http://news.bbc.co.uk/1/hi/sci/tech/4076811.stm

The search for a suitable alternative fuel source to hydrocarbons which can cleanly power our vehicles has touched on various different options.

Fuels which do not "rot" the environment usually bring to mind images of gently humming electric cars, clean hydrogen, natural gas, or hithane - a concoction of hydrogen and methane.

But until a viable mass-scale way of storing and distributing hydrogen effectively is developed, it remains limited in use.

INSPIRATION STEAM CAR
Construction: Tubular steel spaceframe with composite/metal panels
Length: 5.25m
Width: 1.70m
Height: 1.10m
Fuel: LPG (Liquified petroleum gas)
Working fluid: Water/steam
Performance: Maximum speed 200+ mph (320km/h); Initial acceleration: 0.52G
Brakes: Twin front wheel brakes and twin rear inboard rear disc brakes
Steering: Rack and pinion

He does not imagine that steam cars will be the complete road ahead for cars on our streets.

"Gas turbines have been used in the past," he says. "But the problem of turbines is that to be efficient, they have to run at a predetermined speed.

"The very nature of road cars is that their speed changes all the time, so this design would be no good for road vehicles."

But he can imagine the engine design being used in diesel-based commercial vehicles which belch out a large proportion of pollution, like buses and lorries.

"Burning propane is environmentally more friendly than burning diesel. If the technology could be adapted, then it might just be a possibility - it is something we are investigating," he says.
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and a lot more...

I'd certainly go along with the propane part. Extremely clean burning, not expensive, pretty simple to use, available almost everywhere.

There's no real point in using it to power a steam engine though. There's an inherent loss in efficiency when converting propane generated heat into steam, then using that to power the vehicle. For all power conversion systems I'm aware of, its more efficient to get the power directly from burning the propane, as in the typical 4-stroke internal combustion engine, 2-stroke engines, rotaries or turbines.

If they were using the waste heat from the propane engine's exhaust, to turn a steam engine or turbine for a secondary power source, they could actually be coming out ahead.

Glad to hear people are still trying to find a better way though.