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Sunday, August 31, 2014

Ethanol as an engine fuel

Posted by Krzysztof Lis on March 18, 2008

When Otto invented his first internal combustion engine in 1872, people didn’t know how to refine (distillate) crude oil. Because of that, no petroleum-derived fuel were available at that moment. So Otto used spirit (90-95% alcohol, or 180-190 proof) as a fuel for his engine. Ford T’s engine was able to work on gasoline, spirit or any mixture of those two substances. Even now, alcohol is quite a good fuel for spark ignition or even compression ignition (diesel) engines.

Today, ethyl alcohol (ethanol) is not used widely as engine fuel, mostly because of relatively low crude oil prices and low prices of any petroleum-derived fuels. Manufacture of one litre of spirit requires using 8,400 – 11,200 kJ (kilojoules) of energy. Heating value of pure ethanol is only 2/3 of heating value of gasoline, per unit volume.

Anhydrous ethanol (ethanol that contains less than 1% of water) can be easily mixed with gasoline, in any ratio. The mixture of 85% (volumetric) ethanol and 15% gasoline is called E85 and is quite popular ethanol fuel. But most cars run without trouble only on E10 – fuel with only 10% added ethanol.

Why is that? Ethanol has less energy content per unit volume. To release the same amount of energy, you need to use more ethanol than gasoline. Because of that, the engine needs to be altered to give more fuel (carburetor or injection system needs to be tuned). Ethanol contains some oxyden, bound within ethanol particle, so for burning this fuel less air is required than for gasoline. Sometimes it is also needed to alter ignition timing.

Ethanol has also couple of advantages, when compared to gasoline. It has very high knock resistance (measured as Octane Number), so can be used in engines with greater compression than used for gasoline.

Ethanol can be used in diesel engines as well! It is only required that injection nozzles will receive required lubrication. Mixing ethanol with some lubricant (vegetable oil for example) seems enough.

And the most important thing worth noting. Not every ethanol is bioethanol! It is cheaper to produce ethanol by hydration of ethylene than by fermentation of agricultural crops. I must also say, that inputtin of one unit of energy into manufacturing of ethanol from corn in USA gives fuel of only 1.3 unit energy content! For sugar cane this energy balance is more positive, the EROEI is close to 8.

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