Interesting LOP Experience and Not Sure of the Why of It?

[jlunseth]

I have a 231 with the Merlyn wastegate and Turboplus intercooler, as everyone probably knows by now.  After a good deal of engine work I was able to successfully cruise at LOP this past weekend, each time for about two hours.  I noticed some things. 

 

I achieved my LOP settings by using the Lean of Peak function on my JPI.  I would start at a ROP cruise setting I commonly use that the JPI says produces 75% power, hit the LOP buttons on the JPI and then lean it out.  Once on the lean side I found that I had to put quite a bit of MP back in.  I started at around 28" on the ROP side (2400 RPM), that would fall on the LOP side and I would put MP back in until it was 35.5 - 36".  I would lean until the richest cylinder was about -20 d LOP, which would put the leanest cylinders in the -50 to -60 range.  The fuel flow was 11.4 - 11.5, which if my math is correct put me at 75% HP.  (The multiplier for my engine is 13.7 x 11.5 = 157.55 HP, which is 75% of the rated HP which is 210).  The engine seemed fine at this setting. 

 

As is expected with LOP OPS, the CHT's went down by around 75 degrees to the 280-310 range.  The EGT's and TIT were about 100 degrees higher than ROP.  And this is my question, if anyone knows the answer.  Why do the EGT's and TIT go up?  This would suggest that more burning is occurring in the exhaust system once the gases leave the cylinder, than is occurring with ROP operations, although that does not make sense to me.  LOP means there is more O2 than needed to burn all of the fuel, ROP means there is less O2 than needed to burn all the fuel.  One would expect that there would be a more complete fuel burn in the combustion chamber when LOP than when ROP, and less fuel to continue burning in the exhaust system, but that does not seem to be what is happening.

 

Another interesting thing is that the CDT (Compressor Discharge Temp.) went down by around 50 degrees.  This is the temperature of the air leaving the compressor section of the turbo, and the lower temperature indicates that the turbo is running cooler, not hotter. The higher TIT suggests that the temp. of the gases coming into the turbo (which is mainly what heats the turbo) are higher not lower.  So an interesting side effect of LOP OPS appears to be that while the turbocharger is working harder (to generate the 36" MP), the turbo is actually running cooler for reasons I cannot fathom. 

 

Any thoughts?

[danb35]

How far ROP is the setting you started with?  It seems too obvious to mention, but if you started at 125 ROP and leaned to 20 LOP (probably not quite lean enough at 75%), your EGTs and TIT would by definition be 105 degrees higher.

[kortopates]

Although what Dan's says is very accurate, it may not be answering your question. At max cruise power settings we can and usually do operate much further ROP (150F or more) than we can operate LOP simply because power drops off very rapidly lean of peak compared to ROP. But to get to the heart of your question, a lean air-fuel mixture burns slower than a richer mixture, resulting in the higher EGTs/TIT that you are observing; combustion is continuing even as the exhaust valve opens resulting in higher temps downstream. Consequently, our ability to run higher LOP power settings in our turbo's is limited foremost by TIT. Or perhaps more realistically by our tolerance to operate closer to redline TIT. You undoubtedly also noted that your factory massive TIT probe reads between 50-100F cooler than the faster responding hastaloy-X-tip JPI probe which are in theory also more accurate than the factory massive probe. So although the factory probe is still the "legal" 1650F TIT limiting probe, I personally operate by the more limiting JPI probe reading 100F hotter and I also observe a more conservative limit of 1580F as my personal max TIT. Within that limit, I find I can operate LOP only up to 71.7% power (@11 GPH) to stay within that. However, I find I am only losing a couple knots TAS (from GPS testing) so I am not dissatisfied by that. Also I have the same LOP temp range you specified (~40 to -80 LOP). I know of  no real data though that would confirm lower or more conservative TIT management will actually improve turbo longevity. But if you’ve ever seen your turbo operating, you'll see it’s glowing a bright red above 1400F.

 

I personally had not noticed or observed lower CDT's and will remember to compare in a future flight. However, assuming RPM and altitude are unchanged, we know the same mass of air per unit time at the same ambient temperature are being compressed by the compressor, so Boyles law suggest the temperature should be higher rather than lower.

 

However, I wouldn't view a cooler CDTs as indicative of a cooler running turbo temperature. The turbo temperature that limiting to the turbo is TIT, from the hot exhaust gases going through it; which is running hotter LOP than ROP.

[N201MKTurbo]

The EGTs go up because a lean mixture burns slower then a rich mixture. This leaves more heat in the combustion gas when the exhaust valve opens. This also explains why the CHTs go down.

