Pre-heating: Is it necessary or just something to do on a cold morning?

Ben Visser

10/15/2004

 
One of the most frequently asked questions at Oshkosh was "should I pre-heat my engine and when?" I also received a lot of questions about what type of pre-heater to use.

The first point that many pilots do not understand is why you pre-heat an engine. Many think that the only reason is to get the oil warm enough to flow. In aviation, the use of single grade oils, even in the winter, is commonplace. Therefore, it is necessary to get the oil up to a temperature where it will flow freely and quickly to all parts of the engine once it is started. Heating the oil also reduces the drag on the starter so that the engine will start easier.

However, there is another reason for pre-heating your engine — to normalize the clearances in the engine assembly itself. In aircraft, we have aluminum blocks with iron crankshafts. Now the coefficient of thermal expansion for aluminum is greater than that for iron. This means that as your engine cools down, the aluminum crankcase shrinks faster than the iron crankshaft. This reduces the clearance for your main bearings.

Several years ago, the late Peter Tanis pre-measured and then placed an engine crankcase and separate crankshaft outside his shop in Minnesota. The next morning it was about -20°F when he remeasured the crankshaft main bearing journal and the inside-bearing diameter of the case. He found that the case had contracted over two thousandths more than the crank and that, if assembled, there would be little or no clearance for the main bearings. So when you pre-heat your engine, it is important to heat not only the oil but also the engine assembly.

This is important for everyone, but especially critical for people in northern climates who choose to go flying on days when the overnight temperature is below 0°F. If you are in this category, don't just use some heat strips on your oil pan. If you choose to use an electrical pre-heat system as opposed to a forced air system, use a system like the Tanis heater [or Reiff]. The Tanis system has an oil pan heating strip and individual heaters for each cylinder, which will heat the entire engine assembly. This system also has a control to limit the surface temperature of the pan heater [not true, Mr. Visser is mistaken on this]. If you use just a pan heater with no control, the surface temperature by the heater can climb to over 300°F. This in turn can lead to oil coking at the pan surface.

When should you pre-heat? Manufacturers recommend that you pre-heat whenever the temperature is below 20°F. I recommend that you pre-heat whenever the temperature is below 32°F, especially if you are using single grade oils.

Also remember that the colder it is, the longer you should pre-heat to ensure that the entire engine assembly is ready to go.

SOME ADDITIONAL TIPS:

- An engine blanket will greatly increase the effectiveness of these heaters.

- Do not plug these systems in in the fall and let them run all winter. This works great if you fly several times a week and your oil temperature gets up to 180°F on every flight. However, if your plane is used less frequently and has some free water in the oil, the heater will vaporize the moisture in the oil as the plane is sitting. This warm moist air will rise and hit the cold surfaces at the top of the engine (i.e. the camshaft on Lycoming engines) and condense. This will increase the rusting activity in your engine. These problems will be greatly reduced with an engine blanket and cowl plugs.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at Visser@GeneralAviationNews.com.

----- Original Message -----

From: Reiff

To: visser@generalaviationnews.com ; ben@generalaviationnews.com

Sent: Monday, October 18, 2004 12:01 PM

Subject: Preheating article

Mr. Visser,

I just finished reading your Oct 15 article "Pre-heating: Is it necessary or just something to do on a cold morning?".  Overall I think it is a good article, and I always appreciate efforts by the aviation press to increase awareness by aircraft owners of the benefits of preheating engines.  However, I want to point out that there is another preheating system out there besides Tanis that has heating elements on both the oil sump and cylinders.  We have sold such systems for about ten years now, and with our growth rate and volume (average annual growth rate of 23% and over 1200 systems sold last year) it is likely that we are the fastest growing preheater manufacturer, and the largest. 

The article includes the following statement.  "This system [Tanis] also has a control to limit the surface temperature of the pan heater. "  This statement is not true.  Tanis does not use any thermostat overheat control on their engine preheat systems.  We do.  This is just one of the several advantages our system has over competitors that explains why aircraft owners and mechanics all over the world are making the switch to our system.  More information about these advantages can be found at our web site below.

Best Regards,