I’m not a big believer in ground running engines, suspect it does more harm than good. The aerodrome is closed, but my engineer can get access to the hangar without interference.
The article shared in my post above has convinced me otherwise. What do you fear a ground run will do that is not mitigated by the article ?
denopa wrote:
What do you fear a ground run will do that is not mitigated by the article ?
Nothing. The point is that if you do not use a dehydrator, ground running the engine will simply add more moisture in the cooldown cycle after shutdown, which will not have evaporated off in the run cycle, unless you run the engine at high-power long enough (which is clearly not recommended on the ground, unless you have a specific arrangement to allow it safely, like a test cell).
The TwinCessna article you mentioned stated very clearly that moisture will accumulate inside the engine with every single cool-down cycle…unless you dehydrate. So you either dehydrate on the ground with a dehydrator, or in the air with regular flight, or even better: both!
What I understand from the article (especially the sections at the end) is that high moisture doesn’t matter as long as you have oil coverage. Ground runs achieve the latter
It has often seemed to me that a short 30 second engine run would allow oil pressure to distribute oil without distributing a lot of water due to combustion and blow by – you are burning little fuel in 30 seconds. The issue with that might be low oil temperature, viscous oil and splash lubrication limiting oil distribution within a short run but I think the general statement that any kind of ground run is bad may be incorrect.
denopa wrote:
high moisture doesn’t matter
I do not read it that way…and the test therein show otherwise.
From your source:
After 24-days in the high humidity chamber, the lifter baked in the oven with no oil coating had heavy rust and
pitting on all surfaces. The other lifter baked with no oil but alternatively placed in the low humidity chamber had
no sign of rust.
On the other hand all tests in high humidity, regardless of oil coverage and type eventually showed rust after varying amounts of time.
Bottom line is: if you do permanent dehydration AND periodic ground runs, you are better off than with either in stand-alone version. Ground runs will not be detrimental if you use dehydration in between runs, otherwise you will be adding moisture with every cycle. Less so if the run is short, but that is a delicate balance, which the above tests clearly favour for low moisture vs oil coverage if you have to balance between the two.
I think some folks panic unnecessarily about this engine corrosion malarkey.
My Maule, brokered by a straight-dealing contributor to this forum, had been sitting unused in a damp Irish barn for many years. The Lycoming O-360 had logged about 100 hours since new ten years previously and the airframe was covered with the dust of ages. I was apprehensive, but the price was fair so I took what I thought was a bit of a gamble.
Six years and ~700 tach hours later, still on Total DM 15W50, the engine has good compressions, healthy oil samples and never a speck of metal in the filter. Maybe it will pack up soon, but if so I’ll be more inclined to blame mountain flying and glider towing than those ten years by an Irish bog.
On the other hand, a friend of mine kept a new O-360 on straight oil long after the usual 5-10 hour break-in period and through a Scottish winter with little use. Predictably, with no anti-corrosion additives in the oil, the camshaft was toast by springtime.
Antonio, I don’t think cutting my sentence in half is helpful. High humidity doesn’t matter as long as you have an oil film protecting the surface. Since a ground run puts the oil film in place for a number of days, the fact the ground run increases humidity (if it does) is irrelevant.
Apologies, @denopa , I thought the missing wording “as long as you have an oil film protecting the surface…” was clearly implied to anyone who reads your and my posts.
This is not however about proving you (or myself, for that matter) right or wrong, so no intent in the omission: I will (hopefully) not be elected or dismissed because of this: no elections to win here. I am just an aircraft engineer providing an honest opinion for each reader to judge by its own merits.
While the ground run puts the oil film in place for a number of days, the issue is :
a) The actual number of days varies with type of oil in the quoted test. In all cases, regardless of oil types and additives, in the end all samples get corroded within the (admittedly accelerated) 24-day test period: some in 24h, some in 20 days. This does not happen in the dry-chamber test even with no oil to start with.
b) The test was performed with new (vs used) oil. ie no acids or corrosive salts in the oil. This is not something you can say of the oil in your sump unless freshly replaced. Moisture will greatly enhance the corrosive effects of these byproducts.
c) The crankcase will ingest moisture from combustion byproducts blowby, (this will be much less for a short than a long run) , but , also through the case vent as it cools down, this one regardless of run duration, unless some system like a dehydrator is put immediately in place after shutdown to stop the ingress. Initial engine cooldown and subsequent daily temperature changes will condense this moisture inside the engine.
I am wondering how you would factor all of the above in determining the optimum frequency and duration of your ground runs to ensure intended effectiveness, in the absense of a moisture-control and removal system.
I would really like to see fewer engines sent to shop as a result of corroded distribution systems.
