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2001, substantial emphasis was placed on validating the PS AT model, using data from the Toyota Prius and Honda Insight HEVs and the Ford P2000 fuel cell vehicle. Vehicle testing and simulation evaluations are still in progress, and the results to date are encouraging.

Major efforts have been initiated on modeling emissions from internal combustion engines and their exhaus-gas after-treatment systems (catalysts and particulate traps). While this is a challenging technical task, the initial progress is encouraging, and approaches have been developed that should provide useful estimates of vehicle emissions to assess compliance with future emissions standards. It is apparent that the challenge of modeling the effects of engine acceleration and deceleration transients on vehicle emissions with HEV systems is less severe than with stand-alone engine transient modeling because the HEV system reduces the impact of these vehicle drive transients on the engine. However, the HEV internal combustion engine undergoes many start-ups and shut-downs during normal driving. Modeling the emissions produced by these events is especially challenging. The committee is encouraged by this progress in emissions modeling and recommends continuing emphasis on this topic. One important use for a vehicle emissions model is to quantify the trade-off between efficiency and emissions for the various propulsion system options, as discussed in Chapter 5.

Fuel cell HEV system simulation studies of a large sport utility vehicle (SUV) are under way to examine the trade-off in relative sizing of the battery pack and fuel cell. This indicates that a useable fuel cell HEV system model is now available. An especially important fuel cell system modeling area that was not part of the fuel cell modeling review is the liquid fuel reformer system. An effective gasoline-to-hydrogen reformer is a critical component in the most practical shorter-term fuel cell system because it avoids the challenges of developing a hydrogen production and distribution system and the need to store hydrogen on the vehicle. The committee encourages increased effort in modeling fuel cell system-component and overall system performance and especially in the fuel-reformer technology area. Because fuel cell technology is developing rapidly, well-validated system models are important tools for extrapolating from the performance of current prototype system data to likely future system performance.

Progress in cost modeling was not as encouraging as in the areas described above. However, a Cost Analysis Task Group has recently been formed. The committee urges that a framework be developed for using the systems model to assist in cost estimation studies for the internal combustion engine (ICE) and fuel cell HEV systems, and the effort on detailed modeling required to implement effective cost models should be intensified.

HYBRID PRODUCTION VEHICLES

Since the last committee report (NRC, 2000), both Toyota and Honda have introduced HEVs into the U.S. market. An overview of these two vehicles illus-



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