LOP operations

[Ted]

Guess what: it's not 50-100 ROP, it's 25 ROP. Right where Ted said he'd never want to run an engine.

This has been something that I've always wondered about since I got into aviation, and haven't found much in the way of good answers. We all know that's a terrible place to run an engine.

One old timer I talked to said the reason those recommendations were made was that they found most pilots who thought they were running at peak or 25 ROP with a traditional EGT gauge were actually running at peak or LOP. I'm not sure I buy that, but he was around then, I wasn't.

Anyway, so long as you're not at 25 ROP, you're doing better than what Cessna recommends.

 

[Henning]

I didn't want to say anything here until I'd had a chance to check my POH to see what mixture setting Cessna bases their power table values on. It turns out it's not "economy cruise" at all but something called "recommended lean". I guess from the description that it's how they recommend you run your engine in cruise.

Guess what: it's not 50-100 ROP, it's 25 ROP. Right where Ted said he'd never want to run an engine.

But I'm really not sure whether I could still expect 10% less power as compared with "recommended lean", with the mixture set where I'm talking about, which isn't quite "safely LOP" but with the two back cylinders LOP and the front two at peak or slightly, maybe 10-15 ROP. That's the best I can do without roughness. It seems there would be a lot of variables going into a calculation like that and it would depend to some degree on the design of the cylinders.

The number LOP or ROP is meaningless by itself. It represents a combustion condition in that specific engine is all. What that condition will qctually represnt in the way of harm available will be dependent on several factors, most important in this case is the compression ratio of the pistons and ignition / ICP timing (which you can vary the ICP timing in flight with the use of the prop handle.) If your engine is of such a design that it cannot get into detonation as equipped, then there is no risk operating 25ROP.

 

[Ted]

If your engine is of such a design that it cannot get into detonation as equipped, then there is no risk operating 25ROP.

While there may not be a "risk" as far as an immediate catastrophic failure, it is worth noting that doesn't mean that one should operate there if avoidable. The higher ICPs and CHTs aren't going to be good for your engine longevity.

 

[Henning]

While there may not be a "risk" as far as an immediate catastrophic failure, it is worth noting that doesn't mean that one should operate there if avoidable. The higher ICPs and CHTs aren't going to be good for your engine longevity.

Not necessarily true. As long as maximum achievable ICPs are within the design limits of the engine you have what we refer toa Continuous Duty Rating. If that happens with the throttle WFO and mixture set where it can possibly have the most damaging effect, it doesn't matter, you're not damaging anything.

Just because an engine is putting out everything it can doesn't mean harm is being done.

 

[gismo]

This has been something that I've always wondered about since I got into aviation, and haven't found much in the way of good answers. We all know that's a terrible place to run an engine.

One old timer I talked to said the reason those recommendations were made was that they found most pilots who thought they were running at peak or 25 ROP with a traditional EGT gauge were actually running at peak or LOP. I'm not sure I buy that, but he was around then, I wasn't.

Anyway, so long as you're not at 25 ROP, you're doing better than what Cessna recommends.

The story I heard (from APS) is that 25 ROP was dictated by the marketing department of the aircraft manufacturers against the engine mfg's engineers recommendations. 25 ROP was chosen because it represents a significant (and marketable) increase in range without a significant reduction in speed at typical cruising altitudes. I personally suspect that the widespread but incorrect belief that EGTs correlate with valve head temps and that peak EGT represented "worst case" conditions for the valves also led to the 25 ROP compromise.

 

[Ted]

Not necessarily true. As long as maximum achievable ICPs are within the design limits of the engine you have what we refer toa Continuous Duty Rating. If that happens with the throttle WFO and mixture set where it can possibly have the most damaging effect, it doesn't matter, you're not damaging anything.

Just because an engine is putting out everything it can doesn't mean harm is being done.

While that is true, there isn't a hard line above which things will fail quickly and below which they will last forever. It's a sliding scale, and thus you'll be better off if you slide a bit further down the scale.

The story I heard (from APS) is that 25 ROP was dictated by the marketing department of the aircraft manufacturers against the engine mfg's engineers recommendations. 25 ROP was chosen because it represents a significant (and marketable) increase in range without a significant reduction in speed at typical cruising altitudes. I personally suspect that the widespread but incorrect belief that EGTs correlate with valve head temps and that peak EGT represented "worst case" conditions for the valves also led to the 25 ROP compromise.

