Mostly diesel fuel, much of it biodiesel. Fundamental change comes very slowly.
Diesel power is already more and more popular in Europe (with better fuel than in the US). A couple of my friends there have Audi diesels, and there's little noticeable difference in noise, smoothness, or power from gasoline power. As I understand it, diesel pollution problems have also pretty well been solved. I have trouble understanding why hybrids are a better solution than diesel power. Big batteries aren't cheap.
BTW, what about steam power? It's always seemed to me an excellent way to power cars -- for example, maximum torque at zero rpm, so no need for transmissions. One objection used to be that without the engine running while the car is stopped (e.g., at a light) the electrical drains for radio, A/C, etc. would drain the battery too fast, but hybrids have much the same problem and seem to solve it OK. Perhaps one just needs larger batteries, or maybe a small constant speed internal combustion engine.
Chris Earnest
