If you have ever compared diesel fuel and gasoline, you know that they are different. They certainly smell different.
Diesel fuel is heavier and oilier.
Diesel fuel evaporates more slowly because it is heavier. It takes less refining to create diesel fuel, which is why it is generally cheaper than gasoline. Diesel fuel has a higher energy density than gasoline. This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines. For more information on all sorts of engines, check out the links on the next page.