Perch. Somewhere in the mountain of good info there will be articles on LOP. Be aware that Deacon will be a new hero of yours.

Enjoy,

 

[petehowell]

It is John Deakin

Slight spelling correction on the name - but full agreement - the articles are great stuff

 

[airguy]

Agreed - very few people can explain LOP better than he already has.

 

[frankh]

LOP is nice!

An easy way to get there (sorry if this is so obvious...It wasn't to me until I'd flown for a while) is to simply yank the mixture until the engine basically quits and then feed it in a little at a time until the engine just runs smooth.

Check your CHTs they should be around the 295 to 325 range.

If you have a Dynon or similar you can select the lean mode and gradualy richen the mixture from here to find peak EGT and then back off to 50 to 100F LOP.

There are two reasons I do it this way.

1) the engine spends zero time in the 25 to 75deg ROP detonation danger zone.

2) The airplane is too fast for a novice IFR student and I wanted a way to tame the beast (slow it down) and get it set up quickly to a point where I didn't have to fuss with the engine, i.e concentrat on flying and not fussing with the red knob.

My IFR training protocol is to take off, set 2600rpm at 500 feet. At 2500ft (aiming for a cruise alt of 3000) pull the throttle to 20.5 to 21"MP, Dial the prop to 2400RPM. Pull the mixture for a quick and dirty smooth running as described above....This gives me around 100F LOP.

For my long cross county trip I will try to dial it in to 25F LOP, but I have a warm #4 cylinder which may prevent this.

Cheers

Frank

 

[Walter Atkinson]

The advantages of LOP operation include:
1) a cleaner running combustion chamber, cleaner ring lands, valves, and piston heads;
2) a cooler running cylinder, resulting in a stronger cylinder;
3) lower peak pressure pulses on the crank, bearings and lower end;
4) cleaner oil due to lower blow-by;
5) extended range, resulting in greater options when dealing with weather or ATC or changes in plans;

Cleaner and cooler are better. LOP is cleaner and cooler.

The only disadvantage is that you need to have a conforming engine where:
1) fuel:air ratios are balanced,
2) there is a healthy ignition system, and
3) there are no induction leaks.

But, you want that anyway, don't you?

Walter

 

[gmcjetpilot]

Like the "new message" and tone

The advantages of LOP operation include:But, you want that anyway, don't you? Walter

Nice article in kit plane. I like your MESSAGE. It is reasonable and "balanced" with out hyperbole and rhetoric. Just the facts and opinion stated with out prejudice to others opinions. Nice job. Its a choice and not a must.

 

[kentb]

Should I switch now?

I now have 40 tach hour on my engine. I have been running 100 ROP even when running my engine below 75% power. My oil usage for the last 30 hours has been about 2 qts. I will be doing an oil change to regular oil (not mineral oil).

Should I continue to keep thing on the rich side or should I start using LOP operation now? Should I just try to put it at peak egt for now?

Kent

 

[frankh]

Lets see

If you have been running your engine hard and the CHTS have been coming down (and possibly oil temp) then it has probably broken in.

If so I would go for it...If not then run it real hard (like WOT throttle down low) for another couple of hours and then run LOP.

Frank

 

[dan]

Should I continue to keep thing on the rich side or should I start using LOP operation now? Should I just try to put it at peak egt for now?

Imho, you should first determine what the "gami spread" is for your engine (delta in fuel flow at peak EGT for all 4 cyls). Record the data if you can. If that looks good, i.e. the spread is down to a few tenths of gph or less, then imho you can have at it with LOP. If you have a wide gami spread, then you may want to invest in some custom restrictors for your injectors to achieve a lower (ideally zero) spread.

 

[frankh]

Good point Dan

With the greatest of respect to Dan (who has WAAY more experience than me), That would be ideal but I have quite a spread and haven't actually ..er..gotten around to customising the restrictors yet but it runs fine LOP.

The important point is however I can run my richest cylinder at 100F LOP and still have a smooth engine so I don't necessarily agree that you Have to have balanced restrictors....But it will certainly be more fuel efficient.

Dan brings up an important point though in that you have to make sure your flows are balanced enough so that Your richest cylinder is LOP...if you have one that is on the rich side of peak but the other three are LOP there is a risk you could run into detonation on the rich cylinder.

Make sure you have read Guru Deakins articles and understand whats goin on. LOp is perfectly safe providing all cylinders are LOP, if yur flows are so imbalanced you can't do that then balance them first.

Frank

 

[kentb]

Dan, not sure what you mean....

Imho, you should first determine what the "gami spread" is for your engine (delta in fuel flow at peak EGT for all 4 cyls). Record the data if you can. If that looks good, i.e. the spread is down to a few tenths of gph or less, then imho you can have at it with LOP. If you have a wide gami spread, then you may want to invest in some custom restrictors for your injectors to achieve a lower (ideally zero) spread.

My engine monitor records per cylinder temp for CHT and EGT, but I only have one fuel flow for the engine. How do I measure gph per cylinder?

My curse temp are ..........314....316....323....301.

Kent

 

[frankh]

Ahh yes

What Dan was saying is that the point at which each cylinder reaches peak EGT (not CHT) needs to be within almost the same total fuel flow.

I.e lets say you hoofing along ROP, you start pulling the mixture back and eventually the EGT will peak on one of the cylinders at a total of say 8 gallons per hour...You keep pulling the mixture and at say 7.8 GPH the next cylinder will peak its EGT...and so on.

Ideally you want those EGT's to peak at the same flow or within 0.2 GPH I think....Mine is more than that though.

