Electric turbos promise big performance and efficiency gains in the near future

Fri, 08 Aug 2014

An electric turbo system boosts efficiency between 15 and 20 percent, according to Audi.

Turbochargers, like acoustic guitars, use moving air to create magic. And electric turbochargers, like electric guitars, provide the ability to amp up that magic to amazing new levels.

Electric guitars make cool sounds, and do so with a lot more volume than acoustics. Electric turbochargers will make quantifiable gains in both efficiency and power, in addition to harder-to-quantify gains in drivability compared to conventional turbochargers.

An electric turbo system, which includes an energy recovery system to provide power for the electric turbo, boosts efficiency between 15 and 20 percent, according to Audi. It also effectively eliminates turbo lag, the time it takes for turbo boost to build power to levels requested by the driver's right foot. Audi says its electric turbo needs only a quarter of a second to reach full boost.

The company recently demonstrated electric turbo-equipped RS5 and A6 prototypes at an event, where Volkswagen Group technical head Ulrich Hackenberg told Autocar magazine that Audi will release an SQ7 equipped with electric turbocharger technology in 2016.

That would make the SQ7 the world's first production vehicle equipped with an electric turbo, and surely the first of many to come.

Technically they are compound turbocharger/electric supercharger systems.

One quick clarification: turbos, by definition, use exhaust gas to power their compressors. So an electrically driven compressor would actually be a supercharger, not a turbocharger. However, because the electric compressors are used in tandem with turbos, they are commonly referred to as electric turbochargers. Technically they are compound turbocharger/electric supercharger systems.

There are good reasons why many of us could find ourselves driving such cars in the mid-distant future, mostly related to the benefits and shortcomings of traditional turbos. Industry powertrain gurus predict that the gasoline engine will follow the lead of the diesel, with normal aspiration being universally displaced by forced induction, along the lines of Ford's EcoBoost engine family.

Turbochargers, like acoustic guitars, use mechanical means to move air. Guitars' strings vibrate the hollow body, which moves the air inside to make sound. Turbos have a turbine in the engine's exhaust stream to spin a shaft connected to another turbine that compresses air coming into the engine so it makes more power.

Why haven't electric turbos appeared already? Power. Musicians can plug their guitar amps in to a wall outlet for power, but an electric turbocharger needs more power than a conventional 12-volt automotive electrical system can provide.

Around the turn of the millennium, there was the expectation that cars would ditch 12-volt electrics for 42-volt systems to support the many new electronics and electric comfort devices such as seat heaters. That switch never occurred, but the know-how and hardware that had been developed for hybrid-electric cars is suitable for application to the electric turbocharger problem.

"We've been in pre-development for more than ten years," noted Steve McKinley, Honeywell Turbo Technology's vice president of engineering. "It was a matter of waiting for that electric infrastructure to develop on the vehicles."

"You have to have a fair amount of power going into the supercharger," he said. "But when you have other parts of the vehicle being electrified," it becomes possible to use some of that power for the turbo.

Indeed, Audi says that the peak current to the electric turbo in the A6 and RS5 prototypes it demonstrated to the press is 7 kilowatts, about the equivalent of 4.5 hair dryers.

The 48-volt system used by Audi lowers the amperage needed to provide that to a manageable 145 amps, something that would be impossible using a conventional 12-volt system, which would need 583 amps.

The exact voltage depends on the needs of the specific vehicle. "Pick a number, whatever number you like," said McKinley. If the industry settles on a single standard it seems likely to help reduce costs. It is important to stay below 60 volts because that is the threshold at which electricity becomes lethal to people, and non-lethal electric power is much less expensive because it doesn't require the safeguards seen in high-voltage electrics such as employed in hybrid vehicles.

While the electric turbocharger stands to boost fuel efficiency, as sort of a mid-step between conventional engines and hybrid-electrics, early applications are likely to be in performance vehicles, because those are the models whose price can justify the added expense, McKinley observed.

These advanced sports models will surely enjoy improved efficiency as a bonus, but they will also have increased peak power. That's because the drive to eliminate turbo lag has led carmakers to install small, quick-accelerating turbos.

With an electric turbo installed to eliminate lag, the exhaust-driven turbo can be sized for maximum power at wide-open throttle, McKinley pointed out. "You will do things differently in terms of turbo sizing for the high-load, high-pressure situations," he said.

With an electric turbo installed to eliminate lag, the exhaust-driven turbo can be sized for maximum power at wide-open throttle.

Audi's prototype A6 is equipped with a 3.0-liter TDI diesel engine using a single conventional turbocharger plus the electric turbo to produce 326 horsepower and 479 pound-feet of torque. The RS5 has twin turbochargers, as well as the electric turbo, and produces 385 hp and 553 lb-ft, for 0-62 mph acceleration of about four seconds.

Today's production 3.0-liter TDI V6 engine produces 240 hp and 428 lb-ft of torque, accelerating the A6 to 60 mph in 5.5 seconds.

As shown by the Audis, these devices will be built as two separate turbines, connected by plumbing, at least at first. But ideally, the electric boost will be built into a single device that also uses exhaust gas energy, McKinley said.

But the heat associated with the engine's exhaust poses durability challenges to an electric motor. "At this point the benefits are not outweighing the risks of attaching it to the [turbo] shaft."

A possible solution could be a long shaft connecting the impeller and compressor sides of the turbocharger, so that the electric motor could safely live on the compressor side at some distance from the exhaust-driven impeller. Mercedes uses a split turbo design in its Formula One racecar this season, and perhaps not coincidentally, has demonstrated the most engine power.

At least five years out before you see very mainstream adoption." – Steve McKinley

Connecting the electric motor to the turbo's spinning shaft also provides the potential to recapture energy from that exhaust stream by using the electric motor as a generator at part throttle.

"How that translates into vehicles, with small and light components, it might be difficult to propose something like that in a real production environment," McKinley concluded.

When could we see electric turbos in high-volume production cars, as Ford's EcoBoost engines are used currently? "At least five years out before you see very mainstream adoption," McKinley predicted.

That should give people plenty of time to prepare for the change, though it may not sound as dramatic as the arrival of electric guitars was.

By Dan Carney