I mentioned my engine pre-heater in my last post and received a request to explain it in a little more detail. Engine manufacturers and engine overhaul facilities such as Mattituck strongly recommend pre-heating the engine whenever the temperature is below 20° F. and some sources recommend pre-heating when the temperature is below 32° F. However, all the sources I checked agreed that pre-heating before starting in cold temperatures will significantly reduce wear to the engine.

Ben Visser, Staff Research Engineer, Shell Oil:

“Preheating your engine makes a world of difference. It heats the oil so the oil is thin enough to flow through the engine and properly lubricate all of the critical wear surfaces. Preheating also heats the metal parts in the engine. That’s important because aluminum crankcases have a higher coefficient of thermal expansion than iron crankshafts. This means as your engine cools down, the clearance is reduced. And as a result, you may not have sufficient oil film thickness for proper hydrodynamic lubrication at very cold temperatures. In other words, the wear rate is going up. If you’re using [an electric] heater, make sure it’s a system that heats the whole engine, not just the oil.”

I made a small pre-heater for my plane that forces hot air into the engine compartment from below, allowing the warm air to heat the oil sump and then to flow up around the cylinders and out the front of the engine, using the reverse of the route used by the cooling airflow during flight. 

I bought all of the materials for my pre-heater from one of the local home improvement centers like Home Depot. The heat source is a small portable ceramic heater. The sticker says it is a Ceramic Safe-T-Furnace, Model HC-441W (made in China, of course). It has a built-in fan and a plunger-type safety switch on the bottom that will shut the unit off if it tips over. There are both fan speed and heat controls on the front, though I just set both of them to their maximum settings and leave them there. I also have a piece of tape on the bottom of the unit holding the safety switch depressed. As you can see in the photo, the unit is tipped slightly forward and the tape keeps the unit from shutting off.

I used a square to round flange adapter similar to this larger version and tapped it to the front of the ceramic heater to use as a transition to the round ducting, then attached a length of dryer duct to the adapter to direct the heat upward. The duct is a very flexible expandable metallic tubing that is easily molded to any shape that you might need.

I used the pre-heater with bare tubing for a little while, then decided that the unit was transferring too much of it’s heat through the thin-walled ducting, so I bought some self-adhesive insulation and wrapped that around the duct leaving the last 6″ or so uncovered so that it could be squeezed into an elongated oval in order to fit up into the cowl. The adhesive on the insulation wasn’t staying attached to the uneven surface of the duct, so I wrapped the whole duct with aluminum tape that is used by HVAC installers.

I also use 2-3 layers of blankets and carpet remnants over the engine compartment to help reduce heat loss. When I intend to go flying, I plug in the unit, make sure that the fan is operating and then go about my preflight duties. If it’s really cold, I will often make a trip to the local coffee shop for a few minutes and give the unit about an hour to work it’s magic. By the time I return there is a noticeable flow of hot air coming out of the front of the engine and the blankets/carpet over the top of the engine compartment are warm to the touch. The engine always turns over easily during start and the oil pressure indicates quickly.

I don’t know how much the pre-heater cost to make. I do know that I bought more material than I needed for the project because smaller quantities were not available. Two people could probably build units and split the costs. I did notice the other day that you can buy the flexible ducting with insulation already attached. Bigger heaters will, of course, pump out a larger volume of hot air and will do the heating job a bit quicker, but they would also need a larger diameter duct to move the air.

I’m sure other people have made more elegant and more professional-looking pre-heaters, but this one works for me and it seems like it will last for quite some time. It looks like the temperatures are starting to trend higher now, so maybe by the end of the month I’ll be able to put it away for the summer.

Update:I first posted this article on the evening of March 8th after a beautiful, clear day with temperatures in the low 50s (F). It is now 8 am on the morning of March 9th and I am looking at a thermometer indicating a temperature of 22° F with 5″ of new snow on the ground. Maybe I won’t put the heater away this month…