On the first day of its Chevy Volt media drive program, GM officials confirmed speculation that the Volt's 1.4-liter gas engine does help drive the wheels of the car, "in low torque situations," according to Volt chief engineer Andrew Farah. "It's all about efficiency," he said. GM is using a version of its two-mode hybrid transmission that allows for a so-called "power split" mode that connects the crankshaft of the gas engine to the Volt's two electric drive motors by means of a planetary gearset. This allows the power from the gas engine to be blended with the power from both electric drive motors, primarily during high-speed operation, where GM says this mode provides 10-15 percent better fuel economy.

Leaf The Volt transaxle is more complex than many media and onlookers had been led to believe. It employs two electric motors, a planetary gear set, and three clutches, and can operate in four distinct drive modes, switching between them as conditions warrant.

The first mode is the easiest to understand, as it uses the 111 kw traction motors to drive the wheels, with all the electric power coming from the battery. This EV mode delivers 273 lb-ft of torque to the wheels and makes the Volt fairly quick to accelerate from a standing start.

The second EV mode employs both electric motors -- the traction motor and the 55 kw generator – to drive the wheels, with electric power coming from the battery. This mode blends the power by means of the planetary gearset to allow for improved highway efficiency, as both motors are able to run closer to their optimal efficiency.

The third mode happens during extended range driving, when the gasoline engine is operating. In this mode the traction motor drives the wheels, with electricity coming from the battery as well as the motor generator, which is being spun by the gas engine. GM calls this a "weak series" arrangement, as the battery is always the lead power source.

It's the final mode that's come as a bit of a surprise, because it is in this extended range mode where both electric motors and the gasoline engine get connected to drive the wheels. However, GM is still maintaining its stance that there is "no direct mechanical linkage from the engine to the wheels," while explaining that "the Volt is always propelled with electric power delivered by the traction motor."

While there is still likely a semantic argument to be waged here -- whether we call it a plug-in hybrid or an extended-range electric vehicle is a discussion for another day -- the bottom line is that the Volt employs a sophisticated transmission to eke out every efficiency from its drive system.

Watch Bradley take the 2011 Chevy Volt for a spin at GM's secret proving grounds.

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