It's finally time to plug in your snail.
Until recently, electric superchargers might as well have been perpetual motion machines, for sale at the same places as gasoline magnet ionizers and other snake-oil. Power for an electric compressor has to come from somewhere, and e-turbos were a kind of get-power-quick scheme that ultimately left you poorer.
That's starting to change. Step one, as it usually does, involved motorsports. The current crop of Formula 1 cars and Audi's latest R18 use turbos that are a mash-up of an exhaust-driven compressor and an electric motor, allowing them to both harvest and unleash electricity. They're highly complex and expensive ways of storing energy that make more sense for competition than commuting, however.
Before we dive into why electric superchargers finally make sense in the mass market, a little refresher on how they work is warranted. An electric supercharger looks and acts a lot like a turbocharger, and its job is to compress air that is then used to spin up a conventional turbo.
Most of the non-racing applications of the technology are in some respects similar to the older idea of twin-charging: using a supercharger and a turbocharger in conjunction. Volvo's production T6 Drive-E engine, which pairs a conventional engine-driven supercharger with a turbo, is a twin-charged engine. The High Performance Drive-E concept is even more exotic, using one electric supercharger to feed twin exhaust-driven turbos. The e-supercharger takes care of low-speed boost and transitions, while the exhaust-driven turbo or turbos handle the high-rpm tasks they're good at.
Which brings us back to the problem of power supply. Siphoning juice from the 12-volt system won't cut it for several reasons—there's the net-loss issue and the fact that batteries are no good at accepting or supplying lots of power quickly. The answer is a supercapacitor. This is the technology that takes electric supercharging out of the realm of snake-oil fantasy and into legitimacy.
Power recovery can be supplied by a clutched alternator, similar in function to the regenerative braking found in hybrids and EVs, that kicks in only when the car is slowing. To release the recovered energy, electricity is sent from the capacitor through a DC/DC converter for voltage correction and onto the electric compressor. The result is lag-free power delivery from a smaller engine. And instead of trying to power the electric compressor directly from the engine, it instead uses recovered energy. Everybody wins.
Supercapacitors are also longer-lived than batteries in terms of repeated charge and discharge. And while they have low energy density—you can't pack a lot into one compared to a battery of the same size—it's not a worry in this application since the energy is being used in short spurts. It's not moving the entire vehicle, like an EV's batteries—instead, it's just spinning up the turbine for a brief period.
You can already buy a car equipped with a supercapacitor, and it's not some million-dollar hybrid supercar. Mazda's efficiency-focused i-Eloop system feeds a supercapacitor and then uses the power to run the car's various electric components—but not an electric compressor. The aim is improved fuel economy alone, not extra boost, although eliminating the alternator's drag does result in more usable horsepower. The system could, theoretically, power an electric supercharger instead of the Mazda's accessories, if Mazda wanted to go that route.
Sound hybrid-pricey? Supercapacitors are generally more expensive to make than batteries, but costs are coming down. Mazda's system comes as part of a big $2600 tech package on a $27,000 Mazda 3, so it's safe to say the supercap is ready for the mainstream.
The other hurdle concerned programming the controls. Michael Fleiss, Volvo's VP of Powertrain, says making everything in the complex Drive-E concept's three-turbo system work together was the tough part for his team. There's a lot going on in the 450-hp 2.0-liter engine, more so than a "conventional" twin-charged engine. In addition to the complex plumbing, there's wiring to feed and support the supercapacitor. Plus the different turbos need to be cued with wastegates for the exhaust-driven units and a bypass throttle that sits parallel to the e-booster. And it all has to happen smoothly, with no lag, surge, or hiccups.
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Thank the electrical super-geniuses for what will be a more pleasant engine downsizing experience. It's not the zero net loss of perpetual motion, but things are getting closer.