Eco-Friendly Diesel Cars

Courtesy Audi AG

A new breed of diesel cars is staking claim to being green machines.

The smoky, foul-smelling exhaust that once plagued diesel vehicles is no more. High-end models from Audi, Mercedes-Benz, BMW, and Volkswagen have been reengineered to yield better mileage than their gas equivalents while reducing the carbon dioxide emissions that contribute to global warming. And none of them sound—or drive—like a big rig hauling a load down the highway.

What’s new in these cars is the use of a urea fluid, or nitrogenous compound, called AdBlue, which is squirted into the engine’s exhaust stream to scrub pollutants away. “The air coming out is typically cleaner than the air going in,” says Volkswagen Product Manager Bret Scott. Hybrid cars, most famously the Toyota Prius, are the chief competitors for a green badge, and both diesels and hybrids get the same tax break for alternative fuel vehicles. But critics of hybrids complain about squishy regenerative brakes, transmissions that seem to shift aimlessly, poor acceleration, and reduced cargo space due to the cars’ huge nickel-metal-hydride batteries. And because hybrids are so new, little longevity data is available.

By contrast, diesels are recognized workhorses with a life span of as many as 200,000 miles. Audi’s TDI engine, in fact, has been racking up impressive wins on the endurance racing circuit in recent months. Also working in a diesel’s favor is its cost at the pump: The price of its ultra-low sulfur fuel is comparable to regular gas, not premium. And in various long-range test drives, the large, well-appointed Audi Q7 TDI, the BMW X5 xDrive35d, the Mercedes-Benz GL350 BlueTec, and the Volkswagen Touareg TDI had astonishingly infrequent fuel stops—around 600 miles between fill-ups.

As a general rule, diesel mileage for a sport-utility vehicle tends to be four or five miles per gallon better than for its gas equivalent. Take, for example, the Audi Q7 TDI. It gets 25 mpg on highways, which is about 7 mpg more than the Audi Q7 V8. That numerical spread increases with smaller cars. The Mercedes-Benz E320 BlueTec diesel sedan gets 32 mpg at highway speeds, while the V8-powered S-Class gets only 23 mpg. The S400 Hybrid, though, delivers 26 highway miles per gallon.

On the downside, the AdBlue container needs to be replenished every 10,000 miles, but this can be worked into regularly scheduled dealer maintenance. When the AdBlue runs out, the car won’t start. So keeping an extra bottle that costs around $1.50 on hand for emergencies is a good idea, even though its shelf life is only about six months.

A diesel does drive differently, and the sound of its engine, while no longer a tractor-like rumble, is most discernible in the larger cabin of an SUV. The BMW X5 xDrive35d, for example, has a low hum that sounds like a cruise ship coming into port. But what many drivers will find addictive is a powerful engine torque that jumps from the starting gate and instantly delivers a surge of acceleration at passing speeds, an attribute that is perhaps more relevant than either hybrid or gas competitors in a congested 55-mph speed zone where passing is critical. And if the car you pass happens to be a hybrid, you won’t feel very guilty about it either.