Again, from The Economist:
Everyone seems to think that ethanol is a good way to make cars greener. Everyone is wrong
SOMETIMES you do things simply because you know how to. People have known how to make ethanol since the dawn of civilisation, if not before. Take some sugary liquid. Add yeast. Wait. They have also known for a thousand years how to get that ethanol out of the formerly sugary liquid and into a more or less pure form. You heat it up, catch the vapour that emanates, and cool that vapour down until it liquefies.
The result burns. And when Henry Ford was experimenting with car engines a century ago, he tried ethanol out as a fuel. But he rejected it—and for good reason.
The amount of heat you get from burning a litre of ethanol is a third less than that from a litre of petrol. What is more, it absorbs water from the atmosphere. Unless it is mixed with some other fuel, such as petrol, the result is corrosion that can wreck an engine's seals in a couple of years. So why is ethanol suddenly back in fashion? That is the question many biotechnologists in America have recently asked themselves.
The obvious answer is that, being derived from plants, ethanol is “green”. The carbon dioxide produced by burning it was recently in the atmosphere. Putting that CO2 back into the air can therefore have no adverse effect on the climate.
But although that is true, the real reason ethanol has become the preferred green substitute for petrol is that people know how to make it—that, and the subsidies now available to America's maize farmers to produce the necessary feedstock. Yet such things do not stop ethanol from being a lousy fuel. To solve that, the biotechnologists argue, you need to make a better fuel that is equally green. Which is what they are trying to do.
The first step on the road has been butanol. This is also a type of alcohol that can be made by fermenting sugar (though the fermentation is done by a species of bacterium rather than by yeast), and it has some advantages over ethanol. It has more carbon atoms in its molecules (four, instead of two), which means more energy per litre—though it is still only 85% as rich as petrol. It also has a lower tendency to absorb water from the atmosphere....MORE