The purpose of this report is to present a critical assessment of the current state-of-the-art of alternative fuel technologies for heavy-duty transit vehicles. Fuels considered include compressed natural gas (CNG), liquefied natural gas (LNG), methanol, ethanol, liquefied petroleum gas (LPG), hydrogen, and several advanced battery and fuel cell technologies. For each fuel type, results from the recent literature are collected to allow comparison of the combustion methods, emissions, power output, fuel efficiency, range, refueling requirements, development status, safety aspects, and infrastructure requirements of various engine designs. Since many of the engines are under various stages of development, data for operation and maintenance costs are somewhat scarce; however, such information is provided when possible. Short descriptions of problems remaining to be solved are given, but detailed comparisons between technologies are withheld until the Discussion and Comparison section in order to avoid biasing the initial presentations.
The economic implications of the various strategies are then discussed in relation to the technical performance. Since the viability of heavy-duty vehicle operation depends to large extent on future emissions standards as well as the widespread availability of the given fuel, current and proposed emissions standards are listed, along with trends in light-duty and stationary applications involving the various fuels. Most of the work to date has centered on conversion and/or optimization of existing engine designs to accommodate the new fuels rather than the development of completely new engine concepts. Estimates of the capital cost and fuel economy of the various designs have been made, and the feasibility of retrofits versus dedicated engines are presented. The latter is especially important when one considers that poorly optimized retrofits may result in substantially higher emissions and maintenance costs than the original engines. A relative cost/benefit scale is formulated and used to rate the technologies, and the importance of the relative weighting factors in determining the outcome is discussed.