A few issues back I discussed possible fuel handling problems with a proposed ethanol aviation fuel. In this column I would like to discuss energy content.
A few issues back I discussed possible fuel handling problems with a proposed ethanol aviation fuel. In this column I would like to discuss energy content.
In Dr. Dennis Helder’s “”Investigation of Ethanol as a General Aviation Fuel-Final Report”” there is a line that reads “”This 48% increase in fuel flow also occurs at best economy operation.”” One needs to think about a 48% increase in fuel consumption for a minute.
First, the range of every aircraft will need to be de-rated. For example, if your aircraft is presently capable of going 600 miles on full fuel, with the ethanol fuel you will be looking for an airport prior to going 400 miles. That is a rather significant change.
The cause of this increase in fuel consumption is the lower energy content of ethanol. Conventional hydrocarbon fuels contain around 115,000 btus (British thermal units) of energy per gallon compared to ethanol, which contains about 76,000 btus per gallon. This means that the first step for anyone to use ethanol is to re-jet the carburetor. One concern here is if this fuel were introduced, pilots might try a tank full or part of a tank and find themselves flying in a very lean condition. The fuel that Dr. Helder is testing is called AGE-85 (Aviation Grade Ethanol), which contains 80-85% ethanol, a pentane isomerate stream and about 0.5% biodiesel. This fuel would lean out most aircraft engines to the lean side of stoichiometric even at a full rich setting. This would mean a significant decrease in max power. If the pilot mixed 100LL and AGE-85, he could find a mixture that would lean out the engine just enough so that the engine is running at stoichiometric, which would result in peak cylinder head and exhaust temperatures running above allowable limits. All of this leads to the conclusion that if AGE-85 were introduced, it could only be used in aircraft modified for and totally dedicated to using just the one fuel. There could be no cross blending.
The other major concern is engine durability. If one is introducing 48% more fuel into a cylinder, what effect will that have on cylinder wear and crankcase oil dilution? The reason that AGE-85 contains 15% of a hydrocarbon fuel is for starting. Straight ethanol is not volatile enough to facilitate normal easy starting. This is especially true at colder temperatures. So on a cold start and until the engine is warmed up, a significant amount of the ethanol enters the cylinders in a liquid state. This will cause washing of the cylinders, increased crankcase dilution and other possible problems. In earlier work, when high alcohol content fuels were used in automotive engines, high cylinder wear was noted on almost every test. Now, I do not have any data on aircraft engine wear on alcohol fuels. But I think that would be something that should be investigated before they go too much further with their research.
I do not want to sound too negative here. I know that ethanol is a very popular issue with politicians. But I always go back to the old saying that the top three attributes of any new aviation product should be 1. No in flight problems, 2. No in flight problems, and 3. No in flight problems.
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 [email protected].
