Audi is taking another big step forward with the electric biturbo. In this future technology, a secondary compressor boosts the main turbocharger at lower engine speeds.
Diesel engines are well known to deliver great fuel economy and gobs of torque, but the sparkless powerplant is not known for great responsiveness. Audi hopes to change that with a new development we saw at the carmaker’s Future Mobility workshop in the days preceding the 2012 Paris auto show. It’s called the electric turbocharger, and it will change how you think about diesels.
The idea of using an electric motor instead of exhaust gases to drive an intake compressor is not exclusive to Audi. In fact, we think it might show up on the next-generation BMW M3. But this was our first experience actually driving a vehicle outfitted with the technology.
The Audi prototypes use a single exhaust-driven turbocharger nestled in the valley of the familiar 3.0-liter diesel V-6 (as seen in the Q7 TDI and soon in the U.S.-market A8); closer to the front-mounted intercooler is a secondary turbocharger, this one powered by 48-volt electricity. (The car itself runs on 12 volts, with a converter stepping up the extra juice for the blower.) The premise is simple: At low speeds, the electric turbo spins up instantly, providing boost and eliminating the typical lag associated with turbo-diesels.
Behind the wheel, the performance was impressive, with no hint of delay in power delivery. In addition, Audi claims the system increases the low-end power enough that mild acceleration at highway speeds requires less downshifting, which improves fuel economy. Consider us fans of this gizmo.
But the electric turbo isn’t the only area in which Audi hopes to improve diesel performance. Also on hand was a mild-hybrid system called iHEV. It also employs a familiar concept: a combination starter-alternator connected to a larger-than-normal battery that enables frequent stopping and starting of the engine. Specific to the Audi prototype we drove, however, was a 48-volt lithium-ion battery and electric system. As opposed to the electric turbocharger’s setup, here a voltage converter steps down the juice for the rest of the 12-volt systems in the car. The Audi system also runs the starter off the accessory belt drive so that the air conditioning can still operate when the engine is shut down (how the A/C decouples from the engine was a detail lost in German translation). At speeds higher than 19 mph, the engine will shut down completely when your foot is off the throttle pedal. In our limited test drive, that happened quite often, and with the engine not running, the car coasts much more freely than in a conventional vehicle.
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When combined with a third technology, the mild-hybrid system promises to save even more fuel. This last tidbit is Audi’s predictive efficiency assistant (PEA), which uses GPS data and road-speed information to coach the driver on more efficient use of the accelerator. As an example, pretend you’re driving up a hill: Normally, you would apply the throttle to maintain speed until you reach the crest. Based on GPS data, however, PEA knows when the hill ends and can instruct you to lift off the throttle a little earlier. The same can be done when approaching a curve or approaching an area with a reduced speed limit. This helps reduce the engine-on time in the iHEV. It also works with the adaptive cruise control system when driving in traffic.
To us, it seems more than logical that all three of these technologies preview the next steps for Audi’s diesel range. The iHEV system also can incorporate a gasoline engine, and it seems like a logical step beyond today’s stop-start systems to help meet increasingly stringent fuel-economy standards. That the electric turbocharger runs on the same 48-volt system seems more than just coincidence. Throw in the PEA tech, and Audi appears to be setting itself up well for a frugal future.
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