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LOP, Cold Air and Down Low

David-aviator

Well Known Member
I've noticed this in the past, LOP in cold air down low does not work. The engine runs rough. On a hot summer day it does not.

Yesterday it was same, 34F, MP 20" and fuel flow 6 gph. Eased the mixture forward to peak, or just a little rich of peak, and the engine liked it a lot, got very smooth. But going on lean side of peak it does not.

Could have something to do with the density of air, I don't know. It should not matter but it seems it does.
 
Carb Heat

Hi David,

Do you have carb heat or a source of warmed alt air? Carb heat really helps smooth LOP ops for me.

I enjoy your posts from STL!
 
I don't have an answer to why you are seeing this but I will say that I have seen some pretty cold temperatures while flying LOP myself. These incidents have all been while flying 10K or higher but I have seen temps down into the singe digits and teens with no ill effects on LOP operations.
 
I saw this in my 182 days. Warm air, the fuel really does vaporize and goes where the air goes - good fuel/air mixture in all cylinders. Cold air, some fuel stays as liquid droplets, and due to inertia, doesn't turn corners as well as the air, some cylinders now get more, others less, gas.
 
not completely sure how EGT relates to air fuel ratio. However, There is more air in the air when the temps are colder. Cold air is more dense. The molecule are packed tighter and therefore more oxygen packed into the same volume of air. It also cannot hold as much moisture, which displaces the air in warm and humid conditions.

For these reasons, the colder the intake air gets, the more fuel you have to throw in to maintain the same air fuel ratio. This is why all (automotive anyways) fuel injected engine have a MAT sensor. Manifold Absolute Temp. The FI comuuter automatically adjusts the fuel delvery based upon intake air temps. This is also why you notice more power in your engine when it is colder.

Larry
 
David, how LOP were you, and what were the other engine parameters. What engine?

You say down low, I assume altitude, thus a very low DA possibly even a -ve number, yet you say 20"MP.

I can see too possible answers depending on what you were doing and what possible defect you may be about to uncover.

Have you done the GAMI High/Low lean tests lately? What were the results?

I have no problem down low(1000AMSL) on a cold morning running WOT/2500/80dF LOP.

As for folks with carburettors air temp does affect the atomisation of the fuel and carb heat does help.
 
Carb heat will enrichen the mixture. It will do the same thing to the fuel air mixture as pushing the red knob forward.
 
I am no expert or theoretician, but if it runs better rich when cold, could you be running too lean at all temps?
 
I am no expert or theoretician, but if it runs better rich when cold, could you be running too lean at all temps?

My simple mind was programmed on this stuff years ago before we had the instrumentation to fly LOP, etc.

I usually pull the mixture after take off (and at less than 23" MP) until engine runs a little rough and enrich it to get rid of roughness. In cold air it happens sooner, like before reaching peak EGT, certainly after peak on the LOP side.

How come that is happening I don't know. I do know I won't fly around with a rough engine trying to do LOP in cold, dense air. A smooth engine is a happy engine and so is the the pilot happy. :)
 
Are you running wide open throttle? Introducing a little turbulence into the induction system by slightly closing the throttle, or adding alternate air might help atomize that cold fuel.
 
Are you running wide open throttle? Introducing a little turbulence into the induction system by slightly closing the throttle, or adding alternate air might help atomize that cold fuel.
...with fixed pitch prop it is about 20" MP, throttle way back. Summer heat same conditions except OAT LOP works, winter cold it does not.
It's not a big deal, just another little mystery.
 
This is not just a temperature dependent operation. My Superior engine operators document says not to lean the mixture below 3,000 ft. What is not clear is if they mean msl, agl, or density altitude. Turns out it is density altitude.

I fly with a Dynon D10-A and it computes DA for me. That is what I use for leaning. In summer time, the 3,000 ft DA point is sometimes just sitting on the ground. In the winter time, it is sometimes 5,000ft msl.
 
David, can I refer you back to my previous post?

I do suspect that you are trying to do the same setting regardless of the DA, and in winter with colder and denser air if you are trying to set the same fuel flow, it will not work. You are trying to get too far LOP effectively.

This is not just a temperature dependent operation. My Superior engine operators document says not to lean the mixture below 3,000 ft. What is not clear is if they mean msl, agl, or density altitude. Turns out it is density altitude.
DA is what effects the engine. And while Superior print what they print, that is so dummies do not do dumb things at high power, but I can assure you there is no scientific reason not to run LOP at 1000' but "do not lean the mixture" really means do not lean it on the rich side of the curve at high power settings.
 
try lower power settings for LOP

I have a superior 180hp with FI and two mags. I have best luck with 60% power or less set before going LOP. Approach it very slowly and as long as you have good fuel / air distribution you should be able to get 20 to 30 degrees LOP on all cylinders before it gets rough. Good luck...
 
