Understanding the Volt’s extended range mode

By George Bower
In order to understand extended range mode, one must also understand engine maps. The Volt’s engine map looks approximately like this:


Notice the term “SFC” or “specific fuel consumption.” The units of SFC are grams/hr of fuel flow per KW of output (g/kwh). Simply put, SFC is cycle efficiency. The conversion is: (g/kwhX.0119=1/cycle eta).
As you can see from the map, SFC is a function of where you are operating on the map. The Volt’s unique powertrain allows it to operate along a line of peak efficiency. This “operating line” is also shown on the engine map in yellow.
The engine map makes more sense if we cross plot it along its operating line. HP and SFC as a function of engine speed are presented below:


We can learn a lot from this plot. First, we see the engine’s “sweet spot” which occurs at 2,200 rpm. The Volt doesn’t operate exclusively at this one sweet spot as many different power levels are required throughout its operating envelope.
To see some of these operating points, road loads (for a flat road) are overlaid on the map. Note that at 70-75 mph the road load is right on the sweet spot.
Now notice what is happening at 50 mph: Bad SFC. So what is the Volt to do? How should it be programmed to operate at this condition?
There are two options:
1) Run in series mode and charge the battery with the ICE on its sweet spot, or
2) Run with power split engaged AND charge the battery. (Then after charging the battery to the top of its window shut the ICE off and run on the battery for a few miles.)
As it turns out, the Volt chooses option 2. This operating mode has been observed by our own GM-Volt forum expert, saghost (Walter) using the Dashdaq data acquisition device.
Over the years I developed a simplified math model of the Volt. This math model’s core is equations for road load, equations that describe the speed relationship and torque split of the planetary gear set, and also the engine map we have just discussed.
Using this math model, I get a solution for condition 2 (above) which looks like this:


Note at this condition:
1) Power Split is engaged and we are delivering power directly to the wheels from the ICE AND at the same time we are charging the battery.
2) The ICE is operating at its sweet spot.
3) MGB is stopped.
Walter has a nickname for this condition. He calls it “Virtual 6th Gear”
As an old gear head, I think that’s a pretty appropriate term.
Don’t you?
Cheers Volt Heads!!!!


Source: GM-Volt.com