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Audi developing electric biturbo for enhanced performance at lower engine speeds

Components of the electric biturbo system on the 3.0L V6 TDI diesel. Click to enlarge.

Among the technologies it is developing to improve the fuel consumption of gasoline and diesel engines, Audi is working on an electric biturbo system in which a secondary compressor boosts the main turbocharger at lower engine speeds.

Turbochargers are typically driven by energy from the exhaust—i.e., starting from very low revs, the rise in boost pressure and therefore torque becomes gradually greater only as the exhaust energy increases. The electric biturbo makes it possible—independently of the exhaust energy available—to build up charge pressure quickly and achieve high levels of torque even at very low revs.

In advanced diesel development at Audi’s site in Neckarsulm, engineers have built and calibrated a 3.0 V6 TDI with an electric biturbo. It is a combination of a conventional gas-driven turbocharger with a secondary electrically driven compressor—the exterior appearance of which is almost identical to that of a conventional turbocharger.

Instead of a turbine wheel driven by the exhaust gas stream, this is an auxiliary charging stage arranged in line with the gas-driven turbocharger. It integrates a small electric motor that can accelerate its turbine to very high speeds in an extremely short time. The electrically driven compressor is placed after the main turbocharger and intercooler in the charge air path.

In most operating states, the charge air is routed around it via a bypass. However, when the flap integrated in the bypass closes—i.e. when the main turbocharger’s energy output is low—the air is directed into the electric compressor and is compressed there a second time.

The energy required to drive the electric compressor is largely offset by battery regeneration during coasting phases, so that the end effect is essentially neutral with regard to energy consumption.

Charge air path
Charge air path without (left) and with (right) the use of the electric compressor. Click to enlarge.

The flexible and compact charge air path enables two-stage turbocharging with just one turbine that is driven by exhaust gas. The reduced heat demand enables earlier activation of the catalytic converter.

The biturbo 3.0 TDI has a power output of 230 kW (313 hp) and a maximum torque of 650 N·m (479 lb-ft) between 1,450 and 2,800 rpm. Its specific power output is 77.5 kW (105.5 hp) per liter displacement. Combined fuel consumption in the A6 is 6.4 l/100 km (36.8 mpg US), equivalent to CO2 emissions of 169 grams per km (272 g/mile).



I'd rather see 156 HP out of an I3 biturbo.


Great news but it could have been done decades ago?


Decades ago? The euro "yee haw" effect of clean diesel ultra low speed torque development is actually only a couple years old. This has no effect on maximum horsepower. I'd rather see 160 lb-ft at 1450 rpm out of a 1.0L advanced 2 cylinder, or 80 lb-ft out of a 0.5L single. Who cares about maximum power? What we need is a steep power curve out of a hyper efficient burn.


And an engine note that won't drive people crazy.


A brand new redesigned body is very often the same body with an added nickel plate here and there.

Thomas Pedersen

Wow, five comments on this first application of an electric turbo-compressor, and they're all negative. That's sad...

Sure we'd all prefer a smal, frugal, cheap engine with a huge plug-in battery, but none of us are prepared to pay the cost. And even if we were, we'd be the only ones...

Let the rich kids pay the development cost and savour this engine which kicks the Earth backwards as it accelerates a huge car, and with a fuel consumption less than my first car with half the size and 1/3 power.

Electric turbo-compressors have too many benefits to ignore!

I'm excited!


Negative, moi?  I just think that the first application is over-powered, and an engine half the size would be plenty.  Downsizing an I-4 to an I-3 with more power and low-end torque would improve things all around.


The electric biturbo has similar performance as the current biturbo, so one has to look carefully for the progress. Presumably, the transient response will be better. Plumbing is also simplified. Initially, cost will most likely be higher. One could note that the pictures shows the inherent draw back that a V6 engine has when it comes to turbocharging, i.e. the long exhaust pipes before the turbocharger (=large volume) that compromises engine response. An in-line V6 engine does not have this problem. Inward-facing exhaust ports in the cylinder heads on a V-engine (e.g. Ford diesel, BMW gasoline) would solve this problem.

I agree with EP, a small downsized engine would make more sense; at least in an environmentally-conscious world.

Thomas Pedersen

Apart from the Prius, all new cars with hybrid and/or new technology tend to be expensive.

BMW puts their active hybrid with an engine that already has 300 hp. The Mercedes latest hybrid effort is an E-class with the 204 hp, 500 Nm 250 CDI engine, and so on.

