From Snake Oil To Reality: Audi’s Electric Supercharger Kills Lag

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 system is a seeming-jumble of three compressors, two intercoolers, multiple diverter valves, and separate charge paths. However, Audi has refined the system to the levels expected from the luxury automaker.

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.

Here is what a 7kW – or about 9.5-horsepower – electric supercharger looks like. Operating at 48V and drawing 145 amps of current, this particular compressor can reach 70,000 rpm in a quarter of a second and is water-cooled. Only designed to pick up the slack on the low-end and eliminate the lag of single turbo spools up, it excels in that application.

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.

At idle (top, left) the path of the incoming air is simple and straightforward, only flowing through a single turbo. When the throttle is engaged (top, right) a bypass valve closes, diverting half of the intake charge through the electric compressor for a near-instant boost of power. Once the primary turbo has caught up to the pressure created by the electric compressor (bottom, left) the bypass valve is reengaged and the engine operates on the single turbo alone. Once demand arises, the second turbo is brought online (bottom, right) and the engine operates in a "biturbo" 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.

The Audi Valvelift System uses a solenoid to intelligently control lifter engagement on the camshaft and determines whether one or both exhaust valves are engaged. Each exhaust valve feeds a different turbocharger, adding another layer of mechanical elegance to the system.

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.

About the author

Greg Acosta

Greg has spent over a decade in automotive publishing as Senior Editor of Race Pages magazine. In his free time, he is a firearms instructor and volunteer in the police armory.
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