Frank

 

[sf3543]

To figure it out, create a chart to track all of your temps by fuel flow.
Make a page with 5 columns...GPH, egt1, egt2, egt3 and egt4.
Go flying and set up a cruise with a fuel flow you know is rich of peak and 75% power or less. Record the GPH and the temp for each cyl. Lean the engine by 1 or 2 gph and record again. Do this until one cyl peaks and starts to go down, then lean by 1 gph until they all peak and the 4th cyl starts to decline in temp. You are now LOP on all cylinders. You can then look at the peak temp and fuel flow for each cyl. If they are within 2 or 3 gph, when they all peak, you are good to go. If not, you need to balance the injectors. Call Precision Airflow and they will help you out. They helped me by suggesting two new restricters. ($25 each) Now my peaks are within 2 GPH and I run LOP quite a bit.

 

[RBD]

Thanks for the helpful advice everyone. Those articles by John Deakin were more informative than I could have hoped for. Since I brought it up, my father-in-law-to-be (less than two weeks till the big day) is very interested in learning about LOP and I've been tasked with doing the research. Now I have some good stuff for him to take a look at.

Imho, you should first determine what the "gami spread" is for your engine (delta in fuel flow at peak EGT for all 4 cyls). Record the data if you can. If that looks good, i.e. the spread is down to a few tenths of gph or less, then imho you can have at it with LOP. If you have a wide gami spread, then you may want to invest in some custom restrictors for your injectors to achieve a lower (ideally zero) spread.

Thanks for the advice, Dan. This seems to be an important part of the equation that we likely wouldn't have known about. We met a couple of month's ago when I was working out in Santa Ana. You let me come out to Chino and hang out while you changed the oil before the formation clinic that you taught. Brad (RV-4) stopped by for a while, too. Talking to you guys really got me motivated to start saving for my own kit.

To figure it out, create a chart to track all of your temps by fuel flow.
Make a page with 5 columns...GPH, egt1, egt2, egt3 and egt4.
Go flying and set up a cruise with a fuel flow you know is rich of peak and 75% power or less. Record the GPH and the temp for each cyl. Lean the engine by 1 or 2 gph and record again. Do this until one cyl peaks and starts to go down, then lean by 1 gph until they all peak and the 4th cyl starts to decline in temp. You are now LOP on all cylinders. You can then look at the peak temp and fuel flow for each cyl. If they are within 2 or 3 gph, when they all peak, you are good to go. If not, you need to balance the injectors. Call Precision Airflow and they will help you out. They helped me by suggesting two new restricters. ($25 each) Now my peaks are within 2 GPH and I run LOP quite a bit.

Thanks for further explaining the whole "gami" thing. This sounds like a good plan of attack and we'll likely log the EGTs and fuel flows this weekend.

-Ryan Dean
Tampa, FL

 

[Captain Avgas]

Now my peaks are within 2 GPH and I run LOP quite a bit.

Surely you mean 0.2 GPH.

 

[sf3543]

Yes..... 0.2 GPH.
Missed my period!

 

[Walter Atkinson]

Tagging on here to answer several posts.

First, Thanks to George for the kind words about the article. I appreciate it.

A GAMI spread of under 0.5 gph is usually good enough to run smoothly LOP. The lower, the better, but anything under about .3 is pretty good. If the enigne runs smoothly LOP, it means that the GAMI spread is acceptable!

Anyone wishing to do a GAMI Spread can go to the Advanced Pilot Seminars website and download the forms from the TECH page.

In the vast majority of normal cases, if an engine is not broken in by about 5 hours, something is wrong and needs addressing!

If you have any questions, holler.

 

[Mathew Sharp]

Our staff engineer and lead designer put together a nice read regarding engine operation and leaning theory. I can mail copies if anyone would like one in print but here is the .pdf. Pilots Manual to Leaning and Diagnosing Engine Problems.

MTS

 

[Walter Atkinson]

Our staff engineer and lead designer put together a nice read regarding engine operation and leaning theory. I can mail copies if anyone would like one in print but here is the .pdf. Pilots Manual to Leaning and Diagnosing Engine Problems.

MTS

EI makes a very good engine monitor.

I am disapppointed that since 2004, the errors of fact have continued to be present in the Leaning Manual EI circulates. 'Tiz a puzzlement as to why that would not get corrected. There is some good information in the manual, but please be cautious as to which materials you accept as in harmony with the known data. Some of it is unsupported by the hard data.

 

[Jekyll]

EI makes a very good engine monitor.

I am disapppointed that since 2004, the errors of fact have continued to be present in the Leaning Manual EI circulates. 'Tiz a puzzlement as to why that would not get corrected. There is some good information in the manual, but please be cautious as to which materials you accept as in harmony with the known data. Some of it is unsupported by the hard data.

Please, do tell! Not many of us have the expertise you have and thus, might not be up to separating the wheat from the chaff.

Jekyll

 

[Walter Atkinson]

Please, do tell! Not many of us have the expertise you have and thus, might not be up to separating the wheat from the chaff.

Jekyll

I have no deisre to air EI's dirty laundry. We've been through this several times before to no avail. Two simple examples:

1) Pre-ignition: EI says EGT goes up during pre-ignition. Dozens of data files show clearly that EGT goes down. It HAS to. Pre-ignition is like advancing the timing... where EGT goes down! Unless, of course EI has found a new set of Laws of Physics.

2) There is no known science which supports the notion that adding an EGT raw value and a CHT raw value results in any worthwhile number that means anything.

It's just weird.

Let me say that EI is not alone in printing a manual with significant errors. JPI and Insight have errors in their manuals as well.

As the King of Siam said, " 'Tiz a puzzlement. "

 

[Mathew Sharp]

I consulted with Ron in engineering and here is his response:

-------------------
Walter says, "There is no known science which supports the notion that adding an EGT raw value and a CHT raw value results in any worthwhile number that means anything."
-------------------

Bottom line: The CHT and EGT both have an affect on the temperature of the exhaust valve. All science and studies I know of support this assertion.