That seems of equal or greater plausibility to the story I had heard.

 

[Tom-D]

This has been something that I've always wondered about since I got into aviation, and haven't found much in the way of good answers. We all know that's a terrible place to run an engine.

One old timer I talked to said the reason those recommendations were made was that they found most pilots who thought they were running at peak or 25 ROP with a traditional EGT gauge were actually running at peak or LOP. I'm not sure I buy that, but he was around then, I wasn't.

Anyway, so long as you're not at 25 ROP, you're doing better than what Cessna recommends.

Ted, I was around then, And can prove that when you place a EGT, CHT, on the 0-200/0-300/0-320 / or the carb'ed 0-360, and a BMEP pressure tap under the spark plug, you can prove that pulling mixture to lean to lean misfire and then enriching it the engine runs smooth there will be 1 cylinder lean enough to misfire) (lean of peak by 100) and one still in the 50-100 degree range rich of peak.

That is the recommend leaning method in the POHs of the Cessnas, and most of the Piper, non injected engines.

That is why we always loose 1 cylinder before we have any problems with the others.

That is also why both manufacturers recommend that operators use full rich for full power operation, they know the engine does not run cylinder efficiencies close enough to operate LOP at full power.

That is the theory behind the Gami injectors in FI engines, simply because the airflow is never the same.

 

[kgruber]

That is the theory behind the Gami injectors in FI engines, simply because the airflow is never the same.

The airflow is pretty good. It is the fuel flow that varies.

 

[Tom-D]

The airflow is pretty good. It is the fuel flow that varies.

fuel flow is varied, because the holes in the gami injectors are sized to the airflow to each cylinder.

the airflow and fuel flow are both varied by the injector pump to the needs of the power setting.

 

[John Collins]

fuel flow is varied, because the holes in the gami injectors are sized to the airflow to each cylinder.

This isn't true for the big bore TCM IO520/550 engines with the induction manifold with the short runners that connect upward to the intake ports on the cylinders. The airflow is well balanced as evidenced by the smooth running of these engines when ROP.

On these engines the Gamijectors balance the fuel flow to the cylinders. If the injectors are all of the same size, the rear cylinders will run leanest, followed by the middle cylinders being richer, and the front cylinders being much richer. The reason for this is two fold. First the injectors are continuously injecting fuel into the intake chamber above the intake valve, but the intake valve is only open during a portion of the cycle. When the valve is closed, the fuel is drawn by gravity and the low pressure of the intake manifold down into the manifold. The fuel from the rear cylinder is transported forward into the front two cylinders, enrichening the mixture in both of them. In the same way, the middle cylinder's fuel is transported to the front cylinder, enrichening it even further. The Gamijectors compensate for this by having a larger opening in the rear cylinder (papa bear), a smaller opening for the middle cylinder (mama bear), and the leanest opening for the front cylinders (baby bear).

Engines that have overhead induction systems tend to run smoother and need less correction.

When running ROP, air flow controls power as fuel is in abundance (more than needed for combustion). Running LOP, fuel flow controls power as air is in abundance. If we had an air flow meter in our airplane we could use it to measure power when ROP, but alas we don't, so we look up the power settings in tables that have all the factors that contribute to the amount of air flow is being provided to the engine (manifold pressure, temperature, altitude, RPM). On the lean side, if you have a digital fuel flow indicator, you have all the information you need to establish the power setting.

 

[kgruber]

On these engines the Gamijectors balance the fuel flow to the cylinders. If the injectors are all of the same size, the rear cylinders will run leanest, followed by the middle cylinders being richer, and the front cylinders being much richer. The reason for this is two fold. First the injectors are continuously injecting fuel into the intake chamber above the intake valve, but the intake valve is only open during a portion of the cycle. When the valve is closed, the fuel is drawn by gravity and the low pressure of the intake manifold down into the manifold. The fuel from the rear cylinder is transported forward into the front two cylinders, enrichening the mixture in both of them. In the same way, the middle cylinder's fuel is transported to the front cylinder, enrichening it even further. The Gamijectors compensate for this by having a larger opening in the rear cylinder (papa bear), a smaller opening for the middle cylinder (mama bear), and the leanest opening for the front cylinders (baby bear).

Yes.

George Braly termed it "The occult migration of fuel." But he said it only once, years ago, on Avsig. I've never seen the term again, but I like it.

