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The diesels are coming and, in the newest passenger vehicles, the best of them are as clean and as smooth as any gas engine.

Diesels are 30- to 40-per-cent more fuel efficient than gas engines for comparable performance, too.

The German auto makers — Mercedes-Benz, Volkswagen, Audi, BMW — are aiming to be the biggest players, but Honda and Nissan have plans to launch diesel cars in Canada, and others are likely coming, too.

But let's get real. Despite the high hopes of the Germans in particular, diesels are not about to win over the bulk of car buyers in Canada and the United States. That said, diesels may become the single most popular powertrain offering from Mercedes-Benz — thanks to an emissions-scrubbing system called BlueTec. For the record, "Blue" is the new green in marketing-speak.

And diesels are a big piece of Mercedes-Benz's long-term strategy to meet tough carbon dioxide emissions rules in Europe, as well as demanding fuel economy regulations in Canada and the United States. Fleet-wide fuel economy must improve by 22 per cent in Canada by 2020, 40 per cent in the United States.

On a rainy summer day, we are chugging around rural Vermont — truth be told, all of Vermont is rural — for the global launch and test drive of three new diesels from Mercedes — the 2009 ML 320, the GL 320 and the R320 BlueTec.

(No prices have yet been announced, but the outgoing, dirtier versions of this trio in 2008 list for $61,400, $71,500 and $65,000, respectively. Price increases are unlikely in this tough market.)

The three Alabama-built SUVs go on sale this fall as 2009 models and all three are clean enough to be sold in Vermont, which, like California, has the toughest tailpipe-emissions standards in the United States and Canada. In government regulations-speak, they are certified as Bin 5 ULEV (ultra-low-emission vehicles).

In this age of inconvenient truths, it seems so much of our conversation on the auto side now revolves around the technical jargon of pollution and emissions. So here we go: U.S. emission standards are tougher on smog-forming NOx (oxides of nitrogen) and particulates (those little sooty black bits that, unfiltered, come out of diesel tailpipes) than European standards. To get legal in the United States and Canada, Mercedes is one of several to perfect an exhaust after-treatment system. The Mercedes folks call their system BlueTec.

Mercedes had planned to market BlueTec alongside Audi and Volkswagen, but the VW Group companies backed out of the marketing agreement last year and will badge their new diesel engines TDI. The emissions cleanup approach is essentially the same, however.

Audi, for the record, will launch a clean-diesel Q7 SUV early next year and VW's Jetta TDI models are already on the way to dealerships now for a scheduled August on-sale date.

Mercedes is optimistic about diesels in Canada and one reason is that they don't face the kind of uphill battle for consumer acceptance that they do in the United States.

According to research released earlier this year by car buyers' guide Kelley Blue Book, most U.S. consumers believe that hybrids, fuel cells and flexible fuel vehicles are the way forward, not diesels — which are still largely regarded as noisy, smelly and unreliable.

Only 6 per cent of shoppers in the study thought diesel was most likely to succeed as a mainstream vehicle powertrain type, compared with 40 per cent for hybrids, 20 per cent for hydrogen fuel cells and 17 for flexible-fuel or ethanol mix systems.

But after spending the better part of a day driving these three new Mercedes models, the time seems right to move reasonably quickly to fairly clean diesels.

First up, the GL 320. It's a rival to the Acura MDX, BMW X5 and others, and is rated at just 210 horsepower. But that's not the number that matters most.

The diesel churns out nearly 400 lb-ft of torque at a very low 1,600 rpm and that means great performance for those who tow, and loads of off-the-line grunt. The standard seven-speed automatic has plenty of gears to make the most of the available power.

Ah, power. Stab the throttle and 100 km/h arrives in 9.5 seconds, with city fuel economy rated at 11.5 litres/100 km. The $91,000 GL 550, with its 382-horsepower V-8 engine, is rated at a thirsty 16.6 litres/100 km city, 11.7 highway — meaning a 30-per-cent fuel economy gain for the diesel with almost identical torque.

To put it another way, in city driving, the diesel's fuel economy is about equal to the gasoline GL's fuel efficiency on the highway. Impressive.

Mercedes is sharing the same diesel powertrain among these three, so the story is similar for the ML and R diesels.

They are all quiet and rattle-free, nothing like any of the diesels you'll find in an 18-wheeler, and light years ahead of the horrible diesels pawned off on unsuspecting consumers by Detroit-based auto makers in the late 1970s and early 1980s.

Many drivers will be hard-pressed to even notice they're wheeling about in a diesel — they are that quiet and smooth.

The cleaner diesel technology is almost the whole story with these 2009 SUVs, though Mercedes has done a minor mid-cycle facelift to the ML. The GL and R-Class, though, are unchanged.

The latter two are nicely done trucks with loads of sound-deadening materials and acceptable, if not brilliant, driving dynamics. My big complaint with all three is the rather disconnected steering. I found myself constantly making small adjustments to the wheel and this grew irritating.

For the ML, which is the biggest seller of the three, its bumpers have been redesigned and the lights front and rear are new.

The new grille has silver louvers with chrome highlights.

The big change inside is a new four-spoke, multifunction, leather steering wheel with paddle shifters.

For gadget lovers, a new telematics system is capable of integrating Bluetooth cellphones. It also allows passengers to run an MP3 player via the ML's own audio controls.

The cosmetic changes are not the story here. Mercedes-Benz Canada, instead, will spend big to market the BlueTec diesel technology.

Officials say high-end buyers are as keen to be, and appear to be, as environmentally responsible as others. What they do not want to surrender is performance. Mercedes is arguing that the new-generation BlueTec trucks deliver both.

Then there is the economic story. Diesel fuel in Canada and the United States is now pricier than gasoline, but better diesel fuel economy overcomes not only the price issues around fuel, but also the BlueTec price premium ($1,000 has been bandied about, but nothing official is coming from Mercedes).

At least Mercedes does have a history here. About 80 per cent of Mercedes cars sold in North America in the 1970s and 1980s were diesels. If Mercedes has its way, the future of diesels may look a lot like the past.

Consider: Diesels accounted for 61.2 per cent of Mercedes-Benz Canada's sales in June; with the new models joining the lineup, that number looks poised to grow.