This is to set forth the reasoning and logic I used in the concept that the sum of the EGT and CHT can be used as an operating limit for an aircraft engine (list in "The Pilot's Manual for Leaning and Diagnosing Engine Problems" found at buy-ei.com).

Most pilots understand that the maximum CHT listed in their POH is not a temperature at which to operate an aircraft engine continuously in cruise flight. Some say you should run your engine with the CHTs below 400'F. Some say running at peak EGT (while making no reference to the CHT) could burn an exhaust valve. Viewing EGTs and CHTs separately and trying to keep them both at reasonable temperatures can lead a pilot to operating an engine significantly rich of peak EGT, which has its own set of problems.

Much of the information I read as a pilot addresses EGTs and CHTs separately, without discussing their combined impact on the engine. I believe by viewing the EGTs and CHTs together the pilot will gain a much more complete picture of how these temperatures affect their engine. The following covers some of my research on the subject:

1) There seems to be little argument that the intake and exhaust valves are heated by the temperature of combustion. The exhaust valve additionally is heated by the high velocity of exhaust gasses that flow over the valve during the exhaust stroke. This results in the exhaust valve running at a much higher temperature than the intake valve accounting for the lower reliability of the exhaust valve in comparison to the intake valve. Richard Stone (Professor at University of Oxford), in his book "Introduction to Internal Combustion Engines," calculates the energy in the expelled exhaust gases to be higher than the brake horsepower produced by the engine. All of this energy passes over the exhaust valve during the exhaust stroke. Charles Taylor (Professor at MIT), in his books "The Internal Combustion Engine in Theory and Practice Volumes I and II," clearly shows that the flow and temperature of the exhaust gasses has a marked affect on the exhaust valve temperature. He goes on to state, ".... even a small reduction in valve temperature, as from 1400'F to 1350'F, results in a marked improvement in valve life and reliability." Also, Taylor's book (Volume I, figure 8-5) shows a direct and linear affect of the EGT on the temperature of the exhaust valve.


2) As Taylor states, 75% of exhaust valve cooling is through the valve seat and the other 25% is through the valve stem. SAE Technical Paper 920063 (Correlation of Exhaust Valve Temperatures with Engine Reynolds Number in a 1.9L Engine) shows a direct and linear correlation between the cylinder temperature and the exhaust valve temperature. Also, this study showed the exhaust valve operating at approximate 300'F below the EGT. If you search the Internet using the term "exhaust valves and cht" you will find a number of papers covering the detrimental affects of high CHTs on exhaust valve life.

3) There is little question from an intuitive or scientific standpoint that an increase in the EGT and/or CHT will increase the temperature of the exhaust valve. Also, it follows that one could increase one parameter (EGT or CHT) and offset the affects on the exhaust valve by decreasing the other parameter (CHT or EGT).

The hard question for me was: What combination of EGTs and CHTs will produce a safe operating range for most engines? This is a subjective call, but one that can be driven from research and experience. The combination of EGT+CHT at 1850'F comes from a limit on the EGT of 1450'F while running 400'F on the CHT or 1500'F EGT while running at 350'F on the CHT. These are maximum numbers I recommend for continuous operation at cruise power, based on my research, 28 years of experience designing aircraft engine instruments, dyno work and customer feedback. Some engines may be able to operate safely with temperatures above the ones I've set forth; let your own research and experience guide you.

 

[Mathew Sharp]

Here is the rest of Ron's response:

----------------
Walter says, "Pre-ignition: EI says EGT goes up during pre-ignition. Dozens of data files show clearly that EGT goes down. It HAS to. Pre-ignition is like advancing the timing... where EGT goes down! Unless, of course EI has found a new set of Laws of Physics."
----------------

Bottom line: I agree that EGT will go down as ignition timing is advanced, but only up to a point. Beyond that point, advancing the ignition timing will lead to a sharp increase in EGT and possible engine damage.

First I'll describe what I've seen happen when pre-ignition occurs and then explain the physics behind it. These observations were made when I was working at a company called Dynotronix, a company I started in 1976. We were associated with a company that dyno'd big block Chevy race engines. The observations I made years ago have been confirmed by customer feedback on aircraft engines in years since.

1. This is where Walter and I agree. As the ignition timing is advanced 10 to 20 degrees (or when pre-ignition occurs) beyond its normal setting, the EGT will drop slightly and possibly fluctuate. This drop is difficult to detect unless you are watching the EGT instrument closely.

Physics - The first law of thermodynamics states: The total energy (heat and work) in a system remains constant. Applied to an aircraft engine, the heat energy (temperature) produced by combustion is equal to the heat used to produce work (horsepower ~ 26% of the energy) plus the heat absorbed by the cylinders (CHT's ~ 37% of the energy) plus the heat exhausted (EGT's ~ 37% of the energy). These numbers are from Richard Stone's book mentioned earlier; see chapter 12.

At a normal ignition timing (assuming MBT) the torque and horsepower are at their maximum value. That is to say, a change in the ignition time from normal (either retarded or advanced) will result in a reduction in power. As the ignition timing is advanced (or pre-ignition occurs) combustion temperatures and pressures start to increase. Also, cylinder heads are exposed to the combustion temperatures for a slightly longer period of time causing the CHT to increase. The additional energy in the CHT offsets the small reduction in power and produces a drop in the EGT.

Note: I'm assuming normal timing is set to MBT (Minimum advance for Best Torque) and the RPM is not changing. In some engines the timing must be set before MBT in order to deter denotation. For these engines there will be considerably more drop in the EGTs as the ignition timing is advanced or pre-ignition occurs.

2. This is where Walter and I disagree. As the ignition is further advanced (or damaging pre-ignition occurs) pressures and temperatures in the cylinders produced by combustion will rise significantly and approach a point where damage can occur.

