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Understanding LOP Operations - A Summary of John Deakin's Knowledge

nucleus

Well Known Member
This summary of my understanding of LOP operations. It is all based on John Deakin's Pelican's Perch column on AVweb. His LOP know-how is so spread around in those columns that I find it difficult put it all together, so hence this attempt to summarize. He now works at GAMI, and I am grateful to him for all the knowledge and graphs. I will try to put quotes around his words and not mine.

I am big advocate of LOP operations, and even do it at low altitude, like 700 ft MSL, and I hope this summary will clearly illustrate why this is safe and desirable.

Let's start with a graph demonstrating the relationship between mixture, EGT, CHT, ICP and HP:

KeyLOPICPBSFCCHT.jpg


You can see that the Intra Cylinder Pressure tracks very well with the Cylinder Head Temperature. There is also a good correlation between those lines and the Exhaust Gas Temperature.

Key point from Pelican's Perch #65:

"On the rich side of peak, leaner is hotter, but on the lean side, leaner is cooler." That's a crucial concept! Repeat that to yourself, until you "get it.""

Temperature and Detonation:

Pelican's Perch #43:
"We know that combustion temperatures are in the 3,000ºF to 4,000ºF range, EGT "only" run around 1,600ºF, and CHTs down around 400ºF. How can this be? 4,000ºF is more than enough to melt steel, so how does the interior lining of the cylinder survive? Why don't we see hotter temperatures on our instruments? Why doesn't the aluminum piston melt down, when aluminum melts at less than 1,000ºF?
There is a thermal boundary layer, on the order of a millimeter thin or so, that acts as a buffer to protect the steel cylinder walls and the surface of the aluminum piston. Think of it as the thermal equivalent of the aerodynamic boundary layer out on your wing. The metal and the molecules right next to it will be at roughly the CHT reading or a bit higher, the next layers will be hotter and hotter, until the layer next to the combustion event will be at the combustion temperatures. That very thin thermal boundary layer acts as a nice insulation barrier, limiting the rate at which heat can be transferred from the bulk combustion gases into the interior walls of the cylinder head, cylinder barrel, and piston.
The heat transfer is continuous, as the heat moves first through the boundary layer, and then the cylinder wall and is finally carried away by the cooling airflow around the fins on the cylinders. Each intake stroke brings in a cool new charge, which starts the process all over again. There is also a matter of time of exposure. The high-pressure part of the combustion event takes up only about 40 degrees or so of crankshaft rotation, and the very hottest part of that only about 20 degrees, so during the other 700 degrees of crank rotation, cooler temperatures prevail. EGT shows only a number that represents a momentary flash of heat during a small portion of the combustion cycle (when the exhaust valve opens and exhaust gas flows across the EGT probe), and a rapidly dropping temperature at that.
This is NOT the major factor that determines how hot their exhaust valve is during operation. The events that happen a few degrees of crankshaft rotation earlier are much more significant because the temperatures are MUCH hotter than the piddling little 'ol 1500ºF measured by the EGT probe."

"We have nice cool induction air and fuel entering a cylinder;

The cylinder happens to have very hot walls. Those hot walls cause some of that nice cool induction air to start to heat up. And it doesn't all happen uniformly.

Further, shortly after the sparks go off, we have a couple of flame fronts, giving off lots of infrared heat, adding to the continuing heat load being absorbed by some of those little remote pockets of fuel and air that are waiting for the flame front to arrive and consume them;

The unburned mixture is experiencing a very rapid increase in pressure, because of two things: A) The piston is rising rapidly during the compression stroke; and B) the flame front combustion products are creating a huge increase in released energy and resulting bulk gas pressure, all of which is neatly measured on the pressure traces you see in the accompanying graphics.

At least some of those little "local pockets" of unburned combustion mixtures have exactly the right mixture of fuel and air to be just a hair-trigger away from exploding.

And … if the fuel is the wrong octane, or the spark advance was set too soon, or the manifold pressure was too high, or the cylinder head temperature was too high ... then one or more of those little "local pockets" of unburned fuel do just that ... they "explode."
That is what we call "detonation".

Each explosion creates a shock wave that travels at the speed of sound (remember, the speed of sound inside the cylinder, at somewhere near 4000 degrees, is very much faster than at a standard day!) and bounces off the walls of the combustion chamber every 1/5th of a millisecond or so (giving off a 5KHz "ping" that you will not hear in the cockpit). Each of those explosions creates a very sharp rise in pressure and sets off a shock wave, which vibrates back and forth across the cylinder. This shock wave can be just the right amount of additional pressure to cause some other little remote local pocket of fuel and air to, in turn, explode, adding to the problem.

As detonation grows more serious, it will become audible, and this is the pinging you'll hear from that old auto engine on the uphill grade. Remember, you will NOT hear it on an aircraft engine."

Mild Detonation:
DetonationDissection2.jpg


Moderate Detonation:
DetonationDissection3.jpg
 
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Understanding LOP Operations - A Summary of John Deakin's Knowledge #2

Heavy Detonation:
DetonationDissection4.jpg


"How Damaging Is Detonation?
There are newly proposed "standards" that define "light," "medium," and "heavy" detonation. How those are arrived at is far too complex to go into here (which means "I don't know"), but suffice it to say that a little light detonation, even for hours at a time may not be harmful, and in fact, can be beneficial. It does a marvelous job of cleaning deposits off the top of pistons, for example!
The truth of the matter is, most of these engines can operate in the light detonation condition as shown in the graphics for several hundred hours with no detectable damage, PROVIDED the CHTs remain cool and you do not experience a runaway cylinder head temperature during the process.
The problem is how to detect it, and prevent it from becoming worse, because "light" can progress rather quickly into "medium" and worse. It is a "positive feedback" process, with a very negative result! The mechanism that causes it to be self-feeding is that the shock waves from the light detonation tend to begin to "scrub" the thermal boundary layer inside the cylinder. As that happens, the rate of heat transfer increases from the bulk combustion gases into the cylinder. That starts the CHT rising. When the CHT rises, it tends to heat up the incoming charge of new air and fuel a bit faster than the previous crank rotation, and that increases the likelihood of there being more light detonation in the next combustion cycle, which increases the disruption of the thermal boundary layer even more, which heats up ... well, you get the picture. If the cylinder is not really well-cooled, with some cooling reserve, the whole process can snowball to **** in a hurry and you end up in deep detonation trouble."

"I think we can all agree it's better to just stay away from detonation entirely. Much better!"


