Prototype Drive: Audi A6 and RS 5 TDI-e
Story by John LeBlanc
MALMÖ, Sweden — It’s an interesting time in the auto industry for engine geeks. Not since the dawn of the automobile more than a century ago, when cars were powered by everything from gasoline to electricity to steam, have new car buyers had such a variety of powertrains to choose from.
In today’s showrooms, you’ll find everything from naturally aspirated, turbocharged and supercharged gas and/or diesel engines. You can also find varying degrees of electrified motors, from hybrids to pure electric power. Now Audi, with its new TDI-e turbocharged and supercharged prototype diesels, is giving engine geeks even more fodder.
It’s been 25 years since the first Audi TDI — which stands for Turbocharged Direct Injection — engine. And after about 7.5 million TDI-equipped vehicles have been sold around the world and eight wins at the 24 Hours of Le Mans endurance race since 2006, Audi is prototyping a new 3.0-litre six-cylinder TDI-e engine that adds an electric supercharger.
Engines with exhaust-driven turbochargers are now common. And belt-driven superchargers (powered mechanically off the engine’s crankshaft) have been around for decades. But with its new TDI-e engine (based on Audi’s next-generation V6 TDI) the automaker’s engineers have employed an electrically powered supercharger, or what Audi is calling “e-boost”.
Engine geeks know that a TDI engine gets its power from the boost pressure developed by the turbocharger, which is dependent on the energy from the engine’s exhaust. But the TDI-e’s electric supercharger breaks this dependency on high revs. Located downstream from the engine’s intercooler, at very low engine speeds (when the turbocharger is “lagging” behind what the driver is asking for) air is routed to the supercharger, mostly filling the power and torque gap at low revs.
To showcase the flexibility of its new TDI-e powertrain, Audi let us drive two prototype cars for a few laps around Sturup Raceway, just outside Malmö, Sweden. The Audi A6 TDI-e sedan makes 326 horsepower and 480 pound-feet of torque from its single-turbo and supercharged V6 diesel engine. The Audi RS 5 Coupe TDI-e biturbo adds the second turbocharger, is rated at 385 hp and a huge 553 lb.-ft., and marks the first time that a diesel engine has powered one of the automaker’s high-performance RS models.
Compared to Audi’s current, non-supercharged, single-turbo 3.0L V6’s 310 hp and 325 lb.-ft. of torque, the numbers generated from the pair of new TDI-e powerplants are impressive — even if you aren’t an engine geek. However, a better comparison of the RS 5 Coupe V6 TDI-e biturbo may be with the current RS 5 Coupe 4.2 FSI and the naturally aspirated V8 that makes 450 hp and 317 lb.-ft.
With all that available torque, according to Audi, the all-wheel-drive RS 5 TDI-e biturbo is quicker from rest to 100 km/h by about 0.7 seconds than the RS 5 V8’s 4.7 seconds time. Better yet, Audi is confident the new V6 TDI-e biturbo engine will better the current 3.0 TDI’s fuel economy (which sips at a rate of 11.3 L/100 km city and 7.3 highway in the 2014 A6 TDI sedan), and potentially less than the RS 5 4.2’s 13.7 L/100 km city and 9.2 highway estimates. It also helps that Audi’s new 3.0 TDI weighs 192 kilograms lighter than the previous version.
On paper at least, the new twin-turbo/electrically supercharged diesel V6-powered Audi sports coupe is quicker than its gas V8-powered counterpart — and more fuel-efficient than the current V6 TDI. But a short test on the Sturup circuit behind the wheel of the RS 5 Coupe TDI-e prototype proved the new powertrain offers a driving experience to satisfy high-performance enthusiasts as well.
The immediate availability of the RS 5 TDI-e biturbo’s generous amount of low-end torque certainly gets your attention. When you brake in a conventional turbo-diesel, the revs drop and the turbo spools down, so it takes a while for the turbo to provide top power and torque when accelerating again. But by adding the electrically boosted supercharger, the TDI-e biturbo’s two turbos can be spooled up while still braking. That means maximum torque is available almost immediately when I pressed the Audi’s throttle exiting a corner.
Gear heads will undoubtedly enjoy the new V6 TDI-e’s extra power and torque with reduced fuel consumption. But perhaps more interesting is the way the e-supercharger is powered. Regenerative coasting mainly generates the energy needed to power the electric supercharger; the end effect is essentially neutral concerning energy consumption, says Audi. And in addition to the regular 12-volt electrical battery system, the TDI-e powertrain uses a separate 48-volt system, complete with its compact lithium-ion battery in the trunk and power electronics.
Beyond powering the supercharger, Audi sees many uses for the new 48-volt subsystem. The automaker feels it could supply the high-performance electrical needs of the future – like thermoelectric heating elements, electromechanical rear brakes or engine auxiliaries such as oil and water pumps – with obviously more energy than the 12-volt electrical system.
Audi says the new TDI-e engines housed in the A6 sedan and RS 5 bodies are “late-stage” prototypes. I’m speculating that means there’s more than a good chance you’ll find this new type of engine in a production car sooner than later as a replacement for Audi’s 4.2-litre V8 in the next generations of the Q7 crossover, A4 and A5 RS models and maybe even the R8 supercar.