Could Audi’s A1 e-tron Plug-in Hybrid Revive the Rotary Engine?

Posted on: November 18, 2011

By Laurent J. Masson

The Audi A1, the smallest model wearing the Audi badge, was unveiled last year at the Geneva motor show. As you would expect, the manufacturer had many models on its stand. There were production models and two prototypes: one as a sports version, the other a plug-in hybrid. A year later, Audi has built at least 20 more A1 e-tron plug-in hybrids, which are currently part of a pilot electric mobility project in Munich.

Right now, the driving and charging habits of all 20 drivers are precisely monitored, with Audi predicting that the 12-kWh battery should give a 50-km (31-mile) range. The motor makes 75 kW, allowing 0-to-62-mph acceleration in 10.2 seconds. We should note that Audi doesn’t seem to be overestimating the car’s range on electric power—31 miles from a 12-kWh battery seems very realistic. Some drivers may even get more.

The configuration of the car is quite remarkable. Audi could have done it as a pure electric car, but it didn’t. It would have been an easy choice to use a 20-kWh battery pack, but the German engineers chose a 12-kWh pack with a small internal combustion engine behind it. That creates a bit of a problem since the A1 is a small car, only 155 inches long. The Chevy Volt is a packaging marvel with both its electric motor and internal combustion engine (ICE) under its front hood, but it would be impossible to use that same architecture in the smaller A1.

The engineers designed an electric propulsion system to power the A1′s front wheels, which fits nicely under the hood, but there was just not enough free space left for an ICE. It had to be placed elsewhere, with the only other conceivable place being below the trunk’s floor.

No production engine from the Volkswagen group’s part bin could fit there, so the engineers had to design a new engine specifically for this purpose. That was undoubtedly a tough decision to make considering the cost to build such an engine―it would have been much cheaper to simply make the A1 pure EV.

Still, the engineers set out to build a plug-in hybrid, and they didn’t hesitate between a diesel or a conventional Otto-cycle ICE. They chose a rotary. You may know those engines from Mazda. They were originally a German-design, from Felix Wankel. Nearly every manufacturer played with the technology in the 1970s—GM even made a Corvette prototype with a rotary—but Mazda was the only one to build them in significant numbers.

Compared to a conventional ICE, a rotary has some of the same virtues as an electric motor. It makes much less vibration, and it can rev at higher speeds, but its poor fuel economy has prevented it from going mainstream. This may change, and the rotary could see a revival in the near future as a range-extender for plug-in electrics, because it has the strong advantage of a very compact size. Viewed from the top, the rotary inside the A1 e-tron has about the same dimensions as a laptop computer.