Physics - At the same time this advanced pre-ignition occurs the power output (work) of the engine is reduced. Therefore, all of the increase in combustion energy (1000'F or more above normal) and the energy from the reduction of engine power must be accounted for. This energy will manifest itself in a significant rise in the CHT and EGT. This situation can occur very quickly and the damage to the engine can require a forced landing with a seized engine. During this damaging pre-ignition phase of operation I know of no theory or research that supports the idea that all of the increase in combustion energy (over 1000'F) is absorbed by the CHT and the energy in the EGT is reduced.

Taylor, Stone, Heywood and Lumley all have excellent books that cover a great deal of this material, but be prepared for some heavy reading. These are engineering books that cover the theory of internal-combustion engines, targeting advanced college courses.

 

[Walter Atkinson]

I have one question, "If he is correct about EGT having an effect on exhaust valve temperature, why does the exhaust valve get COOLER as the EGT rises across some mixture changes?" There is one range where EGT goes up 25 degrees and CHT goes down 1 or 2 degrees and the exhaust valve cools significantly. So far, he has been unable to explain how that absolutely repeatable fact in the data fits in with his theory.

EGT is no measure of the heat of combustion. That notion is clearly in error. EGT can be quite low with a very high heat of combustion. THINK DIESELS. EGT is NOT a measure of the heat of combustion. The main factor in CHT, internal cylinder pressure, he does not even address. Curious.

I went through all of this with EI as did several others who are combustion savvy engineers about three or four years ago. One gentleman spent a LOT of his time and energy trying to point out the errors and misquotes in the EI manual and even re-wrote significant parts at significant time investment for no charge. There is no shortage of examples. EI is alone in their interpretation of these issues. Life is too short to revist this with them. They are not interested.

His position simply doesn't agree with the ACTUAL measured data.

I REFUSE to argue these many points with EI again as they are convinced they understand this stuff in a way that not one, single other expert does. They mis-apply quotes from Taylor and Heywood and that has been pointed out to them by at least two very sharp experts other than myself.

They can have their point of view.

It's up to each to decide who to believe. Don't believe me, believe the data. The data from all sources is in agreement and is clear that exhaust valve temperature is not related to EGT in any meaningful way. And, that EGT does down during pre-ignition.

I plan no further public discussion on the matter.

 

[Kahuna]

Imho, you should first determine what the "gami spread" is for your engine (delta in fuel flow at peak EGT for all 4 cyls). Record the data if you can. If that looks good, i.e. the spread is down to a few tenths of gph or less, then imho you can have at it with LOP. If you have a wide gami spread, then you may want to invest in some custom restrictors for your injectors to achieve a lower (ideally zero) spread.

My 2 cents and experience on flowing the nozzles are here

Best,

 

[rv6ejguy]

How do you suppose the exhaust valve gets hot? Come on, EGT/ CHT has no effect on exhaust valve temperature? This not only illogical, it is nonsense. The exhaust valve is immersed in flowing exhaust gases and the face is exposed to combustion temperatures. Typical operating temps for the exhaust valve face in air cooled engines using sodium cooling is 1100-1300F, intakes 700 to 800F. (Liston) Quite obviously exhaust flow is heating the exhaust valve.

Most of the heat is conducted away from valves through the seats which run at 500-600F which in turn sink to the head castings running at 400+ degrees.

I can tell you from 30 years of building and dynoing turbocharged engines that higher EGTs exact a toll in exhaust valve life and turbine life.

Just what sort of instrumentation is being used to collect temperature data on valves here?

On my dyno, overly advanced timing does increase EGTs just as overly retarded timing also does. No secret there if you look at the formulas for calculating thermal efficiency and look at brake hp vs. timing.

There is a direct correlation between high CHTs combined with high EGTs and exhaust valve temperature and life.

 

[airguy]

I have one question, "If he is correct about EGT having an effect on exhaust valve temperature, why does the exhaust valve get COOLER as the EGT rises across some mixture changes?" There is one range where EGT goes up 25 degrees and CHT goes down 1 or 2 degrees and the exhaust valve cools significantly. So far, he has been unable to explain how that absolutely repeatable fact in the data fits in with his theory.

I'll chime in here on this one - the rest I don't know enough about to speak to. The total heat input from the combustion event arises from the combustion of the hydrogen and carbon in the hydrocarbon fuel chain with the oxygen available in the cylinder. Due to the nature of the physical reactions between the individual atoms of the molecules of fuel and the radicals formed during the intermediate stages of the reacton, it is true and has been demonstrated that the hydrogen is preferentially pulled off the hydrocarbon molecular chain and combusted first - leaving raw carbon and fractured carbon-carbon chains left to combust after the majority of the hydrogen has combusted. The initial hydrogen-oxygen combustion is very rapid and the carbon-oxygen combustion event is much slower and occurs later in the process. All of the combustion adds heat to the EGT.

If the exhaust valves open at the end of the hydrogen event but before the carbon event has reached maturity, then the carbon is still burning as the exhaust gas leaves the cylinder - resulting in peak EGT OUTSIDE the cylinder, somewhere in the manifold or exhaust system. If the spark is very advanced, the carbon event is in full swing or even finished by the time the exhaust valve opens and substantially ALL the heat energy is present in the cylinder, resulting in much increased CHT's and surprisingly, lower EGT's - since more of the total heat energy has been transferred to the cylinder and piston and lost from the exhaust gas by the time that gas reaches the EGT probe.

The reason "best power" mixtures are rich is due to the rapid hydrogen event, that power can very reliably be harvested while throwing away the excess carbon that cannot be burned rapidly. You supply enough fuel to completely consume all the oxygen during the hydrogen combustion event, and toss the carbon overboard. The carbon simply takes too much time to contribute to a combustion event where maximum power is needed. This is why some people talk about using "fuel to cool the engine" - it's not by evaporation or anything like that - it's that you're only burning the hydrogen and tossing the carbon, developing the same amount of power with more fuel (less efficiently) and letting the carbon be wasted. The heat that would have been generated by that wasted carbon is gone, resulting in less heat per gallon of fuel - hence a "cooler" engine.

