extralite:

***
Perhaps the reluctance to run lean is from what we have all learned from early on about engine. Running lean can cause knocking..pre-detonation and damage to valves.

How are leaded aero engines immune to this?
***

What we ACTUALLY learned (or should have learned) is that running "not rich enough" on the rich side was the cause of the above issues. Running not rich enough does result in high CHTs and high ICPs. It does NOT cause knocking, pre-ignition or damage to the vales, per se, but it does put the engine at a higher risk for those things.

Aero engines are not immune to any of the problems of gasoline, piston engines. It's just that while the observations may be accurate, the assignment of causality is flawed.

FFS. It's because no one's invented a perpetual motion machine yet. :ugh:

Ya can't save increasingly massive amounts of fuel and go increasingly faster. Get it?

It's easy to know your power output when LOP. If you have an 8.5:1 compression ratio, simply multiply 14.9 x FF in gph = HP output. Other CRs have different multipliers. For example, the TC engines with 13.75:1 CRs, have a multiplier of 13.75.

ROP power output is a little more complicated to calculate and the POH numbers are usually generalizations, not actual numbers.

I can promise you that the laws of physics will insist that 70% power ROP and 70% power LOP WILL produce the same TAS. BTDT thousands of times.

As for taking longer when slower, well, sure, but the 25% fuel savings offset the 3% loss in TAS quite nicely. If you need to get there 3% faster, you'd better not accept single vector from ATC. :=

CTRE

What aircraft do you fly and how do you fly it now? What speeds do you get? With this we can talk in real terms for your application.

Happy to do the mental maths to help you out. What I suspect is that if you lose way too much speed you are simply flying too far LOP. This is not uncommon. I can help you with this if you would like to detail what you have and how you are doing it.

This could make for an educational example for others watching.

:ok:

"160 LOP" cleared? 160?

As others have said, 70% power ROP is 70% power LOP. But here's the constraint if you're running a normally aspirated engine: You can't "add" manifold pressure if you're already at wide open throttle. (That's why the turbo-normalised engines are so popular.)

For my part, I'm happy to save about 15 litres an hour in return for about 6 knots TAS.

I fly lots of long legs. To make the maths simple, let's assume it takes me 15 minutes to climb to 8,500', after which I've 450 nms to go. Again to make the maths simple, let's assume it's nil wind and the GS will remain the same in the descent.

I'm going to use the same engine management technique to get to 8,500', irrespective of whether I'm going to choose subsequently to cruise LOP, ROP or at peak. So we can ignore the climb consumption and GS.

450 nms at 160 = 2.8 hours X 60 litres per hour = 168 litres

450 nms at 150 = 3.0 hours X 45 = 135 litres.

I'm happy to spend 12 minutes to save 33 litres. (In reality, flight after flight, the TAS sacrifice isn't as high as 10 knots and the fuel flow difference is greater than 15 litres per hour.)

Obviously there's not much of a difference if all you do is flog around in the circuit or go for 2 hour jollies for the hamburger on a Sunday.

But there's a key point that must be understood here, because it points up why some people get wider TAS differences and narrower fuel flow changes. When I run ROP, I run sufficiently ROP. That means I'm way cooler than 50 degrees F ROP if I'm running ROP.

If, instead, I were in the habit of setting my mixture to 50 degrees F ROP, I would save only about 10 litres per hour and lose about 15 knots' TAS running LOP (I'm rounding here). But I don't like flogging my engine to death at 50 degrees F ROP. So if you want to compare FF and TAS apples with apples, you have to compare sufficient ROP with sufficient LOP.

At normal cruise altitudes, the difference between the Best Power mixture (80dF ROP), and best economy (BSFC(min) which changes with power) is about 3gph for a 3 knot speed loss. We've tested this thousands of times. It's always very close to those numbers. If you are losing more than 3-4 knots TAS, the mixture is leaner than optimal. It's not hurting anything to do so, because all that is happening is a loss of performance.

So, as Lead Ballon has pointed out, that's about 25% fuel savings (with a much happier engine) for a 5% speed loss. What's not to like?

What evidence do you have for this? You would have to carefully inspect valves prior to installation and run hundreds of engines to prove that no valves without machining errors burned.

Or perhaps if you inspected every burned valve and found pre-existing machining errors in all.

