Concerns regarding global warming have increased the pressure on automobile manufacturers to decrease emissions of CO 2 from vehicles. Diesel vehicles have higher fuel economy and lower CO 2 emissions than their gasoline counterparts. Increased penetration of diesel powered vehicles into the market is a possible transition strategy toward a more sustainable transportation system. To facilitate discussions regarding the relative merits of diesel vehicles it is important to have a clear understanding of their CO 2 emission benefits. Based on European diesel and gasoline certification data, this report quantifies such CO 2 reduction opportunities for cars and light duty trucks in today's vehicles and those in the year 2015. Overall, on a wellto-wheels per vehicle per mile basis, the CO 2 reduction opportunity for today's vehicles is approximately 24-33%. We anticipate that the gap between diesel and gasoline wellto-wheel vehicle CO 2 emissions will decrease to approximately 14-27% by the year 2015.

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Abstract-Hydrogen is a means to chemically store energy. It can be used to buffer energy in a society increasingly relying on renewable but intermittent energy or as an energy vector for sustainable transportation. It is also attractive for its potential to power vehicles with (near-) zero tailpipe emissions. The use of hydrogen as an energy carrier for transport applications is mostly associated with fuel cells. However, hydrogen can also be used in an internal combustion engine (ICE). When converted to or designed for hydrogen operation, an ICE can attain high power output, high efficiency and ultra low emissions. Also, because of the possibility of bi-fuel operation, the hydrogen engine can act as an accelerator for building up a hydrogen infrastructure. The properties of hydrogen are quite different from the presently used hydrocarbon fuels, which is reflected in the design and operation of a hydrogen fueled ICE (H 2 ICE). These characteristics also result in more flexibility in engine control strategies and thus more routes for engine optimization. This article describes the most characteristic features of H 2 ICEs, the current state of H 2 ICE research and demonstration, and the future prospects.