 

[dan]

My 2 cents and experience on flowing the nozzles are here

If you're flying behind a fuel injected aircraft engine, you owe it to yourself to read Kahuna's fine web page on LOP tuning. It's a great illustration of how to interpret the data -- and what you're trying to achieve.

Even if you think you'll never run LOP (talk to me in a few years when avgas is pushing 10 bucks a gallon!) with your injected engine, give the test a shot just so you know how well balanced your setup is.

 

[gmcjetpilot]

Yea

Fight!

Settle it like men

Beat it to death

more

keep temps with in recommended limitations. Live long and prosper.

 

[rv6ejguy]

Some of the comments here seem like they come as a veiled war between engine monitor brands and their advocates. The science behind all this stuff was well established in 1945 and has changed little since then. The facts have certainly not changed in that time.

As a point of interest, cylinder pressure has fallen below the compression line by about 100 degrees ATDC on most engines. By the time the exhaust valve opens on most engines with conservative cam timing, the combustion process is long over even if ignition timing was severely retarded.

Many studies have shown that EGT typically falls 60-70C when heavy detonation occurs. I see the terms pre-ignition and detonation used interchangeably often. This is incorrect. Directly and instantaneously, spark timing has nothing to do with pre-ignition. As the name implies, pre meaning BEFORE spark initiation.

 

[Walter Atkinson]

Quite obviously exhaust flow is heating the exhaust valve.

Then please explain why and how the exhaust valve is getting cooler as the EGT is getting higher when the mixture is leaned from 25dF ROP to Peak EGT?

As the EGT is going UP the exhaust valve is getting COOLER. You must reconcile this FACT with your *assumptions* before we can go further in this discussion.

Ref: 1943 NACA study of exhaust valve temperature, 1966 Lycoming data, 2003 Carl Goulet test facilty data--ALL of which confirms the above statement of facts.

 

[zav6a]

Heat transfer

I think both Ross and Walter are correct. Clearly, EGT is grossly related to valve life. If you do not start the engine, EGT and the valve stays quite cool and it lasts forever. Run it at half throttle, EGT is greater, but the valve continues to stay relatively cool and again lasts a long time. Run it at 100 percent or beyond as is done when turbocharging, EGTs are yet higher and there will be greater heat transfer to the valve (and the rest of the cyl for that matter) and it will last less long. My point is that heat load on the combustion chamber walls, valve included, is grossly related to EGT and the amount of fuel burned in the chamber (Ross is right).

If you are talking about a relatively narrow range of operation near 75 percent, then the biggest variable affecting heat flow through the valve (averaged over all four cycles, not just the exhaust cycle) is mixture.

Mixture dramatically affects heat transfer to the combustion chamber walls during the actual combustion event (that is, 25 BTDC to 100 ATDC during which the valve is closed and there is technically NO exhaust). Mixtures slightly rich of peak generate higher combustion pressures and temperatures and the heat is more efficiently transferred to the combustion chamber walls, valve included. Mixtures lean of peak have lower combustion pressures and temperatures and the heat transfer to the combustion chamber walls is less. It also appears that boundary layer effects between the burning fuel and the combustion chamber walls gas operate differently in ROP, peak, and LOP scenerios, driving yet greater contrast in heat transfer than temp and pressure alone would explain.

Even though the EGT is higher in a LOP situation, the valve is cooler (averaged over the four cycles) because it picked up less heat during the actual combustion event (Walter is right).

Not sure who said it, but it is easy to incorrectly equate EGT with combustion temps or for that matter and as this thread indicates, total heat transfer to the valve and head.

I have not reviewed all of Walter's references, but I assume that CHT and exhaust valve temperature are directly proportional. As CHT goes down as mixtures are leaned LOP, it is then inevitable that heat load on the valve would decrease as well.

If all of this is true, I can see where there might be an argument that the fine edge of the seating face of the valve, just those outer few layers of molecules that could react so quickly to gas flowing past during the exhaust cycle, might experience a slightly higher peak temp (in the exhaust cycle only) in a LOP scenerio. Average temps and meatier parts of the valve and the stem would remain more closely correlated to full four cycle temps and be cooler.

Collins wrote a great article in Flying on the Turbo'd Cirrus. He concluded with a statement that there will soon be a huge body of data regarding longevity and LOP operatons. This debate will soon be well indisputably settled.

 

[rv6ejguy]

Even though the EGT is higher in a LOP situation,

Collins wrote a great article in Flying on the Turbo'd Cirrus. He concluded with a statement that there will soon be a huge body of data regarding longevity and LOP operatons. This debate will soon be well indisputably settled.

EGT is lower LOP.

LOP certainly works on engines with good mixture distribution and millions of hours on radial engines cruising LOP in the '40s and 50's supports the longevity issue. LOP has been approved on the Conti 550s for some time as well.

I ask again, what sort of instrumentation is being used to measure exhaust valve temps?

Finally, a few degrees drop in valve temps is relatively meaningless and could be explained by lower combustion temps, lower CHTs and a change in hydrogen to carbon ratios in the exhaust stream or dynamic gas flow effects on thermocouples. Oxygen is climbing LOP while hydrogen is falling.

Today, we do have wideband sensors to correlate EGT vs. AFR were was not available 50 years ago. If you have experience with these, you will learn a lot. Also, if you are versed in port flow dynamics and their effects on thermocouple probes, other possibilities become evident to explain temperature readings which may not actually be accurate.

I stand by my statement that exhaust valve temperature is primarily a function of EGT and CHT.

 

[zav6a]

LOP Valve Temps

Ross

Understood. My comment on LOP EGT being "higher" was in the context of concern of LOP EGT being "high" from a valve life perspective and only relative to the 100 + degree ROP mixtures on the other side of peak that are suggested as "safer".

