Why Ford chose PowerSplit instead of Volt's Serial Plug-In

Chevy Volt and Ford C-Max Energi

There is no doubt that electrification of transportation has started. We are now in the middle of the Perfect Storm of Climate Change, Peak Oil and Foreign Oil dependence that make electrification inevitable. Given this, different auto majors are adopting different strategies. Nissan is starting with pure electric vehicles. GM has started with Serial Plug-In they call EREV. Ford has pure electric and PowerSplit Plug-Ins i.e. C-Max Energi we talk about here.

So, why did Ford decide on PowerSplit Plug-Ins rather than a Serial Plug-In like Volt ?

Sherif Marakby is Ford's director for Electrification Programs and Engineering. In a 2009 interview he talks about why Ford thinks PowerSplit architecture is better than a Serial (EREV) architecture of GM's Volt.

The difference comes down to architecture. The Volt is a series hybrid, meaning that the car is powered predominantly by electricity. The gas engine largely exists to recharge the battery. Ford's plug-in will be a power split hybrid with the gas engine and electric motor working in tandem to propel the car.

According to Marakby, the main reason why PowerSplit is better is that it can use a smaller battery. In a Serial hybrid, the electric motor and thus the battery needs to do all the work in the EV mode (or Charge Depletion or CD mode). The motor needs to be able to propel the car at the maximum rated speed (above 90 mph) and also give all the needed acceleration even at high speeds on the freeway.



GM Volt Power Train


In Volt, for eg., this means the battery & the motor need to be hefty. Volt battery has a capacity of 16 kwh and the motor generates a hefty 149 HP.

In Energi, Ford can use less battery and the motor can be smaller since it doesn't have to provide all the power in high speed and other high demand situations. In other words Energi's electric system needs to be able to handle just the most frequent usages rather than the extreme.

"You don't have to size the battery for the worst case scenario," he said. "The battery will be lower in cost."
"It is a better value," he added later.

The other point to note is that Ford can reuse components developed for other hybrids when using PowerSplit architecture. The big difference between Fusion / C-Max hybrids and Energi, for eg., is the bigger battery and the components needed to charge it from the grid. This enabled Ford to reduce cost by sharing development and manufacturing costs.

"You want to do it on a large scale," Marakby said. "We want to make sure that the technology is real. The durability, the testing have to be done right."

It is interesting to note that Volt didn't meet one of the things Marakby said.

Power split hybrids generally speaking tend to get lower gas mileage than series hybrids

Volt gives about 35 mpg in CS (charge sustaining) mode compared to Prius Plug_ins expected 50 mpg and 40+ for Energi. This is mainly because of the extra weight of Volt's larger battery and the fact that GM seems to have decide to develop a "sporty" car. They used a normal engine rather than one that uses Atkinson cycle - this gives higher power but lower mileage.