Now that the cold weather is upon us, I have received questions about pre-heating and pre-oiling for aircraft engines.
Aircraft engines are pre-heated not just to thin out the oil, but also to expand the main engine crankcase.
The coefficient of thermal expansion for aluminum is greater than that for cast iron. Therefore, when the temperatures dip down well below freezing, the aluminum crankcase shrinks around the iron crankcase and the bearing clearance that is set at normal ambient temperature is reduced. This means that the oil flow and hydro-dynamic lubrication is limited right after startup. This means that the pre-heat process needs to heat the complete engine assembly and not just the oil.
If an external pre-heater is used, it is important that the heat is applied long enough to properly expand the engine assembly. This is especially important if the ambient temperature is below 0°F. The colder the temperature, the longer the pre-heat is needed.
The expansion of the engine assembly is especially important for pilots who use electric plug-in pre-heaters. A lot of these pre-heat systems use just a few heat pads on the oil pan.
These systems have two problems. First they do not properly heat the crankcase. This is fine if you are starting at an ambient temperature of around 20°F, but is not good if the ambient temperature is below 0°F.
The second problem is that many of these units put a lot of heat into a small area. This results in the surface temperature of the pan reaching above 300°F, which can cause coking and degradation of the oil.
The good plug-in pre-heaters have thermostatically controlled heat strips on the oil pan, plus heat elements on each cylinder. The cylinder element can be mounted in the thermocouple site or to a band around the cylinder. These systems work well especially with a cowl blanket to heat the whole assembly.
Another concern with plug-in systems is that many pilots like to leave their pre-heat system plugged in at all times so their plane is ready whenever they want it.
The problem here is that if the plane is at ambient temperature, the engine and oil will cool down at night and heat up during the day. If there is moisture in the oil, which is quite common in winter operations, then the heat on the oil pan will vaporize the moisture. This will rise until it hits the colder camshaft at the top of your Lycoming engine since the cold air enters the cowl at the top of the engine. This will greatly increase the rusting activity of the critical cam and lifters.
If you have a plug-in system with cylinder and oil pan heaters and you have cowl plugs and maybe a blanket, then I don’t see any problem.
A few months ago, my friend Paul McBride had a nice column about pre-oilers and their limited value in Lycoming engines. However, in Continental engines the cam is below the crankshaft, so if you have a pre-oiler, the oil from the crank bearings will usually drip down onto the critical cam and lifter interface. This is also true for many of the older radial engines.
In addition, I have seen ads for drip rails that can be installed in your Lycoming engine that will drip oil on the cam when pre-oiling. This may be an option at your next overhaul if your plane isn’t flown a lot.