Can't imagine a direct measurment method for valve head temps either. Hostile environment. I suspect some sort of indirect measurement or observation.

There is probably a finite element model out there that would resolve the ultimate question of what is the most significant driver of exhaust valve temp - cumbustion in the chamber or the exhausted gases going past the valve. I can't intelligently debate the matter with the knowledge I have!

 

[rv6ejguy]

Temperature measurement on static structures like heads is relatively simple, measurement on a functional, moving exhaust valve are somewhat more difficult. I submit that 2-3 degrees variation on a part operating at 1100F is outside experimental error and therefore insignificant.

Additionally, any actual changes in valve temp with higher EGTs must be due to other factors, perhaps not understood.

 

[osxuser]

In theory, Exhaust VALVE temps should go down the leaner you get, since the fuel burns faster as you lean, and therefore the fuel is burned more completely by the time it reaches the valve in the exhaust stroke. At least that is the way I understand it.

 

[Walter Atkinson]

Temperature measurement on static structures like heads is relatively simple, measurement on a functional, moving exhaust valve are somewhat more difficult. I submit that 2-3 degrees variation on a part operating at 1100F is outside experimental error and therefore insignificant.

Additionally, any actual changes in valve temp with higher EGTs must be due to other factors, perhaps not understood.

Ross:

The valve temps have been measured with direct-applied thermocouples since 1943. It's challenging, but not all that hard to do. It was much more challenging when we measured the spark plug ceramic temps about three years ago... THAT was challenging. It's not as challenging as measuring crankshaft torsional values on a running crank.

Again, I suggest that if you LOOK at the data already collected you will begin to question your assumption that EGT is related to valve temperature in any significant way. Here's a bit of data to consider. If EGT is the issue that drives valve temps, why are diesel EGTs so low and valve temps so high? High EGTs making a valve hotter is *logical*, but it's wrong.

 

[Walter Atkinson]

If I set the mixture at 1500dF ROP and 1500dF LOP, both 80dF below peak, the LOP one will have exhaust valve temperatures about 50 degrees COOLER than the one set ROP. SAME EGT, very different valve temp.

If I set the mixture at 25dF ROP and again at PEAK EGT, the one with the hotter EGT will be running the exhaust valve cooler.

Anyone wishing to assign EGT as a driving force in exhaust valve temperature needs to be able to reconcile these FACTS that are measured, not estimated.

These are hard data points, not my opinion.

 

[Captain Avgas]

In theory, Exhaust VALVE temps should go down the leaner you get, since the fuel burns faster as you lean, and therefore the fuel is burned more completely by the time it reaches the valve in the exhaust stroke. At least that is the way I understand it.

I think you'll find that the flame front is at its highest velocity at about the mix that produces max power (approx 100degrees ROP on a Lyc). Any leaning from there (to peak and beyond to LOP) will produce a progressively slower flame front. Peak EGT provides the most complete combustion...but not the fastest flame front.

 

[Walter Atkinson]

I think you'll find that the flame front is at its highest velocity at about the mix that produces max power (approx 100degrees ROP on a Lyc). Any leaning from there (to peak and beyond to LOP) will produce a progressively slower flame front. Peak EGT provides the most complete combustion...but not the fastest flame front.

Some landmarks:

Peak EGT = even mix of air and fuel. 14.9:1 by weight. By definition, enough of each to fully combust with nothing left over. In actuality, a little of each remains unburned since some molecules never find each other in the mix.

Peak CHT = approximately 40dF ROP. This is the point of maximum flame front speed, highest internal cylinder pressures and highest CHT.

Peak Power = the mixture of approximately 75-80dF ROP. This is the point where all of the fuel does, in fact, find O2 and is completely burned.

Peak Efficiency = BSFC(min) or the mixture whereby the most Hp/pound of fuel is realized. Also known as Best economy. Between about 20dF and about 90dF LOP, depending on the power setting.

These numbers are the same for all spark fired, internal combustion, piston, gasoline engines. These numbers do not change for any particular engine manufacturer, Lyc or TCM.

 

[rv6ejguy]

Anyone wishing to assign EGT as a driving force in exhaust valve temperature needs to be able to reconcile these FACTS that are measured, not estimated.

These are hard data points, not my opinion.

Sorry to disagree, but this flies in the face of thermodynamics and heat transfer theory Walter. EGT and CHT ARE the PRIMARY forces at work concerning exhaust valve temperatures. What you describe here are secondary.

 

[Walter Atkinson]

Sorry to disagree, but this flies in the face of thermodynamics and heat transfer theory Walter. EGT and CHT ARE the PRIMARY forces at work concerning exhaust valve temperatures. What you describe here are secondary.

Ok, then explain how the exhaust valve gets cooler as the EGT goes up from 25dF ROP to peak EGT. Until you explain this fact, your position is unsupportable. I suggest you look at the 1943 NACA study on this, along with Lycomings 1966 data and the 2003 data from the Carl Goulet Memorial Engine Test Facility. All three agree that the exhaust valve cools as the EGT goes up from 25dF ROP to peak EGT.

Had you said, "Temperature of combustion and CHT ARE the PRIMARY forces at work concerning exhaust valve temperatures," I would be in complete agreement with you. I would add that there are actually three factors. Internal cylinder pressure should be added to your two. Temp and pressure *push* the BTUs across the thermal boundary layer and CHT *pulls* the heat away from the valve. (yeah, I know push and pull are not exactly correct, but they explain it pretty well. <g>)

Where this breaks down for you is that EGT is NOT a measure of the heat of combustion. This is a very commonly held misunderstanding. There are mixture changes that result in lower EGTs and a higher heat of combustion and therefore higher valve temps.

I see the light will come on for those who have trouble with this when they finally realize that EGT is NOT a measure of the heat of combustion.

