By George Bower

Does anyone remember “double clutching?” I do. It was a method of shifting a crash box (i.e., non synchromesh) transmission to “synchronize” speeds before a downshift. For example, on a shift from 4th to 3rd gear you would shift from 4th to neutral, let out the clutch and raise engine speed in order to synchronize then push in the clutch and shift into 3rd. If you were really good at it there was no gear grinding and you could actually make the final shift without pushing in the clutch. There are 3 clutches in the 4ET50. In order to save wear and tear on the clutches, the Volt synchronizes the speeds of the input and output side of the clutch before engagement.

This synchronizing was captured by GM-Volt forum moderator WopOnTour (WOT) using the MDI data acquisition tool on an acceleration to 100 mph in Extended Range Mode found here . Each snapshot was then turned into a schematic so it is very easy to see what is going on. Not only do you see the clutches “synchronize” you also see the Volt shift from series mode to Power Split Mode ... Pretty cool stuff and totally unique to the Volt.

In the first snapshot, the Volt is going 50 mph during a slow acceleration. The car is in series mode during extended range operation. It is obvious from the schematic. You can see that the ICE is coupled to the generator and totally unlinked from the wheels. This is the beloved series mode everyone dreams about as being a pure series EREV ... The shift to power split occurs a few snapshots down from here.

First, WOT’s snapshot 1 taken w/ MDI (click on this and following images to enlarge):

Now the schematic:

In the next snapshot we are in transition to power split mode. Clutch3 opens and MGA speed drops to sync w/ ring gear which is at zero rpm (grounded).
First the MDI snapshot2:

And then the schematic 2:

In the next snapshot we are still in transition to power split mode. Clutch 2 closes to link MGA with ring gear which is still tied to ground at zero rpm.
MDI snapshot 3:

Schematic 3:

In the next snapshot 4, clutch 1 has opened, the ring gear is untied from ground and MGA and ring gear speed is increasing to synchronize with ICE speed.
MDI snapshot 4:

Schematic 4:

In snapshot 5, clutch 3 closes linking the ICE to MGA and the ring gear which is transmitting power directly to the wheels. Note however that MGA appears to be supplying the torque and power to the wheels (as opposed to the ICE).
MDI snapshot 5:

Schematic 5:

In the next MDI snapshot 6 taken at 70 mph, the transition to power split mode is complete. The ICE is supplying power directly to the wheels thru the ring gear. Note however that the torque into MGA from the ICE is higher than the torque out of MGA. This implies that MGA is supplying electricity out (Presumably to charge the battery).
Snapshot 6:

Schematic 6:

In the next snapshot 7, the Volt is going 80 MPH in power split mode. The ICE is supplying power to the wheels (with some power being taken off to charge the battery).
MDI snapshot 7:

Schematic 7:

In the final snapshot 8, The Volt is at 100 mph (wow!!), the ICE is supplying power to the wheels. Note however that the torque into MGA from the ICE is slightly lower than the torque out of MGA. This implies that power is being drawn from the battery to MGA and MGA is supplementing the ICE with additional power.
MDI snapshot 8:

Schematic 8:


The Volt is an amazing piece of engineering. There is not one other electric vehicle out their that can do what the Volt does. All the other EV’s are just “one speeders”. No other plug in hybrid allows one to drive all the way to 100 mph in all electric mode ... That however is in EV mode which would be another slide show for another day!

Cheers, Volt heads!!