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Ethanol is a clean fuel and added as an additive to petrol.

Why can't ethanol itself be used as fuel?

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Ethanol clearly can be used as a fuel.

But there are several reasons why it isn't.

First is simple economics. Ethanol costs more to produce than gasoline: oversimplifying only slightly, fermentation is more complicated than simple distillation. Those countries that do use mixtures of ethanol in gasoline do so for "environmental" reasons (which are not always well thought through as being made from plants doesn't make something good if associated production costs are high or alternative uses are better). And "environmental" causes are a good excuse to subsidise favoured groups, like US or Brazilian farmers. In the absence of subsidy, there would be very little use of ethanol as a fuel.

But those are not the only reasons. Ethanol carries less energy than the same volume of gasoline. In other words your mileage will be lower; your fuel efficiency will be lower.

Worse, though, ethanol doesn't have exactly the same characteristics as gasoline. Engines may need to be adjusted so the alternative fuels burn equally well, which is annoying but fixable easily in electronically controlled engines. In addition, Ethanol may be more corrosive to some of the components used in fuel pipes (nothing like as bad as methanol, but clearly worse than gasoline). Ethanol is also much more hygroscopic than gasoline, absorbing significant amounts of water from air. This is another potential cause of contamination in fuel systems and may make corrosion worse.

In short, there are no fundamental reasons why ethanol can't be used. But there are a range of practical issues that make it less economical or inconvenient.

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  • $\begingroup$ But one of the most important reasons is probably that petrol was much, much more easily accessable when motor vehicles started becoming a thing in the early 20th century and therefore 100 years of engine optimation went into petrol engines. But +1 anyhow for a nice summary! $\endgroup$ – Jan Jan 22 '17 at 2:26
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Pure ethanol is used as a fuel in South America. Some vehicles can actually produce more power from ethanol than petrol.

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The argument that ethanol by itself carries inherently less energy, and hence, as a potential fuel source is perhaps not a good power source, I would view as a likely misleading statement, given how it has been actually employed in practice.

In fuel engines, Ethanol is employed as a stand-alone fuel for combustion with Nitrous oxide (N2O). Further, research has even been performed on its use as a prospective rocket fuel (see, Design and Testing of a Liquid Nitrous Oxide and Fueled Rocket Engine). To quote:

A small-scale, bi-propellant, liquid fueled rocket engine and supporting test infrastructure were designed and constructed at the Energetic Materials Research and Testing Center (EMRTC). This facility was used to evaluate liquid nitrous oxide and ethanol as potential rocket propellants. Thrust and pressure measurements along with high-speed digital imaging of the rocket exhaust plume were made. This experimental data was used for validation of a computational model developed of the rocket engine tested.

Futher comments provided by another source:

Under certain conditions more energy is produced by the decomposition than necessary to reach the decomposition temperature -> run away reaction!

$\ce{N2O(g) —> N2(g) + ½O2(g) + 82 kJ/mol}$

Of import by the same source, there are reported explosion of a N2O hybrid motor, to quote:

Incident 4

Design flaw of the injector caused a contamination of the N2O channels with alcohol. “Water hammering“ in the N2O ignited the mixture and destroyed injector and valves.

The author cites a possible explosion trigger from so-called ‘water hammering’ caused by cavitation of N2O combined with organic contamination (including Ethanol) resulting in a “Dieseling” based ignition scenario.

So a limiting factor on implementing Ethanol/N2O fuel engines is likely the need for research to manage safety issues and little to due with a low energetics argument (actually, more like the exact opposite).

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  • $\begingroup$ Your file on EMRTC is corrupted... $\endgroup$ – Zenix Mar 5 at 23:49
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It's not used widely mainly because of production cost and energy density that is lower than petrol. for petrol 34.2 MJ/L for ethanol fuel (E100) 20.9 MJ/L.

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  • $\begingroup$ This low energy density is the reason why ethanol will never be able to be used in a plane. If you want ethanol to replace kerosene in a usual jet, you would have to fill the plane with all the ethanol you are to burn when flying. And there will not give any place for the crew and the passengers. $\endgroup$ – Maurice Mar 5 at 21:08

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