By George S. Bower and Bill Rollins

The new Gen 2 power train gives increased all electric range and improved fuel consumption in extended range mode, lower cost, and it is scalable to almost all other GM vehicles as either a simple hybrid or as an extended range vehicle. At the time of this writing GM has included it in the new Malibu hybrid and potentially the new Cadillac CT6. Undoubtedly there are other vehicles in the pipeline destined for this power train.



The power train consists of many subcomponents of an electrical and mechanical nature: the battery, the inverters, the electric motors, the transmission and the internal combustion engine (ICE). How much did the individual parts improve?


Range Extended Mode




Figure 1: Contributors to Gen 2 Volt MPG Improvement

The first thing we notice from looking at figure 1 is there were significant improvements in almost all of the power train sub systems with inverters topping the list followed by the motors and transmission. The transmission improvements were probably a result of adding fixed gear and high extended range (compound split) modes to the transmission. Also we know that Gen 2 no longer has a pure series mode like Gen 1 which adds efficiency as well. The authors do not have Gen 1 motor maps for comparison but we do have Gen 2 motor maps. The new larger motor B has a very large maximum efficiency island. The maximum efficiency of motor B is 96% and the motor will be operating at this sweet spot around town.
The other interesting thing to note is that the ICE efficiency only improved on the order of 3%. This came as somewhat of a surprise as I had expected around 10% efficiency improvement.


EV Mode



Figure 2: Contributors to Gen 2 Range Improvements

The biggest contributor to the Gen 2 range improvement is the battery which contains 27% more usable energy. In the city, AER increases by more than the 27% battery energy due to efficiency improvements in other sub systems. FTP composite increased more than 27% as well. However, only 22% range improvement was noted in the FTP HWY cycle.

What stands out when looking at the sub system contributors in figure 2? Once again we see the inverters as being the biggest winner with 5-7% efficiency improvement. We see a 3% reduction in motor losses in the FTP city cycle. However, we note a slightly worse situation in Gen 2 in the motor and transmission losses in FTP HWY. We have improved 12V accessory loads but a slightly worse vehicle drag and tire losses. This implies that the Gen 1 city EV rating of 101 MPGe should improve for Gen 2, however, the HWY EV rating for Gen 2 could be less than the Gen 1 rating of 93 MPGe.

The above scenario can be explained by examining the individual motor maps in the Gen 2 Volt.


Figure 3: Larger Motor B Map

We see from the motor map that motor B has great peak efficiency (96%) and a very large efficiency island centered at lower speeds and torques which occur during city driving


Figure 4: Smaller Motor A Map

The smaller motor A, on the other hand, has its peak efficiency island at higher speeds where motor B does not. Thus when we use both motors they complement each other. As motor B falls off in efficiency, motor A can be added in to compensate.

As noted earlier, we have a slightly lower efficiency in Gen 2 in the motor and transmission department during HWY operation. The following is the author’s speculation as to why. As said earlier we do not have Gen 1 motor maps for a direct comparison so we must speculate. We do have Gen 2 motor maps however. All the gear ratios in the PG set and final drive have been spelled out by GM. We know the tire sizes so we can easily calculate the motor speeds for any vehicle speed.


Figure 5: Motor B Speeds as a Function of Vehicle Speed.

We see from figure 5 that at lower vehicle speed, around 30 MPH, motor B is sitting on its sweet spot at 96% efficiency. This is why motor losses improved in the city cycle. However, as vehicle speed increases we start to fall off the end of the island….ie motor efficiency is decreasing. By 70 MPH motor efficiency has fallen 6-7% and is below 90%. This would explain why Gen 2 motor losses are worse than Gen 1 in EV mode during HWY driving.

In EV mode, the new Gen 2 transmission runs a fixed gear ratio resulting in higher motor speeds than Gen 1 where the motors could mix.

Figure 6: Gen 1 Motor Speeds, Steady 60 MPH

Figure 6 shows Gen 1 motors operating at low speeds compared to Gen 2. Motor A is running at 1100 RPM and motor B is running at 2900 RPM. This is based on Dashdaq data acquisition tool in the author’s 2012 gen 1 Volt. For Gen 2 at 60 MPH, motor A spins at 6200 RPM while motor B spins at 6600 RPM.The authors are speculating then that higher motor speeds in Gen 2 are what is causing the slightly worse motor losses in Gen 2 in the HWY cycle.


Conclusion


Sometimes GM is accused of taking the high tech, high cost approach when solving a problem or inventing a new product with the Gen 1 Volt being at the top of the list. We can see from the article today that there were many tradeoffs involved in designing the Gen 2 Volt.

Could GM have gotten higher AER out of Gen 2 if they had used the Gen 1 transmission design? The opinion of the authors is that yes they could have gotten slightly better AER in the FTP HWY cycle with the old Gen 1 transmission. However they had to make tradeoffs. Life, like engineering, is full of tradeoffs. Nothing is ever black or white. In the case of Gen 2 Volt some small sacrifices had to be made in order to meet cost bogies for Gen 2. However,GM also developed a new transmission that can be used throughout their product line as either an EREV, like the Volt or, as a simple hybrid in the new Malibu: an equitable trade in the author’s opinion.

Reference: SAE Paper 2015-01-1152: “The Next Generation Voltec Extended Range EV Propulsion System”, Conlon, Blohm et all General Motors dated 4/15/2015