Something very big is starting this month. Just how big it will grow to be anyone's guess.

Volkswagen begins selling a diesel-powered Jetta TDI station wagon with a starting price of $25,775, fuel economy rated at 6.8 litres/100 in the city, 4.8 on the highway and a range, on one tank of fuel, approaching 1,000 or 1,200 km, depending on how you drive.

Imagine driving from Toronto to Thunder Bay without filling up. Go ahead, take a road trip from Calgary to Vancouver on one tank of diesel fuel in this Jetta. It's possible. And the TDI is as clean as any normal gasoline car sold and certified today, though Toyota's hybrids are still 50 per cent cleaner.

Many more of these cleaner diesels are coming, too. Audi, VW's luxury arm, Mercedes-Benz and BMW all plan to expand their range of cleaner diesels in the coming months.

The Germans are stretching their lineups will new diesels because they can. Nissan is also going to launch a diesel version of its new Maxima sedan in 2010, Subaru is planning a diesel Legacy and Honda's Acura division, while being coy about the specifics, will introduce its new clean-diesel engine likely in next year's version of the TSX.

The Germans are pushing hardest, though. They have the technology, refined in Europe where diesels account for 50 per cent of passenger car sales. They also have brands strong enough to command the extra cost of a diesel power train -- $2,000-$3,000, and more in the future as emissions regulations get stiffer still and more expensive technology is needed to meet legal requirements.

With gas prices averaging about $1.29 a litre across Canada now (diesel fuel averages about $1.42 a litre), diesels are starting, of course, to make some sense to consumers. Fuel economy is 25-40 per cent better than gasoline, carbon dioxide emissions are lower and highway fuel economy beats gasoline-electric hybrids hands down.

Diesel engines also deliver excellent torque - the twisting force that matters at your car's wheels when you take off. , Better torque is well suited for larger cars, sport-utility vehicles (SUVs) and pickups, too.

Not only that, diesel engines are robust, lasting up to 500,000 km or more, and the extended fuel range means you'll need to fill up less frequently. All good.

Of course, not everything is rosy in diesel-land. Diesel engines are more expensive to build than gas engines and the emissions systems needed to clean them up are hugely costly.

In many of these so-called "clean diesels," the latest emissions controls required to meet regulations demand regular refills of urea to make the emissions treatment systems work properly. If you don't fill up, the car or SUV is programmed not to start after a certain number of tries. With Mercedes-Benz, it's 20.

There's more, too. Within two years the cost to make a diesel clean enough to meet emissions regulations in Canada and the United States will make clean diesels as expensive as a gasoline-electric hybrid with its two separate but sometimes integrated power trains.

Then there is the price of diesel fuel. It is higher than gasoline now and will stay that way as long as there is such limited diesel fuel refinery capacity in North America.

Also, for many buyers, diesel's dirty image is a huge turn-off. Diesels tend to conjure up images of 18-wheelers clogging the 401 today. For those with longer memories, diesel cars are nothing more sooty, smelly, foul, black-cloud-belching rattle traps that broke down regularly in the late 1970s and '80s.

General Motors vice-chairman Bob Lutz, the Swiss-born product boss who overseas everything GM does to make new cars and trucks, says the new advanced emissions systems needed to clean up diesels adequately by the end of this decade will add a $3,000-$4,000 premium to a diesel engine over a comparable gasoline one.

At last month's London auto show, Lutz got very specific about how expensive diesels are going to be by 2010, when stricter emissions regulations come into effect.

"You will be paying a very substantial price premium even over today's diesels," he said to reporters covering the show. "You will have the privilege of paying another $3,000 to get a 20 per cent fuel saving to buy a fuel that is 20 per cent more expensive per gallon than normal pump petrol. You have to start asking yourself, where's the benefit?"

Nonetheless, auto supplier Robert Bosch says diesels will own roughly 15 per cent of the light vehicle market by 2015. A study from J.D. Power and Associates suggests diesels will account for 13.4 per cent of the passenger car and light truck marketplace by 2015 in Canada.

So diesels are going to grab a much, much larger share of the marketplace. Then again, they have a long way to go.

The Power Information Network reports that this year diesel sales account for just 2.3 per cent of the market. Most people are still buying gas-powered cars (91.5 per cent), with hybrids and flex-fuel vehicles (2.0 per cent and 4.2 per cent) making up the rest.