[jlunseth]

Well duh on the EGT's, of course if I run at say -20 LOP v. 120 ROP, the EGT's are 100 degrees warmer when LOP.  I should have realized that.  Thanks Dan.  But what I am really trying to puzzle out is what is happening in the exhaust system.  Since LOP involves a mixture that contains more O2 than needed to burn the available fuel, and ROP is the opposite, I would have thought most of the fuel would be burned in the cylinder when operating LOP, so the exhaust stream would be cooling as it moves down the collector to the turbo.  TIT would hypothetically be less than or equal to the EGT's, not more than the EGT's and by the same amount as when ROP.  But that is obviously not what is happening, there is some continued burning in the exhaust manifold both LOP and ROP.  The collector is quite long, it runs all the way  around the front of the engine.  So the first cylinder (the one furthest from the turbo) is several feet away.  But I suppose the last cylinder(s) are constantly dumping exhaust gases into the collector and they may keep the temp up. 

 

Why the CDT would be cooler though is a puzzle.  I realize that TIT is the foremost operating limit of the turbo.  I am lucky, my TIT is at 1580 at 75% (at least right now it is, wait til the spark plugs get older or something else changes), but it is the exhaust gases, primarily, that heat the turbo.  I am puzzled why the CDT would be lower.  Maybe I do not have enough observations and it was the particular day conditions.  I was also at low altitude, between 4-6,000 feet, that would help with cooling of the turbo.  I have not tried LOP in the flight levels for some time, that is next and we will see what happens.

 

N201MKTurbo, both ROP and LOP slow the flame front.  ROP does it by reducing the available O2 and wetting the fire with non-burnable fuel, LOP by reducing the available fuel.  It is probably true though that at an equal degree setting (say -20LOP v. +20ROP) the ROP will burn faster.  But slowing the flame front is exactly what we are doing when we run ROP, in order to stay out of detonation or high ICP's, we just use excess fuel to do it when ROP, rather than lack of fuel when LOP.  At least, that is what they say at APS.

[Awful_Charlie]

Most interesting observation.  Looking back at some JPI data, for mine the difference in CDT is barely detectable when leaning from 200 ROP to 20 LOP - is the Merlyn a differential controller? If not, is the IAT after the throttle plate, or between the turbo and throttle?

[jlunseth]

The Merlyn senses differential pressure on the two sides of the throttle plate as I understand it.  I do not know where my IAT probe is located.  However, I have two separate probes/functions, IAT and CDT.  The CDT is required by the Type Cert. and is before the intercooler and immediately after the turbo outlet, the IAT is after the intercooler.  The reason for two is that to use the Turboplus charts one needs to know the difference between CDT and IAT, and then use that with the chart to determine what indicated MP is needed, to develop the equivalent MP for reference to the POH tables.  I generally don't use all that stuff, especially not LOP where the tables are irrelevant.  This is probably different from your Bravo.  The intercooler is an aftermarket item on 231's, so the aircraft was not instrumented from the factory with the intercooler in mind.

 

Also, I don't think your figures of no drop in CDT when leaning from 200 ROP to 20 LOP are comparable to mine.  For one thing, I think you only have an IAT readout, not so?  And that is after the intercooler has done its work, cooling the turbo air down.  Second, I generally run my 231 at 100 ROP, so my comparsion is 100 ROP to -20LOP, and that generally is yielding a drop in CDT of 35 degrees or more.  At least, that is my observation so far in my LOP experiments.  I am doing another long flight (to MT) later in April, so will have a chance to test everything out again, this time some of it will be in the flight levels, at least that is the plan.

 

I have done 275 kts. sustained cruise (KPIA to KFDK) at 100ROP, 12.5 GPH, imagine the same at 11.5 GPH in LOP.  Pretty amazing if it works. 

 

I should download my JPI data and see what it says.

[bd32322]

Hopefully this will answer your question:

Combustion happens in two steps:

1) fuel hydrocarbons and oxygen react and form CO and H2O

2) CO then forms CO2 in the presence of more O2 and H2O from step 1

If you have a rich mixture, all the O2 is used up in step 1.

In LOP operation, you get more energy from step 2 than step 1. There is more O2 hanging around instead of being burned with the hydrocarbons. Unfortunately some of this or maybe a large part of it goes to waste as the exhaust valves open.

[N201MKTurbo]

The heat at the outlet of the turbo compressor is caused by compression not being conducted from the turbine side. If the turbine was stone cold it would make very little difference in CDT.

Read the following article about mixture and flame speed.

http://urcar-engine.blogspot.com/2012/01/factors-affecting-flame-speed.html

[carusoam]

What is the relationship between...

CDT and TIT. Do they peak at the same time?

(1) The turbo pilot's main interest is CDT, the temperature of air entering the engine. Knowing this, he can avoid engine issues such as pre ignition.

