Monday, September 26, 2005

Sacred Cow: Hydrogen fuel cells, Part I

In the wake of The Sunday Times determining in a real-world test that the petrol Citroen C1 is the most fuel efficient car in Britain, more efficient than the Toyota Prius by a good margin and even slightly better than the diesel (how did that happen? Short answer: more powerful engine is less stressed and uses less fuel), I would like to take on the matter of the much-vaunted fuel cell car. I come here not to praise hydrogen, but to bury it; and unlike Marc Anthony, I mean it.

For the last half a decade at least, car manufacturers, engineering students and the automotive media have been collectively ranting and raving about the glorious future of the fuel cell vehicle. Everyone and their grandma is making a prototype - even Lada! Theirs actually cost $30,000 to build, compared to millions for other car makers - but only because they used the fuel cell generator developed for the Soviet space program. Incidentally, that thing was designed and tested to withstand forces of up to 100g. That's a hell of a lot more than you get in a car crash.

The idea behind fuel cell vehicles is two-fold. The primary benefit is zero toxic exhaust. A free-standing hydrogen molecule consists of two atoms; so does a free-standing oxygen molecule. Hydrogen and oxygen are very keen on bonding with each other (which is why there is so much water around), so what happens when you mix hydrogen and oxygen molecules is, and this is grade school chemistry:
2H2 + 02 => 2H20 + 2e
Two molecules of hydrogen combine with one molecule of oxygen and make two molecules of water. They also release two electrons. (Each oxygen atom releases one molecule; it originally bonded with the other oxygen atom because they were both one electron short - so they designated two electrons to be used by each one in turn, forming the bond - but now each of them have two electrons available courtesy of the hydrogen atoms. If you want to know why most atoms are short of electrons and have to form molecules, talk to your chemistry teacher, or google.) These two electrons then travel down some conductors and make an electric motor work.

The upshot is, you put in hydrogen, suck in some air that contains about 21% free-standing oxygen, and get water out the other end. The water then comes down as rain or whatever, into the rivers, etc; if you take some water and introduce a lot of electrons - by running an electric current through it, dunking a couple of wires basically - you get hydrogen and oxygen again. So you can always get more fuel, and as an added benefit, your car doesn't kill baby pandas.

The other benefit of fuel cell cars is what happens after you get the electricity out. Electric motors are really much better than internal combustion engines.

  • They're small, so you can put one on every wheel and have an AWD car without all the bulky transfer case nonsense - just lay some wires. (By the way, you can make one hell of an electric offroader - without the need for driveshafts, you can make the ground clearance really high, and it's easy to direct all the power to the wheel that's slipping the least at the moment.)

  • When you brake, instead of using discs and pads you can throw the electric engine in reverse and actually charge the batteries using the car's inertia!

  • Electric engines don't need to gather revs - they always produce maximum torque, right away. If you don't need all the torque it's capable of - give it less electricity; in fact, give it exactly as much as you need, and save the rest. And in the winter, clever computers can work out how much torque to give each wheel without it spinning on the ice. In general, electric engines make the car go zoom and only spend as much energy as they can use.

  • The power losses from running some electricity down some wires are quite small, especially since you're not running it very far in a car. The losses from running torque from an internal combustion engine to the wheels are, by comparison, enormous - especially in AWD cars.

Looks extremely nice, doesn't it? Electric engines are efficient and make it easier to design a car, so engineers love them, and fuel cells don't produce any nasty stuff, so baby pandas love them. Unfortunately, there are problems here. These I will be explaining tomorrow, in Part II.


Anonymous said...

Monday, September 26, 2005
More powerful (which usually means larger) engines are more efficient?

That's pretty much contrary to everything I've ever heard about car efficiency. In fact it goes contrary to that article (the one about the C1), which is talking about cars that are laughably anemic compared to what we're filling the roads with her in North America these days (68bhp? Our super-ultra discount cars like the Chevy Cavalier pack about 130hp, and I'd say the average car these days is from 180 - 270HP).

This car may be more fuel efficient than the Prius (with a combined 150HP, and a tubthumpin 300+lb/ft of torque), but I'd bet the Prius would smoke it in measures like acceleration (which are the point behind getting the super powerful cars, given that we don't have an autobahn here).

Dennis (:-))

Flasher T said...

That's a very good question, Mr. Forbes. So good in fact, that I'm going to make a separate post out of it. :)


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