There's no doubt that the Toyota Prius is the most well-known green car in the world, a fact that's as often attributed to the car's distinctive styling as it is to the car's impressive fuel efficiency. The latest 2010 model pushes that efficiency to a new level with a combined 50 mpg rating here in the States. Up next is likely a plug-in version of the Prius, almost certainly with a lithium ion battery.
Of course, there are ways to improve the fuel efficiency of the Toyota Prius besides adding a plug. According to Toyota R&D head Tasatami Takimoto, though, the options that the company is investigating doesn't currently include a diesel powerplant. He says:
We already make a diesel hybrid truck in Japan, but in order to mass-market diesel hybrids we need to make the exhaust emissions much cleaner, and that requires development. You have to combine this clean emissions technology with the hybrid system cost, so that means that, currently, it's a very high-cost powertrain for us. So for Toyota, it will take many more years.