LOP, Cold Air and Down Low
 

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!

No carb or carb heat with AFP system. Interesting you get smooth LOP with some heated, less dense air.

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?

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. :)