I doubt that either has been done. This is the sort of blanket statement that turned me from a LOP believer 10 years ago when I first read John Deakin's columns to an APS sceptic now.

i have heard this argument quite a few times, "running LOP burns valves!" yet how can a temperature, thats exactly the same if on the Rich side of peak, cause burnt valves, when the same temp on the rich side wont?

peak is peak, it can go no higher! (thats why its the peak)
so 50 deg lower on the RICH side is exactly the same as 50 deg lower on the lean side.. so it cant be temperatures on the lean side that cause burnt valves.

Andrewr …….Is that all it took? :confused:

If you ask a few of the leading engine builders around who actually know their stuff, and I am not one of them, the likes of Andrew Denyer, David Paynter, Bill Cunningham, the Barret's you will find the truth.

They find the exact same failures in ROP run cylinders, so how could it be LOP ops? They also comment on the deposits not helping and may in fact be accelerating guide wear.

I hope you can restore your faith in science and data now.

I am sure Walter can add to this if he reads this.

Andrew, I can give you anecdotal evidence. I had a O300 rebuilt and balanced with all new cylinders fitted. This engine would not run rough when it was leaned out, it would simply loose power. When I had favourible conditions I would often see fuel flows around 26 litres per hour with the throttle wide open. Within 250 hours, all cylinders had been off due to leaking exhaust valves. Some were fixed under warrantee, some were fixed by a shop that specialised in vintage motorcycle repairs. (Taken there by my LAME)

I did not change the way this engine was operated and went on to do aother 600 odd trouble free hours in it. The new owner, who I keep in contact with, has had no issues either. Apart from the valve issues this has been an exceptional engine.

All evidence thus would tend to point at poor cylinder manufacturing.

Ah, but is it? Remember what EGT is measuring - the temperature of a probe inserted at some point down the exhaust pipe. It is at best measuring the average EGT at that point.

I would expect that if damage to valve sealing surfaces occurs, it would be most influenced by the gas temperature at the instant the valve begins to open and the temperature of the first perhaps <1% of gas to exit. The average gas temperature in the exhaust is not necessarily indicative.

Measurements I would be interested to see:
- Instantaneous EGT in the exhaust port through the combustion cycle vs measured EGT as the mixture changes
- Temperature of the valve head vs EGT as the mixture changes

Let me get this straight, Andrew

If someone makes the blanket statement that running LOP causes burnt valves, that's OK.

If someone makes the blanket statement that manufacturing defects cause burnt valves, that's not OK.

If you wish to ignore the science and data that prove that ROP is where you can give your engine the hardest beating you can give it, go forth and give it that beating. If your engine happens not to suffer burnt valves or other problems, you can tell yourself that it's because you don't run LOP.

Funny thing too is that the burnt valve problem is confined almost exclusively to CMI engines and not Lycomings. Clearly nothing to do with manufacture and everything to do with LOP?

Ignorance is indeed bliss.

The alternative is to register on a blog like BeechTalk, and do some research about what's happening in a first world GA country. The poor quality control of CMI cylinder manufacture is widely understood and discussed in detail, and the SOP of people considering cylinder replacement is to send the replacement to a competent engine maintenance organisation first, to get the valves installed properly. Those people are not doing that for fun or because their astrologer advised them to.

And how I wish I had done just that with my old O300. The pain of pulling cylinders between and during annual inspections is not something I wish to repeat.

I'm quite happy with the statement that either MIGHT cause burnt valves but would be looking for evidence for both. It seems quite likely to me that there can be more than one cause.

The statement that LOP does not cause burnt valves, and burnt valves are caused by manufacturing defects is more definite and I would be looking for evidence to support it.

Incidentally, I found a flight engineer's notes on operating the Pratt & Whitney R-4360 (one of APS's beloved radials):

R-4360Ops1

From those notes, when leaned at cruise power the ignition timing was advanced from 20 to 30 degrees which favored longer valve life.

Also, ignition timing advance maintained peak pressure at the most effective point as the mixture was leaned. Why then do APS say that it is a good thing to delay the peak pressure at cruise power (which is the same effect as retarded ignition timing)?

Everybody else I can find (including R-4360 operations) say that you want to advance the timing at cruise power compared to maximum power. APS want to effectively retard it...

Another thing arising from that document - APS tell us over and over that your Lycoming is effectively the same as a big radial. In this case it's obviously not - if you can't advance the ignition when LOP, that is a significant difference.

