From the June 2016 issue
Not long ago, the only info about electric supercharging in these pages was in the classified section next to a male-enhancement ad. Only one of those products actually worked.
Electric supercharging, long rumored but never fully realized, is finally happening. Audi?s upcoming SQ7 TDI pairs an electric supercharger with sequential turbos on the SUV?s 4.0-liter diesel V-8. It?s a first for a production vehicle.
As with a conventional centrifugal supercharger, an e-supercharger uses a traditional compressor wheel but drives it with an electric motor rather than a crank-driven belt. E-superchargers draw their power from batteries or capacitors, which can be charged via regenerative braking or, in the case of the SQ7, a beefy generator and a 48-volt sub-system.
The biggest benefits of e-supercharging are power and response, particularly at low engine speeds. Because an e-supercharger?s ability to create boost is not coupled to exhaust energy or engine rpm, it offers flexibility not found in alternatives. Though traditional turbocharging remains a more efficient means of adding power, it has drawbacks such as lag.
As engines downsize and pressurize, e-supercharging offers the ability to size a compressor for a power target without sacrificing low-rpm drivability. It does so by filling in the torque-less void below the turbo?s threshold for creating boost. This is exactly how Audi is using the Valeo-supplied electric supercharger in the SQ7 TDI.

Turbo...