Condensation and corrosion.
I'm a believer in NOT starting the car every week over the winter, preferring instead to leave it charging and untouched.... (on the basis that starting cold introduces wear, so might as well introduce wear once rather than every week).
Neil,
Your instinct to avoid starting a car and letting it run for a short while, once a week during the winter time is correct, but for the wrong reason. The biggest problem that occurs from starting a car, letting it idle for a short while, and then shutting it down, is corrosion and oil contamination with water. This problem is especially acute when humidity is high and the temperature the car is stored in is low. Unlike most automobile engines, aircraft engines are routinely hangered for long periods of time and proper procedures for such storage are well-known in the aviation community.
The amount of water that air is able to hold is directly proportional to the temperature of that air. When a piston engine is started it begins to warm up immediately. When it is shut down, the air temperature in the ventilated crankcase and in the cylinder's, rapidly rises to match the temperature of the surrounding metal (between 180° and 200°). Because that air is so much hotter than the surrounding air, it wicks moisture out of that surrounding air like a sponge. Then, as that high moisture content, hot air cools, it loses it's ability to contain that moisture, and condensation forms. Not just a trace amount of condensation, a lot of condensation.
It takes about 30 minutes of engine operation for the condensation in engine oil to evaporate. Aircraft engines that have been started once a week by well meaning owners without being flown are sometimes found to have an inch or more of water sitting in the bottom of their crankcases! The colder the outside temperature air is, the greater the water build up is when an engine is started and allowed to run for just a short period of time.
A visual example of this condensation process can be observed every time you make yourself a glass of iced tea. The surface of the glass rapidly cools to exactly 32°, matching the temperature of the liquid inside. If the temperature in your kitchen is 72°, the surrounding air will hold a lot of moisture. The air touching the surface of your 32° glass however, cools to the surface temperature of the glass and at that lower temperature it is unable to hold the moisture it easily contained at 72°. Condensation forms on the outside of the glass, leaving a wet ugly ring on your nice wood coffee table.
Another example is the vapor trail you see when a jet flies high overhead. The white streaks coming out of the jet engines that hang for a considerable period of time as long white lines in the sky, is not smoke. That white line is a cloud. The extremely cold air at that high altitude holds very little moisture. When that cold air passes through a jet engine, it immediately warms to well over 100° and rapidly wicks any available humidity out of the surrounding air. That hot humid air then rapidly chills and just as rapidly loses it's ability to hold that moisture. The frozen condensation that results forms the long thin cloud that will hang in the air for some time.
An iced tea glass with a temperature differential to the surrounding air of only 40° can generate a ton of condensation that you can see drain down on to your coffee table. The winter time temperature differential between the air inside your engine and the air outside your engine could be 180° or more. The amount of condensation water generated is huge. Again, this is no big deal if the engine is run for 30 minutes as that condensation will burn out and leave the engine. But a succession of starts with brief runs (heat cycles), will allow that water to continue to build up. It never evaporates because it sits underneath a thick layer of your engine oil (the water is heavier than the oil). And water is not a good engine lubricant, especially at startup.
This is probably a lot more than you wanted to know. I'll shut up now.
Cheers. :cheers
Chip
