Why is it bad to run the engine without flying?

[Saville]

From what I read and what people - whose opinions I greatly admire - tell me, it's bad to start the engine, run it for a few minutes (5-10), even at 1000 or 1200rpm, and then shut it down...just to circulate some oil and do a small bit of charging on the battery.

I'm not aware of all the reasons why this is a bad idea (just some) and I'd appreciate an education.

Thanks!

[Low Pass]

Broken in, or not? Two different situations. For the broken in engine, as long as you didn't get CHTs too high, what's the harm? Get the oil up close to normal temp, cycle the prop a time or two (if CS). Heck of a lot better than letting condensation and acid collect in the engine over time. Just be careful running it on the ground.

[Saville]

Low Pass:

Well broken in.

And yes I should have said run long enough to:

get CHT's and EGT's into the green
get oil temp into the green.

[DragonflyAero]

I have always heard that it is because a short run will not get the oil hot enough to eliminate any water condensation that results from combustion so you can end up with water in oil rusting internal engine parts.

[Saville]

Dragonfly:

That's the reason I heard...I'm wondering if there are any more.

thanks!

[Auburntsts]

I don't think you don't want to run it just for a few minutes. One of the byproducts of combustion is water. Lycoming states you should run an engine at least 45 mins and get the oil temp up to 180 deg minimum to boil that water off.

As for running on the ground it's all about temp management. The cowling and baffling isn't designed to provide adequate cooling on the ground.

Also I doubt running the engine a few minutes will really put much of a charge the battery and probably won't even replace the capacity used up in the start.

Bottom line is I think it's OK occasionally I just wouldn't make it a habit as I think the cons out weigh the pros.

[rightrudder]

In a DVD that came with my Lycoming engine, the narrator states that about 1300 rpm results in the best ground cooling.

Of course, cowling-off runs should be kept brief (unless you have a plenum setup) because the rear cylinders won't have adequate cooling.

[Mel]

Air cooling systems on aircraft are designed for "in flight" cooling. Not running on the ground. Running on the ground does not give equal cooling for the cylinders. If you run the engine long enough for the oil to get hot, the cylinders will get inadequate cooling differential.
If you run for a shorter time, you will get condensation in the oil.

Bottom line, if you can't fly it, leave it alone.

[maniago]

As mentioned its pretty much just a water issue. Realize that aircooled motors are really sloppy piston ring to wall vs a water cooled so youre gonna be putting more water vapor in the oil than you might think. And it takes a fair amount of time for that water to boil off and make it overboard. How much time? Who knows. But the engine mfgs probably do have the best handle on it. Pickle the motor or fly, and as little as possible in between IMHO.

[Saville]

Thanks to all.

It seems that water vapor is the main issue, which makes sense.

I'm mostly thinking about Winter times (as I sat here yesterday at -33 with the wind chill). During Spring, Summer and Fall I can fly 2-3 times a week no problem. That keeps the battery charged and the oil circulated. It's only the Winter where I might have longer delays between flights.

Early in January I hadn't flown for 2 1/2 weeks. When I tried to start the engine to taxi over to the hangar in which my annual was going to be done, the prop turned almost one rev and stopped. The battery was gone.

Prior to that attempt I had not gone a week without flying prior to the annual. The engine started just fine every time.

The battery is a Gill G-25 flooded battery. I do not know the age of the battery.

So I pushed the plane over to the hangar and recharged the battery and when the annual was done, the plane started up just fine.

So I'm thinking about long delays between flights in the Winter on the order of 2-3 weeks. I saw two issues:

1) corrosion in the engine

2) Battery life (due to the above non-start).

To help with #1 I added Camguard.

To help with #2, I just bought an Odyssey PC925MJ AGM battery, along with an Odyssey charger. That battery is supposed to have more cranking power and be able to last longer between charges - especially at low temps. The MJ is also supposed to help with that at low temps.

Also, as I fly acro it's better to have an AGM than a flooded battery.

I'm hoping these two things will alleviate the worry I have about going 2-3 weeks without flying in the Winter time.

The only other Winter aspect I have to deal with is engine pre-heat and I have the Reiff Turbo cylinder heaters and oil sump heater on hand ready to install. That plus a smartphone switch and I think I'll have Winter under control. Just in time for the end of Winter

Any other thoughts along these lines are welcome.

Thanks!