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4T : Reviews & Road Tests

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16 October 2012


It’s fair to say Audi learned a lot about turbo lag in the ‘80s. Go ahead and google ‘Walter Rohrl footwork video’ (or watch HERE) and you’ll see why. Back then, a single turbo was used to boost power… a single turbo that first had to be spooled up by exhaust gases. The bigger the turbo used, the more power. However, the bigger the turbo also meant the longer the wait for said power. As a result, drivers like Rohrl had to keep the revs up in order to keep power on tap. For the Gary Cooper of rally car driving, this was hardly a problem though turbo lag is less than optimum in daily driven cars where a range of driver experience naturally occurs.

By the launch of the B5 S4, Audi found a way to dial back turbo lag. Switching to a V6 instead of the old I5 allowed engineers to pair two smaller turbos to each bank, each spooling up power quickly. The stock B5 S4 boasted plenty of punch without all of the fancy footwork. In their current engines, Audi has refined their biturbo designs to such a degree that they boast power delivery much like of engines with higher displacement and more cylinders, all while increasing efficiency by a significant margin. Today’s 4.0 TFSI in the S6, S7 and S8 is exemplary of this. So too is the 3.0 Bi-TDI (biturbo TDI) found in the SQ5 and A6 sold in Europe.

In particular, the 3.0 Bi-TDI is one hell of an intriguing engine. This performance diesel mill from Audi boasts 313 hp (230 kW) and 479 lb-ft (650 Nm) of torque between 1,450-2,800 rpm. Even as a diesel, this engine is a proud member of the 100+hp per liter club at 105.5 hp per liter, yet in the A6 it is still capable of 36.75 mpg (6.4 liters per 100 km). These numbers are impressive to say the least, and explain why it was also chosen to power Audi’s first ever diesel S-car.

As impressive an engine like the 3.0 Bi-TDI is, there’s always room for improvement and Audi engineers are experimenting with one particularly intriguing new technology. Recently we had a chance to drive an A6 Bi-TDI development mule boasting a very interesting new component fitted to this motor – an electric biturbo.

The idea of an electric biturbo is quite simple. On an engine like the 3.0 Bi-TDI, an electrically driven compressor effectively replaces the smaller turbo. While it looks much like a normal turbocharger, this compressor is not reliant upon exhaust pressure in order to ‘spool up’ and thus allows the engine management to choose when and how much boost to implement independent of engine speed.

This electric turbo is configured by integrating a small electric motor that is capable of spinning its turbine up to very high speeds in the blink of an eye. This compressor is positioned ‘down wind’ of the main turbocharger and intercooler in the path of the charge air. Under most conditions, charge air is routed around the electric turbo via a bypass, however when the charge air is low in the larger conventional turbo then the bypass closes and air is directed to the compressor.

We’re explained all of this as we steer a white A6 S-line out onto roads around Munich for a very short test drive. Given this car is a development mule, it has been fitted with some interesting software and switch gear – most importantly one particular switch that allows one Axel Schwarz, an Audi engineer and our co-pilot, to activate or de-activate the electric turbo. The setup makes it possible for the car to run either augmented with the new component or deactivated and solely with a single turbo.

Steering to an empty straight of road, the car feels much as we’d expect a normal A6 3.0 TDI to feel. It’s got no lack of power, but it’s also not particularly urgent in its acceleration. We fire off a standing start with just the conventional turbo in operation. The car pushes on at a brisk rate, and the impression of a normal US-spec 3.0 TDI is reinforced.

We stop again and our co-driver flips the switch. This time the launch is more brutal and immediate. It’s not like an Audi TDI is a slouch off the line anyway but there’s notably more urgency in this case. For reference, we have driven both the SQ5 and the A6 with the Bi-TDI and this car feels decidedly more urgent off the line.

Our passes are kept to low-speed runs. Given this is technically a development prototype, Herr Schwarz intends to keep this test drive short and sweet. He explains though that off-the-line acceleration is not the only place the electric turbo benefits this engine.

As TDI owners can attest, the 3.0 TDI is no slouch off the line. Cruising at highway or Autobahn speeds though, a diesel is optimally running low revs and transmissions like Audi’s 7-speed S-tronic or 8-speed Tiptronic keep the revs low and efficiency high. Hit the accelerator at cruising speed though and the TDI can feel anemic until it builds up boost. Here again, the electric turbo is said to make a big difference.

We inquire with Axel Schwarz about acceleration numbers compared to the 3.0 TDI or 3.0 Bi-TDI A6. He claims they’ve not yet tested the car in that way, and while we’re skeptical of that statement the translation is likely something more like, “those numbers aren’t approved for publishing just yet.”

Whatever. The car feels quite fast off the line. We just wish we could test it at high-speed as well. Unfortunately, that’s not part of the test loop.

Regarding electric turbos, Audi isn’t the only company experimenting with this technology. BMW filed a patent in 2011 and thus showed their intent for a similar application. There’ve been rumors that Subaru may also employ a similar setup with the next WRX STi. For now Audi isn’t divulging plans for its own production implementation though looking at current engine configurations it is not hard to guess. We’d put our money on the 3.0 Bi-TDI going first, and this technology should also do great things also for the 4.0 TFSI. Theoretically even engines like the 2.0 TFSI fitted with a larger turbo could benefit from such a setup in order to minimize lag and yet still allow for a larger turbo that would boost the small engine into big displacement power numbers.

Back to the situation at hand, our time with the A6 quickly comes to an end. We head back to the restored factory building near Munich from which Audi has staged this test drive. Though minor technology and one born more for efficiency, you can’t deny the smart performance gains of an electric biturbo.

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