Engine Downsized, Not Expectations

You would think it would be easy to improve an engine aimed at performance enthusiasts, especially one as wonderfully decadent as Audi AG's 4.2L V-8.

The engine is so good, a dash more horsepower and improved fuel economy likely would have been enough to keep many hardcore fans happy for years.

Like many driving enthusiasts, Ward's editors were infatuated with the V-8's gushing torque and sonorous exhaust note. Testing it in the Audi S4 and S4 Avant, we named it to the Ward's 10 Best Engines list three years in a row from 2004 to 2006.

But we fell out of love in 2007, mainly because of the engine's gas-guzzling ways. Fuel economy of 15/21 mpg (16-11 L/100 km) city/highway are not numbers an auto maker would want to benchmark for a C-class sport sedan.

Audi engineers were keenly aware of this issue and decided some years ago (Audi won't say exactly when) that changing consumer tastes, toughening emissions requirements and volatile fuel prices dictated a bold approach to the 4.2L's successor.

So they set out to create a direct-injection forced-induction V-6 that could outperform the magnificent naturally aspirated V-8 and still deliver superb fuel economy.

Audi spokesman Christian Bokich says the decision to move from a V-8 to V-6 was not controversial in-house because it fit the auto maker's long-term powertrain downsizing strategy. Plus, market research showed the S4 customer base was ready to accept change.

“Audi is pursuing an engine downsizing strategy when appropriate for the market segment,” he says. “With the S4 and S4 Avant, it was decided that a move from a naturally aspirated V-8 to a charged V-6 was appropriate for the segment.”

To the surprise of many, including skeptical Ward's editors, the transition from V-8 to V-6 succeeded beyond almost everyone's expectations. Ward's 10 Best Engines judges came staggering back from test drives of the new 3.0L TFSI supercharged DOHC V-6 grinning like they had been out on a bender.

But this time, they not only were jabbering excitedly about the engine's power and intake and exhaust sounds, but also its excellent fuel economy.

Despite the efficiency-sapping effects of the S4's all-wheel drive, we found a light foot could lead to 30 mpg (7.8 L/100 km) on the highway. With a 6-speed manual, the S4 officially is rated at 18/27 mpg (13-8.7 L/100 km).

The V-6's 333 hp is slightly less than the V-8's 340 hp, but it offers 23 lb.-ft. (31 Nm) more torque than the larger V-8 and the new car is faster off the line, sprinting to 60 mph (97 km/h) in 4.9 seconds, 0.4 seconds faster than the V-8.

Several Audi powertrain engineers in Germany quizzed by Ward's via email say they consider the combination of a 27% fuel economy improvement and faster acceleration their most significant technical accomplishment.

“Achieving either one or the other of these two goals would have been a challenge,” one engineer says.

The S4 sports the first supercharger used in decades at Audi. The auto maker likes to say it has a long history in supercharging since the 1930s, but it has been focused on turbocharging since the 1970s.

The decision to choose supercharging over turbocharging is rumored to have been the subject of heated arguments within the company, but insiders contacted by Ward's say the judgment process was all very civil.

“During the development phase, we tested the 3.0L engine in both bi-turbo and supercharged configurations. We found the supercharged version was superior in terms of starting performance, responsiveness and packaging,” he says.

“The supercharger was so compact that we could fit it easily into the 90-degree V of the cylinder banks in place of the intake manifold. A comparative, scientific development process such as this leads to rational debate rather than argument!” the insider says in an email.

The compressor, itself, is an Eaton Corp. Roots-type blower. It features two 4-vane rotary pistons that counter-rotate at a speed of up to 23,000 rpm, with an air gap between them measuring just a few thousandths of a millimeter.

The rotors can deliver 2,205 lbs. (1,000 kg) of air per hour and force it into the combustion chambers at a boost pressure of up to 11.6 psi (0.8 bar).

Two aluminum water-to-air intercoolers connected to their own separate coolant circuit are integrated into the housing.

Extensive sound damping reduces the level of noise generated by the compressor to a minimum so drivers can enjoy all the good sounds the engine makes.

The air path from the compressor to the cylinders also is very short, which means torque is built up quickly, even more than on a naturally aspirated engine of the same displacement.

Audi also credits its direct gasoline injection technology, known as FSI, for much of the engine's efficiency. For one, it allows the compressor to be located behind the throttle valve. At loads below supercharging level and when coasting, its rotors are free-running and the amount of power required to drive them is low.

The high compression ratio of 10.5:1 plays a big part in the engine's efficiency as well, Audi says, because the intensively swirled fuel cools the combustion chamber, reducing the tendency to knock.

The injection system also is a fundamentally new design. The common-rail system with 6-hole injectors sprays fuel directly into the combustion chambers at a pressure of up 2,175 psi (150 bar). The injectors' fast response permits up to three fuel injections per combustion event.

Hitting lofty performance and efficiency targets and deciding whether to supercharge or turbocharge were not the only major challenges engineers faced.

“We also had to completely re-engineer the basic engine,” says one. “That included a volumetric flow-controlled oil pump with pressure control, a reduced-friction chain drive and a reduced piston ring pretension.”

There were manufacturing changes, as well. A honing process was improved to optimize various technical properties, Audi sources in Europe say.

Engineers always are reluctant to talk about future improvements, but the several Ward's contacted say they are looking at numerous ways to make the already lightweight engine even lighter. Magnesium currently is being tested in numerous applications in place of aluminum, says one engineer.

Chain drive, valvetrain and bearing parts also are being optimized to reduce friction and reach the future demands of start/stop functionality, adds another.

And of course, a major focus in development will be to further upgrade the efficiency of the engine while also enhancing performance, or what Europeans frequently call “dynamism.”

As 4-cyl. and 6-cyl. engines continue to get dramatically better and the pressure to improve efficiency and emissions increases, the question being asked more and more is whether auto makers such as Audi soon will decide to abandon their big engines with eight cylinders or more.

After all, Audi already decided to offer only a 4-cyl. engine in its standard A4 model in the U.S., and now the high-performance version sports only two additional cylinders.

But Audi says it is not yet ready to take that step, and neither are its customers. For instance, spokesman Bokich says the same market research that shows S4 customers would accept a 6-cyl. engine also told the auto maker that prospective buyers of more upscale models still want higher cylinder counts.

“While there is still customer demand at the highest levels of our product portfolio, we will continue to offer such engines,” Bokich says. “This year, for example, we will launch the RS 5 Coupe with a V-8 and the R8 Spyder with a V-10.”

“We will only downsize the engines in these top-of-the-line vehicles when customers demand it.”