"Only about 4,000 Chevrolet
"Only about 4,000 Chevrolet Volt data points were recorded, since they excluded trips taken when temperatures were below -4C (25F), since the Volt runs its combustion engine to augment battery thermal management and hence is not in EV-only mode."
2011 Volts and most 2012 Volts have no engine temperature setting, so it will cycle on and off at 25F or colder ambient temperatures.
The 2013 and newer Volts and some 2012s by default are set at 35F but have a "very cold" option in which the engine will only cycle on and off at temperatures of 15F or colder.
The ONLY reason why the engine does turn on when you still have charge in the battery is to keep the safety companies happy with the speed in which the windows have to defog at. The engine only helps with heat coming through the cabin vents, it does NOT heat the battery directly whatsoever, no engine coolant line or heat exchangers from the engine coolant goes into the battery pack. The battery pack has an 1800 watt resistive heater.
So, there is no need for the engine to come on, let the seat warmers keep you warm and perhaps some light electric heat through the cabin vents.
Also, you can DISABLE the engine in cold temperatures entirely by replacing the ambient temperature sensor with a fixed resistor so the car will always read a value above 35F all the time. It shouldn't screw up anything. There is a seperate air sensor for the engine system.
Google "Dealer resetting the ERDTT to the lower value on 2012s and earlier",
at the bottom post it has a link for a pdf showing instructions on how to do this. Replace sensor with a 1/2watt 15Kohm 5% tolerance resistor. User 'somms' of gm-volt figured this out. If you ever have to service the volt, swap the resistor for the sensor again so no warranty concerns.