Of all the columns I’ve written for this magazine, my August 2016 column titled What Stalled the Wankel? has received the most reader feedback. Obviously this topic struck a chord with many readers and, perhaps not surprisingly, the Wankel engine is in the news once again.

Toyota has acquired a 5% stake in Mazda, the only automaker to take the Wankel engine into mass production. In particular, Toyota is interested in developing a Wankel-driven generator for its self-driving vehicle platform (1). Toyota announced the e-Palette concept at the Consumer Electronics Show in Las Vegas earlier this year (2). While intended as an electric vehicle, Toyota is aware that current battery technology would not permit the vehicle to have sufficient power and range for intended applications. 

To extend the range of the vehicle, Toyota is considering an on-board generator powered by an internal combustion engine, an arrangement first mass-produced by General Motors in the Chevrolet Volt. The Volt uses a conventional piston engine, while Toyota is planning to use a Wankel engine.

By producing rotary motion directly, the Wankel does away with much of the hardware required by a piston engine to convert reciprocating motion into rotation. Hence, to provide a given amount of power, the Wankel is smaller, lighter and less mechanically complex.

Recall, however, that the Wankel has several disadvantages. The combustion chamber of the Wankel has a large surface area-to-volume ratio, resulting in high heat loss through the walls. And the shape of the combustion chamber is not ideal for efficient burning, also resulting in inherently lower fuel economy.

These effects can be minimized, however, by designing the engine to run at or near its peak efficiency at all times. Since the engine is not directly connected to the drive wheels, but only powers a generator to make electricity, the engine can always run at one speed and load with excess electricity being routed to on-board batteries. By not having to deal with changing speed and load, the engine can achieve reasonable efficiency.

The main tribological problem is the apex seals. Recall that, unlike the rings in a piston engine, the apex seal moves only in one direction, has no natural means of direct lubrication and must seal both the radial and axial directions. All these challenges still exist when the engine runs a generator, but, again by running at just one speed and load all the time, wear becomes considerably more predictable. Even start-up wear could be minimized by warming the motor to near operating temperature via an electric heater before starting.

Will the combination of more predictable, near steady-state running conditions and over half a century of development by Mazda be enough for Toyota to succeed with Wankel? We should know the answer fairly soon. Mazda’s North American president and CEO Masahiro Moro recently said he expects to see a rotary engine in the Toyota e-Palette in “a few years (3).”

REFERENCES
1. Available here.
2. Available here.
3. Available here.