Could Plug-in Hybrids Revive the Rotary Engine? Consider Audi A1 e-tron.
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. Not many people looked at it back then, but we should now because Audi has built 20 of them. The Audi A1 e-tron is still far from a production model but at least it has left the shop floor. Those 20 cars are now part of a pilot electric mobility project in Munich.
The definition of the car is quite strange. They could have done it as a pure electric car, but they didn't. It would have been the 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 brought a huge 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 was impossible to use that same architecture in the A1. The engineers have designed an electric propulsion system to power the front wheels, and it fits nicely under the hood, but there is just not enough free space there to fit an ICE on its top or on the side. It has to be elsewhere, with the only possible place 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 necessary budget. It would have been so much cheaper to make a pure EV.
The engineers kept on their plug-in hybrid idea, 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. Every manufacturer tried the technology in the 1970s, even GM 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 shares some virtues with 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 may see a revival as a range-extender—because it has a 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. That's with the generator mounted at its end, which provides 15-kW at 5,000 rpm. It doesn't rev any higher to maintain decent fuel economy, though we don't have much information about that.
It will come next year after 12 months of testing. Right now the driving and charging habits of all 20 drivers are precisely monitored, with Audi explaining that the 12-kWh should give a 50-km (31-mile) range. The motor makes 75 kW, allowing 0-to-62-mph accelerations in 10.2 seconds. We note that Audi doesn't overestimate the car's range on electric power—31 miles from a 12-kWh battery seems very realistic. Some drivers may get more from it. More important, the test will show if the rotary has some future as a range-extender. We know now that the fuel economy of the Fisker Karma isn't very good with its ICE running, and the Chevrolet Volt is only average there. Maybe a a low-rev rotary get better results.
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