Is this creative problem solving, or utter madness? Diesel fuel, gasoline’s much-maligned stepsister, could be welcomed back into the energy fold if the auto industry has its way. Diesel fell out of favor in the U.S. two decades ago because it is smelly, pollution-causing, and carcinogenic. But faced with increasing demands for better fuel efficiency, automakers are hoping to revive diesel engines, which are more efficient than their gasoline-driven counterparts. Proponents argue that diesel technology is far greener than it used to be, and point to Europe, where some 30 percent of vehicles rely on diesel fuel. Critics say diesel exhaust is still bad news for the environment and public health.