I have a superior 180hp with FI and two mags. I have best luck with 60% power or less set before going LOP. Approach it very slowly and as long as you have good fuel / air distribution you should be able to get 20 to 30 degrees LOP on all cylinders before it gets rough. Good luck...

At that low power setting that is what I call a well set up engine. At 60% power you want to be 10dF LOP with some maybe out to 20-30 but the last to peak need only be 10.

Going to 20-30 is going to cost you speed. But that will increase your range through less parasitic drag ;)
 
I have a superior 180hp with FI and two mags. I have best luck with 60% power or less set before going LOP. Approach it very slowly and as long as you have good fuel / air distribution you should be able to get 20 to 30 degrees LOP on all cylinders before it gets rough. Good luck...

Could be I am not patient doing this...have you done it at 30F? The engine seems to get rough before EGT peaks.
 
Could be I am not patient doing this...have you done it at 30F? The engine seems to get rough before EGT peaks.

Rough before peak, sounds like your injector restrictors aren't balanced. It would appear that your cylinders are peaking at different flow rates. The roughness is caused by one or more cylinders that are leaner than the others.

Have you done any of the testing defined by AirFlow Performance or GAMI?

It will make a world of difference.
 
Rough before peak, sounds like your injector restrictors aren't balanced. It would appear that your cylinders are peaking at different flow rates. The roughness is caused by one or more cylinders that are leaner than the others.

Have you done any of the testing defined by AirFlow Performance or GAMI?

It will make a world of difference.

Good point, I have not balanced the injectors.
Most of first 100 hours was learning to land the darn thing, getting around to details of late. :)
 
Yesterday it was same, 34F, MP 20" and fuel flow 6 gph. Eased the mixture forward to peak, or just a little rich of peak, and the engine liked it a lot, got very smooth. But going on lean side of peak it does not.

I'd bet with Bob here. You have Airflow Performance fuel injection; Bendix/Performance would be the same:

At high fuel flows, the fuel divider's diaphragm and piston are fully raised, and equal division of the fuel supply to the individual cylinders is a function of the restrictor size installed in each nozzle.

Given standard 0.028" restrictors, fuel division becomes a fuel divider function as fuel flow drops into the 6 to 7 GPH range (the exact value depends on the nozzle sizes). Dividers generally do a good job, but sometimes not as good as division by nozzle; the fuel delivered to individual cylinders can unbalance slightly.

You can check by recording a GAMI spread at 24/24 and some specific LOP setting, say 20F LOP. Now repeat at low MP and low fuel flow, and see if your GAMI spread widens. A poor GAMI spread at 24/24 says you need to balance flow by swapping restrictors for slightly larger or smaller sizes. If you have a tight spread at high fuel flow, but it widens at low flow, it's a divider spool function.
 
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DA is what effects the engine. And while Superior print what they print, that is so dummies do not do dumb things at high power, but I can assure you there is no scientific reason not to run LOP at 1000' but "do not lean the mixture" really means do not lean it on the rich side of the curve at high power settings.

David is right, in that the manufacturers instructions really don't have anything to do with altitude. Let's look at specific examples.

Subject is an IO-360 angle valve at 20 degrees timing, running with the pro-detonation thermal conditions (very high CHT, oil temp, and intake air temp) required by certification.

Mixture sweep at 2700 RPM and full throttle, i.e. 28.5 MP. Note that as the mixture is leaned (right to left on the chart) it gets into light detonation from about 50 ROP to 50 LOP.



2700 RPM and 26.8 MP. It doesn't matter if the 1.7" MP reduction is due to altitude gain or throttling. The result is the same; no mixture position results in detonation. At this undersquare RPM/MP, you can do anything with the mixture knob, despite being at about 81% power.



2400 RPM and full throttle (28.6"), an oversquare setting hard against the "Maximum Manifold Pressure For Continuous Operation" line on the left side of a Lycoming power chart. At fuel flows higher that 70 lbs per hour there is no detonation, but you most surely would not want to lean below 70...not even a big pull to somewhere LOP. Under these conditions you would need to be more than 100 LOP to stay out of detonation. Point here is that merely being LOP is no blanket guarantee of safe operation; there are other factors in play.

 
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