 

[Tom-D]

This isn't true for the big bore TCM IO520/550 engines with the induction manifold with the short runners that connect upward to the intake ports on the cylinders. The airflow is well balanced as evidenced by the smooth running of these engines when ROP.

On these engines the Gamijectors balance the fuel flow to the cylinders. If the injectors are all of the same size, the rear cylinders will run leanest, followed by the middle cylinders being richer, and the front cylinders being much richer. The reason for this is two fold. First the injectors are continuously injecting fuel into the intake chamber above the intake valve, but the intake valve is only open during a portion of the cycle. When the valve is closed, the fuel is drawn by gravity and the low pressure of the intake manifold down into the manifold. The fuel from the rear cylinder is transported forward into the front two cylinders, enrichening the mixture in both of them. In the same way, the middle cylinder's fuel is transported to the front cylinder, enrichening it even further. The Gamijectors compensate for this by having a larger opening in the rear cylinder (papa bear), a smaller opening for the middle cylinder (mama bear), and the leanest opening for the front cylinders (baby bear).

Engines that have overhead induction systems tend to run smoother and need less correction.

When running ROP, air flow controls power as fuel is in abundance (more than needed for combustion). Running LOP, fuel flow controls power as air is in abundance. If we had an air flow meter in our airplane we could use it to measure power when ROP, but alas we don't, so we look up the power settings in tables that have all the factors that contribute to the amount of air flow is being provided to the engine (manifold pressure, temperature, altitude, RPM). On the lean side, if you have a digital fuel flow indicator, you have all the information you need to establish the power setting.

It's called "volumetric efficiency" and the fuel must be compensated for it or you can't have all cylinders running at the same EGT, how the manufacturers deal with it is a matter of design.

 

[Ted]

Tom,

There are a number of planes that recommend 25-50 ROP as the leaned out best economy operating point. Those are the ones that Lance and I are referring to.

Some Navajo POHs even recommend LOP operation at certain power settings...

 

[Tom-D]

Tom,

There are a number of planes that recommend 25-50 ROP as the leaned out best economy operating point. Those are the ones that Lance and I are referring to.

Some Navajo POHs even recommend LOP operation at certain power settings...

To get any engine to run that close, Volumetric efficiency must be compensated for, it's simply a matter of how.

Carburated engines are impossible to do that.

 

[Ted]

To get any engine to run that close, Volumetric efficiency must be compensated for, it's simply a matter of how.

Carburated engines are impossible to do that.

When the POH states to run at a particular EGT, it's using a single-point EGT as reference. Obviously, this isn't representative of the whole engine. When these planes were built, engine monitors didn't exist. How many people had anything more than a single EGT or single CHT for temperatures? Virtually zero.

Those of us who know the details of engine operation know that's not going to be representative of all the cylinders. The people who wrote the original instructions were trying to make it simple.

 

[Dave Siciliano]

I was in Ada last week and gave this to George.


Dave
=====================================================
Posted on another board.

Many of us recognize the wonderful contribution George Braly has made to GA and especially those of us that have Beechcraft and have learned to fly them LOP. I purchased my TN A-36 just over ten years ago and was fortunate to attend what I seem to recall as the first APS seminar in Ada in September of 2002. One had to be a real believer in what George, Tim, Walter and John were preaching back then as it wasn't mainstream.
After I attended that class, visited Ada and chatted with those folks and let them work on my plane, I was able to run it well past TBO and, John Foose who purchased it from me ran it several hundred additional hours before replacing the engine.

Since then, I purchased a P Baron and ran it from 900 hours to over 2200 (1600 hour TBO) using a hobbs meter on the squat switch (more tach time), Over the years, George's advise has saved me a ton of money, helped me become a much better pilot regarding engine operating procedures and he's just become a long time mentor and friend.

In one of the many discussions we've had over the years, we realized we were both big fans of T. E. Lawrence (Lawrence of Arabia). We chatted about how unique he was; the insight he had and how I identified with him having been a former Special Forces officer.

About two years ago, I was contacted by a foundation that was reprinting a few hand bound copies of the 1922 edition of his book: The Seven Pillars of Wisdom. I ordered one for George and was able to deliver it in person when in Ada today.

Best,

Dave

 

[Henning]

While that is true, there isn't a hard line above which things will fail quickly and below which they will last forever. It's a sliding scale, and thus you'll be better off if you slide a bit further down the scale.

1/2 HP/ cubic inch is a real good line you can use for that determination.