|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 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).