Like kudzu, zebra mussels, and purple loose-strife, diesel engines are poised to invade the American landscape. The environment is certainly conducive. Federal CAFE regulations and California's threatened CO2 restrictions demand a quantum leap in fuel economy of the sort that diesel promises. Rising oil prices are driving consumer demand for improved thrift, and the Green groundswell is encouraging renewable fuels like biodiesel. Over a dozen brands have announced plans to sell diesels here, but astronomical diesel-fuel prices and pending SULEV emissions standards are formidable threats. On these pages, we'll share what we know about the individual species populating this diesel invasion and examine the technology's fitness for survival.

Modern diesel engines have a lot going for them. The fuel packs about 11 percent more energy per gallon than gasoline. Compression-ignition combustion has inherent thermal-efficiency advantages due to its higher-expansion ratio, lean and unthrottled operation, and natural affinity for turbocharging. Diesels can't rev as fast as gas engines so their peak-power figures are usually lower, but their torque curves resemble Ayers Rock, so they pull like Clydesdales. To withstand higher combustion pressures, diesels are built stronger to last longer than gasoline engines.

On the downside, the heavy-duty engine construction, ultra-high-pressure direct-fuel-injection systems, and the emissions gear that's now required to scrub exhaust clean of the NOx and soot that diesels inherently produce can add nearly as much cost to a vehicle as an electric hybrid system does. (For now, ammonia carried on board in urea tanks or generated by a two-level catalyst from excess diesel fuel neutralizes NOx, and a particulate trap grabs the soot and burns it off.)

Maintaining the fuel and emissions-controls systems erodes the operating-cost advantage diesels once boasted, and the fuel has a serious image problem to overcome. Only about half of all filling stations sell it, and suburban commuters don't like tanking up at truck stops. It's slimy and smelly if you get any on you, and the American public still remembers the bad old pokey diesels that smoked and stank and sounded like a blender full of ball bearings. Test drives of modern diesels will cure most of the reputation issues one customer at a time.

Perhaps the toughest hurdle diesel faces at the moment is the alarming fuel-price discrepancy relative to gasoline. For years, diesel cost less than gasoline except during cold winters when demand for the closely related home-heating oil caused price spikes. But diesel has usually cost more since September 2004, and, in the first quarter of 2008, the discrepancy has shot up to historic highs, averaging as much as 20 percent (70 per gallon) above gasoline. That erodes the economic argument for diesel, pushing the "payback" mileage well into six figures for many vehicles, at least at current diesel prices.