Pre-Ignition

"Pre-ignition is the ugly cousin of detonation. It is never beneficial, and can cause failure in seconds. As the name implies, preignition occurs before the normally anticipated spark event. Pre-ignition starts a fairly normal flame front, but early, often very early. This forces the pressure higher before top dead center, and that rapid rise in pressure instantly drives the temperature up. At top dead center, the pressure from pre-ignition can reach 1500 to 2000 PSI, rather than a normal maximum of 900 to 1100 PSI. That extremely excessive peak pressure causes all kinds of havoc. Among other things, it can damage internal components, like valves, spark plugs and rings, and it will quickly eat its way through the aluminum piston head.
Pre-ignition is most often caused by some projection, some sliver of metal, or something inside the combustion chamber that can be heated white hot during the combustion event, and that spot must remain at very high temperature all the way through the subsequent power, exhaust, intake, and compression strokes. This projection acts just exactly like the glow plug in a model engine.
One cause is a "helicoil tang." Cylinders are made of soft aluminum. Helicoils are steel inserts, into which the spark plugs can be screwed. After inserting the helicoil it is not unusual to have small, sharp slivers of metal, which must be cleaned off carefully.
A damaged spark plug can lose its capacity to transfer heat from the ceramic and then the ceramic insulator and tip can become unacceptably hot inside the cylinder (somewhere up around 1600F, rather than a normal 1100 to 1200F).
When this happens, the spark plug itself can become the source of the pre-ignition event.

For completeness here, I'll mention that carbon deposits in the combustion chamber can heat up and cause preignition, but bluntly, if the engine is run properly, there should not be any carbon deposits. These come from operating exclusively on the rich side for many hours."

One thing I take from this is that as long as my CHTs are low, I am very unlikely to cause any damage to my engine.

Pelican's Perch #65:

The Dangerous Red Box

Just where is that "red box" I keep talking about? Some rough numbers, good (that is to say, BAD) for most of these engines -- these are "no fly zones," DO NOT set the mixture between them:

Red Box = No Fly Zone

At and below about 60% power, there is no red box. Put the mixture wherever you want it.
At about 65% power or so, 100ºF ROP to Peak.
At about 70%, 125ºF ROP to 25ºF LOP.
At about 75%, 180ºF ROP to 40ºF LOP.
At about 80%, 200ºF ROP to 60ºF LOP.
All those numbers are approximate! Please don't start splitting hairs, here!

You probably don't want to run your engine between those mixture settings. If you do, you are running very high peak pressures inside the combustion chambers, and that peak pressure is occurring too close to top dead center."

Okay, now you have Deakin's take on what we want to avoid, let's look at normal ops:

"Outside the Box

At 65% power, use richer than 100 ROP, or leaner than peak EGT.
At 70%, use richer than 125ºF ROP, or leaner than 25ºF LOP.
At 75%, use richer than 180ºF ROP, or leaner than 40ºF LOP.
At 80%, use richer than 200ºF ROP, or leaner than 60ºF LOP."


Summarized in Graph Form:
KeyRedBoxGraph1LOP.jpg
 
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Understanding LOP Operations - A Summary of John Deakin's Knowledge #3

KeyRedBoxGraph2LOP.jpg


Pelican's Perch #66:
We use these charts in our seminars, and they are a variation of those showing "the red box." Ok, so these are triangles, so sue me!

Both charts assume WOT ("Wide Open Throttle"), and a fairly high (or full) RPM.

The left edge represents sea level, and note that to stay outside the red box at sea level, you must be somewhere around 250 degrees ROP (slightly off the chart), or you must be very close to 100 LOP. Both are completely safe settings for the engine, with the LOP setting running much cooler, but producing less power.

Reminder, this assumes full throttle, full RPM (or nearly so)! We are fooling around with the mixture only, here.

Move across the chart to about the 5,000-foot level, and note the red box has gotten smaller, because altitude has taken away a lot of MP, and there's no recovery from that in a normally aspirated engine. Now our red box is around 30 LOP to about 90 ROP. At roughly 8,000 feet, the red box goes away, and that's the area where you can't get 65% due to altitude.

Again, let me remind you, these are approximations; don't be a slave to precise numbers! Don't yell at me because I say "80" in one place, and "90" in another. That's "close enough"!

The same chart can be used for illustrating the climb, if you climb as I suggested a couple of columns ago. Leaning to that target EGT as you climb produces pretty much what you see on the second chart, just rich of the red box. Once you get to your altitude (the example is at 4,500 feet), do the "big mixture pull," and set LOP. The green lines show a good area to be in when running LOP, up to about 9,000 feet altitude. At and above that point, you probably want to switch over to ROP to keep the power up as much as possible." [I don't do this until above 11,000 depending on temp]


Pelican's Perch #64:

"Parking the Engine

It is worth noting at this point another crucial concept we teach in the seminars. We call it "parking the engine." Any time we adjust the engine controls, we want to leave the engine in a situation where it will not, if left alone or neglected, change by itself into a "bad" setting. That way, if we get busy, or have to concentrate on ATC, another airplane system, a radio setup, or a baby barfing, the engine will be safe. In the climb I have just described, the worst that can happen is that you forget to lean, the engine slowly goes richer and gets cooler. The engine and its controls are "parked."

A reminder here, "On the rich side of peak, leaner is hotter, but on the lean side, leaner is cooler." That's a crucial concept! Repeat that to yourself, until you "get it."

Some like climbing LOP. This works, as the engine doesn't know what the airplane is doing; all it cares about is fuel, air, spark, and cooling airflow. But if you set up a LOP climb in a normally aspirated engine and forget it, the mixture will gradually go richer and richer, perhaps getting too hot. LOP climbs violate our general principle of "parking" the engine safely. It doesn't mean you can't do it, or shouldn't do it, it just takes a little more care. The alarms on the JPI [or something modern!] make this a much more feasible operation, and it will save a little fuel, perhaps enough to give you just a bit more range to make that long non-stop.

Don’t be anal about adjusting the climb mixture, please? Many people fiddle it to death, and that’s not necessary. Once you make an adjustment, it will be just fine for a thousand feet of climb, or more. Tweak it again, and it’s good for another thousand feet or two. There's a subtle (and graceful) distinction between tortuously trying to set it within a degree or two, and just setting it so that it drifts into what you want. It doesn't matter if the climb CHT runs 320, 340, 360, or even 380. If it's cooler than that, and you're still ROP, lean it a bit. If the CHT is sneaking up slowly, wait a bit, leave it alone, and altitude will take care of it.

Note: There are some notoriously bad engine installations in which the cooling air flow baffling is so poorly designed, installed, and maintained that holding CHTs under 380 to 400°F on a normally aspirated engine in cruise is a problem. If yours is one of these, you should spend some serious time and effort to get that corrected. There are people that understand these issues and who know how to get it done properly."


IntraCylinder Pressure Graph Comparing LOP vs ROP at same HP:
CylinderPeakPressureLOPvsROP.jpg

While both ROP and LOP slow the flame front, at the same HP LOP reduces both peak cylinder pressure and pre-TDC pressure.

Pelican's Perch #65:

"Set RPM for Smoothness

I have one more consideration, smoothness. I'll go for a smooth engine every time, and some airplanes have specific RPM settings where they seem smoothest. You also don't need to be at some exact multiple of 50 or 100! 2534.202 RPM is a perfectly valid setting! It amazes me how some pilots will work so hard to get an RPM at exactly some "even" setting, especially those with digital tachs, accurate to 1 RPM. If it helps, put a piece of masking tape over that final digit! Heck, if you're LOP, cover up the whole tach!

A few engines just won't run smoothly LOP, no matter what you do. Bearing in mind that "smoothness" and "roughness" are VERY subjective terms; a little roughness from small cylinder-to-cylinder power variations won't hurt the engine at all. It MAY have a long-term effect on the airframe, accessories, instruments and hemorrhoids.

