''During the course of the 21st century, new fuels and vehicle technologies will have to bedeveloped to address these sustainability challenges. The challenge will be the transition to the use of renewable energy sources in transportation," Dr Hans-Wolfgang Luke, Director Technology Shell Global Solutions, told an international conference on sustainable transportation here.

He said in a paper on ''Future Fuels for Sustainable Mobility'' that questions of energy supply security, local emissions and Greenhouse gas emissions all need to be balanced against the desire of a growing global population to have access to affordable and efficient mobility systems.

He said these objectives could take decades to achieve. "In the meantime, a more diverse mix of transport fuels and vehicle types is expected, as new technologies are introduced alongside improved versions of existing technology," he added.

This could be through the use of renewably produced hydrogen, probably powering fuel cell vehicles, or increased use of new advanced biofuels, in highly efficient internal combustion engines.

''For the next 20-30 years it is likely that liquid fuels will continue to dominate in transportation, with the availability of cleaner gasoline and diesel being extended to enable the wider introduction of more advanced and efficient internal combustion engines,'' he remarked.

He added that there would be increased use of new blending components, including biofuels and synthetic fuels from gas and other feedstocks.

''The initial penetration of hydrogen as a transport fuel will be based on its production from natural gas. Gaseous fuels such as LPG and CNG will continue to have important niche applications in some markets.