No, not the £50 eBay specials that use the electric motor from a food blender, I mean proper electrically driven forced induction systems used by vehicle manufacturers. The technology debuted earlier this year on Audi’s SQ7, but why is the industry now moving towards electrically sourced boost?
To answer that, lets first quickly look at how traditional, mechanically driven superchargers work. A belt, connected to the engine’s crankshaft, either drives a turbine (known as a centrifugal blower) or a positive displacement pump (Roots or Twin screw are the most common) to produce boost. There are two key drawbacks to a mechanically driven supercharger: it continuously drains power from the engine, even when the vehicle is stationary, and the speed at which the supercharger rotates (and therefore boost) is directly linked to engine speed.
So why the move to electric superchargers? The simple answer is emissions. Just like carburettors and distributors became extinct as the control of fuel and ignition became digitised, offering much greater levels of efficiency, feeding air into the engine is now one of the final combustion elements to be controlled electronically.
There is also a distinct move towards 48-volt architectures and away from the traditional 12-volt supply. This switch has made electric forced induction more viable as the higher voltage systems are necessary for the high-power motors that are used.
Audi’s SQ7 uses its electric supercharger to provide transient boost levels to support two traditional mechanical turbochargers. Providing full boost in less than 250 milliseconds, the electric supercharger completely eliminates turbo lag, meaning that the mechanical turbochargers can be better sized and optimised. The result is 429bhp and 664lb ft from a 4.0TDI with a claimed combined cycle of 39.2mpg and 190g/km of CO2. Impressive numbers considering the performance available.
The next generation of electric superchargers promises to provide even better figures. A prototype vehicle from MAHLE Powertrain is being developed using an electric supercharger technology from British engineering firm Aeristech. Unlike Audi’s product, Aeristech’s version can provide continuous levels of boost, meaning it can operate over a much wider rev range. In practice, this means that the mechanical turbocharger can be even bigger to provide very high levels of boost, and therefore power, without a hint of turbo lag.
MAHLE claims that its 1.2-litre, three-cylinder engine produces an incredible 256bhp and 232lb ft. To put this into perspective the new Bugatti Chiron’s engine produces 185bhp per litre, whereas the little three cylinder produces 213bhp per litre, and has no turbo lag.
So that’s it, electric superchargers are the future then? Not necessarily. As with any new technology, the cost of these systems are higher than traditional turbocharging and current systems do require vehicle manufacturers to adopt a 48-volt architecture. There is also some debate as to whether engine downsizing, a key reason to adopt any form of complex and expensive forced induction system, will continue, with significant changes to emissions testing expected in the near future.
As with a lot of innovative solutions only time will tell if electric superchargers will be the next big thing. It is hard to argue against it with the industry heading the way it is and the exceptional results from the first generation systems.
Oh, and in case you were wondering why it is termed an electric supercharger instead of an electric turbocharger. Electric turbochargers use the exhaust stream to spin a turbine that is connected to a generator, this energy can then be stored or sent directly to the electric motor attached to the compressor. But maybe we will look at this in more detail in another article.