Swinging his truck door open, the driver obligingly steps onto the cab seat and reaches for the roof. Extending himself upward, he slings a handkerchief over the exhaust stack of his late-model diesel rig. In mere moments, black fumes begrime a section of the white square with soot. "This good?" he asks, handing down the fluttering fabric. Nodding, I thank the man and retrieve the hankie. A short stroll away from his idling truck and its fellow 18-wheelers parked in this New Jersey Turnpike rest area sits their newborn brother, a Mercedes-Benz E320 Bluetec sedan. With a turn of the key, its diesel engine springs to life. Moments later, I kneel behind the car and cover its tailpipe with an unsoiled patch of cloth. It remains nearly spotless, even after a full minute.

As the so-called handkerchief test shows, the words "clean diesel" are no longer a contradiction in terms. Diesels have long been regarded as among the dirtiest of power plants, a reputation that lingers because so many decades-old examples of this durable technology still work the roads today. But the E320 is the vanguard of a new wave of diesel cars, SUVs and pickups that release far fewer air pollutants without compromising the engine's traditionally excellent fuel economy. Powered by a 3.0-liter V-6 engine, the E320, for instance, gets 36 miles per gallon (combined) and can travel as much as 780 miles between fill-ups.