Forget the smog-spewing, noxious-gas belching stereotypes. A whole new generation of cleaner diesel cars is on its way to showrooms, with Volkswagen, BMW, and Mercedes-Benz all offering 2008 models in the United States. To prepare for the forthcoming wave of oil-burners, we got our hands on a diesel-powered European-spec Audi A6 and put it through some rigorous testing to see how it measures up to its gasoline counterparts.
Our 2007 Audi A6 SE 3.0 TDT Quattro test car is not available in the United States. However, aside from the turbo diesel direct-injection engine and a few minor interior differences, the car is cosmetically indistinguishable from the SE 3.2 FSI Quattro gasoline model that is sold here.
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Our main objective in our test with the A6 TDI was to see how it measured up against the traditional stereotypes associated with diesels of being noisy, dirty, and slow. For our first test, we used a sound meter to measure the car's engine noise relative to that of a comparable gas engine. As suspected, the A6 was significantly louder by every measure (hood closed at idle; hood closed at 2000rpm; hood open) than the gasoline engine.
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We approached the emissions test in true scientific fashion with a damp piece of paper towel, which we held over the exhaust for one minute. According to Audi, the diesel engine puts out less carbon dioxide than its gasoline counterpart (figures on the nitrogen oxide emissions were not provided). For emissions treatment, the car packs an air-gap insulated exhaust manifold, dual catalytic converters, and water-cooled exhaust-gas recirculation
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Somewhat surprisingly, the A6's exhaust did not leave a single trace of visible pollution on the towel.
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Encouraged by the paper-towel test, we moved on to a more detailed examination of the car's emissions. Senior Editor Wayne Cunningham put curiosity before personal comfort by getting a lungful of A6 diesel exhaust. As of this writing, he is still alive.
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The spec sheet that came with our test car said that it had a zero-to-62 mph time of 7.1 seconds. In manual shift mode, our performance computer clocked us at 6.95 seconds to 60 mph. This is at least as fast as the gasoline-powered 3.2-liter V-6 Audi A6. While performance may be on a par with Audi's gasoline V-6, fuel economy is more than 30 percent better in the diesel A6, which gets an average of 34 mpg, according to Audi.
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The A6 SE TDI's 3-liter diesel engine features common rail direct injection with piezo injectors, and a VTG intercooled turbocharger. It makes 233 horsepower and 332 pound-feet of torque compared with 256 horsepower and 244 pound-feet for the comparably sized SE 3.2 gasoline direct-injection model. The diesel A6 is nearly 200 pounds heavier than the gasoline model, with the extra weight being attributable to the extra engine components, exhaust treatment systems, and noise vibration and harshness (NVH) damping.
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An interesting option that the Europeans get is a central-console mounted Audi telephone handset, which requires drivers to insert their (GSM) SIM cards for operation. While the phone is an attractive piece of hardware, we can't quite see the point of removing your SIM card when the car also has Bluetooth, enabling a wireless connection to your cell phone. Maybe it's part of the Vorsprung durch Technik that we just don't understand.