I agree with all of you that it'd be better with a smaller engine, I just found the general sentiment rather drab around a simple technology that is sure to drizzle down to smaller engines soon.

It has been a long time we have heard about these electric turbos and all their benefits. I find it exciting to see it in action. However, I saw a comparison today showing that the torque is not much greater than on the regular strong Audi 3.0 TDI. But the electric turbo allows Audi to attach a larger turbo with more air output at high revs yielding more power. And the max torque now comes at lower revs. (1450 vs 1750)

Where is the Ricardo/Ford Focus 1.0 with electric turbo?


@Thomas Pedersen
No, the torque does not come at lower rpm with the electric biturbo. The “standard” biturbo provides 650 Nm at 1450 rpm, exactly the same as is claimed for the electric biturbo. The gain in this respect would consequently be nil. This would hardly motivate the use of more “exclusive” technology. Note that the working principle of a “normal” biturbo is quite complicated. In some sense, it is sequential if you compare the operation at low and high speed. At low engine rpm, it operates as one small turbocharger. If variable turbine geometry is used, the limiting factor is not exhaust energy or turbine efficiency. Instead, compressor pumping will limit charge pressure and engine torque & power. As a conventional turbine can already provide enough power to reach the limiting line for compressor pumping (the compressor does not care if the power comes from a turbine or an electric motor), no gain can be achieved by using an electric biturbo. The main advantage, as I can see it, would be in better transient response for the electric biturbo. I am surprised that Audi did not try to extract more power from the engine, e.g. by using other means (increased injection pressure, improved cylinder heads or any other technology) than the conventional biturbo. How are they going to market the new engine? Would it replace the conventional biturbo or would both be produced in parallel? One could discuss how much bigger price tag customers would accept for only minor improvements. Or else, maybe Audi plan to increase power and torque before introducing the technology on the market.

The state-of-the-art technology in this field (and in so many other cases) is BMW. The BMW triturbo of similar size (3 liter) as the Audi engine provides almost 400 hp. The example illustrates where you can find really advanced technology.

Roger Pham

Perhaps the best application of this technology lies in high-end high-power German electric hybrid designed to cruise the Autobahn at high speed and efficiency extendedly yet can deliver very high city fuel efficiency.

For this, a GDI Miller cycle can be used, with high expansion ratio to maximize cruise efficiency. However, Miller cycle at lower loads delivers low exhaust gas pressures insufficient for rapid spool up of the even 2-stage bi-turbo, hence, an electric supercharger would be vital for high-acceleration city driving. At Autobahn speeds, the turbocharger takes over for extended driving at high speed and high power, yet, still higher efficiency than an Otto-cycle engine driven in the same vehicle at the same speed, due to the high expansion ratio of the Miller cycle. That's the beauty of this technology!

Roger Pham

I must hasten to add that a turbocharged diesel engine at lower equivalence ratio is quite analogous to a Miller-cycle gasoline engine. The exhaust pressures just isn't that much for rapid spooling up of the turbine wheel. Raising the equivalence ratio to deliver higher exhaust pressures will run the risk of high particulate matter and NOx emission, which will be quite stressful for both the SCR and DPF. Perhaps a way to better preserve the expensive emission control system is to rapidly deliver boost pressure in order to maintain an equivalence ratio ideal for low ehaust emission and to prolong the life of the emission control system. The engine will also run cooler, happier and will last longer, too! Electric boost to supplement the turbocharger is vital for Low-Temperature Combustion engines to stay within the LTC parameter while still maintaining fast acceleration.


@ Roger,
It is often said that turbochargers for gasoline engines have to work with exhaust gasses with temperature that is 100-150 degC higher the gasses from diesel engines, so they need to use more expensive materials, and are perhaps likely to last less than turbos for diesel engines.
Atkinsonized gasoline engines, with high expansion ratio are supposed to produce gasses with lower temperatures, than non-Atkinsonized ones.
Any idea how much less hot those exhaust gasses in Atkinsonized gasoline engines are?
Could they use turbochargers for diesel engines?

Roger Pham

Very good point, MG.
About 110-130 degrees C cooler when comparing an Otto-cycle engine at CR of 10 to an Atkinson-cycle engine with CR of 13. I don't see why they can't use diesel's turbocharger.


Please take a look at previous post:
There is nothing new or original in this technical solution. It is just a careful implementation of what has beeen done before. There are many companies working in the same direction and I do not think that Audi's solution is the best one. Instead of redesigning the system and/or optimizing it they just added an additional part

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