Audi Developing Electric Bi-Turbo System

Volkswagen AG's Audi unit has developed a prototype electric turbocharger system to provide low-speed boost in conjunction with higher-speed conventional turbocharging.

Audi has been evaluating the bi-turbo system with its 3.0-liter diesel. The combination delivers 313 hp and 479 lb-ft of torque and, when fitted in an A6 large sedan, nearly 37 mpg on the European test cycle, according to the company.

Conventional turbochargers boost air intake with a fan driven by exhaust gas. Boost pressure, and thus engine torque, builds as engine speed increases. Adding a small electrically driven turbocharger can provide high boost at low engine speed, which virtually eliminates the low-speed lag in power assist typical for a traditional turbocharger.

Audi's system bypasses the electric turbo under normal operating conditions. But when extra power is required at low engine speed, a bypass valve closes. This diverts the charge from the gas-driven turbo through the electric turbocharger, where it is compressed further.

The arrangement heats the catalytic converter quicker during a cold start, thus helping to reduce emissions, according to Audi. The company says the energy required to drive the electric turbo unit is mostly offset by electricity gained by a battery regeneration system that kicks in when the vehicle coasts.

Audi has not indicated when the bi-turbo system will go into production. The company takes credit for introducing the first diesel with direct fuel injection, turbocharging and electronic control when the Audi 100 sedan debuted in 1989. Such TDI engines today provide double the power and 70% more torque than naturally aspirated diesels of that era.