We think of diesel motors as industrial powerplants—noisy roughnecks that puff black smoke and should be screened off from polite society. But what makes diesels so useful in heavy-duty trucks, tractors and generators can also benefit passenger cars, even prom queens like this 2014 Audi A6.
Diesels don’t have spark plugs; they ignite their oil fuel by super-heating it through compression in the engine’s cylinders. This extra squeeze makes a lot of torque and gives the diesel motor the highest efficiency of any combustion engine. Replacing a gas engine with a comparable diesel often improves a car’s fuel economy by at least 10 MPG.
But the characteristic diesel clatter—and the exhaust fumes, the extra weight and cost of a motor that can survive extremely high internal pressures, and the oil’s tendency to sludge up in cold weather—made diesels less than ideal for everyday cars. In Europe, really high fuel prices and then clean-air laws spurred carmakers to address these problems, one by one. Now, just about every non-Detroit car is available over there with some sort of diesel under the hood, with three, five, six or even 10 cylinders.
You may have noticed that gas (and diesel) prices are getting high here too. So is concern over carbon spewed by internal-combustion engines. Last week, we calculated that, yes, diesel savings in fuel costs and pollution output are real—at least with a VW Passat TDI sedan that averaged an eye-popping 47 MPG.
This week’s ride got “only” 36 MPG in the same conditions, but an Audi A6 TDI (Turbo Diesel Injection) is bigger, heavier and much more powerful than that VW, with way more power-hungry accessories and systems, and its quattro all-wheel drive adds more weight yet.
At $68,000, this A6 TDI was also two and a half times more expensive than its corporate cousin, the Passat, and $2,400 more than a gas A6. So don’t think you’re going to finance a luxury car just on savings at the pump.
The diesel A6 does go five to 10 miles farther per gallon than its various gas siblings. That’s terrific, but the most remarkable thing about this car is how Audi has disguised its diesel-ness. Any clatter is noticeable only at idle and only from outside the car. Inside, if it weren’t for the lower rev limit—about 4700 RPM—on the tachometer, we might think this was a healthy gasoline V-8, not a smallish oil-burning six.
In an Oscar-worthy supporting role is Audi’s superb Tiptronic automatic transmission. Not only can it shift almost unnoticeably, its two top speeds—there are nine in all, counting reverse—are overdrives. At 75 MPH the engine is not far off idle, ticking along at only 1500 or so RPM. It’s not all just cream-and-silk, either; put the pedal down and the A6 TDI responds, forcefully and right now, with a gusher of torque.
The ride seems almost harsh at first, but, in good German fashion, the faster you go, the smoother the A6 TDI gets. For all its heft and luxury, this is an athlete, a world-class drivetrain in an equally outstanding chassis.
As well, being an upscale German sedan, the A6 can be blessed (or burdened) with additions from a long menu of convenience, performance, luxury and safety options. Ours had $10,000 worth, ranging from 3G Mobile Wi-Fi to a top-view camera. And then there was Active Lane Assist: Drift toward a dividing line, and all by itself the steering tugs the car back into its lane, hands-free. It’s just the thing for cellphone-addled ADD sufferers. I found it a bit creepy, so I shut it off.
With federal Corporate Average Fuel Economy standards rising sharply—to 35.5 MPG by 2016, 54.5 MPG by 2025—carmakers have to improve their lineups’ efficiency, so the A6 TDI is just one of four new diesel models Audi is shipping to America for 2014. Diesel-powered Audis have won again and again at Le Mans, so it’s no surprise that the company is going this way rather than embracing electric power. At least for now.