bons, nitrogen oxides, and particulate matter, are also reduced. In fact, because DME meets and surpasses the California Air Resources Board emissions standards for automotive fuel, it is considered an ultraclean fuel.

At present, the preferred route and more cost-effective method for producing DME are through the dehydrogenation of methanol from synthesis gas, which is a mixture of CO and H₂. The basic steps for producing DME are as follows:

1. Syngas production either by steam reforming of natural gas or by the partial oxidation of coal, oil residue, or biomass.

2. Methanol synthesis using copper-based or zinc oxide catalysts.

3. Methanol dehydrogenation to DME using a zeolite-based catalyst.

The produced DME fuel is not suitable for spark-ignition engines because of its high cetane number, but it can run a diesel engine with little modification. DME has properties similar to those of GTL diesel, including good cold-flow properties, low sulfate content, and low combustion noise (Yao et al., 2006; Arcoumanis et al., 2008; Kim et al., 2008).

The principal advantage of using DME as an automotive fuel is that it is clean burning and easy to handle and store. But as with other potential alternative fuels, the primary challenge facing the use of DME is the lack of an infrastructure for distribution. Other disadvantages include low viscosity, poor lubricity, a propensity to swell rubber and cause leaks, and lower heating value compared with conventional diesel.

Hydrogen, like electricity, is an energy carrier that can be generated from a wide variety of sources, including nuclear energy, renewable energy, and fossil fuels. Hydrogen also can be made from water via the process of electrolysis, although this appears to be more expensive than reforming natural gas. Used in vehicles, both hydrogen and electricity make efficient use of energy compared with liquid-fuel options on a well-to-wheel basis. As generally envisioned, hydrogen would generate electricity in a fuel cell, and the vehicle would be powered by an electric motor.⁹ Developments in battery technology that might make plug-in hybrid-