Walter

 

[Kevin Horton]

If I set the mixture at 1500dF ROP and 1500dF LOP, both 80dF below peak, the LOP one will have exhaust valve temperatures about 50 degrees COOLER than the one set ROP. SAME EGT, very different valve temp.

If I set the mixture at 25dF ROP and again at PEAK EGT, the one with the hotter EGT will be running the exhaust valve cooler.

Anyone wishing to assign EGT as a driving force in exhaust valve temperature needs to be able to reconcile these FACTS that are measured, not estimated.

These are hard data points, not my opinion.

Very interesting. I wonder if the explanation has to do with mass flow. If the EGT remains the same, the heat transfer to the exhaust valve will vary with mass flow of the exhaust gases. Higher mass flow leads to higher heat transfer leads to higher exhaust valve temperature.

The two cases cited above have the same EGT, and presumably the same air mass flow, as that is driven by the rpm. But the rich case has higher fuel flow, so the total mass flow will be higher (total mass flow = air mass flow + fuel mass flow, as mass is neither created nor destroyed in the combustion process). Maybe the mass flow in the rich case is enough higher than it is in the lean case to explain the higher exhaust value temperature.

 

[rv6ejguy]

Ok, then explain how the exhaust valve gets cooler as the EGT goes up from 25dF ROP to peak EGT. Until you explain this fact, your position is unsupportable. I suggest you look at the 1943 NACA study on this, along with Lycomings 1966 data and the 2003 data from the Carl Goulet Memorial Engine Test Facility. All three agree that the exhaust valve cools as the EGT goes up from 25dF ROP to peak EGT.

Had you said, "Temperature of combustion and CHT ARE the PRIMARY forces at work concerning exhaust valve temperatures," I would be in complete agreement with you. I would add that there are actually three factors. Internal cylinder pressure should be added to your two. Temp and pressure *push* the BTUs across the thermal boundary layer and CHT *pulls* the heat away from the valve. (yeah, I know push and pull are not exactly correct, but they explain it pretty well. <g>)

Where this breaks down for you is that EGT is NOT a measure of the heat of combustion. This is a very commonly held misunderstanding. There are mixture changes that result in lower EGTs and a higher heat of combustion and therefore higher valve temps.

I see the light will come on for those who have trouble with this when they finally realize that EGT is NOT a measure of the heat of combustion.

Walter

Walter, you are not reading what I'm writing here. I never stated that EGT was an indication of combustion temperatures. I stated that EGT (heating) and CHT (quenching or cooling against the seat and guide) are the primary factors in exhaust valve temperature. They are not the only factors since the valve head is exposed to combustion heat sitting on its seat. If combustion temps are lower, less heat will be transferred into the valve head. There are many other minor factors which could affect valve temperatures. Since intake valve temps are several hundred degrees lower, it is obvious that EGT has a higher impact on valve temps than combustion temps when both valves are closed. If combustion temps were the primary factor in valve temps as you say, intakes and exhausts would run at the same temps. I'm sure your data does not show this to be the case.

To say that EGT does not have a major effect on valve temp is simply incorrect seeing as it is immersed in high mass flow 1600F exhaust gases when open. You are drawing erroneous conclusions from your data.

Pressure has no effect on temperature at a fixed volume. Theoretically the EGT changes as it flow through the port due to cross sectional variations at the seat, guide and boss, and cooling of the head casting. This is obvious if you have taken local velocity measurements on a flow bench at different points. So what? Of interest to me as an engine builder to improve port flow and hp, not of much use to most other people.

As experimenters or scientists, we often draw incorrect conclusions from the data presented just as we sometimes get questionable data from incorrect methodology. I've seen my share of both over the years.

Not to to belabor the point. The original post asked for information on LOP operation which was forthcoming from others here. On this, we both agree, with proper instrumentation and understanding of the concepts, combined with an engine which has good mixture distribution, LOP operation is a viable way to save fuel and engine wear. The finer points of exhaust valve temperature is essentially meaningless to most pilots. I won't waste any more space on this forum discussing this.

 

[Walter Atkinson]

Very interesting. I wonder if the explanation has to do with mass flow. If the EGT remains the same, the heat transfer to the exhaust valve will vary with mass flow of the exhaust gases. Higher mass flow leads to higher heat transfer leads to higher exhaust valve temperature.

The two cases cited above have the same EGT, and presumably the same air mass flow, as that is driven by the rpm. But the rich case has higher fuel flow, so the total mass flow will be higher (total mass flow = air mass flow + fuel mass flow, as mass is neither created nor destroyed in the combustion process). Maybe the mass flow in the rich case is enough higher than it is in the lean case to explain the higher exhaust value temperature.

Kevin:

No. The mass airflow is the same in both cases--same MP and RPM. Higher heat transfer does not lead to higher EGT. We need to get away from the notion that EGT is having any effect on CHT or Valve temp or temperature of combustion. It doesn't necessarily.

EGT is nothing more than the heat left after the expansion of the 3800 degree combustion gases. PV=nRT.

Walter

 

[Kevin Horton]

Kevin:

No. The mass airflow is the same in both cases--same MP and RPM. Higher heat transfer does not lead to higher EGT. We need to get away from the notion that EGT is having any effect on CHT or Valve temp or temperature of combustion. It doesn't necessarily.

EGT is nothing more than the heat left after the expansion of the 3800 degree combustion gases. PV=nRT.

Walter

I agree that the mass airflow is the same in both cases. But, the fuel mass flow is higher in the rich case, so the total mass flow (i.e. the total of air mass flow + fuel mass flow) will be a bit higher. How much difference would there be in fuel flow between the two cases you quoted (80dF ROP, and 80dF LOP)?

I'm not suggesting that valve temperatures depend on EGT alone. Clearly the temperatures of the valve seat and valve guide would also be major factors. These two items would have some relationship to CHT. What would the CHTs be in the two cases?