The arrival of the Jetta and so many other modern diesel passenger vehicles gives consumers a growing range of choices. We'll see how big it all gets starting with these new diesels coming to market in the next 6-8 months:

Audi Q7 3.0 TDI
On sale: Early 2009

Power comes from a turbocharged, 240-hp, 3.0-litre, six-cylinder diesel engine. This SUV is very strong out of the gate and very quiet - even in cold-weather starts below -20 C. Up to now, Audi has been selling gasoline versions of the Q7 only in Canada, though diesel accounts for half of worldwide Audi sales.

Audi A4 3.0 TDI
On sale: Early 2009 (not officially confirmed)

The A4 3.0 TDI shares its engine with the Q7 3.0 TDI. In the smaller, lighter diesel A4, fuel economy and overall performance should be excellent. So far Audi Canada has not confirmed the diesel A4 is coming to Canada, only that anew A4 hits showrooms later this year in the fall. The U.S. is getting the A4 diesel, however, so Canada surely will, too.

Mercedes-Benz GL320 Bluetec
On sale: Fall, 2008

This new diesel GL320 CDI is not the current non-Bluetec model sold today. This is the clean one, the one that meets emissions regulations in all 50 U.S. states - California and Vermont included - and in Canada, as well. So GL320 Bluetec is a "50-state" diesel, with improved technology. Bluetec pricing hasn't been announced.

Mercedes-Benz R320 Bluetec
On sale: Fall, 2008

Like the GL Bluetec, the R320 Bluetec is a "50-state" diesel. The new technology might give a boost to the otherwise slow-selling R-Class wagon.

Mercedes-Benz ML320 Bluetec
On sale: Fall, 2008

This ML320 Bluetec is the third member of this made-in-Alabama sport-utility trio with the new clean diesel technology. The higher torque of a diesel engine offers significant benefits for SUV customers. It makes it easier to tow a boat or trailer, or for occasional off-road driving.

Mercedes-Benz E320 Bluetec
On sale: Now

Here we have the first "50-state" diesel from Mercedes-Benz. In Canada, the diesel E-Class accounts for more than 50 per cent of all E-Class sales. It's been a huge success since arriving in November, 2007, ahead of other Mercedes-Benz Bluetec models. At 210 hp and 400 ft.-lb., the diesel has less power, but more torque than the E350, at 268 hp and 258 ft.-lb.

Volkswagen Jetta TDI
On sale: August, 2008

With budget-minded buyers looking for ways to save money at the pump, this Jetta diesel has winner written all over it. It helps that VW has a long history of selling diesels successfully in Canada. VW will have no trouble selling every one available from Germany.

BMW 335d
On sale: Fall, 2008

BMW is putting a strong and cleaner diesel engine in its facelifted 3-Series this fall. On-date sale looks to be November.

BMW X5 xDrive35d
On sale: Fall, 2008

The diesel-powered BMW X5 xDrive35d is the other new diesel coming this year from the Bavarian auto maker. A diesel in this large SUV is a good idea and will help lift sales.

THE DIESEL ADVANTAGE

Unless you know what you're looking for, there is little to distinguish a diesel from a gasoline engine. Both use pistons, valves and electronic fuel injection.

Here's the difference. Gasoline engines ignite their fuel with a high-voltage spark; diesels -- compression-ignition engines -- light the air-fuel mixture with the heat generated by squeezing the air in the cylinders to a far greater degree.

This is the diesel advantage, a compression ratio of nearly 20:1, compared with a maximum of about 12:1 for gasoline. This means that diesel engines extract more power from their fuel, improving fuel economy. In addition, diesels go more kilometres per litre because the fuel contains 12 per cent more energy a gallon.

By burning less fuel, diesel engines produce up to a third less carbon dioxide than gasoline models. On the other hand, diesels without proper emissions controls produce very high levels of smog-forming nitrogen oxides and carcinogenic soot.

To clean diesels up, manufacturers have turned to new ultra-low-sulfur fuel, cleaner-burning engines and a suite of emissions equipment. Filters trap sooty particulates while catalysts use ammonia to convert nitrogen oxides into harmless nitrogen and water in the exhaust.

BMW, Mercedes and Audi all have on-board tanks of urea, an ammonia-rich solution, which is injected into the exhaust to neutralize smog-forming pollution.

To ensure consumers don't let the urea run dry, Mercedes has a dashboard alert to warn consumers about falling levels of urea in the tank. If the urea falls below a certain level, drivers will have just 20 starts before the engine will no longer start. The system re-sets when the tank is filled and your new diesel Mercedes should start again.