A detailed look at regeneration

By George S. Bower

The Route

 
 

Some may have read a previous article I wrote describing the modeling of my daily route between my home in Tonto Basin, Ariz. and Payson, Ariz. described earlier. This route involves climbing and descending a fairly steep, 5.6-percent grade. When I modeled the route I assumed 80 percent regeneration efficiency.
 

 

Calculated regeneration efficiency from the data is 83.8 percent, somewhat better than was modeled.

Volt captures enough energy during descent to drive another four miles on battery alone with ICE shut down

 
 

 

The route profile with significant milestones is shown in figure 1. The descent was 1,700 ft in altitude in 5.6 miles for a 5.6% grade. Total energy recovered from the descent was 0.96 kwh. This captured energy was then used by Volt to travel another 4 miles with the ICE off.

Volt shifts to single motor and then to 2 motor

 

This was the interesting part, and came as somewhat of a surprise to me. The shifts are presented in figure 2. (The descent was in Drive not Low gear shift position). There’s a lot of excruciating details in the frames presented here. As a general overview, one can see the steady state condition prior to the descent in Frame 1 (extended range mode, power split engaged). In frame 2 we begin transition to single motor. In frame 4, 0.9 seconds later, we are in single motor and have begun regeneration. Volt then completes the shift to 2 motor in frame 11, 10.5 seconds later and continues in 2 motor mode for the remainder of the 6 mile, 6 minute descent.

Important to note:

1) Despite taking a seemingly long 10.5 seconds to get into 2 motor, regen is initiated quickly in only 0.9 seconds.

2) All shifts (clutch closings) occur at zero slip resulting in NO CLUTCH WEAR.

Detailed discussion of the shift

 
 

Frame 1 shows the PG set configuration just prior to the descent. Volt is in extended range mode with power split engaged and MGA is charging the battery to rebuild its 0.5-kwh buffer.
Transition to single motor begins in frame 2. Clutch 3 opens releasing the ICE from the PG set. Interesting that the ICE holds constant speed AT NO LOAD as other shifting occurs within the PG set.
After opening C3 and releasing MGA from the ICE, MGB speed is driven up which forces MGA speed down. The purpose here is to get MGA/ ring gear speed to 0 rpm so we can close clutch 1 and ground the ring gear (at which point we will be in single motor).

In frame 4 after 0.9 seconds, we get our first regen. At this point we have driven MGA/ring gear to 0 rpm and closed C1.
 

 

In frame 5, C2 opens freeing MGA from the ring gear so it can speed up and grab the ICE which is still running at 1,850 rpm, no load. This speed increase is shown in frames 6 and 7.

In frame 7 MGA has synced w/ the ICE at 1,855 rpm and locked on by closing C3. By frame 8, MGA/ICE have spooled down to 0 rpm and the ICE SHUTS DOWN. Note that while all of this is happening we are regenerating at 27 A.

In frame 9, MGA latches onto the PG set (again at 0 slip) by closing C2. C1 opens to release the ring gear from ground so MGA/ ring gear are free to speed up to the final 2 motor configuration.
Transition to 2 motor is complete in frame 11. In this configuration we are at 1,220 rpm and 2,730 rpm on MGA and MGB in 2 motor versus 5,286 rpm for pure single motor shown in frame 8.
Volt chooses 2 motor for the remainder of the descent.

In summary, Volt shifted to single motor and began regen in only 0.9 seconds, then completed transition to 2 motor in 10.4 seconds where it remained for the remaining 6 minute regen.

The 64 million dollar question

 
 

You might ask:

Why doesn’t the Volt just shift right to 2 motor? Why bother shifting to single motor first if the final configuration is 2 motor?

I don’t know … It just does.

As forum moderator and Volt Guru Walter (saghost) says: “The Volt is full of surprises.”

I would concur on that statement.

So I will leave it up to all of you Volt enthusiasts: Why does the Volt operate in this way?

Source: GM-Volt.com