High Power N/A ops LOP vs Full Rich

[Shadrach]

With all of the talk on MS lately about winter time, high power, LOP operations, I thought I'd do a quick analysis today during post MX ops check flight (new Skytec starter and Concord RG-35A).

I took off from a field elevation of 706ft, conditions as follows.

OAT was 40df,

Baro was 30.04

DA -400ft.

It was brisk and clear with pretty high winds but was perfect to run relatively high MP at WOTRAO down low. I departed KHGR with takeoff flaps and watched the VSI quickly wind out to ~1600fpm. At 2000 ft, I opened the Ram Air, closed the cowl flaps and brought the prop back to 2500RPM, MP gauge was showing ~29"MP. I flew for about 5 mins in this configuration at full rich. CHT on my richest cylinder stabilized at 306df while full rich, EGT was 1186DF. I went LOP and enrichened to peak which was 1433df and then leaned to 1373df (60LOP). CHT fell precipitously into the 290s and dropping. At this point I decided to look for the LOP setting that yeilded a CHT of 306df (stabilized full rich CHT). The magic LOP EGT number to get back to the full rich CHT was 1406df or 27df LOP, but even enrichening to 15df LOP only yielded a CHT of 312df.

As an interesting aside, the CHTs spread among all 4 was less than 20df on the lean side of peak. At full rich the CHT spread was in the neighborhood of 40df

My take is that the notion that you must be 50 LOP for high power ops with our little lycs is overkill. 27LOP produced the same CHT as 247ROP (full rich). Anything leaner than 27LOP was cooler than than full rich.

KIAS deltas were difficult to establish as it was a bit choppy and the needle was jumping about. I was indicating between 153KIAS and 160KIAS through the whole flight. I doubt that we were actually making 160KTS true at any point. If it had been smoother, I'd have made a 4 way speed run.

Has anyone had a similar experience?

[DS1980]

Hey Ross,

This is spot on. I've talked to one of my aviation mega-savant friends extensively about how lean to run and he said anywhere from 35-50 LOP, with his preference being 35 LOP. He flies out of Vegas, so your 23 LOP on a cold day is valid. He always adds a comment about CHTs being the deciding factor. I think when we finally get all details about LOP operation, we will find we were making it too complicated. Balanced fuel flow, run the engine lean enough, and let the CHTs be the deciding factor. Running LOP is simply asking the engine to do less work, and the engine doesn't care where it's ran as long as it's lean enough to allow acceptable CHTs.

 

I also like how you added "with takeoff flaps."   This one goes to 11.

[scottfromiowa]

Fuel Flow at each?

[Shadrach]

Fuel Flow at each?

Wish I could tell you Scott, but I've not upgraded my JPI yet. All I can say is that KIAS was in keeping with, or better than book numbers at 90%+ power. My bird is supposed to top out at 182 mph balls to the wall at sea level. I was between 178 and 185mph indicated for the whole flight.

Maybe I'll take some video today...

[201er]

Ross, how do you know CHT is a valid measure here? I think despite equal CHT, ICP at 27LOP are going to be more than at full rich. And what about detonation margins? You're probably running 85% power with high ICPs...

[Shadrach]

Ross, how do you know CHT is a valid measure here? I think despite equal CHT, ICP at 27LOP are going to be more than at full rich. And what about detonation margins? You're probably running 85% power with high ICPs...

2 things:

1) why would you believe ICPs to be higher on the lean side of peak if CHTs are lower?

2) the attached graph (credit Deakin) seems to contradict your assumption.

[201er]

The graph exactly demonstrates my point. 50LOP ICP = 250ROP ICP. So to keep the ICPs the same as at full rich at full power, you'd have to go 50LOP. You chose to go to 27LOP. I'm questioning if that is actually producing higher ICPs and putting you at higher risk of detonation than being at full rich.

[DS1980]

The only way to have acceptable CHTs but high ICPs is to have some sort of artificial cooling. If Ross left cowl flaps open on such a cold day,  I could see the argument. But with cowl flaps closed it's safe to say that CHTs represent ICPs. Regardless, hard to argue with 27 LOP and 306 degrees CHT. That's excellent.

 

I see what you are saying about 250 ROP vs. 50 LOP, but I believe that the combustion event on the lean side of peak isn't as violent as on the rich side of peak. Although I can't remember the flame front off the top of my head.

[jetdriven]

I've ran as much as 11.0 GPH LOP which is 83% power.  You have to be below 2000' or so DA to get away with this, otherwise its right at peak or slightly rich of peak EGT.

[Shadrach]

The graph exactly demonstrates my point. 50LOP ICP = 250ROP ICP. So to keep the ICPs the same as at full rich at full power, you'd have to go 50LOP. You chose to go to 27LOP. I'm questioning if that is actually producing higher ICPs and putting you at higher risk of detonation than being at full rich.

