[ad#post_ad]It seems very likely that the GM patent application "Output Split Electrically-Variable Transmission with Electric Propulsion Using One or Two Motors" describes the Chevrolet Volt transmission (which GM has called the Electric Drive Unit). The file is a pdf and you’ll need the free Adobe reader to view it:


Here is a snapshot of the title page:

I will apologize in advance to those of you who are less technically inclined. This posting contains a lot of engineer speak. I’ve tried to keep it as straightforward as possible, but this is a complex device. If you want help understanding how a planetary gearset works here is a link to help:


Figure 1 of the patent application is here:

It shows the internal combustion Engine connected via clutch C3 to Motor/Generator A which connects via clutch C2 to the Ring gear (labeled C) of a planetary gearset . The Ring gear also connects via clutch C1 to a stationary member of the transmission case. Motor/Generator B is connected to the Sun gear (labeled A) of the planetary gearset. The planet Carrier (labeled B) of the planetary gearset is connected to the Final Drive which, though this is not shown, is connected through the differential to the drive wheels of the car. (Don’t worry about the second, dashed, copies of Engine and M/G A, they just show an optional alternative configuration without clutch C3.)

The nature of a planetary gearset is that when the speeds of any two of the Ring, planet Carrier, and Sun gears are known the speed of the third is determined.

Figure 3a of the patent application is here:

It shows a truth table defining the available operating modes of the transmission. In the first three columns C1, C2, and C3 there is an X when that clutch is engaged or a blank when that clutch is disengaged. The rows of the table define six operating modes (plus a seventh, transition, mode).

In mode 1: 1 Motor Electric-only the Ring is held stationary and there is a fixed gear ratio from M/G B to the Final Drive.

In mode 2: Series we still have the fixed gear ratio from M/G B to the Final Drive. In addition, the Engine is connected to M/G A so that M/G A can start the Engine and the Engine can then drive M/G A to generate electricity for the battery and/or M/G B.

In mode 3: Output Split the Engine and M/G A work in unison to drive the Ring gear. This torque is blended with that from M/G B to the Final Drive. (This mode is just like the Toyota Prius’s Hybrid Synergy Drive.) Note that GM seems to have stated that the Volt never mechanically couples the Engine to the wheels. That would imply that mode 3 is never selected in the Volt, even though it could be. I don’t know what GM has really done in the Volt. We will presumably find out by the time the first Volts are delivered to dealers.

In mode 4: Neutral the Final Drive receives no torque since the Sun gear can spin freely.

In mode 5: Neutral / Battery Charge the Final Drive receives no torque, but the Engine is attached to M/G A and can drive M/G A to generate electricity for the battery.

In mode 6: 2 Motor Electric-only M/G A and M/G B are driven simultaneously and their torque is blended for delivery to the Final Drive. The combination of speeds of M/G A and M/G B determines the speed of the Final Drive. Alternately put, the speed of M/G A determines the gear ratio from M/G B to the Final Drive. This is the Electrically Variable Transmission (EVT).

You may ask how these six modes relate to the Charge Depleting (CD or Electric) mode of the Volt and to its Charge Sustaining (CS or Extended Range) mode. Here is my take:

In CD mode the 1 Motor Electric-only and 2 Motor Electric-only modes are used to optimize the electrical efficiency of M/G A and / or M/G B at the required road speeds. It is all about using the EVT to get the most miles of All Electric Range from the charge available from the battery. No gas is used.

In CS mode the Series mode is used to have the Engine, over time, drive M/G A to generate just enough electricity to keep the battery state of charge within the buffer range while M/G B is driving the wheels through the fixed gear ratio when the Ring gear is held stationary. As I said earlier, the Output Split mode could be used in CS but GM may have chosen not to do this for some reason.

Of course, there is no proof that the Volt's transmission is what this patent application describes, but the timing (filed September 10, 2007) is right and it fits the statement by GM public relations guy Rob Peterson that the Volt has a planetary gearset and a number of clutches. It also fits the statements Volt powertrain engineer Alex Cattelan made in her interviews with Lyle last November: http://gm-volt.com/2009/11/09/engineering-design-of-the-chevy-volts-two-electric-motors/ . And there is NASAman’s statement from Bob Lutz that "the Volt will have a transmission like no other" In addition, several people who've ordered Volts have posted the following description from the myvolt.com order tracking site:


In conclusion, if the patent application does describe the Volt's transmission it means that the Volt is NOT a single speed only transmission. It is an Electrically Variable Transmission (EVT). In addition, if the clutches C2 and C3 are engaged simultaneously there is a mechanical power path from the engine to the wheels. It may be that GM has chosen to never engage C2 and C3 simultaneously, but they could if they wanted to. (I don't know if that would be an infringement of Toyota's Hybrid Synergy Drive patents; I'm an engineer, not a patent lawyer.)

For more discussion of this patent see the following thread in the Engineering forum:


Lyle tells me GM has said it plans to publicly unveil this system soon after the cars are out there, so we should get a definite resolution of all these questions by then. In the meantime it sure is fun to speculate.