Solving the Volt’s Extended Range Combined Mode Power Split

Remember when GM finally announced that the Volt did transmit power directly from the internal combustion engine (ICE) to the front wheels in extended-range (AKA charge sustaining) mode?

What a furor it caused! Some were outraged …

But GM did it for a reason: fuel efficiency!

How much of the road horsepower actually goes directly from the ICE to the front wheels at 70 mph? All of it, or just part of it?

We sometimes hear people refer to the Volt’s transmission as being a continuously variable transmission (CVT). Is it really a CVT? The answer is yes, it is continuously variable, but not in the same sense as most CVTs which have a continuously variable torque ratio.

The Volt’s transmission does not have a variable torque ratio but instead it has a continuously variable speed ratio. The torque ratio across the planetary gearset (PG) is fixed.

Consider the following schematic of the Volt’s powertrain in extended range (CS) mode:

Power from the ICE goes into the PG thru the ring gear. Power also goes into the PG thru the sun gear from the main traction motor (MGB). Power comes out the PG through the carrier and is transmitted to the wheels via a 2.16 gear reduction set in the final drive. The power coming out is simply equal to the sum of the power going in minus some small gear losses. The torque is split at a constant ratio depending on the tooth counts of the ring, carrier and sun gears within the PG. In the Volt’s case, the torque split is as shown in the following schematic:

This split is based on knowing the ratios between the ring and carrier with the sun gear locked and the ratio of the sun to carrier with the ring gear locked as shown below.

We know the speed and power out of the PG. The power out is simply the road load and the carrier speed is based on the vehicle speed, tire size and the gear reduction set. We can calculate the torque at the output of the PG based on the following equation: HP=torque (ft-lb) X speed (RPM)/5252

Using the 69/31 percent torque split we then have the torques at the input side of the PG. All we need now is the input side speeds in order to calculate the power split.

In order to figure the speed splits at this 70 mph condition we need to pick an ICE speed which feeds into the ring gear of the PG. I picked an ICE speed that results in the best specific fuel consumption (SFC) from the ICE at 27.3 horsepower (20.4 kw) which is the road load for this condition.

The speed that gives the best SFC at 20 kw out of the ICE is 2,300 rpm. That speed was chosen from a plot of horsepower versus engine speed for peak cycle efficiency (I used a Prius engine map).

Using the selected speed of 2,300 rpm for the ICE and 1,930 rpm for the output (carrier) speed we can solve for the sun (MGB) speed from the PG speed equation derived above. We also can split the torque using the 69/31 percent torque split. Using those speeds and torques we can calculate the power split.

The resultant power split is shown below:

As you can see, the resultant power into the PG almost exactly equals the optimum power for best SFC out of the ICE at 27.3 horsepower road load. In this condition in order to get better SFC out of the ICE we need to add another 4.5 horsepower to the ICE.

To do so, we simply add 4.5 horsepower worth of generator load. So here we have a near perfect match between the ICE and the road load – 82 percent of the power generated from the ICE is being transmitted mechanically directly to the wheels. The remaining 18 percent power is being converted into electricity by the generator and then being fed to the main traction motor.

Conclusion

The Volt is very efficient. It has the potential for Prius mpg in CS Mode with power split engaged.

Source: GM-Volt.com