If you find that this roughness hurts your ears and your head (from your spouse beating you upside the head, or screeching in your nearest ear, "DON'T DO THAT!"), then perhaps you need your subjectiveness adjusted to more nearly match your spouse's.

While you're fiddling around, trying to find a power setting that will give you the range/speed you want, be aware that small changes in RPM do not affect the mixture ratio very much (fuel pump RPM changes with engine RPM), so you don't need to twiddle the mixture at every RPM change. Once you're where you want, if you want, do one final check for peak, and set the mixture to the appropriate setting.

Sound complicated? It's a lot harder to write and read about it, than to just do it! It's complicated here, because I'm trying to give you the reasons, the logic, and the science behind it, along with a few bad jokes. With a few practice runs, while thinking about it, these procedures become second nature, are very easy, and you're using science for power settings, rather than witchcraft, old wives' tales, and the "knowledge" of a 300-hour CFI. The single most common comment I get from folks who ride along with me is, "But you're not doing anything!" As in anything worthwhile, it takes a bit of practice to get used to it. Take along a safety pilot, the first few times, to watch for traffic, or even have him fly while you learn your engine."


So what I do at low altitude with a fully warm engine is keep my mixture between the green lines of the "Red Triangle" chart. This means the lower you are the more important to run further LOP, at sea level this means at least 90 degrees LOP, I usually go about 100.
 
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Understanding LOP Operations - A Summary of John Deakin's Knowledge #4

"Run It the Factory Way?

We get a few comments from people who report they used to run as the factory says, and usually enjoyed good engine life, some even making TBO. That's true, but there's more to the story.

Most pilots have used some variation of the 65% power setting for decades. All the magazines show the data at 65%, the power charts all seem to suggest 65% is "the way to go." Everyone is comfortable with "good old 65%."

Folks, note that's right on the edge of our "red box"! We suggest that you can safely set the mixture anywhere you please at 60% power, and 65% is probably "close enough." Remember, just because we teach the concept of the "red box" doesn't mean the engine will instantly explode if you get near it, and we cannot begin to state with assurance EXACTLY where the red box begins and ends. There's room for error here, and we're trying to be conservative in this. We DON'T want pilots to look at our "red box," and say too themselves, "Well, if 125 ROP is the edge of the 'red box,' I'll add a margin and use 150."

These are "fuzzy numbers," folks! You may be able to operate well within the "red box" in some cases (65%), but if you do, be aware you may not be treating your engine with quite the care it deserves.

So, what does "The Factory Way" (65%, 50 ROP) get you? At 50 ROP, we suggest you run at 60%, and certainly not more that 65%. We can say, then, that at 50 ROP, 65% is your LIMIT on power. At that setting, you will probably be burning up to 3 GPH more fuel than you need to. You will be running "dirty" enough to eventually foul your valve guides and encrust the tops of your pistons with deposits. Finally, you will be making enough carbon monoxide to be lethal if you develop an exhaust leak into the cabin.

Until recently, speaking in broad general terms, we operated the whole fleet that way, industry-wide. Have you ever looked at the "for sale" ads in Trade A Plane, and the engine numbers in them? How often have you seen "1,200 since new, 600 since top overhaul." If TBO is 1800 or so, WHY did they do that "top overhaul" at 600? Ever think about that? Now, how many of those ads that don't mention "top overhaul" are about engines where the mechanics/owners/operators just sorta forgot to report the removal of one or more jugs during that run, for valve work, or a damaged piston? How many of those "events" just get quietly forgotten when it comes time to make that required entry in the logs? Certainly, no one would EVER omit such work intentionally! I would be aghast at such dishonesty! Sure I would. Riiiiiight.

So, if most of the ads have that verbiage in one form or another, and there are significant numbers that have had the same problems without recording them, I can only leave it to the reader to make the judgment. Have we been operating these engines properly? I think not.

Do we really NEED to do more of this, to prove a point? No, that experiment has been ongoing since World War II, and the results have been dismal at best, even with "conforming" engines.

We have become so accustomed to "early top overhauls," or frequent jug work, that we forget these engines should make TBO and beyond, assuming they are "conforming" engines to start with, and they are operated in any reasonable manner. We overhaul a jug here, and another there, maybe one per annual (when the compression test is done), and we shrug it off as the normal cost of doing business. It isn't.

In spite of all that, we still think ROP operation is a viable part of any pilot's bag of tricks, provided it is rich enough, and used properly. Climbs are probably best done well ROP, for example. For normally aspirated engines cruising at altitudes above about 9,000 feet msl, 80 ROP will produce the most possible power, and 50 ROP will produce almost as much power, but on less fuel.

But for normally aspirated engines cruising below about 9,000 feet (and for turbos cruising at all altitudes), LOP operations, properly done, will give the needed power, will burn less fuel at the same power, will operate with lower peak combustion pressures and temperatures, and therefore run cooler (in all parts of the engine, valves included). They will run cleaner, making no deposits on the pistons or the valves, and without making measurable carbon monoxide, with less stress on the engine for any given power. It will remove that 60% or 65% limit, and allow normal (LOP) cruise at higher power settings, even up to 85%.

It seems intuitively obvious that all that will extend the service life of the engine, but we'll probably never see hard data for that. For everything else, there is hard data, already."

"Summary

Use full power on all takeoffs, with a "rich enough" mixture. FORCE your mechanic to set that fuel flow high enough to get roughly 1300 degrees EGT or a bit less, and CHTs in the low to middle 300s.

If you have that full-power mixture set properly, determine your "target" EGT right after takeoff, and lean in the climb to keep that same absolute value on the digital EGT. As you climb, that EGT will drop a little. Lean until it comes back up. Drops a little, lean and bring it back up.

For cruise, first determine your range needs, and set a power setting to maintain the AIRSPEED that will do the job you want. Set WOT, then the RPM and mixture you need to maintain that. LOP is much better, if it will do the job, but use ROP if needed. Cruise outside the red box, or at worst, on the fringes of it.

For descent, use mixture control and RPM to get the desired descent, switching to slightly rich of peak EGT, if you wish to keep the CHTs up. (Remember, 50 ROP on the EGT may be the same absolute value of EGT as 50 LOP, but the CHT will be much hotter when 50 ROP!)

None of this takes a lot of effort, once learned. Most of it will fry your CFI's mind, but it may do him some good in the long run, and force him to examine the issues. You may teach him something.

Finally, and very sadly, unless you have a truly enlightened check pilot or examiner, do it the old way when taking a check ride. It'll feel like you're abusing your engine, but once won't hurt a thing. You will never profit from doing "something different" with the FAA or a check pilot on board.

Be careful up there!"

Well, that is my summary, I hope I did him justice.

Hans
 
This thread needs a Sticky!

........just like the ones Paul Dye started on EFIS's.

Great work Hans
2thumbs.gif
 
Allow a moment of devil's advocacy......

First, the scary "red box": The good shops run your brand new engine at full rated HP on a dyno, near sea level, well within the box (80-150 ROP). Why don't they detonate into oblivion?

LOP advocates claim longer top end life. What is the nature of the actual failures?
 
