Going into the second half of 2008, the Volkswagen Group is making a major push into the US light-duty vehicle market with new clean diesel engines that meet Tier 2 Bin 5 (T2B5) emissions standards for all 50 states. Volkswagen dealers will receive TDI versions of the Jetta and Touareg while the premium Audi brand will be kicking off the festivities with the Q7.

Both the Q7 and Touareg pick up the group's 3.0L V6 TDI for their diesel debuts - powerplants available in the European market since both models debuted earlier this decade. Diesel systems supplier Bosch provided Diesel Forecast with a 2007 Audi Q7 TDI from its demonstration fleet recently, which provides some insight into what to expect when the US model debuts around the end of this year.

The 3.0L diesel V6 in the Q7 we evaluated peaks at 233hp at 4,000 rpm while the torque curve is flat at an impressive 369lb-ft from 1,750rpm to 2,750rpm. The US specification variant will be largely the same and should be close to these numbers. The V6 has a Bosch common rail injection system that helps provide clean, quiet performance.

The Q7 is substantially larger than its platform mates: the Volkswagen Touareg and Porsche Cayenne. In contrast to the Touareg, the Q7 spans nearly 6 more inches between the axles, while the bumper to bumper length is 13 inches longer. Audi's largest vehicle also exceeds other rivals, such as the BMW X5 and Acura MDX by 9-10 inches. In fact, the Q7 comes up just 2 inches shy of a Chevy Tahoe in overall length. However, the American SUV stands 6.4 inches taller than the Audi and is discernibly more roomy. The unit-body German competitor does have a 500lb mass advantage compared to the body on frame Chevrolet in conventional drive form. When compared with the two-mode hybrid Tahoe, the Q7's weight advantage grows to 800lbs. 

The main difference between the test unit and the models set to appear at US Audi dealers lies in the exhaust gas aftertreatment system. The current European models are equipped with a diesel particulate filter (DPF), which will be carried over to US models. The 2009 models also will be equipped with a selective catalytic reduction (SCR) system to bring nitrogen oxide emissions down to the extremely low levels required by the T2B5 standard. 

The SCR system injects a urea solution into the exhaust stream that will be converted to ammonia by the heat of the exhaust gases. The ammonia reacts with the NOx in a special catalytic converter, transforming it into nitrogen gas and water.

The common-rail injection system allows for up to seven individual fuel pulses per ignition cycle. Spreading out the fuel delivery as the piston moves down during the power stroke allows engineers to more precisely control the rate and temperature of combustion. This helps to both reduce the diesel's characteristic knocking sound as well as reducing the production of NOx. The SCR system then helps to eliminate 90-95 percent of the remaining NOx.

Were it not for the rather garish Bosch Clean Diesel Power graphics all over the side of the Audi, most people would never realize it was a diesel. At idle there was virtually no noticeable clatter and under acceleration, it exhibited a pleasant growl. For an SUV that weighs in at over 5,000lbs, the performance of 3.0L V-6 is impressive. The acceleration feels stronger throughout the range than the larger heavier Tahoe Hybrid. With half the displacement of the GM V-8, the torquey diesel provides the Audi with a 7,000lb towing capacity, exceeding the Tahoe by half a ton. 

The interior of Q7 also feels much more finely crafted than most SUVs and in particular the Tahoe. The surface of the dashboard has a minimum of seams and a soft touch feel. The gauges are large and legible and the controls are fairly well laid out. Audi's take on the famous, or infamous depending on your point of view, BMW i-Drive type main control knob goes by the name MMI or Multi-Media Interface. The MMI knob sits directly behind the shifter surrounded by four buttons.  The menu-driven interface is common to all recent Audis and is much easier to navigate then the BMW interface. 

The steering wheel has a thick rim wrapped in leather that provides a nice grip for controlling the direction of the Q7. For an SUV, the Audi has fairly quick direct steering although as with most vehicles in its class there isn't a lot of feedback. The power and torque of the diesel engine are transmitted through a six speed automatic transmission to all four wheels via Audi's Quattro all wheel drive system. A Torsen center differential normally sends 50 percent of the drive torque to each axle, although the split can be adjusted with up to 75 percent going to either end.

The Q7's all independent suspension provides surprisingly nimble ride and handling for a two and half ton vehicle, but no one will ever mistake this beast for an A4. Compared to the similarly sized Lambda crossovers from GM (Saturn Outlook, GMC Acadia, Buick Enclave) the Q7 feels less ponderous and Audi quotes the acceleration at 0-62mph in 8.5seconds. For drivers in a more sporting mood, paddle shifters on the back side of the steering wheel allow the driver to sequentially toggle the transmission up and down through the gears. 

Unfortunately, like most such setups the response of the transmission to the drivers commands can be somewhat sluggish. The laggard shift response can result in gear-changes at inopportune times in the middle of corner. Fortunately, few drivers of the Q7 likely to push it through curves hard enough for a mid-corner gear-shift to pose a dynamic problem.

The Q7 TDI to Tahoe hybrid comparison is particularly valid considering the premium "green" SUV customer they are courting. The Tahoe is slightly larger inside than the Audi, but the German has a more pleasant environment.  In spite of how quiet the Audi diesel V-6 is, it cannot compete with the hybrid when it's running in electric drive mode. One of the classic reasons American consumers have shied away from diesels is the smoky smelly exhaust. That is no longer an issue. Even at wide open throttle, there is never any black sooty smoke from the Q7, nor is there any smell.  In fact, most drivers would be hard pressed to tell the difference between a gas and diesel fueled Q7. 

At this point Audi has not announced how much extra the Q7 TDI will cost over the price of a gas fueled variant. The current 3.6L V-6 Q7 starts at $42,500. As fuel prices continue to climb, efficiency is becoming increasingly important. For this type of vehicle, the Q7 does surprisingly well managing 23mpg in the time we drove it. The four-wheel-drive Tahoe hybrid starts at $10,000 higher and only managed 20mpg on both the city and highway testing during EPA certification.

MAY 2008