Turbochargers, on their own, are fairly simple devices. Escaping exhaust gasses spin a turbine, allowing the compressor side of the turbo to increase the amount of air entering the engine via the intake manifold.
More air allows for more fuel, which in turn produces greater power over comparably-sized engines that aren’t force-fed. Turbocharging generally has inherent limitations (like lag), which is why manufacturers adopt solutions like twincharging (using a supercharger in conjunction with a turbocharger) or using multiple turbos.
In twin-turbo setups, a small turbine (with low mass) is generally used to boost power at low engine speeds. At higher speeds, a secondary turbo, with a larger turbine, provides more power at high end. Though usually effective, such systems add complexity and cost to the price of a car and its upkeep.
Audi is working on a better way to achieve the same results via an “electric biturbo” setup. Its design relies on an electrically powered compressor to quickly supply low-rpm boost, coupled with a conventional turbocharger for more power at top end. In Audi’s testing, torque build-up occurs more quickly than with conventional biturbo systems.
Power required for the electric compressor can be supplied by regenerative coasting and braking, meaning that the electric turbo draws no power from the engine. Since less exhaust gas is used to spin a turbine, the catalytic converters come up to temperature more quickly, ensuring cleaner emissions.
While Audi’s electric biturbo system is still in the development and testing stages, it certainly sounds promising. BMW has already patented its design for a similar electrically-assisted turbocharging system, and even Subaru is said to be testing the technology for its next generation WRX models.
We say it's only a matter of time before such systems become the norm.