It has been called the invisible energy source that moves the world. With little hullaballoo, it hauls our groceries and building materials over Interstate highways, shunts laden trains from one side of our continent to the other, powers our navies of warships and cruise liners, even generates unseen hordes of electricity that eventually trickles down into iPods.
But in North America at least, if you mention "diesel" in polite company, the vocal response is likely to register along the chromatic scale from "ughhh" to "eewww."
The Euros know better. They've embraced diesel-powered personal transportation to such an extent that some 50 percent of non-commercial vehicles are diesel-powered on the Continent and in the United Kingdom.
Modern diesels are clean, quiet, thrifty. By their fuel efficiency, they represent an important antidote to the politics of petroleum. By their ever-improving cleanliness, they vaccinate against uncompromising environmental virulence.
And yet, despite the simple elegance of the diesel process that makes power without fire or spark, despite the alchemy now available that converts active pollutants into water and nitrogen, diesel-powered vehicles in the United States continue to wear reputations that range from the eccentric to the villainous.
Meantime, misplaced hopes are being spent in heedless praise for "magic-bullet" technologies that are either economically misleading or technologically unproven. Gasoline-electric hybrids get sexy press, but they still can't pay for themselves economically; and no one has yet answered satisfactorily what will become of the acres of discharged, heavy-metal, acidly toxic battery packs that will inevitably accumulate if personal transportation goes all-hybrid all the time.
And "hydrogenists," as they deserve to be called, rhapsodize about a mobile utopia of fuel-cell transport; but the smart money says we'll be suntanning at beachside resorts on Mars before a safe, reliable, convenient and national liquid-hydrogen fuel supply network is established.
Self-proclaimed Futurists and self-appointed Priests of Trend never noticed that diesel has shed its grimy image as an ugly duckling. The wide-spread availability of low-sulfur, clean-burning diesel fuel is now the law of the land in the U.S. (years after its adoption in Europe).
Ingenious "scrubbing" technologies now neutralize both of diesel fuel's remaining Achilles' Heels: particulate soot is trapped in filters, and smog-forming nitrous oxides are catalyzed into inert water and nitrogen. An extant technology championed by DaimlerChrysler, called Bluetec, injects nitrogen-rich and ammonia-like urea into the exhaust stream from a refillable cannister to accomplish this transformation.
Not to be outdone, Honda is readying a rival system that uses an engine's own internal combustion chemistry to produce on-board and self-contained the necessary ammonia for the same job.
These are real technologies, existing and imminent and sexy as hell if you like to fantasize about traveling in roomy, practical vehicles for more than 700 highway miles between fill-ups. This is already possible in offerings like the new 2007 Mercedes-Benz ML 320 CDI sport/ute.
By 2008, Merc's already clean diesel-SUV will be Bluetec-equipped and, thereby, clean enough for sale in all 50 states. By 2009, Honda's i-CDTi "self-scrubbing" diesel will first be appearing as a 2.2-liter turbo.
But there's no need to wait to be amazed by the manifest attractions of the invisible energy source that moves the world. The above-mentioned ML 320 CDI is on sale now, delivers 18 mpg/city, 30 mpg/highway (even while weighing almost 5,000 pounds), and costs just $1,000 more than its gas-powered sibling the ML 350. (Hybrids, by comparison, cost from $2,500 to $5,000 extra.)
Moreover, Mercedes' new diesel SUV can perform real work beyond merely flitting out for a new pair of Birkenstocks. With 398 foot-pounds of he-man torque, the ML 320 CDI can tow up to 5,000 pounds. It can stream confidently into freeway traffic thanks to acceleration on the order of 8.5 seconds zero-to-60. It seats five and totes anywhere from 20 to 72 cubic feet of stuff. And it's still capable of over 700 miles per tankful!
And then there are updated features Mercedes has made throughout its line of new M-Class SUVs. The trailer-sway control feature is one. Improved side and head airbag coverage for front and rear occupants is another. A magically smooth seven-speed transmission is so perfectly suited to the diesel's torquey power characteristics that gear shifts feel seamless.
Suspension tuning has softened ride comfort without promoting handling vagaries. An all-time all-wheel-drive system dubbed "4Matic" tackles civilian (i.e., on-pavement) traction challenges as admirably as before. It's off-road gumption that's been improved with the addition of Downhill Speed Regulation for heading down, and Start-Off Assist for going up.
Passenger room in the second row is also improved, so that three can travel there in more comfort than before for over 700 miles on one tankful. Mercedes' controls for adjusting cockpit climate remain easy to understand and use. But an infernal telematics system integrating audio system, telephone and optional navigation is still a forest of hard-to-navigate screens and buttons whose identities mutate. As other manufacturers have proven Acura and Audi spring to mind it doesn't have to be this hard.
The ML 320 CDI is sized right and behaves nimbly in urban and suburban settings. Perhaps its one quirk in real-world circumstances is the geometry of its hood. Specifically, its descending slope leaves a shorter driver in some doubt about the exact location of the front corners.
Accordingly, there is a bit of driving-by-Braille to be endured until one's mental curb-feelers adapt to the ML's footprint. Or not, in the case of those ML's with scuffed bumpers.
But all of the improvements to Mercedes' M-Class for 2007 are mere refinements compared with the revolutionary gesture, finally, of installing diesel power. What's it gonna take to win converts to the clever, clean, economical even sexy accomplishments of diesel's "Big D," if not the ability to travel in a large, practical vehicle for over 700 miles per tankful?
4-door SUV, 5-pass.; 3.0-liter DOHC V6 w/ CDI turbo-diesel; "4Matic" AWD, 7-sp. auto; 215 hp/398 ft.-lbs.; 18 mpg/city, 30 mpg/highway w/ diesel; curb weight: 4,817 lbs.; cargo space: 19.5-72 cu. ft.; std. equip.: 4-wheel ABS disk brakes & ind. suspension, 17-in. wheels, front/side/head airbags; base price: $43,680