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Fuel cell vehicles promise to become, in near future, competitive with conventional cars in terms of performance, efficiency and compliance with emission reduction schedules. However, many steps still have to be done, and a series of fundamental choices, such as high vs. low air pressure system options remain unresolved. Modeling can be a powerful instrument to evaluate different components or plant layout, and to predict the dynamic behavior of a fuel cell system. The first part of this paper illustrates the implementation of a direct engineering dynamic model of a load-following fuel cell vehicle. The modeling techniques, assumptions and basic equations are explained for each subsystem, with special attention to the air supply system, whose dynamic simulation was one of the primary targets of this work. Some of the simulation results are presented in the second part.

The effect of variations of compression ratio (CR) and injection pressure (IP) on the emissions and performance of a small displacement common rail off-road diesel engine was evaluated. The operating point corresponding to the 5th mode of the ISO 8178 - C1 test cycle (intermediate speed / full load) was considered, since it represents one of the most critical operating conditions as far as exhaust emissions are concerned. The main effect of a reduction of the compression ratio, for a fixed injection timing, was found to be, as expected, an increase in NOx emissions along with a decrease of PM emissions, with a substantial redefinition of the PM-NOx trade-off curve; the choice of a proper value for the start of injection can therefore lead to a better compromise among pollutant emissions, although remarkable variations in BSFC and combustion noise must be taken into account.