Good point Dan. To expand on that, it has been argued by some that it is not even possible to cause a naturally aspirated Lycoming to detonate. While I?m sure that there are some extreme examples (a high compression engine on mogas, perhaps), the fact that our fleet is not falling out of the air on a regular basis has a lot more to do with engine design than our collective skill at engine management. Let?s face it, we?re pilots ? if there is a way to screw something up, we?ll find it! I don?t know for sure, but perhaps detonation in most of our engines is like the boogey man ? mostly a figment of our imagination ? and the only thing we really need to worry about is CHT management?

For the record, does anyone have firsthand knowledge of a N/A Lycoming failing due to detonation?
 
For the record, does anyone have firsthand knowledge of a N/A Lycoming failing due to detonation?

Several years ago, a fellow at my airport had a Glasair III equipped with an extremely high compression ratio IO-540 angle valve (K-series, I think) engine. He used to race it. I helped him change his cylinders out when he "downgraded" to lower compression, and most of the pistons had busted ring lands with chunks of aluminum falling out, holes melted all the way thru the sides of the piston in the grooves behind the compression rings, the compression rings came out in a bazillion fragments, the valves had been badly overheated but none of the piston tops had shown any signs of being melted. The engine was still running before it was top-overhauled, but I'd imagine a compression check would've been horrible.

The pistons reminded me of my second hot-rod car I had as a teenager... a 1970 Ford Torino Cobra with 429CJ engine (11.3:1 CR). After buying the car, I drove it for most of one summer, and it blew about a quart and a half of oil out the tailpipes per hour of driving. It would still turn mid-13 second quarter mile times in that condition. I pulled that engine apart and found 5 of the 8 pistons were busted up all along the compression ring lands, with a couple holes melted thru the top compression ring grooves in two of the pistons. All the compression rings were cracked into countless little segments, and looked like little oblong roller bearing pieces where they has been turning in what was left of the grooves. Obvious extensive damage due to detonation and/or pre-ignition. I rebuilt that engine, boring it .030 oversize and putting 11:1 pistons back in it. It ran much better, very scary strong, after the rebuild, and I still have that engine today... 3 decades later. Too bad it's far too extremely heavy to even think about some sort of aircraft use for it. :p
 
and to make a statement "a little roughness won't hurt your engine a bit" really?? 2000 hrs of roughness isn't going to hurt anything else? A lot more to an engine than just the cyl and pistons - bearings do not like vibrations
 
...Several years ago, a fellow at my airport had a Glasair III equipped with an extremely high compression ratio IO-540 angle valve (K-series, I think) engine. He used to race it...

A big, high compression 6 in an airplane that has to be going just below the speed of sound to cool properly requires a high degree of attention. I can expect that something like this would see higher instances of trouble.

...But how about our "run of the mill" 8.5 CR engines? If you can keep the CHT in check, is it even possible to hurt it with the mixture? It's possible we are spending way too much time worrying about an event that is so rare in practical terms to be statistically insignificant.

I guess my point is, if it was possible to hurt an engine by mismanaging the power settings, 172?s and their student pilots would be falling out of the skies left and right. The fact that they aren?t leads me to believe the risk is far less than most of us are willing to accept. :confused:
 
I also think its probably fairly difficult to get an 8.5:1 or lower compression Lyc to detonate while running 100LL avgas... unless you had all of the worst circumstances happening simultaneously of extremely high CHT, leaned the mixture into the very peak of the "red box", and pulled the CS prop all the way back to low rpm while pitching the plane into a hard climb to lug it and wide open throttle and had your magnetos or EI spark advance way too high. Not impossible to get such an engine to detonate, mind you, but you'd probably have to work at it.

The RV-8 I fly has 9.2:1 pistons in it, and that's probably the highest you'd want to run with 100LL fuel. I think that it would be quite a bit easier to induce detonation in this engine, that extra 0.7 points is probably enough to get it near the brink, and I think perhaps it does detonate a wee bit during the few seconds that you pull the mixture back to kill the engine when its on the ramp, and still quite hot after a flight. The reason why I suspect this is... if you're old enough to remember the high compression V-8 car engines from the late 1960's thru 1971, and remember how some of them would "diesel" when you shut the engine off while running gasoline with octane rating too low for the engine, the exhaust while dieseling would have a very characteristic pungent, eye-burning odor to it -- the odor of nitrogen oxides that are only created under detonation circumstances in most piston engines... well this 9.2:1 ECI Lyclone engine makes that very *exact*, very unique, unmistakable same smell in its exhaust when you pull the mixture to kill it under the above circumstances. The very first time I smelled that odor coming from this plane's exhaust, it instantly brought back car memories from my childhood and teenage years. I have no doubt that this engine would "diesel" forever when hot if you simply switched off the mags and didn't cut off the fuel supply. And dieseling *is* detonation keeping the engine running ;)
 
I guess my point is, if it was possible to hurt an engine by mismanaging the power settings, 172?s and their student pilots would be falling out of the skies left and right. The fact that they aren?t leads me to believe the risk is far less than most of us are willing to accept. :confused:

I'm not terribly confused about it - I think that with a STOCK (not high compression, not hopped up, N/A) Lycoming, it is pretty tough to ruin it unless you are wide open at sea level and lean it untiul it squawks....with this summer's temperatures!

While I agree with almost everything that Deakins has written, it is important to remember that he was writing to the big engined (often turbo-charged) crowd...a diferent audience. We simple folks with simple engines aren't ruining engines on a regular basis, best I can tell.

Paul
 
well this 9.2:1 ECI Lyclone engine makes that very *exact*, very unique, unmistakable same smell in its exhaust when you pull the mixture to kill it under the above circumstances. The very first time I smelled that odor coming from this plane's exhaust, it instantly brought back car memories from my childhood and teenage years. I have no doubt that this engine would "diesel" forever when hot if you simply switched off the mags and didn't cut off the fuel supply. And dieseling *is* detonation keeping the engine running ;)

Curious how you can smell the exhaust that's underneath you blown by a propeller? I've shut down my RV thousands of times and never smelled anything.

Michael is spot-on about the inability to forcefully cause detonation on a normally-aspirated Lyc. I've tried, with 87 octane mogas that I ran for over 1000 hours and couldn't make it do it.
 
I also think its probably fairly difficult to get an 8.5:1 or lower compression Lyc to detonate while running 100LL avgas... unless you had all of the worst circumstances happening simultaneously of extremely high CHT, leaned the mixture into the very peak of the "red box", and pulled the CS prop all the way back to low rpm while pitching the plane into a hard climb to lug it and wide open throttle and had your magnetos or EI spark advance way too high. Not impossible to get such an engine to detonate, mind you, but you'd probably have to work at it.

