Op-Ed: More All Electric Miles Equal Less Range Extender Need For 2nd Gen Chevrolet Volt
I think we would all agree that, as AER increases, the need for a range extender decreases. This is depicted graphically in figure 1.
If we have an EV with 200-300 miles range, no range extender is required: witness Tesla Model S. If we have an EV with 100 miles range, a small range extender will suffice: witness BMW i3.
How does the Volt fit into this picture? We need to put some numbers to the words. (Engineers love numbers..yes?)
What Does Voltstats Say?
GM pitched the Volt by saying: “We will just make the battery big enough to cover the daily driving distance of the average driver of 40 miles.”
But how many miles does the average fleet driver really drive in a day?
Referring to figure 2, we see a high concentration of drivers at the 50 mile mark using the range extender. Also, referring to figure 3 we see an average fleet MPG of 135 miles/gallon.
In figure 4 the AER/ER split is calculated that gets 50 miles total range and 135 MPG. The breakdown of the average fleet driver is: 36 miles on electricity and 14 miles on gas for a total of 50 miles. The 36 mile AER is probably less than 40 because some of the fleet miles are in cold weather.
Using this average driving cycle, we can now derive partial derivatives (influence coefficients) for the effect of AER and ICE cycle efficiency on fleet fuel consumption.
Figure 5 shows that adding only 5 miles of AER has a HUGE effect on fleet fuel consumption equaling a whopping 36% savings in fuel.
How does this compare with a more efficient range extender? Often discussed is a turbo 3 cylinder for Volt Gen 2. It is expected that the turbo 3 would have approximately 10% better cycle efficiency than the current Volt ICE.
The comparison of the influence coefficients is presented in figure6.
Five more miles of AER decreases fuel consumption by 36% while going to a Turbo 3 cylinder decreases fuel consumption by only 10%. The influence of AER is nearly 4 times as great as the influence of the Turbo 3.
Keep in mind that one of the goals for gen 2 Volt is to reduce costs. GM’s turbo 3 undoubtedly will be a pricy piece of equipment compared to the current 1.4, 4 cylinder. Is this costlier range extender justified? I say no. One would be better off by increasing AER instead.
But you say: “How much does another 5 miles AER cost? Batteries are exensive. Wouldn’t it be cheaper to go with the turbo 3 than with expensive batteries?” Again I say no. The cost of another 5 miles AER is zero. All we do is switch to Spark EV chemistry which is more robust and has a higher cycle life. We increase DOD to achieve the added 5 miles of range. Spark EV DOD is 80% versus 65% for the Volt. That is a 23% increase and it is worth an equivalent percentage increase in AER of 9 miles. Spark EV battery article found here (http://gm-volt.com/2013/08/02/spark-ev-versus-volt-battery/).
A more expensive range extender is not justified.
Volt Gen 2 would benefit from a smaller, lighter pure series range extender. Not only do we save money on the range extender, we save money on the transmission. We can eliminate the costly 2 mode transmission we have now.
We could have a Gen 2 Volt that is:
- Lower cost
- Lower weight
- Higher performance
- More Interior room
It is hard to argue against that combo.