AlanHo says "........No significant nasty emissions other than CO2. "
I agree that particulates, unburnt hydrocarbons, oxides of sulphur and nitrogen etc should be eliminated from car exhausts, but CO2 is benign and it's wrong to label it as a pollutant (or nasty). US navy submarines tolerate up to 8000ppm in their air supply (as opposed to the 400ppm in the atmoshpere) and the navy guys do just fine. Plants also love lots of CO2 as any botanist will tell you. It has been suggested that CO2 is the "control knob" for the planets surface temperature but I'll remain sceptical for now.