Supercharged Audi V-6 Excels in Power, Efficiency, Refinement

The Ward’s 10 Best Engines competition has recognized outstanding powertrain development for 18 years. In this installment of the 2012 Behind the 10 Best Engines series, WardAuto looks at the development of the supercharged 3.0L Audi V-6.

For the last 18 years, WardsAuto editors have systematically evaluated dozens of engines in a variety of vehicles and have carefully chosen the 10 they see as best of the best in a U.S. market packed with outstanding engines. It is far from easy for any engine (or electrified powertrain) to make the list, and far harder to make it more than once.

Yet the uniquely wonderful supercharged V-6 from Audi has earned its third straight Ward's 10 Best Engines trophy for 2012. This time it was tested in slightly less-powerful form in Audi's midsize A6 sedan, where it delivers a still-healthy 310 hp at 4,300 rpm and 325 lb.-ft. (441 Nm) of torque from 2,500 to 4,500, rpm through an 8-speed automatic gearbox.

Last year's award-winning, somewhat freer-flowing version jet-propelled the smaller S4 with 23 more horses and the same prodigious torque.

According to WardsAuto judges, the direct-injection V-6 has earned a third straight 10 Best award because it, "continues to impress with stunning power, refinement and efficiency."

As far as performance goes, the V-6 propels the 4,045-lb. (1,835-kg) A6 from rest to 60 mph (97 km/h) in a strong 5.3 seconds. On the efficiency side, it returned a respectable 22.4 mpg (10.5 L 100/km) average over 258 miles (415 km) of heavy-footed test driving. The official rating is 19/28 mpg (12.4-8.4 L/100 km) city/highway.

Editor Christie Schweinsberg sums up its refinement by saying the ’12 A6 test car is "perhaps the most perfectly tuned vehicle in terms of NVH I've ever driven."  

The 3.0L TFSI V-6 is derived from Audi's 3.2L naturally aspirated V-6, which leaves the U.S. market as of the ’13 model year in the Q5 cross/utility vehicle. The 3.0L TFSI is boosted by a belt-driven supercharger instead of a more common exhaust-driven turbocharger because engineers wanted instantaneous response, and because they needed to move the forced-induction plumbing into the valley between the 90° cylinder banks without making the engine too tall to meet European pedestrian-impact requirements.

While turbocharging harnesses otherwise wasted heat energy, by using engine exhaust to drive a compressor that boosts the engine’s intake charge to pump up its power and torque, a supercharger uses an engine-driven compressor to boost intake air. The former's primary advantage is that it works mostly at higher rpms and essentially loafs in normal driving. The latter's benefit is that it is more compact and provides instantaneous response.

Audi engineers tested both strategies during the engine’s development before deciding on supercharging, not only for its better launch performance and responsiveness but also for its tighter packaging. Unlike an intercooled turbo system, it was compact enough to fit snugly between the cylinder banks.

"This now is our only gasoline V-6, so one of the key priorities was to have a single engine suitable for use in a wide variety of different vehicles," says Audi of America product communications manager Mark Dahncke.

Besides the A6, it also powers the A7 4-door "coupe," the high-performance S4 sedan and Cabriolet and S5 coupe, as well as the Q5 and Q7 CUVs and the ’13 A8 luxury sedan.

"It's the first V-6 we've offered in the A8, and it's faster than the 4.2 L V-8," Dahncke says.

The engine also is offered in other Volkswagen Group vehicles, including the VW Touareg, Porsche Cayenne and Porsche Panamera Hybrid.

Fitting the supercharger into the V might not seem such a difficult challenge, Dahncke says, except that European pedestrian protection laws require a substantial amount of clearance between the top of the engine and the inside of the hood.

Also, to keep the intake air path as short as possible to maximize acceleration response, engineers packaged the intercooler between the compressor and the cylinder heads, which required changes to the heads, fuel lines and the intake manifold. Using a combination of special materials and heat treatment, they also strengthened the aluminum crankcase to handle the supercharged engine's higher combustion pressures.

Another challenge in the engine’s development was ensuring the new TFSI V-6 would work equally well with 6-speed manual and 8-speed automatic transmissions, as well as with Audi's quattro all-wheel drive, and that it could handle high towing loads in the Q7. And while doing all that and more, it had to meet both U.S. ULEV2 and European EU5 emissions rules.

The compressor is an Eaton roots-type unit with two water-to-air intercoolers integrated into its housing and two 4-vane rotary pistons that counter-rotate at up to 23,000 rpm to deliver 2,205 lbs. (1,000 kg) of air per hour to the combustion chambers at pressures up to 11.6 psi (0.8 bar). The system uses common-rail six-hole injectors capable of three injections per combustion event at pressures up to 2,175 psi (150 bar). Efficiency-enhancing features include a pressure- and volumetric flow-controlled oil pump, a reduced-friction chain drive and lower piston-ring friction.

Dahncke says the "T" in TFSI designates forced induction, not necessarily turbocharging as in Audi's 4-cyl. 2.0L TFSI. "There was some brand value in the T," he says. "So instead of coming up with another nomenclature, the company decided to stick with that and apply it to both supercharging and turbocharging."

Despite the trend toward continued engine downsizing, Audi still sees a bright future for its powerful V-6, even in Europe.

"It already is a relatively fuel-efficient engine,” Dahncke says. “The fuel-economy ratings on it are at minimum competitive, if not best in class. It delivers very good performance, and we have been able to fit it so well into so many of our products. It has excellent bandwidth as both a performance engine and a performance-oriented luxury car engine, and it does both very well."

But there always is room for improvement, Dahncke says. “We will continue to look for ways to improve everything about this engine, including how quickly it warms up." One way to do that, he adds, may be with segments within the cooling system to help it use the energy more efficiently for faster warm up, one of the most crucial hurdles in emissions tests.

A new stop/start system on the ’13 A6, A7 and A8 3.0T will boost fuel efficiency a bit in city driving, and electrification of the water and oil pumps are future possibilities, as are further friction reductions and combustion-efficiency improvements.

"We are not out of innovative ideas to improve this engine going forward," Dahncke says. "Let us surprise you."