The RV-8 I fly has 9.2:1 pistons in it, and that's probably the highest you'd want to run with 100LL fuel. I think that it would be quite a bit easier to induce detonation in this engine, that extra 0.7 points is probably enough to get it near the brink, and I think perhaps it does detonate a wee bit during the few seconds that you pull the mixture back to kill the engine when its on the ramp, and still quite hot after a flight. The reason why I suspect this is... if you're old enough to remember the high compression V-8 car engines from the late 1960's thru 1971, and remember how some of them would "diesel" when you shut the engine off while running gasoline with octane rating too low for the engine, the exhaust while dieseling would have a very characteristic pungent, eye-burning odor to it -- the odor of nitrogen oxides that are only created under detonation circumstances in most piston engines... well this 9.2:1 ECI Lyclone engine makes that very *exact*, very unique, unmistakable same smell in its exhaust when you pull the mixture to kill it under the above circumstances. The very first time I smelled that odor coming from this plane's exhaust, it instantly brought back car memories from my childhood and teenage years. I have no doubt that this engine would "diesel" forever when hot if you simply switched off the mags and didn't cut off the fuel supply. And dieseling *is* detonation keeping the engine running ;)


I think you might have some issue with the engine you might want to look at. I have 10 to 1 pistons and the engine shuts down instantly with either the mixture or the ignition. It was recommended to me that if I intended to restart the engine within a few minute on very hot days to shut it down with the ignition for a easy restart. This seems to work. Never any smells different or otherwise. I do have a FI system with a fuel return. Even very hot starts have never been a problem.

George
 
When I taxi up to our gas pumps with the canopy open I can smell the exhaust upon shutdown due to the way the prevailing wind blows in a circle at this spot and the nearby hangars which affect the propwash and winds. Also can smell the NOx when someone else is flying and I'm standing outside near the plane at shutdown.

I've never actually shut the engine off with the mags myself, only with the mixture cutoff. We've got the ECI constant flow return FI and it shuts down quickly. I only suspect that it would likely diesel if shut off with the mags. On an extremely hot day, even my old low compression Cherokee would try to diesel if I was running auto fuel when I shut it off with the mags, but it never made the characteristic NOx exhaust smell.
 
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Nucleus, you mentioned cruising WOT. What if cruising at 3500 feet above sea level? I normally operate LOP at most altitudes but I usually use 24MAP and 2400 RPM and about 50LOP. How does that sound? Reading lots of articles I had the idea that if I were to fly low and LOP I should limit power limiting MAP. Does it make sence? If I keep WOT low wouldn't I get more power than I should risking to damage the engine?

Is it ok to run at peak EGT when up high (9000 feet high or so)? What about run at peak down low but limiting MAP say 21 or 22?

Thank you

Moura
RV10
IO540
Hertzel CS.
175hours in 12 months.
 
Nucleus, you mentioned cruising WOT. What if cruising at 3500 feet above sea level? I normally operate LOP at most altitudes but I usually use 24MAP and 2400 RPM and about 50LOP...


Look at the graph in post #2 and #3... as long as you are leaner than 50, even at sea level and WOT, you are out of the "red box".

Also, this picture from another thread showing 3500 MSL, WOT and 2320 RPM where the engine was quite happy:

14jn76u.jpg
 
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WOT at 3500

Nucleus, you mentioned cruising WOT. What if cruising at 3500 feet above sea level? I normally operate LOP at most altitudes but I usually use 24MAP and 2400 RPM and about 50LOP. How does that sound? Reading lots of articles I had the idea that if I were to fly low and LOP I should limit power limiting MAP. Does it make sence? If I keep WOT low wouldn't I get more power than I should risking to damage the engine?

Mauro, the meme about LOP being dangerous down low is part of what led me to start this thread.

One of the things I did as I was training myself to do LOP was to print out a copy of this chart
KeyRedBoxGraph1LOP.jpg

and keep it in the cockpit with me. Of course there are a lot of variables for each aircraft, but as long as you have have good instrumentation on each cylinder, it is safe. Safer than 50 ROP in my opinion.

This chart is likely over cautious by the way. Deakin's main focus is big turbocharges engines. I run 9:1 pistons in my 6 and I stay very cautious. I have killed (car) engines with detonation before. I don't want my engine to detonate at all. To me cautious means run even leaner at low altitudes, like 90-120 degrees LOP at 500 feet MSL.

Hans
 
So it seems to me that as long as you have cool CHT you are safe. Most of the time I am running LOP with CHTs around 330, except cylinder 6 that is always around 360 due to oil radiator.
I do run LOP when low but always throttle backback to 24MAP before doing so. I want try WOT next time. What about prop. Flying low at high RPM wouldn't allow for a way too high power? Say 26MAP and 2600RPM LOP.
Sorry if my questions (or myself) sound stupid. I just have read so much about it and still it is not totally clear to me when going LOP low and at high RPM. BTW I Fly and RV10 with IO540D4A5.
 
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First, the scary "red box": The good shops run your brand new engine at full rated HP on a dyno, near sea level, well within the box (80-150 ROP). Why don't they detonate into oblivion?

Why not? Think about it for a minute. I run like this every flight for a few minutes some times..........they do not detonate into oblivion in a mater of seconds. The Dyno runs are for short periods of time not hundreds of hours.

What will kill your engine really quick is pre-ignition. The biggest problem with operating at near detonation is high CHT and high valve temps, combined with any poor manufacturing tolerences and thus short top end life.

My engine is the standard IO540, hard to kill, but I understand they have a much lessor detonation margin than say the IO320's and 360's. Its not much different, but if you look at which engines get MOGAS STC's from Petersen's

http://www.autofuelstc.com/autofuelstc/pa/Approved_Engines.html

No IO540 D4A5 in that list :(

And that I understand is due to detonation margin.

Watch what happens when your mixture cable gets pulled out of sync with the stops because the bracket flexes with the engine mounts sagging.....and it pulls the mixture off the stop during the takeoff. :eek: When you see a couple of CHT's climbing better than the others just like your VSI, that is detonation. Did our engine die straight away, no, but we never let it go for very long until we found the problem. Had we flown that way with a single point CHT we would never know until one day a couple of hundred hours later we had a problem.
 
So it seems to me that as long as you have cool CHT you are safe...

It seems that this is what most of the experts are saying... Also, remember that Deakin is discussing big bore, turbo engines, and many of our low compression Lycs can't detonate even if you try.
 
Good point Dan. To expand on that, it has been argued by some that it is not even possible to cause a naturally aspirated Lycoming to detonate. While I?m sure that there are some extreme examples (a high compression engine on mogas, perhaps), the fact that our fleet is not falling out of the air on a regular basis has a lot more to do with engine design than our collective skill at engine management. Let?s face it, we?re pilots ? if there is a way to screw something up, we?ll find it! I don?t know for sure, but perhaps detonation in most of our engines is like the boogey man ? mostly a figment of our imagination ? and the only thing we really need to worry about is CHT management?

For the record, does anyone have firsthand knowledge of a N/A Lycoming failing due to detonation?

And you make a good point, Michael. Perhaps there are few instances of failure due to detonation because CHT is a limiting factor in high power situations - pilots are using fuel flow to augment cooling.

All the data assembled here at the beginning of this thread is most impressive, I've read bits and pieces of it over the years but have not seen it all so well presented. Thanks, Hans.

I like Mike Busch's take on all this regarding the red box, it is a bit more simplified. He recommends using take off EGT as the reference and leaning to it during climb. Maybe Deakins does the same, I have not read the entire post.

This is interesting stuff.
 
Deakin calls it TARGET EGT, its the only way to climb :)

And yes the low compression as you call it, or standard LYC can be run in detonation.