160 vs 150 knots should require about 20-25% more power.

How much fuel do you save? Are you sure that it is more than if you just reduced the throttle/rpm until you were at 150 knots while operating according to the engine manual?

My primary reason for running LOP is not to save the fuel, although that's a pleasant bonus. (The rounded fuel savings calcs are in my post, confirmed by many hundreds of hours of real world ops.)

My primary reason for running LOP is to save the engine.

I don't particularly care what the engine manual and POH say about settings. The engine and prop are rated to run at 2,700 and full power continuously, so I know that anything less than that isn't going to be prohibited. (There are no RPM range limitations.)

What I do know is the CHT for each cylinder, the EGT for each cylinder, the FF and TAS, +/- not much, at each point on the lean curve. I therefore know where on that curve I could give the engine the hardest beating I could give it, and when I'm not at that point.

There is no point in playing with MP of a normally aspirated injected engine like mine, unless I'm in the circuit or doing some low and slow sightseeing. I set wide open throttle at the start of the take off run and don't touch it again until joining the circuit at my destination. I'm a simple person and don't like unnecessary complications.

Yes Jabba,

I want to know how you're getting 170 kt TAS at 10k LOP when 'book' figure should be closer to 160kt ROP.

..And why I only get about 154kt LOP on the -7!

Haydn

PS: Andrew, it's worth reviewing and marking the signifance of the brown curves in that set of curves that Jabba frequently posts.

HC
That was well spotted. As you can see there was a fair amount of westerly wind, and some wave surfing going on. It would have been about TAS164 just prior to that. Don't get too upset by that. ;)

I have seen and understand the charts, but it's hard to estimate the significance when there is no scale. I see the theoretical increase in efficiency but without a scale you can't tell whether it's 10%, 1% or 0.1%.

All I know is that people keep posting their "fuel savings" from running LOP, and when you do the calculation it's no better than what you would expect from the speed reduction.

Here's the test we need to see:
Set your LOP 150 knot cruise and note the fuel flow.
Then set the recommended best economy setting and adjust the throttle for the same 150 knots (same rpm). How much does the fuel flow differ?

I know, this gives the highest temperatures and pressures etc...

BUT:

Temperature and pressure is what turns the prop around so it's not surprising that you get the best economy where temperatures and pressures are maximized for a particular fuel flow.

The important question is whether they are temperatures and pressures that the engine is designed to handle. If you are well below the temperature limits, and well below full power, temperatures and pressures should not be an issue.

Andrewr

I demonstrated this just recently, and these numbers are from memory as I did not video it, but here we go;

We were at something like 2500' and maybe 2450-2500 RPM and around 75% power LOP at 49LPH (about 40dF LOP). I then went full rich, and adjusted the MP to get me the same %age power which would have been around 24" and set appropriately 180 or so ROP, which ironically on a well set up Bendix fuel servo was on the full rich stopper or a bee's whisker off it.

Hey presto, same power, same speed, and a whopping (from memory OK) 70-75 LPH, and if I am wrong it was not far from it.

Now that is massive you say. Yep sure is and the higher the power the bigger the fuel spread because you need more so much more fuel to achieve the same peak pressures or at least keep them down.

You need to see this on the Carl Goulet Memorial Dyno test stand to fully appreciate this. No other way to describe it.

Now if we get to say higher altitudes and compare 65% and we are 100-125dF ROP and say 10dF LOP, the percentage of extra fuel is not so massive, for example on a IO520 that might be 47LPH compared to 58LPH. On my IO540 that is 41-42LPH to 53-54LPH (off the top of my head).

I think this is worth repeating, and Walter will no doubt agree……….You need to see this on the Carl Goulet Memorial Dyno test stand to fully appreciate this. No other way to describe it.

So Andrew, your view is that provided the higher CHTs are under redline, it's OK. You are of course free to choose to operate engines that way (but you'll never get to touch mine).

For my part, I choose to operate my engine where it is put under the least stress to achieve the performance I need. I don't need scales on those curves to know where those settings are, because the data out of my engine monitor over many hundreds of hours (and many thousands of other monitors over millions of hours) and actual TAS and fuel consumed are objective facts.

I still reflect on that poor kid PIC and his pax on the Whyalla Airlines Chieftain. I still wonder whether the engine failures might have been avoided if the mixture had been left sufficiently rich of peak rather than leaning it to a mixture rich of peak that meant the engines were given around about the hardest pounding they could be given. (CHTs still under redline, mind you Andrew. Just before they got very cold.)

