Top. Drivetrain of the RS 5 TDI concept with biturbo. Note the position of the high- and low-pressure exhaust gas turbochargers and the electrically driven compressor (downstream of the intercooler). Middle. Drivetrain of the A6 monoturbo TDI concept. Note the position of the single exhaust gas turbocharger and the electrically driven compressor. Bottom. Elements of the 48V subsystem. Click to enlarge.

The electric turbocharger is downstream of the intercooler. In most operating states, it is bypassed. At very low engine speeds and thus correspondingly low exhaust gas energy at the turbocharger, the bypass valve closes and the air is routed to the electric compressor. This can be flexibly and compactly integrated into a variety of forced induction concepts. The new technology enables a spontaneous development of power never before seen when starting off and at low rpm.

Charge air path with and without the use of the electric compressor in the A6 TDI study. Click to enlarge.

In steady-state—no additional impetus—the monoturbo produces a constant 240 kW (326 hp) and 650 N·m (479.4 lb-ft) of torque, the latter between 1,500 and 3,500 rpm. The electric compressor fills the gap in the torque curve below this range and provides for fast response and excellent elasticity. Acceleration from 60 to 120 km/h (37.3 to 74.6 mph) in sixth gear is reduced from 13.7 to 8.3 seconds.

The modified V6 biturbo in the Audi RS 5 TDI concept produces 283 kW (385 hp), with peak torque of 750 N·m (553.2 lb-ft) is available between 1,250 and 2,000 rpm. The electric compressor provides for tremendous power when starting off. If the driver stays on the accelerator, 100 km/h (62.1 mph) is reached in roughly four seconds. Boost pressure is available immediately after each change of gears thanks to the intelligent interplay between the two turbochargers.

The strengths of the electric biturbo lie exactly where they make the most sense in everyday driving, Audi says. It eliminates the need for constant downshifting, keeping engine speeds low. Sporty drivers will appreciate the passing power and immediate delivery of power when exiting a curve. The electric biturbo is suitable for use in many Audi model series as well as with gasoline engines, in principle, Audi said. It will soon enter series production in the TDI sector.

The energy required to drive the electric compressor is largely generated by recuperation during coasting phases, so that the end effect is essentially neutral with regard to energy consumption. It is supplied with power via a separate 48-volt electrical system, complete with its own compact lithium-ion battery in the trunk and power electronics. A DC/DC converter provides the connection to the 12-volt electrical system.

The new 48-volt subsystem offers major advantages, Audi suggests. It can supply the high-performance electrical consumers of the future—thermoelectric heating elements, electromechanical rear brakes or engine auxiliaries such as oil and water pumps—with more energy than the 12-volt electrical system. Higher voltage means lower currents, allowing for smaller cable cross-sections and thus reduced weight. Audi plans to introduce the 48-volt electrical subsystem to multiple model series shortly.

Electrification. The electric biturbo also represents the first step in Audi’s planned electrification of the TDI engine.

New hybridization components will soon be launched on the market, Audi said at the Audi TDI – Tech Workshop 2014. In the future there will be a tailored solution for every customer and requirement, up to and including a TDI with plug-in hybrid technology. When it comes to fuels, the brand is committed to sustainably produced Audi e-diesel (currently, synthetic diesel produced by Joule, earlier post), which enables CO₂-neutral driving.