Go try it some time with all EGT and CHT probes and a good display, it will make your eyes :eek:
 
"John Deakin is a 35,000-hour pilot who worked his way up the aviation food chain via charter, corporate, and cargo flying; spent five years in Southeast Asia with Air America; 33 years with Japan Airlines, mostly as a 747 captain; and now flies the Gulfstream IV for a West Coast operator.

He also flies his own V35 Bonanza (N1BE) and is very active in the warbird and vintage aircraft scene, flying the C-46, M-404, DC-3, F8F Bearcat, Constellation, B-29, and others. He is also a National Designated Pilot Examiner (NDPER), able to give type ratings and check rides on 43 different aircraft types."

I see Mr. Deakin is a very experienced pilot but what about his engineering/test background? Maybe this short bio doesn't include his technical expertise or background - anybody know?
 
...And yes the low compression as you call it, or standard LYC can be run in detonation...

There are quite a few experts who would disagree with you... Not under "normal" circumstances anyway.

Also, while I'm not among the "experts", I do know that I have never seen detonation as evidenced by runaway CHT on any Lycoming I've flown - and I have done some dumb things with the mixture control over the years.

Just my (nearly) undeducated opinion here, but I'll bet if the engine has an STC or is otherwise capable of operating on the watered down Mogas, it's nearly impossible to induce detonation in that engine on 100LL.
 
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My 2 cents

I read this article several months ago and was all psyched to go try LOP. I fly a -7 with a Superior IO-360 and C/S prop. I don't have GAMI injectors.

I usually run 65% and lean to 25 ROP on the first cylinder to peak. My field elevation is 3300 and CHT's are 340 in cruise.

My findings: it was very difficult to get all cylinders LOP. Some were, others were right at peak or ROP. And as soon as you change altitude everything changes. I would spend so much time staring at the EFIS I was afraid I was going to fly into something!

I don't like an engine that is borderline "rough" running. He says it may not hurt anything but I don't like the feel. The fuel economy ain't worth it. For me.

I said forget it and went back to my old way.

As for carbon build up and fouled plugs....... I add Decalin Runup to every tank of 100LL and lean the engine on start up and taxi and of course cruise.

On inspection of the lower plugs recently they were burning very well with no lead balls on the electrodes. This after idling for an hour at OSH waiting to depart.

LOP ain't for me.

Darren
 
...I don't like an engine that is borderline "rough" running...

Ignition quality has a huge impact on "roughness". Before the dual Pmags I could not even get to 30 LOP without noticable rough operation. Without any other changes, I can do 150+ LOP now (before stumble)
 
I don't have GAMI injectors.

Darren

Lucky for us Lycoming opertors, we don't need GAMI injectors on a Lycoming. Lycoming injectors have removable restrictors that come in different sizes. You can search injector matching for the info on how to do it.
 
All it takes is a call to Don at AFP and run their test cases. Send them your data and they will provide a recomendation on how to balance your injectors.
 
Since Don from AFP was mentioned, I did replace my restrictors and they were perfect. Only replaced 3 for 25 dollars (I think that is what I paid) each.
But you know what puzzles me, Don mentioned he would never run not even an inch above 24MAP when LoP!! Should I be confused?
Moura
 
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Since Don from AFP was mentioned, I did replace my restrictors and they were perfect. Only replaced 3 for 25 dollars (I think that is what I paid) each.
But you know what puzzles me, Don mentioned he would never run not even an inch above 24MAP!! Should I be confused?
Moura

Yep, and for me the number is 23. That's the thresh hold into the dreaded red box or 75% power.
 
Yep, and for me the number is 23. That's the thresh hold into the dreaded red box or 75% power.

For those that believe a "red box" even exists for our engines, remember that "power" is MP and RPM... You can still run big MP, but use low RPM to drag the % power out of the red box if you choose... Look at the picture in post #17. Thats nearly 26 inches of MAP; happily LOP at low RPM.
 
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For those that believe a "red box" even exists for our engines, remember that "power" is MP and RPM... You can still run big MP, but use low RPM to drag the % power out of the red box if you choose... Look at the picture in post #17. Thats nearly 26 inches of MAP; happily LOP at low RPM.

What is best in that case (flying low), high MAP and low RPM, low MAP and high RPM or somewhere in between such as 24/24?
 
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"Best" is a tough subject...

WOT (high MP) produces the best VE from an engine...

High RPM without MP makes lots of friction and noise...

While I'm no expert, it sure seems like MP is king... After all, the main proponents of this LOP thing are the turbonormalized guys. Those engines will pull 30 inches of MP from the ground up to the flight levels, so if high MP was bad, they would certainly not spend to money to make it. Mooney (and others) have offered "ram air" systems; many of the aftermarket cowls offer the same - all with the stated benefit of increased MP. Nobody would install/use this equipment, then negate the benefits by partly closing the throttle, would they?

If I'm not in an extreme hurry, I'll crank the RPM way down and run it WOT even down low. In fact, the only time my engine is not WOT is in the pattern, or really just loafing around sighteeing. If I'm going from A to B - it's on the stop, wide open. I use RPM and mixture for power output.
While there are some MP/RPM combinations that may cause resonance issues with certain prop/engine combinations, I doubt you can actually "lug" an aircraft engine enough to cause damage. The power output would be so low as to be impractical for flight.
 
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Not sure if I can get this to format correctly but I did a test flight today in my RV6 and recorded some data points. Those who have high compression engines may be interested. The engine is a ECI IO360 with their fuel injection and dual lightspeeds. The Pistons are 10 to 1.

All data points are at 8500 feet, 68F OAT, WOT and 2500 RPM with a RV200 prop. All speeds are knots.

FF 10.7 CHT/EGT 332/1260 358/1299 354/1245 363/1287
120 ROP 154 IAS 183 TAS

FF 8.9 330/1380 360/1423 353/1363 365/1410
peak 149 IAS 177 TAS

FF 8.0 309/1337 338/1379 332/1328 346/1379
50 LOP 147 IAS 175 TAS

FF 7.5 288/1317 317/1344 312/1312 323/1343
80 LOP 143 IAS 170 TAS

Glancing at the numbers a couple of things surprised me. The first was that my CHT's were not really different from 120 ROP to peak. It may be that I did not allow enough time for the temps to stabilize at the 120ROP setting. The peak numbers I see all the time and are right on. I am going to repeat that part another day and see what happens. CHT's dropped rapidly LOP. The other thing was that IAS loss was only 11 knots from 120 ROP to 80 LOP however fuel flow was down 2.2 gallons. I was actually trying to run 100 ROP so did not hit the numbers quite on but 154 is pretty much best power IAS at 8500.
I would appreciate it if anyone is seeing anything in these numbers that would suggest any issues with LOP ops on my aircraft. Getting ready for the first couple long cross countries and plan on LOP ops unless of course I chicken out. As mentioned in past threads I have very little piston engine experience and still trying to get a grasp on how to best operate my engine.

George
 
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Not sure if I can get this to format correctly but I did a test flight today in my RV6 and recorded some data points. Those who have high compression engines may be interested. The engine is a ECI IO360 with their fuel injection and dual lightspeeds. The Pistons are 10 to 1.

