Here's a cool idea that you may or may not have in the back of your mind—why not a petrol-fueled diesel-combustion-cycle engine? Advantages would include cleaner burning of the gasoline fuel combined with greater efficiency.
The folk at the U.S. Argonne National Laboratory are working on the technology, as well as industry groups investigating such technology as HCCI (homogeneous charge compression engine).
But rather than me attempting to fill you in, here is a blog just posted by Steve Ciatti, a mechanical engineer at the Laboratory, about their research.
The only comment I would offer is that I disagree with his statement, "There’s really no significant improvement that can be achieved with either one [gasoline or diesel combustion alone]." It seems to me that with each industry meeting, such as Convergence coming up in Detroit in October, there is always some news of improvements to these internal combustion engines that design engineers, thanks to microelectronics-enabled controls, can continue to wring out of these high power-density systems.
Why not add a comment here and let us know your opinion?