In concept form, the Chevrolet Volt plug-in hybrid used a small three-cylinder, but when the car goes on sale in 2010, a 1.4-liter four-cylinder engine will lie underhood.

The premise of the engine - to serve merely as a range extender for an otherwise electric car - will stay the same when the Volt reaches production, but engineers added the extra cylinder in the pursuit of finesse and fuel economy.

According to Larry Nitz, executive director of hybrid engineering at GM, the I-4 not only delivers better fuel economy (thanks to higher torque numbers), but runs smoother than a three-cylinder ever could.

"The objective is to keep the engine off," Nitz told Green Fuels Forecast, "and when the engine comes on, you don't want to know it's on.  You want it really smooth, and a four-cylinder is smoother than a three."

Nitz says the 1.4-liter I-4 is approximately the same size and weight as the three-cylinder engine, a turbocharged 1.0-liter unit, and helps reduce the Volt's complexity.

 Source:  Green Fuels Forecast