Most promising green technologies number five: diesel

Click the new Mercedes diesel for a high res gallery

Sure, diesels are already widely available all over the world. One reason is that diesel engines are simply more efficient than gasoline engines. Consider the fact that a diesel engine is capable of running on biodiesel and green-minded drivers can make a strong argument in diesel's favor. Perhaps what is most important, though, is that diesel vehicles would require no major changes to the current energy infrastructure, meaning that diesel-powered vehicles could be one of the simplest possible ways to reduce the amount of oil that the world consumes. Holding them back, though, are costly exhaust after-treatment technologies that are needed to put the clean into clean diesel. Since these systems are less expensive than some of the rest of the items on our list, diesel engines fall right in the middle of the pack.

And so we move to Number 4.