The heat is on: Our experts agree on the best oil temperature
A few issues back, my good friend Paul McBride was asked what was the best oil temperature for an aircraft engine (Just what is normal? Oct. 15 issue). Since then, I have received a few questions about the same subject, but from the lubricant perspective.
Although I hate to admit it in public, Paul was right. The best oil temperature for most opposed, certified aircraft engines is about 180°F under normal cruise conditions. (Higher oil temperatures can be expected during climb-out.)
This is based on several pieces of data. First, the oil throughout the engine is at various instantaneous temperatures. Usually the hottest oil in the engine is the oil coming off the underside of the piston.
We have determined that on most non-turbocharged engines, this instantaneous oil temperature is usually about 50° hotter than the temperature of the oil going into the engine. If we add the 50° to an engine oil temperature of only 160°, we see that the oil never gets over the 212° mark, which is the boiling point of water. By running at 180° during cruise, the oil should be hot enough to boil off the normal condensation in a one-hour flight.
On the high end, the maximum preferred oil temperature is around 200°. This guideline is based on data that show the instantaneous temperature of the oil coming off the pistons for many turbocharged piston engines is about 75° above the temperature of the oil going into the engine. This means that if you are running an oil temperature of 240° during cruise, the oil is actually reaching 315° at some point in the engine. This is OK for a short time during climb-out, but if your engine operates for extended periods at this temperature, it can lead to coking and an increased level of deposits in your engine.
Remember that these are just guidelines and there are some exceptions. For example, if you have an aircraft that does not have a CHT or EGT gauge, the oil temperature is the only temperature indication that you have. Say that your engine oil temperature normally runs at 160° under cruise conditions on an 80° day. If it suddenly starts to run at 180° under the same conditions with no other change being made to the aircraft, this could be an indication of one or more cylinders running too hot or some other problem. You should check to make sure that your oil temperature is not only at the proper level, but also that it is consistent under the same conditions.
A final point is one that Paul also noted, and that is oil temperature gauge calibration. Many of the airplanes flying today have gauges that have not been checked in 20 or 30 years. In addition, many of the gauges just have a green band from around 100° to 240°. I would recommend that you remove your oil temp sensor and place it in a container of oil or water. Place the container on a hot plate with a good referenced thermometer in the liquid to check the temperature. Now heat the container up to 180°. When the temperature in the container stabilizes, check the gauge. I also recommend that you paint a small mark on the face of your gauge so that you can easily see where your oil temp is relative to your 180° mark. Now you can start worrying with confidence about how to get your oil temp up to 180° in the winter and/or down to 180° in the summer.
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.
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