What's the best way to start a cold engine?

Ben Visser



8/5/2005 

What's the best way to start a cold engine? I've recently received several questions about this, with some asking about pre-oiling, hand propping or even squirting a small amount of oil into each cylinder before cranking.

All of these procedures may help a little, and they shouldn't hurt anything. However, the biggest problem with cold starts, especially after long storage, is getting oil to the cam and lifter interface. Of all the wear points in an aircraft engine, the highest load point is the cam and lifters. The rod and main bearings carry all the load, but they are plain bearings that operate under hydrodynamic lubrication, which means that there is a film of oil between the bearing and the crank at all times. In addition, a film of oil is usually trapped between the bearing surfaces, so there is some residual lubrication on the initial startup.

When the cam lobe starts to open the valve, the unit loading at the small contact area is significantly higher than that in the rod or main bearings. In fact, the load is well above the limit for hydrodynamic lubrication, which means that there is some metal to metal contact, especially during startup. At the interface, there is rolling and sliding contact that is lubricated by splash lubrication coming mainly off the rod bearings.

At startup, the first few swipes of the cam lobe are the source of most wear on an engine in good condition. Once wear has started and the mating surfaces become rough, then wear will progress, even during normal operation. This means that the key to long engine life is to properly lubricate the cam from the start.

Pre-oiling was developed for radial engines and works well because it forces oil into the lifter bores and the oil then drips down on the cam prior to starting. In an opposed engine, the oil from the lifter bores does not drip onto the cam, so will have little effect on initial start up lubrication of the cam/lifter interface. There will be more benefit in a Continental engine than a Lycoming since the oil from the rod journals may drip onto the cam. If you have a Lycoming engine, you will need to install a drip rail above the cam in order to see a major benefit to pre-oiling.

If you squirt oil into the spark plug holes before starting, it may lubricate the cylinder walls some, but this will have little benefit. Likewise, hand propping has little, if any, benefit. When you hand prop, the loads are the same on the cam/lifter interface. Even taking the plugs out and cranking the engine will not reduce the wear on the cam and lifter.

So what is the best procedure? The first step is more frequent use. When a plane sits for long periods of time, especially in a humid climate, the coating of oil tends to run off and the surface rusts. Then on startup, this rust acts as a grinding compound to increase the wear rate.

So when you start your engine, just follow the normal procedures. If the temperatures are below freezing, preheat. Preheating warms the oil so that it will flow to critical wear surfaces as soon as possible. You may also consider using multi-grade oil in the winter to improve flow rates even more. Once your engine starts, keep the rpm down. One needs to run the engine long enough to ensure proper oil flow to all parts of the engine before taxiing out.

Some people will idle the engine long enough to get the oil temperature into the green. I usually recommend that you idle the engine only long enough to see the oil temperature gauge move. Then just start your taxi, and by the time you reach the run up area, your oil should be warm enough.

A final point, CHANGE YOUR OIL OFTEN. I cannot stress the importance of frequent oil changes enough, especially for low usage aircraft. The only way to remove the rust particles from the oil is to drain the oil and put in new. Remember, oil may be expensive, but it is a lot cheaper than a new cam and lifters.



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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