(2) The secondary interest is the TIT, the measurable peak temperature that the turbine reaches in the exhaust stream. Knowing this he can avoid damaging the turbine.

(3) The tertiary interest is EGTs, having all cylinders operating at LOP. It makes sense for best fuel burn and low cyl temps?

(4) CHTs, the temperatures that are monitored for best cylinder health.

So how does the turbo pilot set the mixture to run LOP? By experience using the big pull? Close monitoring of TIT, CDT, EGT or CHT?

Complex enough to make me happy to be a flat lander. How did this get done in the days prior to good instrumentation?

Best regards,

-a-

[bd32322]

What is the relationship between...

CDT and TIT. Do they peak at the same time?

(1) The turbo pilot's main interest is CDT, the temperature of air entering the engine. Knowing this, he can avoid engine issues such as pre ignition.

(2) The secondary interest is the TIT, the measurable peak temperature that the turbine reaches in the exhaust stream. Knowing this he can avoid damaging the turbine.

(3) The tertiary interest is EGTs, having all cylinders operating at LOP. It makes sense for best fuel burn and low cyl temps?

(4) CHTs, the temperatures that are monitored for best cylinder health.

So how does the turbo pilot set the mixture to run LOP? By experience using the big pull? Close monitoring of TIT, CDT, EGT or CHT?

Complex enough to make me happy to be a flat lander. How did this get done in the days prior to good instrumentation?

Best regards,

-a-

 

 

I am a non-turbo operator - but the big pull requires no experience - at least in the non-turbo model.

In the turbo, it would be similar - pull to roughness, enrichen to correct fuel flow for the power desired and to get smooth operation. Watch all temp readings and see if within acceptable range. If not and you cannot lean the mixture any more without causing roughness, you have no option to go rich again - maybe do a big push

[jlunseth]

I don't think that you could do it that way in my turbo, maybe someone else's with a different controller, but not in the 231 and especially not in the 360-GB.  Maybe there is someone who has more intestinal fortitude than me who can chime in.  The problem is that the MP will fall as you pull to the lean side, and from my experience it needs to be 34.5 - 36" once you are LOP.  34.5 - 36" in the 360-LB with an intercooler is 100% HP when ROP.  You would have to start higher than that, say 38" or more, in order to end up at the desired setting.  So you would overboost the engine in order to do the big pull and wind up where you want to be.

 

Not saying you can't do a big pull, but it requires starting at something less than >100% HP, 75% HP ROP works for me.  Then you can pull to the lean side and push in MP once you are there.  And since the MP and fuel flow are linked, there will be more adjusting of the FF and MP to get things right.  Wish it were as easy as just pulling one lever, but it isn't in my plane.  It is not that hard though.

[bd32322]

I don't think that you could do it that way in my turbo, maybe someone else's with a different controller, but not in the 231 and especially not in the 360-GB.  Maybe there is someone who has more intestinal fortitude than me who can chime in.  The problem is that the MP will fall as you pull to the lean side, and from my experience it needs to be 34.5 - 36" once you are LOP.  34.5 - 36" in the 360-LB with an intercooler is 100% HP when ROP.  You would have to start higher than that, say 38" or more, in order to end up at the desired setting.  So you would overboost the engine in order to do the big pull and wind up where you want to be.

 

Not saying you can't do a big pull, but it requires starting at something less than >100% HP, 75% HP ROP works for me.  Then you can pull to the lean side and push in MP once you are there.  And since the MP and fuel flow are linked, there will be more adjusting of the FF and MP to get things right.  Wish it were as easy as just pulling one lever, but it isn't in my plane.  It is not that hard though.

 

 

Okay, like you mentioned, all you have to do to counter the lowered MP is add one more step:

 

Push throttle in to recover MP and enrichen mixture for smoothness. Its conceptually the same as a non-turbo model, the turbo adds one more step of MP loss. You are just avoiding the area of engine operation where you should not be by doing the big pull.

 

The above turbo operation is also done in a NA aircraft when one is loitering around LOP with low power and then deciding to go into cruise LOP - without going ROP. The steps would be:

1) Increase RPM from loitering RPM to cruise RPM - this just moves peak combustion pressure to happen later in the crank angle or more degrees after TDC - which is not an issue.

2) Increase MP to desired cruise MP - usually WOT for non-turbo cruise

3) Enrichen mixture to cruise fuel flow or until there is no vibration - verify LOP by EGT behavior.

 

Steps 2 and 3 seem to be the same as what you do in your turbo LOP. Increasing MP in step 2 does not exceed 75% power limits or something because your power is being dictated by your fuel flow at this point and not by MP.