All data points are at 8500 feet, 68F OAT, WOT and 2500 RPM with a RV200 prop. All speeds are knots.

FF 10.7 CHT/EGT 332/1260 358/1299 354/1245 363/1287
120 ROP 154 IAS 183 TAS

FF 8.9 330/1380 360/1423 353/1363 365/1410
peak 149 IAS 177 TAS

FF 8.0 309/1337 338/1379 332/1328 346/1379
50 LOP 147 IAS 175 TAS

FF 7.5 288/1317 317/1344 312/1312 323/1343
80 LOP 143 IAS 170 TAS

Glancing at the numbers a couple of things surprised me. The first was that my CHT's were not really different from 120 ROP to peak. It may be that I did not allow enough time for the temps to stabilize at the 120ROP setting. The peak numbers I see all the time and are right on. I am going to repeat that part another day and see what happens. CHT's dropped rapidly LOP. The other thing was that IAS loss was only 11 knots from 120 ROP to 80 LOP however fuel flow was down 2.2 gallons. I was actually trying to run 100 ROP so did not hit the numbers quite on but 154 is pretty much best power IAS at 8500.
I would appreciate it if anyone is seeing anything in these numbers that would suggest any issues with LOP ops on my aircraft. Getting ready for the first couple long cross countries and plan on LOP ops unless of course I chicken out. As mentioned in past threads I have very little piston engine experience and still trying to get a grasp on how to best operate my engine.

George
I would be suspect of your temperature data. At least in comparison to what I have seen with my engine (ECI IO-340 with 9.0:1 CR). I typically see:
  • CHT's in the 375-400 and EGT's in the 1250-1350 range when running around 100 ROP.
  • CHT's in the 290-320 and EGT's in the 1350-1400 range when running around 50 LOP.

When analyzing your performance runs, your data shows that when you compare the differences between the 120 ROP run and the 80 LOP RUN:
  • your 2.2 gal/hr decrease in fuel flow is yielding a change of 29.91%
  • your 13 KNOT speed decrease is yielding a change of 7.10%.

So these numbers are the basis of the decision you have to make. Is it worth a 7% decrease in speed in order to see a 30% decrease in fuel burn?

My last comment pertains to your statement about ". . .unless of course I chicken out." This I don't understand. What are you afraid of?
  • When reading this thread or other threads dealing with LOP operations are you still disbelieving those who are posting that they are successfully running LOP?
  • Are the expert opinions, stating the likelihood of damaging our small engines is really very small, not convincing enough?
  • Is your own data not convincing you that running LOP is a realistic goal you can achieve?
If your temperature readings are accurate then you surely do not need to worry. As has been posted here and elsewhere, the key is to keep the CHT temps down. If you are successfully doing this there should be little, if anything to worry about.

Live Long and Prosper!
 
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My big concern is with the compression. I have yet to find someone running 10 to 1 who also runs LOP. I agree the data is compelling however the engine manufacturers still all seem against LOP ops in high compression engines. Most however seem against it because they don't have any real data.
I don't know about the temps. I did not build the aircraft however the builder was from Arizona and spent a lot of time working on the engine cooling with a custom cowl and plenum. Below is a quote from his website that leads me to believe the indications are correct. Your aircraft seems to have a bigger drop when LOP. What may be a factor is perhaps I did not give the temps enough time to stabilize. I will look at that when I take a long trip. The plenum could also be a factor in the lower drop I am seeing. The top numbers I do like however.

"The plenum took almost 35 hours to complete but the benefits were; CHT down to 430 degrees on the first flight of my new ECI engine in Phoenix OAT 95 degrees. When the rings seated, about 2 hours later, the CHT went down to 360 degrees."

360 seems to be around the max I am seeing. The other day with the takeoff temp at 99 degrees I saw closer to 380 however I was climbing at 100 knots for the first 3000 feet. I am very pleased with the CHT temps. It was not the builders first RV and he really understands cooling and engines. Even though I did not buy the aircraft from him directly he spent a great deal of effort discussing the aircraft and its construction with me. Seems to be the norm with the people in this community. Everyone is beyond helpful.

Edit. I just realized you might be talking about the difference in EGT's more then CHT's. Not sure why we would see such different data points as far as drop ROP to LOP.

George
 
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George's data seem comparable to what I see on my Aerosport Power (ECI parts) IO-360B1B. I dont see how you can argue against LOP with reduced CHTs and reduced fuel flow. As long as you are monitoring all cylinders, dont be afraid to play with the mixture knob and see how it changes things. If its running rough, the temps are elevated, or your speed has dropped below an acceptable level, adjust the mixture! Roughness and temps will tell you if your engine is happy or not

erich
 
My big concern is with the compression. I have yet to find someone running 10 to 1 who also runs LOP. I agree the data is compelling however the engine manufacturers still all seem against LOP ops in high compression engines. Most however seem against it because they don't have any real data.
I don't know about the temps. I did not build the aircraft however the builder was from Arizona and spent a lot of time working on the engine cooling with a custom cowl and plenum. Below is a quote from his website that leads me to believe the indications are correct. Your aircraft seems to have a bigger drop when LOP. What may be a factor is perhaps I did not give the temps enough time to stabilize. I will look at that when I take a long trip. The plenum could also be a factor in the lower drop I am seeing. The top numbers I do like however.

"The plenum took almost 35 hours to complete but the benefits were; CHT down to 430 degrees on the first flight of my new ECI engine in Phoenix OAT 95 degrees. When the rings seated, about 2 hours later, the CHT went down to 360 degrees."

360 seems to be around the max I am seeing. The other day with the takeoff temp at 99 degrees I saw closer to 380 however I was climbing at 100 knots for the first 3000 feet. I am very pleased with the CHT temps. It was not the builders first RV and he really understands cooling and engines. Even though I did not buy the aircraft from him directly he spent a great deal of effort discussing the aircraft and its construction with me. Seems to be the norm with the people in this community. Everyone is beyond helpful.

Edit. I just realized you might be talking about the difference in EGT's more then CHT's. Not sure why we would see such different data points as far as drop ROP to LOP.

George
It sounds to me that your engine is happy running LOP.

I think most engine manufacturers are not willing to accept LOP operations because of the lawyers. They are afraid there is too small of a margin for all of the variables associated with installation and operation when running LOP. Without a pilot understanding what he is doing and without proper instrumentation to monitor there could potentially be problems. However, in our experimental world most everyone I know flying LOP has a great wealth of information readily at their disposal. Of course, we are fortunate in the experimental world to have this detailed instrumentation. The certificated GA world is not so blessed. I am sure these manufacturers are thinking in terms of all of those GA aircraft that may be woefully inadequate when it comes to engine monitor instruments. So they are going to err on the side of conservative views on this issue.

I forgot to mention what engine I am running in my previous post (I have edited the previous post to reflect my engine information). I am running an ECI IO-340 with 9.0:1 CR pistons. As far as the temps on my engine, I am definitely seeing a decrease in CHT temps when I run LOP. I usually will not see CHT temps above 325 and usually they are closer to 310 during cruise. When I run ROP the CHT are closer to 375-380 with the hottest pushing toward 400 in some instances. There is a drastic difference in CHT temps LOP vs. ROP with my engine. Not sure why.
 
