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Fueled Cooled Engine?
 
Author: reproduced with permission of Clay Ramskill

Our model engines have been called this- and with some reason!
We all know the consequences of taking to the air with our needle valves set too 'lean'; we get airborne, the engine puts out great power but eventually sags, then dies. Actually, the little motor didn't just die because it didn't have enough fuel to run on. What REALLY happens is that the engine runs too hot at the lean mixture setting, and SIEZES due to excessive internal friction.

Generally this seizure occurs at the flash point of the oil we are using in the fuel. The flash point is the temperature at which the oil burns, at about 400 degrees for most synthetic oil. When the oil burns, it doesn't lubricate, and the friction in the engine goes up dramatically, causing even more heat, causing even more of the oil to burn, causing... well, you see the point. So what we have to do is run the engine with a rich fuel-air mixture, ensuring that all of the fuel that goes through the engine does NOT get burned! That extra fuel will keep our engine cool. How?

Most all of us did a little experiment in High School, converting water to steam. We heat up a pan of water to the boiling point- and then must supply a LOT more heat to get the water to turn to steam. The same principle applies to the alcohol we're burning in our engines. If there's too much of it, it won't all burn; but the rest is turned into a gas (alcohol steam?), absorbing and carrying away considerable heat energy from the engine. Alcohol, by the way, is very 'good' at this process. Pour a bit of fuel on your arm on a hot day - you can instantly feel the cooling effect as the alcohol evaporates into the air, carrying away some of your body heat.

Oil in your fuel also carries away heat, assuming that it doesn't burn. Although the oil doesn't convert to gas, it will still carry away some of the engines heat as it passes through. So, the more oil in your fuel, the cooler your engine will run, not only from the extra lubrication, but also from heat transfer into the exhaust. Humid air also gives a cooling boost- the water vapor doesn't burn, and carries some heat out of the engine.

OK, we've all had the 'run your engine a bit rich' bit drilled into our heads at every opportunity. Aside from embarrassing dead stick landings, what's the big deal? After all, the engine will start right back up, and runs fine. True. But go back up to the 'seizure' part of this article. Note that the seizure comes from lack of lubrication. Every time we let our engines get too lean, we are shortening its life span, from extra wear.

Getting that little bit of extra power, by going a click or two leaner, may well be costing you. Think about it - run rich, 'waste' some fuel = bucks. Run lean, seize engine often equal BIG bucks!


 


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