 

[Walter Atkinson]

1) I stated that EGT (heating) and CHT (quenching or cooling against the seat and guide) are the primary factors in exhaust valve temperature.

2) Since intake valve temps are several hundred degrees lower, it is obvious that EGT has a higher impact on valve temps than combustion temps when both valves are closed. If combustion temps were the primary factor in valve temps as you say, intakes and exhausts would run at the same temps.

3) To say that EGT does not have a major effect on valve temp is simply incorrect seeing as it is immersed in high mass flow 1600F exhaust gases when open.

4) Pressure has no effect on temperature at a fixed volume.

1) You still have not explained how the exhaust valve gets cooler as EGT is going up. Until that fact is addressed, any theory that says that EGT heats the exhaust is not viable. The EGT is NOT what's heating the valve in the above case, it's actually COOLING it. If the exhaust valve is cooling as the hot gas is getting hotter and flowing past it, it must be cooling it! <VBG> That problematic fact must be reconciled.

2) The intake and exhaust valves are both exposed to the same 3800 degree heat of combustion. The intake valve is exposed to a HUGE flow of cold air as it is open. The metal around the intake valve seat is cooler than that around the exhaust valve. No wonder it runs cooler. The metal around the exhaust seat is hotter. No wonder the exhaust valve is not cooled as well--it's sitting on a hotter seat.

3) OK, as the gas is getting hotter, why is the valve getting cooler as the mixture is moved from 25dF ROP toward peak EGT? If you're right it oughta be getting hotter, but it's getting cooler. Please explain that.

4) Internal cylinder pressure does have an effect on the BTUs transfered into the cylinder, head, valves, and piston. High pressure means more heat transfer across the thermnal boundary layer. THIS effects valve temperature (along with CHT) more than anything else.

Think about this:

Diesels run very high head and valve temps and must shed much more heat than gasoline engines but have very, very low EGTs. Why the higher valve temps with lower EGTs?

The answer is in Boyle's Law and the expansion of the combusting gases. Based on one mixture, EGTs are what they are based on the compression ratio and the timing. The earlier the thetaPP, the lower the EGT, but the higher the exhaust valve temperature!

The heat of the exhaust valve is related to the temperature of combustion, the internal cylinder pressure and the cooling effect of the valve seat/head contact. Yes, it cools less with hot gas going by it than the intake valve that has cold gas going by it.

Many are surprised to find that the exhaust valve is often hotter than the exhaust gas and is actually being cooled by the exhaust gas going by it!

Walter

 

[Walter Atkinson]

I agree that the mass airflow is the same in both cases. But, the fuel mass flow is higher in the rich case, so the total mass flow (i.e. the total of air mass flow + fuel mass flow) will be a bit higher. How much difference would there be in fuel flow between the two cases you quoted (80dF ROP, and 80dF LOP)?

I'm not suggesting that valve temperatures depend on EGT alone. Clearly the temperatures of the valve seat and valve guide would also be major factors. These two items would have some relationship to CHT. What would the CHTs be in the two cases?

Kevin:

Oh, I see what you mean about the total mass of the fuel and air. Inconsequentially different. Not even a blip in the noise. The heat of vaporization of any extra fuel is completely lost in the noise.

The CHTs are different. The LOP CHT is cooler.

Here is a fact I just looked up from the data.

Two different mixtures with the same mass airflows and the same exhaust valve temperature. Consider peak at 1550dF.
An EGT of 1425dF (125df ROP) and an EGT of 1525dF (25dF LOP) result in the exact same exhaust valve temperature.

How can that be if EGT is the driving force in exhaust valve temperature?

 

[rv6ejguy]

1) You still have not explained how the exhaust valve gets cooler as EGT is going up. Until that fact is addressed, any theory that says that EGT heats the exhaust is not viable. The EGT is NOT what's heating the valve in the above case, it's actually COOLING it. If the exhaust valve is cooling as the hot gas is getting hotter and flowing past it, it must be cooling it! <VBG> That problematic fact must be reconciled.

2) The intake and exhaust valves are both exposed to the same 3800 degree heat of combustion. The intake valve is exposed to a HUGE flow of cold air as it is open. The metal around the intake valve seat is cooler than that around the exhaust valve. No wonder it runs cooler. The metal around the exhaust seat is hotter. No wonder the exhaust valve is not cooled as well--it's sitting on a hotter seat.

3) OK, as the gas is getting hotter, why is the valve getting cooler as the mixture is moved from 25dF ROP toward peak EGT? If you're right it oughta be getting hotter, but it's getting cooler. Please explain that.

4) Internal cylinder pressure does have an effect on the BTUs transfered into the cylinder, head, valves, and piston. High pressure means more heat transfer across the thermnal boundary layer. THIS effects valve temperature (along with CHT) more than anything else.

Think about this:

Diesels run very high head and valve temps and must shed much more heat than gasoline engines but have very, very low EGTs. Why the higher valve temps with lower EGTs?




Walter

I won't address the valve thing here any more but Diesels DON'T shed more heat than a gasoline engine! Diesels have higher thermal efficiency than gasoline engines therefore shed less heat because more energy is extracted from the fuel and goes to the crankshaft. Look at radiator volumes and areas on Diesels, look at Diesel aircraft rads, look at warm up times for Diesel cars in cold weather, lack of heater output. Finally valve and seat materials are virtually identical to gasoline engines and valve life is exceptional on good Diesel engines.

EGTs have a huge impact on CHTs or my 30 years of cylinder head development and engine development and dyno testing were all in error. A mere porting change has dropped CHTs as much as 60F at WOT even while increasing hp. Any time you have 1600F gas in contact with an exhaust port, you have heat transfer. Restrictive porting is unquestionably a factor in high CHTs on many engines. Work on Corvair engine over 25 years ago proved this.