Another point that seems to be missed with those in "fear" of LOP is that detonation is a chemical thing that can be eliminated both with too rich and too lean a mixture. If Deakin's chart is right, then 50 or more LOP will NOT support detonation, regardless of MP, RPM, CR or any other reasonable variable we are likely to find in our engines.

If I'm reading this stuff right, too lean kills a bit of power, but it also seems to kill ALL detonation. The only part that really requires skill is jumping across the evil red box quickly enough to not do any damage. And even with the big turbo engines, you still have plenty of time, apparently.
 
What is best in that case (flying low), high MAP and low RPM, low MAP and high RPM or somewhere in between such as 24/24?

In the context of detonation, any of the above is fine. Only the high MAP, low RPM case has limits.

I do think you can detonate a 8.5 -8.7 CR normally aspirated Lycoming. I just don't think the "red box" for those engines is anywhere near as large as the oft-repeated Deakin graphic would suggest. The graphic is a great illustration of general principles, but the numbers are not gospel. For our little engines I don't think they are even close.

Lycoming offers guidance (or did) on operation limits due to detonation. It's right there on the old power charts; look for the line titled "Limiting Manifold Pressure For Continuous Operation".

Here's a sample chart for an O-320 (an illustration; find and use the chart for your engine). The assumption is best power mixture, meaning the limit line is a worst case. Any combination of RPM and manifold pressure to the left of the line is acceptable at any mixture, since richer or leaner than best power is less likely to detonate, not more. You could probably move to the right of the line if you canceled detonation with a very lean or very rich mixture.



And the first question I proposed back in post #6, about engines on the dyno? Look at the chart....given CHT within limits, you can't detonate it at 2700 RPM. Without a turbo there isn't enough manifold pressure.

So, the second question. If detonation risk is low, then what is the failure that brings on early cylinder replacement?
 
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Good info Dan,

The thing I find interesting is the fact that despite the warnings of this power chart, many pilots are in the habit of pulling the prop back right after takeoff for a variety of reasons (noise, "easier" on the engine, etc), yet still maintaining WOT and best power mixture. There are plenty of times that an engine is operated at high enough MP and low enough RPM to be to the right of the limit line of this chart... Heck, there are plenty of fixed pitch applications that run in this zone for a large percentage of each flight. For all the people that do this, where are the failures?
 
Detonation Experience - what it looks like

I had a problem with an electronic ignition several years ago that resulted in detonation. It was certainly moderate detonation, if not heavy. I did a short flight and shut down, then restarted again and took off. CHTs were probably just under 400 degrees at the start of the takeoff roll. By a couple hundred feet in the air, the CHT alarm was going off and the #4 cylinder CHT was climbing rapidly through 430 degrees. I don't remember the exact temps or rate temp change, but it was readily obvious something was wrong. I pulled the power way back and came back to land. Temps came back down with the power reduction.
I checked the cylinder. It was very clean, but the plug looked undamaged. The inside of the cylinder and piston looked okay. I've got another 400 hours on the engine now.
To summarize, detonation and its attendant CHT increase will not go unnoticed with a full engine monitor and good alarms.

Seb Trost
RV-7A
Boulder City, NV
 
So, the second question. If detonation risk is low, then what is the failure that brings on early cylinder replacement?

Does it have to do with lack of carbon build-up on the pistons, which acts as an insulator, thereby protecting them from intense heat when ham fisted pilots don't stay away from the "red box"? It seems I read something along those lines in the Sacramento SkyRanch Manual a few years ago.
 
Detonation?

By a couple hundred feet in the air, the CHT alarm was going off and the #4 cylinder CHT was climbing rapidly through 430 degrees.

Is a CHT of 430 degrees indicative of detonation? Doesn't Lycoming say that 500 degrees is the max temp for continuous operation?
 
With regard to the LIMITING MAN PRESS FOR CONTINUOUS OPERATIONS note on the performance charts, it appears the limitation applies only to engines with a CS prop.

Engines certified with a fixed pitch prop do not have the limitation. The only time they are right of the limitation line is for take off. My engine isn't even on the chart at 28" MP and 2180 RPM, but as soon as RPM is up to 2300, it doesn't matter, it would be right of the line if there were one.

So, I guess the bottom line is Lycoming does not recommend continuous ops at say 27" and 2000 RPM (which of course is impossible with a FP prop.) That may explain an old procedure to reduce MP to 25" after take off. It put the operation left of the line no matter what the RPM.

(Except of course with the H6 Subby. With it, it was WOT and manage power with RPM. The ECU responded appropriately, you could cruise around at 1700 RPM, WOT and be burning 5-6 gph. There are good things to be said about modern engine ECU timing and fuel control not possible with a Lycoming.)
 
Is a CHT of 430 degrees indicative of detonation? Doesn't Lycoming say that 500 degrees is the max temp for continuous operation?

I think in this situation what he means is that the CHT rising rapidly above the norm was caused by detonation. The detonation could have started with the CHT's much lower but drove the temps up rapidly. The detonation itself was caused by the ignition issue.

George
 
Is a CHT of 430 degrees indicative of detonation? Doesn't Lycoming say that 500 degrees is the max temp for continuous operation?

Nope. Max continuous for high performance cruise is 435, for economy cruise it is 400 and the never exceed limit is 500.

Lycoming says 400-435 is OK for cruise, Mike Busch says for long engine life 400 should be the red line and the norm for cruise 360. I buy into that. At 400 the cylinders are at half strength.

Without a knock sensor, I don't think it is possible to detect detonation, these things are so noisy. If you hear the familiar pinging like in an old stick shift auto, it is detonation. Even with a knock sensor, it might not work because there is a lot of normal knocking going on all the time. How would it know what is normal and what is detonation? :)

There are a couple simple techniques to set the engine at less than 75% power. With a fixed pitch prop, it is 23" or less MP. With a CS prop the 23" will work but RPM enters the equation also, if it is set 2300-2400, you are safe. At less than 75% power, the engine can not be harmed by leaning, it might quit if you're too aggressive but you can not harm it. :)

Seriously, if the engine does quit due to over leaning, push the mixture forward and it will spring back to life immediately. Some times when flying into cold air on a cross country, it will sound like it is ready quit, push the mixture in a bit and problem over.

The premise of detonation occurring is a stretch with these engines unless one drags it around on the right side of max MP line. I don't know if Lycoming meant that line to be a detonation line or not, probably not since its limitation is not for continuous ops. If it were a detonation line it would say never exceed.
 
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"So, the second question. If detonation risk is low, then what is the failure that brings on early cylinder replacement?"

I'll take a shot at that question:

Valve failure caused by poor valve geometry, angles not matching with the seat, valve not concentric with the seat leading to the valve not fully transferring it's heat to the head.
 
Rate of increase was the issue here

Is a CHT of 430 degrees indicative of detonation? Doesn't Lycoming say that 500 degrees is the max temp for continuous operation?

The temperature was increasing more than 1 degree a second (give or take, no stopwatch going at the time). The rapid increase was the point, not a particular value. It may have come apart well before getting to 500 degrees.

Seb Trost
RV-7A
Boulder City
 
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