 

Sorry, I posted the wrong graph. See attached (credit GAMI Ada, OK).  The first graph I posted was a "conceptual" graph and I don't think it could possibly represent all engines under all conditions.  This graph shows actual cylinder pressures and verifies that what I think I was experiencing is totally possible. Note that Peak EGT shows lower ICP and a gentler power pulse than full rich.  As to your concern about ICPs being higher at 27LOP than full rich spite of 27LOP showing lower EGTs, I think that it's not likely if all other parameters remain equal.  If your airspeed is approximately the same, the OAT is exactly the same and MP remains constant, how on earth could higher ICPs translate to the same or lower CHTs?  

 

Byron, care to pose the question over on BT to see if Walt, John or George will chime in?  They've been kind enough to reply to private emails in the past, but I don't want to wear out my welcome.

 

[201er]

Ok, but then why do they show the red box with 50LOP being the same as 250ROP? Based on red box, 27LOP is bad. I'm not saying it is, but I want to know what makes it ok?

 

[carusoam]

Ross,

GAMI did these graphs using IO550s?

Not that it would matter much. Timing (20d BTDC) is probably the more important part. Moving the spark/graph up to 25d BTDC, the effect could be quite large. Especially when operating at 75-100 dROP. Surprisingly, they left the 50dROP line off the chart?

Best regards,

-a-

[carusoam]

Mike,

I would see being in the redbox as being bad. Specifically when we don't have direct data that indicates where we are at any time. Unfortunately, we can't measure and often can't feel pre ignition issues.

However, the box is theoretical. It covers an area of engine safety that avoids pre-ignition in a usable way.

Being safely outside the box is improved by altitude, CHT, ignition timing, and mixture...

Decreasing engine safety nearer or into the red box often improves performance. The operator must use knowledge to his advantage to do this.

Risk of pre-ignition comes by

(1) increasing manifold pressure. Chemical reactions occur faster as pressure increases. (Low Alt, or TC'd engines)

(2) increasing CHTs and EGTs. Any glowing ember in the cyl. can ignite the fuel mixture prior to normal spark timing.

(3) increased fuel flow - more fuel burning in the same cylinder, obviously leads to higher pressures.

(4) increasing the timing BTDC starts the burn further before the cylinder crests the top of the stroke.

(5) mixture, getting this "right", the burn rate is quick and peaky. I would have expected it would occur at peak EGT, but realistically it occurs at 50dF ROP.

(6) Bringing the timing back starts the burn faster, more exhaust gas volume is produced before the cylinder reaches TDC.

5 of 6 of these are directly controllable... #2 is challenging to control, at least the glowing ember part....

That's my take, avoid the red box by flying at higher altitudes, because I can't afford any more engines than one.

Best regards,

-a-

[Cruiser]

One concern that needs clarification about CHTs being used for power settings.

In the winter the additional cooling effect from low temperatures (compared to summertime) can produce low (acceptable) CHTs in the power range that is still producing higher ICPs than desired. So the idea of using CHT as the ONLY indicator for engine performance may be misleading if external cooling very high.

 

BUT this is NOT what Ross was doing......... He was comparing the ROP settings that resulted in a CHT to the LOP settings that gave him the same CHT. This is very different and not an approach to engine management. 

I am not surprised that the Lycoming O360 actual values are somewhat different, the graph is a generalization and the actual APS data is from a high power Continental. I would also expect that the same engine under different timing settings will be somewhat differenet as well. Byron has experimented extensively with this....... 

[Shadrach]

Ok, but then why do they show the red box with 50LOP being the same as 250ROP? Based on red box, 27LOP is bad. I'm not saying it is, but I want to know what makes it ok?

 

Both of the graphs you are referring to are "conceptual" graphs designed to keep new users from doing anything detrimental.  Have you heard the APS/GAMI folks use the term "fuzzy" red box?  This is in their words, the way they account for the fact that there is no way to measure ICPs in flight and that it changes depending on conditions and application. I would also submit that GAMI/APS are "For Profit Entities" with "For Profit Entity" liability risk...if their advice can be proven to be detrimental.  If you were in that position, would you not make your margins as big a possible.

 

I agree that CHTs do not give a full picture of engine ops when taken alone.  However, full rich is the best we can do to slow the combustion event with fuel to keep CHTs in check and it is deemed safe (if not dirty) by everyone from GAMI to APS to Lycoming and Continental. If you know what your engine's CHTs are full rich under specific conditions, how could the same CHT or cooler have higher ICPs on the lean side if all other parameters are the same as they were when full rich? High pressure = high ICPs = High CHTs.  I do not see how it is possible that a setting with the same of cooler CHTs can be more abusive then another all other things being equal. If anyone can, I'm open to learning how...

[carusoam]

As Tom points out, a low CHT could be a little misleading... We get more fuel/air mixture into our N/A engines in the winter and we see lower CHTs as well.

I would expect that I could be producing over 100% BHP with low winter DAs.

As Ross points out, all things being equal, might not be the case.

Cool cylinders are nice for not wearing out early, but it may not point out how far you are into the fuzzy red box...

This is just my humble opinion that I share,

-a-

[Shadrach]

Ross,

GAMI did these graphs using IO550s?

