Range extenders, small gasoline engines that produce electricity to run electric cars after their battery packs discharge, will likely see huge innovation in the coming years.

While the Chevy Volt and Fisker Karma both use off-the-shelf GM engines as range extenders, many companies are working on tiny, radically more efficient engines to turn generators in battery-powered cars.

German parts supplier KSPG showed a novel range extender at last months Detroit Auto Show: a small V-twin engine that uses a pair of generators to counterbalance its uneven firing pulses and inherent vibration.

Summertime tests

This summer, the company will test its prototype range extenders in Germany, mounted in a fleet of Fiat 500s adapted to electric drive.

The short crankshaft of the V-twin lets KSPG mount the engine on its side under the load deck of the Fiat 500, within the space that formerly contained the spare tire.

KSPG deliberately designed the engine-generator package as a module with minimal interfaces, making it easy to include or omit during vehicle assembly.

"Where can we specialize?"

The range extender project began 18 months ago, when KSPG looked at what it could do in the field of vehicle electrification.

As a traditional supplier of parts for internal combustion engines, executives realized KSPG had little hope of competing effectively in lithium-ion cells, electric traction motors, or power electronics.

Range extenders to alleviate "range anxiety" and extend the usability of electric cars, however, are a component with no established vendors.

And the limited, specialized duty cycle of a range extender makes a combustion engine designed from scratch far more efficient than one adapted from a small conventional engine.

Small, simple, modular

The specs were simple: an 800cc engine with 30 kilowatts (40 horsepower) of power and 66 Newton-meters (49 pound-feet) of torque to drive its pair of 15-kW generators.

And, it had to produce little or no vibration and running noise with the engine switched on.

Each cylinder has just two valves, operated by pushrods, and KSPG chose to forego direct injection for simpler port fuel injection.

A single, remote-mounted radiator cools the engine, alternators, and power electronics, and the entire package weighs roughly 130 pounds.

Slower than battery mode

KSPG admits, however, that the Fiat 500 will be slower in range-extending mode than running on battery power. The electric conversion uses a 45-kW traction motor, so the 30-kW output of its engine cannot supply it with maximum power.

"We hope the customer will accept that lower performance," said KSPG's chief technical officer, Hans-Joachim Esch.

That's contrary to the design goal of both the Chevy Volt and Fisker Karma, which were designed from the start to offer the same acceleration in either electric or range-extending mode.

Still, KSPG left some room for future expansion, with a 40-kW version of the engine also under discussion.

Challenging engineering

Designing an engine to run an electric-car generator brings several major challenges. Controlling noise, vibration, and harshness is crucial, because battery electric vehicles are so quiet.

KSPG 800cc V-twin 30-kW range-extending engine

KSPG 800cc V-twin 30-kW range-extending engine

KSPG engineers evaluated different configurations. The V-twin was relatively compact, with good inertial characteristics, but its output torque was very uneven--leading to constant vibration.

Cleverly, the engineers chose to use a pair of alternators running at twice the crankshaft speed to counterbalance the power pulses. According to KSPG, the vibration transmitted to the car's structure is effectively zero.

The next challenge was intake and exhaust noise, which were far more intrusive compared with the silence of battery electric drive.

The engine has huge resonators to baffle the flows of intake air and exhaust, effectively muffling the sound--which is, usefully, behind the passengers and not up front where an engine would sit.

KSPG is hardly the only company developing dedicated range extenders; it faces competition from both Lotus Engineering and Mahle.

"We believe ours will be pricier, but better on the Number One priority: noise, vibration, and harshness," said Esch.

"We shall see."

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