Think diesel, and three words may come to mind: Dirty, slow and unreliable.

Audi Auto Group wants to change this perception with its latest fleet of diesels, but it may be hard to win over a U.S. market familiar with large buses, trucks and old European imports that chugged along noisily emitting a thick, dark cloud of emissions.

Alternative energy vehicles have become the new trend, with cars like the Prius selling faster than Japan-based Toyota (STOCK QUOTE: TM) can build them. But even though hybrids are a respectable option, many European manufacturers already have proven technology within their diesel fleets.

Take Audi, for example, which by building fun-to-drive vehicles with robust miles-per-gallon numbers and uncompromised performance has created a compelling argument for new diesel technology.

Through the lowering of harmful pollutants, diesels are Audi's answer to the short-term energy problem. The facts do not lie.

Turbo Direct Injection engineer Zaccheo Giovanni Pamio said the TDI engine uses "the most fuel-efficient technology in the world."

Pamio says Audi's fleet uses 30% less fuel on average than it did in 1995, suggesting that TDI engines are the root of this turnaround.

Pamio says Audi has doubled the engine's power while reducing particulates a stunning 98%.

With the introduction of clean diesel technology to the U.S., Audi hosted the Audi Mileage Marathon from Oct. 5-20. The four-stage rally consisted of 184 international journalists and Audi executives in what proved to be a challenging 4,800-mile endeavor.

The event allowed an inside peek into the new, U.S.-bound Q7 TDI alongside other diesel-powered variants in Audi's European lineup.

The seven-passenger Q7 TDI sport utility vehicle's 225 horsepower motor has 405 pounds per foot of torque and manages 0-60 mph in 7.9 seconds. However, it is not a lightweight, checking in at over 5,100 pounds. At more than 5,100 pounds, this SUV is estimated to get 17 miles per gallon in the city and 24 on the highway, with up to 30 miles per gallon reported during the Audi marathon.

In contrast, the gasoline variant clocks in at 14 city and 16 highway miles per gallon. On top of these numbers, the Q7 TDI is an ultra-low-emission vehicle -- think: Honda (STOCK QUOTE: HMC) Civic.

All that aside, diesel is hard to find and expensive to buy. Currently, diesel is priced anywhere from 50 to 90 cents more per gallon over premium gasoline. But don't brush off diesel yet. Diesel engines can get impressive gas mileage, provided the driver maintains a stable speed, keeps revs-per-minute low and leaves the transmission in top gear.

Large, diesel-powered variants do make sense for the long-distance commuter, but gas mileage suffers in stop-and-go traffic. You might get slightly more bang for your buck with a diesel in New York City, but it might not be enough to offset the initial cost of the vehicle and the premium for diesel.

Where new, clean diesel variants could pay off and win over the American market will be in the smaller model offerings. In Audi's case, the new A4 was producing stunning fuel consumption. Average driving easily yields gas mileage of 37 mpg or better. Driving economically yielded over 44 mpg.

The problem: Audi is only bringing over the Q7 TDI, the big 5,100 whopper. Plans for the A4 TDI and A3 TDI remain unclear; however, if those vehicles were available stateside, Audi would definitely have one buyer on the list.

Diesel is substantially cleaner than it once was, and it can provide impressive gas mileage and a thrilling ride. If this is what you are interested in, the diesel is for you.

Keep an eye out for more incoming diesel variants. Volkswagen Group, alongside Audi, has imported several TDI models, and BMW is releasing a diesel for the 2009 model year, as well.