With Audi V-6, Who Needs a V-8?

The Ward’s 10 Best Engines competition has recognized outstanding powertrain development for 17 years. In this last installment of the 2011 Behind the 10 Best Engines series, WardAuto looks at the development of the supercharged 3.0L Audi V-6.

To turbocharge or supercharge, that is the question.

As auto makers buckle down to squeeze more efficiency from existing and future vehicles, boosted smaller-displacement engines are replacing larger naturally aspirated ones at an unprecedented rate.

Witness Ford’s EcoBoost efforts, which will have gasoline direct-injection turbocharged 4-cyl. and V-6 engines available in 90% of the auto maker’s North American vehicles by 2013.

Turbocharging harnesses otherwise wasted heat energy by using engine exhaust to drive a compressor that boosts the engine’s air intake to pump up power and torque.

The primary advantage of turbocharging is it works mostly at higher rpms and loads and essentially loafs in normal driving, including Environmental Protection Agency emissions and fuel-efficiency test cycles. The result is a have-your-cake-and-eat-it combination of exhilarating on-demand performance and small-engine fuel economy.

By contrast, supercharging uses an engine-driven compressor to boost intake air. Superchargers generally are more compact, less complex and less costly than turbocharging. They also provide instantaneous response because they do not require time to spool up like exhaust-driven turbos.

But superchargers also are less efficient because they are directly attached to the engine and create parasitic drag at all engine speeds.

So why did Audi choose supercharging instead of turbocharging for the TFSI V-6?

Audi engineers tested both twin-turbo and supercharged configurations during the engine’s development. They decided on the latter not only because of its better launch performance and responsiveness but also for packaging reasons.

Unlike an intercooled turbo system, the supercharger is compact enough to easily fit into the 90-degree “V” between the engine’s cylinder banks, saving precious space in the engine compartment.

While this is Audi’s first mechanically supercharged production passenger-car engine, Audi’s ancestral company, Auto Union, used Roots-type blowers on its Silver Arrow racing cars in the 1930s.

Between 1934 and 1939, Auto Union racked up numerous Grand Prix wins and a string of world speed records, Audi says.

The supercharger in Audi’s current 3.0L V-6 is an Eaton Roots-type unit with two water-to-air intercoolers integrated into its housing. Two 4-vane rotary pistons 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 result is 333 high-spirited horses and a hefty 325 lb.-ft. (441 Nm) of torque, good for 4.9-second 0-60 mph (97 km/h) bursts in Audi’s sport-compact S4 sedan.

That’s nearly a half-second quicker than the previous V-8 S4, mainly because the boosted V-6 has 23 lb.-ft. (31 Nm) more torque available over a broader range, even though the V-6 has seven less horsepower.

The V-6 also trounces the V-8 in fuel economy, providing a respectable 18/27 mpg (13-8.7 L/100 km) city/highway, compared with the V-8’s woeful 15/21 mpg (15.7-11.2 L/100 km).

Audi engineers say this muscular boosted V-6 is substantially re-engineered relative to the 3.2L naturally aspirated engine on which it is based. Changes include efficiency-enhancing features such as a pressure- and volumetric flow-controlled oil pump, reduced-friction chain drive and lower piston ring pretension friction.

The new common-rail system incorporates 6-hole injectors capable of three injections per combustion event at pressures of up to 2,175 psi (150 bar).

“Audi has a lot of experience with turbocharged 4- and 6-cyl. engines,” says an engineer involved in the development.

“The 2.7L twin-turbo V-6 in the (A6 and S4) was built until 2001. It had a cast-iron crankcase and 5-valve cylinder heads. In 2004, we introduced the new generation of V-6 engines with four valves per cylinder and an aluminum crankcase.”

What were the top design priorities?

“Our main objective was to develop a versatile engine that can be used where appropriate within the Audi model range,” the engineer says.

The versatility mandate required the engine to be very compact, with few changes for vehicles destined for a variety of markets.

The engine had to meet key market requirements in the U.S. and China, deliver strong acceleration and refinement and be optimized for all-wheel drive and automatic transmissions.

And of course, it had to meet U.S. emission limits as well as Europe’s EU5 ULEV2 requirements.

Audi engineers tell WardsAuto that after thoroughly evaluating both forced induction strategies, supercharging prevailed by providing the best balance of performance and efficiency in this application.

What were the top development challenges?

The first major issue, Audi engineers say, was to create room for the compressor in the V between the cylinder banks. That’s not as easy a task nowadays because European pedestrian protection laws require significant crush space between the hood and hard engine surfaces.

“Then we wanted to keep the air intake as short as possible to maximize acceleration response,” says an insider. “Therefore, we chose to locate the intercooler between the compressor and the cylinder head, (which) required some changes to the cylinder heads as well as fuel and air components below the compressor.”

“Last but not least, we needed to strengthen the aluminum crankcase to handle the higher combustion pressure,” he says. “We opted for a combination of materials and heat treatment to withstand the stress and ensure the increased pressure is controlled.”

While this engine initially was built in small volumes only for the S4 and A6, it now is found in the S5 and performance models of the A7, A8 and Q7.

The engine also is used in other brands belonging to Volkswagen, Audi’s parent, including versions of the Touareg, Porsche Cayenne and Porsche Panamera Hybrid.

As good as it is today, engineers say there still is room for future improvement as European carbon-dioxide emissions limits and U.S. corporate average fuel economy requirements accelerate.

“In the latest generation V-6 TFSI, which began production in mid-2010, we have reduced engine friction to significantly reduce fuel consumption. And we are not out of innovative ideas to improve this engine going forward. Let us surprise you,” the insider says.

In other words, Audi is confident there is plenty more that can be done to create more fuel-efficient future generations of the 3.0L V-6 without sacrificing its grin-inducing performance. But it’s not going to tip its hand just yet.