advantages in emissions and small advantages in fuel economy over gasoline/diesel, their
properties and benefits have received significant attention over the last decade, and there is a large
body of literature on their costs and benefits. The one exception to this is hydrogen, which often is
portrayed as the zero emission fuel of the future. Hydrogen’s ability to fuel current and future
automobiles is considered in this section.Alternative engine technologies considered for the future include gas turbine and Stirling
engines. (In this context, the two-stroke engine is considered as a “conventional” engine type, as it
is similar in operating principles to four-stroke engines). The gas turbine engine, in particular, has
received increased attention recently as a power source for hybrid vehicles. As a result, the
potential for the gas turbine and Stirling engine in nontraditional applications is also discussed
here.Hydrogen
Hydrogen is viewed by many as the most environmentally benign fuel, because its combustion
will produce only water and NOX as exhaust components, and its use in a fuel ceil produces only
water as a “waste” product. Because hydrogen, like methanol, must be derived from other
naturally occurring compounds at substantial expenditure of energy, fuel economy evaluations of
hydrogen vehicles should consider the overall energy efficiency of the hydrogen fuel cycle. Even if
hydrogen is produced using electricity from photovoltaic cells, it maybe more efficient to use the
electricity directly for transportation rather than through the production of hydrogen, depending
on the location of the hydrogen production.
Because hydrogen is a gas at normal temperatures and pressures and has very low energy
density, it has serious storage problems on-board a vehicle. There are essentially four different
ways to store hydrogen, which are as a:
. compressed hydrogen gas,
. cryogenic liquid,
. reacted with metals to form a hydride, and
. adsorbed on carbon sieves.
Compressed hydrogen gas can be stored in high-pressure tanks (of advanced composite
material) at pressures of 3,000 to 6,000 pounds per square inch (psi). To store the equivalent of
10 gallons of gasoline, a tank at 3,000 psi must have a volume of 150 gallons, and the tank weight
is approximately 200 lbs.^137 Doubling the pressure to 6,000 psi does not halve the tank volume
because of increasing tank wall thickness and the nonideal gas behavior of hydrogen; at 6,000 psi,
137 Daimler-Benz, “Hydrogen: An Alternative Fuel,” n.d.