Believe there is a note in the TCM tsio360 manual and I have heard it elsewhere.
The heater may not have a thermostat and if left on too long it can possibly "burn" the oil. Also the cold surfaces on top of the engine can promote condensation and increased moisture in the oil.
This is not for all sump heaters and does not apply to cylinder head heaters.
If left in all the time then the temp should be controlled and proper blankets over the engine cowls to insure even distribution of heat thru the block.
I would expect the ceramic heaters to do a far better job of heating as it does not promote temp differentials between sump and crankcase housing. No doubt the crank/prop would be a good source of "cold".
Perhaps this is a good explanation // see below for part of the article on corrosion.
http://www.amtonline.com/<br />
sea...%5C1002_01.htm
I will dig up more if need be.
Bob
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By Gary Schmidt
The debate in the industry has centered around the issue of the sources of these elements as well as the issues of when and why the metal is vulnerable to the other elements necessary for the corrosive process.
First let’s analyze where these elements necessary for corrosion originate. No. 1, moisture: When you burn a pound of fuel, you get about a pound of moisture as a result. Most of this is expelled out of the exhaust stacks but some enters the crankcase via "blow by." Some moisture comes from the air that the engine takes in through intake and it is mixed with the combustion moisture. One other minor source is the moisture in the air that is drawn in through the engine breather tube as the engine cools after shut-down. The aircraft engine absorbs this moisture and doesn’t release it unless your oil temperature gauge gets over approximately 180 degrees F. If you have your oil temperature high enough, the moisture vaporizes and exits by way of the crankcase breather.
Naturally, because moisture is a key element to the corrosion process, engine manufacturers emphasize that aircraft located in coastal areas and areas with humid climates are more susceptible than aircraft located in arid regions. The fact is that there are very few areas of the world where the air is so dry that you do not need to worry about corrosion. Everyone needs to consider corrosion regardless of their geographic location.
Now, where do the corrosive agents come from? This is another area where you will get differences of opinion. In addition to the small amount of corrosive agent found in air, the primary source is the result of the combustion process. When fuel burns, the exhaust gases are corrosive. Some of these gases enter the crankcase via the "blow by" and the corrosive agents are deposited into the crankcase and the oil. You can observe the corrosion caused by exhaust by looking at your aircraft belly behind the exhaust stack. Rivets in this area often corrode.
When an engine oil is exposed to heat, a process called "oxidation" is started. When oxidized oil is mixed with moisture as discussed earlier it often forms an acid which attacks metal surfaces. This is a concern relating to certain sump pump preheaters which may overheat and degrade the oil as it heats the engine. Another problem with these heaters is that as they heat the oil, driving the moisture out, the vaporized moisture will rise and again condense on cooler parts of the engine. This condensed moisture will begin the corrosion process on that engine part.
Another suggestion for operating your engine is to keep the engine running lean or at least do not operate it "full rich" more than necessary. A rich mixture results in unburned fuel left in the cylinder. This unburned fuel with its corrosive agents leaks by the rings and ends up in the oil sump. A properly leaned engine also helps increase oil temperature.
Avoiding "sump only" engine preheaters is another recommendation. These heaters tend to vaporize the moisture in the oil and allow it to condense in the cooler parts of the engine which, in turn, provides the moisture necessary for the corrosion process. Engine pre-heaters that heat the entire engine evenly avoid the condensation problem. Buldoc, the engine rebuilder, also strongly recommends not leaving any engine preheater on for lengthy periods, i.e. days or weeks. Clearly warm air holds more moisture than cold air.
There is another little known product from Tanis Aircraft Services that is designed to deal with engine moisture and corrosion. It is a product patented by Peter Tanis to blow moist air out the inside of an aircraft engine. It connects to the oil breather tube and blows air through the engine out of the oil cap. It works best when the air inside the engine is warm and it has absorbed the moisture created by recent fuel combustion. The humid air is blown out and replaced with dryer outside air. If the outside air is cool, all the better. As it enters a warm engine, either warm by recent operation or warmed by a preheater, the cooler air will absorb more moisture as it warms. The use of the aerator is not recommended if the aircraft is stored in areas where the air is already very humid or where it may contain corrosive agents such as salt which is likely in some coastal regions.
About the author
Gary Schmidt recently purchased Tanis Aircraft Services. You can contact Gary at (320) 634-4773 or by e-mail at
Info@tanair.com.
Above is from the URL posted
bob