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An auto mechanic friend claims to have a gizmo that makes his vehicles run at least partially on water. He swears it’s true and has about five test vehicles running with this thing now. It will work best on vehicles with carburetors—fuel-injected vehicles need tweaking of the computer chips. He’s got one on an old VW Bug and says he gets about 80 mpg—he’s trying to win a prize for getting over 100 mpg. He installed one in a large diesel truck that originally got about 8 mpg; it supposedly now gets 20 to 22 mpg with lots

more power. My friend says the gizmo uses electricity from the alternator to split water molecules into something called “Brown’s gas” that gets input into the intake manifold. Is this true or another myth? —Walt Bruun, Glen Ellyn, Ill.

I’ll tell you one scientific reaction involving Brown’s gas that you can take to the bank: It makes my blood boil. Where miraculous fuel economy schemes are concerned, tricksters abound, preying on marks who distrust “the authorities” and can’t tell good science from the pseudo kind. Some mutter of brave souls silenced because they knew too much—like the late Stan Meyer, inventor of the magical “water fuel cell” (ultimately shown to be bunk), who fans claim was poisoned in 1998 by operatives of the government and/or the oil companies.

The device you’re talking about is similar to Meyer’s but places the emphasis on hydrogen, thus piggybacking on the “hydrogen economy” meme President Bush brought to public attention in his 2003 State of the Union address. Newspapers and magazines subsequently devoted acres of unskeptical column space to on-board hydrogen-generation and -injection technology—in 2005, for example, Wired wrote that big-rig truckers were getting major improvements in fuel economy and power from hydrogen electrolysis systems.

Here’s what happens. The gizmo is hooked up to a standard internal combustion engine. Like your pal says, it draws power from the car’s electrical system to split water into a mixture of hydrogen and oxygen—or so-called Brown’s gas—which gets fed into the engine and burned along with the usual gasoline/air mix. Alleged result: big gas savings!

But how? On the most basic level, the technology makes no sense. Let’s walk slowly through the process:

1. Your car engine burns gasoline or diesel fuel to power the wheels and, among other things, your alternator, at about 20 to 25 percent efficiency.

2. Your alternator generates electricity at about 60 percent efficiency.

3. You take said electricity and use it to turn water into hydrogen and oxygen at about 70 percent efficiency, tops.

4. Then you burn the hydrogen and oxygen, or just the hydrogen, in your engine at about 98 percent efficiency.

In short, you’re converting fuel A (gasoline) into fuel B (hydrogen) which then helps power the car. Net efficiency of this complicated process: 10 percent. Efficiency of an ordinary car engine (see step 1 above): 20 to 25 percent. Conclusion: Hydrogen gizmos are a fool’s bargain.

Advocates claim using hydrogen as a fuel increases combustion efficiency. The problem is that in modern engines combustion efficiency is already close to the max—95 to 98 percent under optimal conditions in a gasoline engine and 98 percent or better in a diesel engine. Understand, this refers strictly to how thoroughly the fuel burns in the cylinders. Overall engine efficiency is, as seen, much lower, due to heat loss through the engine block and out the tailpipe. Switching fuels won’t change that.

So why do hydrogen injector users report improvements? The same reasons people often swear by iffy technology—lack of appropriate comparisons, sloppy record keeping, wishful thinking, a sample size of one. The fact that fuel economy is partly a function of driving habits no doubt also plays a role. If you simply pay more attention to your speed when driving, you can often increase mileage even without a miracle device.

To be sure, a little water can improve internal combustion engine performance under some circumstances. Water injection helped WWII aircraft engines put out more power by reducing knock. BMW has been trying to increase fuel economy and power by using exhaust heat to power what’s in effect a small steam engine attached to an internal combustion engine. A Honda hybrid uses a similar concept to turn a generator to recharge the battery packs while cruising. Although some bugs remain to be worked out, a six-stroke engine using water injection for power and cooling shows promise. Bear in mind, though, that water isn’t being used as a fuel in any of these cases. Hydrogen injection, meanwhile, is for the birds. If you really want to improve your fuel efficiency, check your tire pressure. It may not be sexy but it works.

—Cecil Adams

Is there something you need to get straight? Take it up with Cecil on the Straight Dope message board, straightdope.com.