How important is it to preheat an engine?

Ben Visser

1/5/2007 


How important is it to bring up oil temperature before takeoff? So asks Larry Lowenkron, who uses a multi-viscosity oil.

What's important is not what type of oil you use, but rather the reason for and amount of preheating prior to takeoff.

The primary purpose for preheating an engine is to ensure that the entire engine assembly is warm and at proper clearances. The coefficient of expansion for aluminum is higher than that for iron or steel. This means that when an engine cools down after a flight, the aluminum pistons and crankcase decrease in size more than the iron crankshaft and cylinders. Now when the engine is started at very cold temperatures, the clearance of the main bearings is less than specifications call for and the clearance of the piston to cylinder walls is more than the specs call for. This change in clearances is not a major problem at 40°F. But when it gets down to 0°F or below, it can become a major problem. This is why preheating is so important. Most people think they preheat their engines to just warm up the oil so that it will flow to all parts of the engine. Therefore they think they can reduce the amount of preheating when a multi-viscosity oil is used compared to that needed with single-grade oil. In actuality, the improvement in flow for the oil is only a secondary concern. The primary concern is if the clearances are near zero, then metal to metal contact can occur in the main bearings. This will lead to increased wear and clearances at operating temperatures, which can result in lower oil pressure and eventually premature replacement of the bearings or the engine. The pistons also are affected because at really cold temperatures the excessive clearance can cause slapping of the piston skirts, which can result in increased wear and reduced engine life.

This is why it is so critical to properly preheat an engine prior to startup. I recommend that you preheat whenever the temperature is below freezing (32°F). I know that engine manufacturers usually say that preheating is only necessary at temperatures below 20°F. My concern is the temperature history of the engine prior to start-up. For example, say an engine sits all night at 0°F. In the morning the temperature climbs to 20°F just before takeoff, so many pilots will just jump in and go with no preheat. The air temperature may be 20°F, but the engine parts are still well below that temperature. If you use the 32°F target temperature, it gives you an extra margin of safety.

The other concern is how the engine is operated after start-up. The biggest problem is if the engine is started and immediately pushed to full throttle or high loads.

In a car you should start it, wait for the oil pressure to come up and then drive away slowly. Never push it to full throttle until it is fully warmed up. If your aircraft engine is above freezing, or if it has been properly preheated, then the oil temperature at takeoff is not too critical no matter what type of oil is used. Under these conditions, I would recommend idling the engine for a minute or so and then start your taxi. Now go through your run-up/mag checks, etc. By now your oil temperature gauge should be starting to move off the low end peg. Once it does, your engine is ready for takeoff. If you did not preheat and your engine was well below freezing when started, then you need to adjust this procedure to ensure that the temperature of the critical parts are normalized prior to takeoff.

While some airplanes have a CHT, most small aircraft do not have any temperature indicating instrumentation other than the oil temp gauge. This makes the oil temperature critical to your operating procedures. If you have a CHT, this would be a better indicator of when your engine is ready for flight.

Always remember that proper preheating is critical to your engine life even if you use a multi-vis oil. Also remember that a preheating system that just heats the oil is not doing a proper job. There are a lot of electrical heating systems that just put a heating pad on the oil pan. If you have an electrical preheating system, make sure that it has heating elements on the cylinders as well. In addition, you should either blanket or at least plug the cowl to ensure that the entire engine assembly is heated.

Multi-vis oils give you an extra margin of safety in that they provide improved flow at low temperature, but they are not a replacement for proper preheating and recommended operating procedures.



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.

This article re-printed from: