critically dependent on fuel octane rating; diesel engines are similarly dependent on fuel cetane ratings. Many other fuel characteristics, determined by the hydrocarbon mixture, are also important for proper engine operation over a wide range of ambient temperatures. Both exhaust and evaporative emissions are affected by fuel composition, and, in some geographical locations, special "reformulated" fuels have been mandated to reduce emissions. In addition to modifications of basic fuel compositions, additives have been introduced to ensure satisfactory performance, for instance, to improve storage characteristics or reduce the formation of engine deposits.
For the PNGV program, the greatest uncertainty about the CIDI engine meeting its performance targets is associated with emissions, both NOx and particulates. One way to reduce particulates in the exhaust is to lower the sulfur content of the fuel. As particulate emissions standards become more stringent, lower fuel sulfur levels will almost certainly be required, as well as other measures to control particulate emissions. Lower sulfur content will also be necessary if a petroleum fuel is used as the feedstock for an onboard reformer for a fuel cell, and lower sulfur levels will be beneficial for NOx treatment technologies. Reducing sulfur content increases the complexity and cost of refining processing. The magnitude of the increase will depend, among other things, on the sulfur content of the crude oil being refined. Nevertheless, a fuel with low sulfur content could be introduced in much less time than a completely new fuel.
Another way to produce low-sulfur fuel is to start with natural gas and synthesize a liquid fuel through the Fischer-Tropsch process. Natural gas is also the starting point for producing DME, a fuel with the potential to reduce both NOx and particulate levels in CIDI engines. The PNGV program is in the process of testing these and other types of fuel in CIDI engines. Because engine performance may vary with different fuels, joint development and evaluation by the transportation fuel and automobile industries will be important. The potential and time frame for making these fuels widely available must also be assessed.
Fuel cells require hydrogen with low (ppm) concentration levels of CO and sulfur compounds to function efficiently. To avoid energy losses associated with an onboard reformer, the widespread availability of hydrogen will be a critical factor in the practicality of using fuel cells as energy converters. One possible option would be to produce hydrogen in large-scale plants at central locations and distribute it in pipelines. Another would be to convert hydrocarbon fuels into hydrogen at local fueling stations. Substantial energy losses and emissions can occur at various points in the process of making hydrogen available for automotive fuel cells. Sensible trade-offs for this fuel and power plant combination will require that the entire system, from the wellhead to the vehicle wheels, be analyzed. However, trade-offs would require specific criteria, such as reduced petroleum consumption or total energy expended, in addition to the vehicle energy consumption goal of PNGV.