I agree with Frenetic that cellulosic methane has an interesting potential as a vehicle fuel; but only with the right drivetrain and the right engine. Cellulosic methane (after the CO2 is removed) can be mixed with natural gas and be delivered to fuel stations using existing pipeline infrastructure. To offset the 28 Quads of petroleum used by the US transportation fleet annually, you would need about 14 Quads of methane and a methane-fueled vehicle that was twice as efficient as the piston/crankshaft gasoline engine. Natural gas could be used as a bridge as cellulosic methane facilities were incrementally developed. Methane has one big advantage over gasoline; its autoignition temperature is 1000F versus gasoline’s 495F. A high autoignition temperature allows a higher compression ratio, which, in the Otto cycle, translates to higher efficiency. The highest compression ratio engine is a free piston engine because it has no piston rods or crankshaft. A high compression ratio, free piston motor/generator, such as Pempek’s (www.freepiston.com), can maximize methane’s efficiency potential. The free piston motor/generator recharges lithium ion batteries, as in the Chevy Volt’s drivetrain. You then get the “at the wheels” efficiency benefits of an electric car but use methane fuel to recharge while driving. In effect, you have an engine/battery combination that functions exactly like a fuel cell–except it runs on methane, not hydrogen. The drawback to methane is its energy density. A Honda Civic GX has only about half the range of the gasoline model. But if you double the “at the wheels” efficiency of the vehicle, you have a comparable range even with a less energy dense fuel. For a vehicle, you have to rethink the fuel, the drivetrain and the engine and they must all be complementary. A methane-fueled free piston electric drive vehicle has that revolutionary potential.
