In-Depth with Lotus’s Electric-Vehicle Range Extender

With its Range Extender engine, Lotus Engineering is asking, “Why use a car engine in an EV?” The firm first showed off the powerplant in an Evora concept at the 2010 Geneva auto show and later announced it would produce the setup in partnership with Spanish automotive supplier Fagor Ederlan.

The idea is simple: If maximizing efficiency is the goal, then the device spinning the generator should be optimized for the task. First-generation range-extended EVs like the Fisker Karma use off-the-shelf engines that were designed to directly drive a car’s wheels, not to turn a generator. In the case of the Karma, this task falls to a turbocharged four-cylinder making an impressive 260 hp and previously employed in the Pontiac Solstice GXP, among other cars. You can imagine that such an engine is not at its best when making electricity. So how would you design an engine for the express purpose of making electricity?

Vertical orientation

If you are Lotus Engineering, you keep it simple. Both the range extender’s intake and exhaust manifolds are integrated into the cylinder head, which reduces the number of parts. There is no variable valve timing, which adds weight, cost, and complexity. The generator housing is mounted directly to the block, which increases stiffness and reduces vibration. And the engine is optimized to run at low speeds, with a long, 95.5-mm stroke.

The 1.3-liter inline-three’s final output figures (they’ve varied slightly throughout its development) are 51 or 74 hp, the latter with the addition of a supercharger. Both power peaks occur at 3500 rpm. Peak torque is 79 or 111 lb-ft, both at 2500 rpm. The generator in Lotus’s example (automakers could alternatively use their own unit) puts out 35 kilowatts with the naturally aspirated engine and 50 kilowatts with the supercharged unit.

The numbers might not seem too impressive, but consider the size of the engine: just 20.1 inches long, 16.9 inches wide, and 24.0 inches tall with the generator attached. That’s about the size of two pieces of carry-on luggage, and it weighs between 112 and 128 pounds. Lotus also designed the engine to work on its side with only minimal design changes. In this configuration, it’s small enough to fit under a seat. Although we find that scenario unlikely—it might get a little hot—we won’t be surprised at all if we see this engine in a production car somewhere in the world.

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