Previously we heard rumors that GM might use a 1.4 L non-turbo 4 cylinder engine as the Volt's range extender.  Last week GM CEO Rick Wagoner confirmed it was under consideration.

Now GreenCarCongress' Mike Millikin reports the decision has been finalized as per personal communication with Larry Nitz who is GMs VP of Hybrid Powertrain Development.  Nitz also clarified that the engine will be naturally aspirated.

In the original Volt concept model the plan called for a 1.0 L 3 cylinder turbo engine.

Keep in mind, the only function of this E-85 capable engine is to fire up when the battery discharges to a level of 30% state of charge after 40 miles of EV driving.  When it runs, it will match the demands on the battery that are called for by the electric motor.  It will sustain the battery at that 30% state of charge, and will turn off at times (see graphic above).

The goal will be to get the driver home in the charge depleted state.  This way the driver can get the most benefit from charging.

Source ( GreenCarCongress ) thanks to Karik in the forum for the tip!

Graphic from MotorTrend, thanks to RB and Exp_EngTech for pointing it out!