We exhale carbon dioxide. Cars burn fuel and exhaust carbon dioxide. Plants need carbon dioxide to make sugars. And we can either eat sugars or ferment sugars to make fuel. It's a renewable cycle.

It's other kinds of exhaust gases that are really the problem: smog forming nitrogen oxides, sulfur, soot, carbon monoxide. Incomplete burning of fossil fuel cocktails produce these pollutants.

Ethanol is a particularly good fuel whether it's burned in a conventional engine or used by a fuel cell. It burns cool and as a pure chemical has a very narrow flash point, thus an engine can be finely tuned for burning ethanol very completely with a minimum of pollution. And next years biomass crop will absorb all the CO2 exhaust.

Ethanol even has a major advantage over hydrogen. It is a liquid at room temperature, meaning it can be stored in a featherweight plastic container. The storage container adds zero complexity and weight to the vehicle.

The advantage of fuel cells might be even greater efficiency, and still fewer pollutants (nearly room temperature operation). But they cost more. Direct Ethanol Fuel Cells therefore have a decent prospect for use with expensive items with low power consumption: portable electronics, and perhaps model cars and aircraft (expensive).

Check out Iron Bay Models. Originally marketed by Byron Originals, their flagship 1/7th-scale twin-engine ducted fanjet F-14 has steerable retractable landing gear, airbrakes, strobe and landing lights and can reach speeds upwards of 170 mph. Having a wingspan of 7 feet and length over 12 feet, this model will rquire it's own hangar (leave the car in the driveway!) Only for the hard-core hobbiest..... $7000 US.

Turbine conversions are available if you have the cash (If you own this model in the first place). Many photos are missing from their website, but I can tell you that the F-15 Eagle and F-18, and P-51 Mustang are equally hot.

Fuel-cell conversion? Nah. I wanna see flames in the exhaust ducts!