For what seems like an eternity, we’ve seen claims of horsepower gains from gimmicky products calling themselves “electric turbos” or “electric superchargers” and amounting to nothing more than repurposed 12-volt computer fans on the low end, and leafblowers on the high end. We’ve even seen other automotive media outlets take the novel concept ad absurdum, and plumbing multiple industrial leaf-blowers into the intake tract of a small-block V8 in the search for alternative forced-induction.
Absurdity aside, German automaker Audi has not only done the engineering and design work to properly package and execute the industry’s first production electric supercharger, but they have incorporated it into an extremely involved and complex system of forced-induction aimed at not only absolute power, but seamless application of that power with elimination of turbo lag, releasing the technology to the public in the 4.0-liter V8 TDI engine powering the SQ7 luxury SUV.
In the video above, the system is shown through slick animations, while in the video below, Jason Fenske of Engineering Explained does a great job of breaking down Audi’s intensely complicated system that involves a heap of electrical components, three separate compressors, and a cutting-edge cam and valvetrain system that revolutionizes how exhaust is fed to the two traditional turbochargers in the system.
The Heart of the Matter
At the core of the Audi anti-lag electric compressor system is the electric supercharger unit and its dedicated electrical system. Operating at four-times the normal automotive voltage, a dedicated 3,000-watt generator feeds into the 48-volt battery to supply the electric compressor unit, which consumes 7,000 watts, or about 9.5 horsepower.
The compressor unit is able to reach 70,000 rpm impeller speed in only 0.25-second through massive influx of electrical current, making its “boost” almost instantaneous. As Fenske points out, that compressor is not designed to provide all of the engine’s compressed air, but in fact only to supplement the first turbocharger’s output while it spools up.
The Entire System
First let’s take a look at the induction system as a whole. At idle, the air comes into the main air intake, and flows into the first turbocharger. From there, the compressed intake charge flows through the dual (split) intercoolers, then splits off into the separate intake manifolds, as the Audi 4.0-liter V8 diesel is arranged in a “Hot-Inside-V” configuration.
The exhaust gasses are filtered through only the single turbocharger at idle through the ingenious Audi Valvetrain System, which only activates a single exhaust valve at idle and the low-to-mid RPM range.
When a the throttle is pressed, a bypass valve on the outlet of one of the intercooler cores engages, funneling the pressurized output through the electric compressor to supplement the intake tract pressurization until the single turbocharger is completely spooled up and can take over the complete pressurization duties, at which time the bypass valve disengages and allows the engine to operate in just a monoturbo configuration.
Once RPM and engine load increase to the point that the system feels the second turbocharger’s input is warranted, the Audi Valvelift System engages the second exhaust valve, which feeds the turbine side of the second turbocharger. Simultaneously, a blockoff plate is opened, allowing the second traditional turbocharger’s compressor to feed the intake tract, providing the full twin-turbo—or “biturbo” in Audi parlance—operation.
While exceedingly expensive and complex, Audi’s system takes linear power on demand to an entirely new level, providing three separate intake tract pressurization configurations, each of which can reconfigure in the literal blink of an eye.
Limited as it Might Be
You might be wondering, if it works in this application, does that mean we’ll be seeing applications that are pressurized entirely by electrically driven compressors? The answer to that, is simply “no”. As Fenske points out in his video, the amount of complexity and power required to supply only a supplementary amount of boost is incredible.
Fenske also points out that you would always have to have a separate electrical system for the compressor, as trying to get the power needed to drive even the current supplemental compressor system, requires over four times the output of a conventional automotive 12-volt system.
If you were to run a compressor large enough to provide the entire system pressurization, you would probably require an electric motor in the neighborhood of five to ten times as powerful, at a minimum. That’s an incredible amount of energy to be exchanged to simply run what is supposed to be a power-adder. However, Audi has not only proven that an electric supercharger can be feasible, but has incorporated it into one of the most interesting engine systems we’ve ever seen.