You know what some journalists do when we get behind the wheel of a test vehicle... we hammer the throttle, shift at redline, late-apex every corner, and hope the manufacturer doesn't measure tire-tread depth after the car is returned. Different behavior wouldn't be in-line with The Official Journalist Creed.
Well, the creed is more a set of guidelines than rules, so our responsible behavior whilst participating in the Audi Mileage Marathon can be excused. During our three full days behind the wheel of a 2009 Audi Q7 fitted with the 3.0-liter turbo-diesel, we were easy on the throttle, coasted a lot, and worked to maximize fuel economy, not speed. The results were impressive, as we achieved almost 31 mpg during one jaunt from Mammoth Lake to Monterey, California. Furthermore, when we drove with complete disregard for efficiency, we still couldn't push our average mileage below 26 mpg. This kind of fuel economy matches that of many smaller sedans and is similar to what we observed when driving the 2009 Chevrolet Malibu Hybrid.
What makes the new Q7 TDI's mileage remarkable is that it comes from a full-size, luxurious SUV that seats seven, tips the scales at more than 2.5 tons, and has fourteen cupholders. This isn't your typical eco-weenie penalty box on wheels - it's a leather-lined and fully equipped vehicle with just about every feature you could want. Voice-activated navigation, Bluetooth, and an iPod adapter, for instance, are available and were fitted to our test car.
These features, however, don't separate the 2009 Q7 from any other highline SUV; Audi's fifty-state legal 3.0-liter TDI V-6 does. The turbocharged (T) diesel with direct injection (DI) joins two gasoline engines in the Q7 lineup: a 3.5-liter V-6 and a 4.2-liter V-8. The diesel highlights Audi's approach to improving fuel economy and emissions, and the engine recently earned the stringent fifty-state Bin 5/LEV II certification.
The 3.0-liter turbo-diesel produces 221 hp and 406 lb-ft torque. The single, variable-geometry turbo resides in the valley of the cylinder vee. Maximum boost is a staggering (by gasoline engine standards) 37.7 psi. Cylinder heads made from a high-strength vermicular graphite iron weigh fifteen percent less than traditional cast iron and utilize four valves per cylinder. Fuel is delivered through a pressurized common rail to piezo injectors at 29,000 psi.
With this kind of pressure, the injectors optimally provide fuel over the entire power stroke to maximize efficiency and minimize noise. The traditional semitruck-like diesel sound comes from the fuel charge igniting all at once. The 3.0-liter TDI delivers and burns each fuel charge over a brief span of time, thereby reducing engine noise.