[kach22i]

https://www.audi-technology-portal.de/en/mobility-for-the-future/audi-future-lab-mobility_en/audi-future-engines_en/electric-biturbo

Electric biturbo

With the electric biturbo, Audi is taking another major step forward in its TDI engines. In this forward-looking technology an additional compressor assists the turbocharger in the lower rev range.
Almost a quarter of a century ago, Audi delivered a major boost to diesel engine development worldwide. 1989 saw the debut in the Audi 100 of the first direct injection compression ignition engine with turbocharging and electronic control, since when the TDI has enjoyed an impressive and ongoing success story.................

All turbocharged internal combustion engines share one characteristic – that turbochargers are driven by energy from the exhaust gas. For this reason, the charge pressure, and thus torque, does not begin to rise sharply at the lowest end of the rev range until exhaust gas energy increases.

The electric biturbo, however, offers a significant improvement. Specialists from Audi’s Advanced Diesel Engine Development department in Neckarsulm have built and calibrated a 3.0 TDI with this configuration. The conventional turbocharger operates together with a supplementary, electrically driven compressor. This facilitates a rapid build-up of charge pressure and high torque from the very lowest revs, independent of the available exhaust gas energy...............

........bypassed under most operating conditions...........

The energy required to drive the electric compressor is derived largely from recuperation under trialing throttle conditions, making it consumption neutral......

Click the link to read the whole thing - worth it.

Audi also did a gas engine with similar tech.

Audi electrifies the track with electric biturbo TT clubsport
C.C. Weiss May 13, 2015
http://newatlas.com/audi-tt-clubsport-electric-biturbo-concept/37425/

After we saw Audi's electric "turbo" on last year's RS 5 TDI concept, we knew it was just a matter of time before it made a return. It resurfaces this year on the 2.5-liter TFSI of the all-new Audi TT clubsport concept car. Developed for Volkswagen's annual Wörthersee event, the clubsport attacks the track with 600 hp and a big, brutal aerodynamics kit........................

The electric compressor cuts the lag that turbochargers reliant on exhaust gas experience. With the ability to rev right up to max rpm, the e-turbo gives the car pep from low engine speeds, allowing the conventional turbo to be designed specifically for high charge pressures. The electric compressor is powered by the 48-volt electrical sub-system that Audi is developing for eventual production. The system stores energy in a small, recuperation-charged lithium-ion battery mounted in the trunk and is wired into the traditional 12-volt system by a DC/DC connection.

From what I recall the math never worked before.................guess the duration and application along with the charging changed all that. However, I bet it's not cheap or easy to do which I believe was the original attraction of the concept.