There's been a lot of discussion on here about the merits of running LOP and ROP to varying degrees but I'm interested to know what you guys think about doing neither ie running at peak EGT? I notice in the POH for the PA-32R-300 which has an IO-540 engine, the performance charts for economy cruise, range and endurance, are all based on 'mixture leaned to peak egt'. The performance cruise chart says to use 'mixture leaned to 100deg rich of peak EGT at 75% power and below'.

These are based on standard issue ex factory Lance with single probe EGT.

Any comments on the merits of these performance charts..? All I've ever heard of is leaning to peak EGT, then either leaning further to LOP or leaning less to ROP but never staying at peak EGT.

This thread is full of misinformation.

**Ah, but is it? Remember what EGT is measuring - the temperature of a probe inserted at some point down the exhaust pipe. It is at best measuring the average EGT at that point.***

It is NOT measuring the average EGT. It is measuring the EGT minus the refractory period cooling between pulses of hot air.

***I would expect that if damage to valve sealing surfaces occurs, it would be most influenced by the gas temperature at the instant the valve begins to open and the temperature of the first perhaps <1% of gas to exit. ***

That is proven to be false. EGT does NOT affect valve temperature. We have hard data to prove that. The data came from a 1943 top secret NACA report done during WWII.

***Measurements I would be interested to see:
- Instantaneous EGT in the exhaust port through the combustion cycle vs measured EGT as the mixture changes
- Temperature of the valve head vs EGT as the mixture changes***

Both of those are available. The first answer is that the two temperatures track rather closely--as in immeasurably different in delta

The second is that valve temperature does NOT track EGT. This is 1943 data, not ours. Lycoming verified it in 1966. We ran the test confirming this about 15 years ago. Funny how the physics is everywhere the same. The hottest valve temperature is found with a mixture of 40dF ROP. As the EGT is going up toward peak, the valve temperature is getting cooler. Anyone who suggests that EGT affects valve temperature must reconcile this fact. In addition, there are times when the exhaust gasses blasting past the valve are actually cooling the valve. These were surprises we learned while MEASURING these things.

BTW, EGT is not a measure of the temperature of combustion. It is the result of the expansion of the 3800dF combustion temperature. That's why low compression engines have higher EGTs and higher compression engines have lower EGTs.

***Also, ignition timing advance maintained peak pressure at the most effective point as the mixture was leaned. Why then do APS say that it is a good thing to delay the peak pressure at cruise power (which is the same effect as retarded ignition timing)?***

Because the fixed timing is "averaged" in our flat engines to be "acceptable" across the mixture spectrum. The R-4350, along with some other radials had the timing retarded at max power to control ICPs and CHTs, and more importantly to stay barely outside the detonation margin. This retarded takeoff timing at max power was unacceptably retarded at cruise powers, so they had a way to alter timing to make it work efficiently in both high-power and cruise-power configurations.

One must be careful in comparing these engines and their engineering requirements to our GA engines.

The effective timing on our engines is too far forward at high powers ROP and slightly retarded when in cruise. If we retarded the timing during takeoff, the engine would produce more power and the CHTs would run lower, but if we did that, the timing would be so far retarded in cruise as to be very inefficient. It's a trade-off. We do not advocate delaying the peak pressure beyond 16dATDC. That is optimal.

'Tis amazing how often one gets misquoted by internet experts.

***Temperature and pressure is what turns the prop around***

That is an over-simplified and basically incorrect statement. I can demonstrate many instances where a lower combustion temperature and a lower peak pressure produces more power than higher temps and pressures.

There are two enemies of metal: heat and pressure. It's good to control them.

Mean cylinder pressure relates to HP.

Peak cylinder pressure relates to longevity.

It is quite possible to have extremely high pressures with very little to zero power being produced. ??pre-ignition??

The Lycoming recommendation to be 100dF ROP at powers above 75% is suboptimal. One should be richer than that if longevity and engine health are your goals.

The Lycoming recommendation to run AT peak EGT for economy cruise is based on two facts: 1) that their engines' F:A balance is not good enough to run smoothly LOP, and 2) the difference in efficiency between peak and BSFC(min) is relatively, but measurably small. This recommendation was made for one purpose... to keep the customer service phone from ringing off the hook when their engine ran rough LOP.