Not that it would matter much. Timing (20d BTDC) is probably the more important part. Moving the spark/graph up to 25d BTDC, the effect could be quite large. Especially when operating at 75-100 dROP. Surprisingly, they left the 50dROP line off the chart?

Best regards,

-a-

 

Timing certainly has an effect. Referring specifically to the engine in the graph... The engine is running Max RPM (2700 IIRC) at full rich the peak pressure pulse is happening very close to or right on the ideal crank angle of about 17ATDC. At peak the pressure pulse is happening a bit later. The take away here is that along with ignition timing, mixture has an affect on the speed of the flame front and the peak pressure, in fact it's the only thing we can control. What they don't demonstrate in the graph is that the crank angle at which PP pulse occurs can be altered by changing RPM.

[Shadrach]

As Tom points out, a low CHT could be a little misleading... We get more fuel/air mixture into our N/A engines in the winter and we see lower CHTs as well.

I would expect that I could be producing over 100% BHP with low winter DAs.

As Ross points out, all things being equal, might not be the case.

Cool cylinders are nice for not wearing out early, but it may not point out how far you are into the fuzzy red box...

This is just my humble opinion that I share,

-a-

Maybe not, but on the same day, same OAT and same altitude, 2 settings that produce identical CHTs and very similar Airspeeds should be producing the same ICPs.  If your cooling capcity has not changed, how could you have higher ICPs at the same or cooler CHTs.

 

Over 100% rated is a fact of life in the winter. I took off yesterday to try and get some video (too bumpy) and the DA at my airport (elev 706) was 1700ft below SL and baro was 30.42.  There is no question that the engine was making more than rated power.

 

You may be right in that I might have been running LOP in the fuzzy red box. My point is, that if that is the case, than full rich is also in the fuzzy red box...

[DS1980]

One concern that needs clarification about CHTs being used for power settings.

In the winter the additional cooling effect from low temperatures (compared to summertime) can produce low (acceptable) CHTs in the power range that is still producing higher ICPs than desired.

As we don't know yet the effects a cold day has on the relationship between CHT and ICP specifically to our make/model, we don't know if this is accurate, although it probably is and makes sense. With cowl flaps closed and the engine heat soaked, how far apart is the spread? How close is it on a warm day? These are the questions that would help a ton in answering a lot of engine management concerns.

 

I think with Ross seeing 306 CHT, the ICPs were acceptable. Maybe even a little higher than "normal."

[Shadrach]

One concern that needs clarification about CHTs being used for power settings.

In the winter the additional cooling effect from low temperatures (compared to summertime) can produce low (acceptable) CHTs in the power range that is still producing higher ICPs than desired. So the idea of using CHT as the ONLY indicator for engine performance may be misleading if external cooling very high.

 

BUT this is NOT what Ross was doing......... He was comparing the ROP settings that resulted in a CHT to the LOP settings that gave him the same CHT. This is very different and not an approach to engine management. 

I am not surprised that the Lycoming O360 actual values are somewhat different, the graph is a generalization and the actual APS data is from a high power Continental. I would also expect that the same engine under different timing settings will be somewhat differenet as well. Byron has experimented extensively with this....... 

 

Tom,

 

Why is it not an approach to engine management? Here in the Mid-Atlantic, and certainly in the mid-west 2500 to 3000 ft is very much a usable altitude when West bound fighting winds.  On a day like yesterday when conditions mean the DA at 2000MSL is ~300ft, why not use full rich CHTs as a target for LOP ops? It would likely save a significant amount of fuel without giving up much speed...

 

Another thing I'd like to add. After establishing that 27 LOP gave the same CHT as full rich, I went to 50LOP and the CHT drop from there was  pretty minimal as I recall... went from 306 to the high 290s.  

[Cruiser]

I think what you are doing is an excellent idea. Something I had not considered.

 

Without FF you can establish a comparison to ROP settings and use the LOP CHT as a guide to equivalent power. Pretty slick actually.

 

My intent of saying it was not engine management was related to the need to establish this relationship ROP/LOP CHT first, then once you have a table or chart developed you could use that.

[carusoam]

We should visit with APS in person, for a Mooneyday.

Do they have any arguments for or against running "over square" for extended periods of time? I would expect that the ICPs would increase and lead to exhaust escaping past the rings. If this were a problem, the oil would indicate an issue by getting pretty dark quickly.

-a-

Cris, are you still with me on the APS visit?

[jetdriven]

There is no argument against oversquare ops, in fact the Lycoming 360 operators manual allows for such power settings at 1800 RPM and 24". Those are all ROP settings as well, so LOP at those values is less ICP

[201er]

On a day like yesterday when conditions mean the DA at 2000MSL is ~300ft, why not use full rich CHTs as a target for LOP ops?

Could be that equal CHT does not mean equal ICP. The combustion event is shifted so the ambient temp in the cylinder is lower. I'm not sure that this guarantees the ICP is lower too. I admit I understand fairly little of this. But something about running full power 27LOP just doesn't seem right to me.

[DS1980]

But something about running full power 27LOP just doesn't seem right to me.

All comes down to altitude and % HP. At sea level this is no bueno. At 5000 feet there's nothing wrong with it.