Gentlemen:
There is a lot of misunderstanding and misinformation being posted in this thread. While there are certainly others who understand this well, the posts by Lead Ballon and Jabber are the posts with accurate information that can be supported by documented, repeatable, hard data.

DO NOT believe me, Lead Ballon, Jabber or any one else. Believe the Data. We have seen the data and have drawn conclusions based on that, not what we or anyone else might "think" to be right.

'Tis important to consider that without data, all you are is another person with an unsupported opinion.

Look, it's not a difficult concept. Start with the understanding that all else being equal, power varies with mixture, and that there exists some optimum mixture where power is at a maximum. It seems a pretty simple concept that for a given desired power output there must also exist two points, on the rich and lean side of the peak, where power is equal to that desired output. and for those two mixture stetting which yield equal power output, the cruise airspeed will be the same. And if the airspeed is the same, and one setting has lower fuel flow, then the fuel burned per mile is going to be less. I'm not sure which part of this you can't get your mind wrapped around.

Probably because by advancing the timing, the mixture is more completely burnt before the exhaust valve opens. Leaning the mixture slows down the burn, provided that detonation doesn't occur. Advancing the timing point compensates for that slowing. An open exhaust valve is subjected to greater temperature rise than when closed; A) because both sides of an open valve are exposed to the heat of the burning mixture and B) the edge of a closed valve valve dissipates heat through its metal seat.

Shy Torque

Probably? Well it might seem to make sense, but it does not work as described. The best suggestion I have is read carefully what Walter wrote above. :ok:

Lead Balloon

Scammed? A little defensive there I think LB. I have asked for clarity and data. This is a public forum and so it is completely fair for advice being offered here to be scrutinised here. Of course if you don't like that, too bad. You can always take your own advice to "move on". But I am here to stay :ok:

And, I have responded in an attempt to clarify and have provided some data. An internet forum precludes the presentation of very much data because of the bandwidth alone. I have presented over 16 hours of data dozens of times over 16 years and not one of the thousands who have looked at that data have questioned the accuracy of it. Anyone may chose whether or not they wish to take advantage of the opportunity to see and study that data, but to demand that it be provided free and at their whim seems a bit unreasonable.

Oggers
Don't you take any notice of that nasty LB. Someone still loves you. Donald Trump loves poorly educated people. He said so on national TV.

you mean Donald Drumpf?

Little Donald has done very well for someone brought up in poverty in a poor family without a billion $ to their name. How many of you could be where he is today if you were only left a lousy $44million to start your investment career?

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Can the ppruners tell me if this man speaks truths?

He also tells us to go to APS.

Hm? That's not quite correct.

When ROP, power is determined by mass airflow. FF has little effect in the normally used ROP mixtures. Once they get overly rich, yes, power decreases.

When LOP, power is directly related to FF. Mass airflow plays no part.

The reason there is a Best Power mixture has nothing to do with FF. It is the mixture where the mass airflow is adequate to burn all of the fuel. That does NOT occur at stoichiometric (AKA, Peak EGT). It occurs at approximately 75-80dF ROP.

Mike has a lot of it right. He's been to the APS class three times and refined a lot of his knowledge thereunto, appertaining. He is honest in acknowledging APS and we appreciate his recommendation.

Where we disagree with Mike is on his recommendation to lean based on CHT. This is a very flawed concept. In addition, we do not think his approach to maintenance is optimal where dispatchability is concerned. Other than those two issues and few minor peccadilloes, he's got a lot of good information.

You're the only one talking about fuel flow. Yes, mixture ratio and fuel-flow are pretty tightly correlated, but you're interjecting words I didn't use.

Here's what *I* meant: If you had a power meter on your engine*, and you started with the throttle set and the mixture at full rich, then you started gradually pulling the mixture toward lean, you would see the indication on your power meter rise, peak, then fall as the mixture control moved toward Idle Cut Off.

I'm not sure what you mean about power not being a function of fuel flow exactly, and I'm not saying that you're wrong, but I think you may be making a semantic distinction that's not relevant to my statement.



* I flew for a number of years in an airplane which had power meters on the engines. For a constant RPM, a torque meter is a power meter. I have watched the needles rise then fall many, many times.

Lycoming say leaning on the ground is a no no in this flyer. What do the APS guys think of their logic?

Lycoming Flyer