In the segment of the study presented, the Argonne team modeled four PHEV configurations: an input power split with a fixed ratio between the electric machine and the transmission (e.g., Camry hybrid) PHEV with a 4 kWh and an 8 kWh pack; and a series hybrid (extended-range electric vehicle, e.g., Volt) with a 12 kWh and a 16 kWh pack. These were compared to a conventional HEV, using the same hybrid drive as the power split PHEV, and a conventional combustion engine vehicle.

For drive cycles, the Argonne team used the Kansas City data collected in 2005 by the US EPA, which instrumented more than 100 drivers and collected their driving statistics for one day.

The PHEVs used different control strategies:

• EV/CS (Thermostat) strategy for the series configuration. Here the controller drives as long as possible using battery energy, depleting the state of charge (SOC) from 90% to 30%. The engine turns on only if the road load exceeds the power capability of either the battery or the motor. Once the battery reaches charge sustaining mode (regular hybrid mode), the engine is to regulate the SOC.

• Load engine power strategy. An SOC-based power threshold is used to urn the engine on. As a result, the engine can be turned on during charge depleting mode. To maximize charge depletion, the engine only provides the requested wheel power without recharging the battery.

• Optimum engine power strategy. Similar to the Load Engine strategy, the engine is turned on based on a threshold. Here, however, the controller attempts to restrict the engine operating region close to its peak efficiency. As a result, the engine might be used to recharge the battery during charge depletion.

Basic comparison of fuel consumption showed mean values of:

• Basic conventional: 6.6 L/100km (35.6 mpg US)
• Hybrid (HEV): 4.69 L/100km (50.2 mpg US)
• Split 4 kWh: 3.27 L/100km (71.9 mpg US)
• Split 8 kWh: 2.32 L/100km (101.4 mpg US)
• Series 12 kWh: 1.50 L/100km (156.8 mpg US)
• Series 16 kWh: 1.23 L/100km (191.2 mpg US)

The larger the battery, the more fuel saved. However, what we also noticed was that the delta for consumption is not linear. The fuel we save by going from 4 to 8 kWh is much greater than the fuel saved going from 12 to 16.

—Aymeric Rousseau

On top of the significant gain achieved by using a standard HEV compared to a conventional vehicle (27.6%), the 4 kWh configurations adds an additional 20%. The gains from adding further battery capacities decrease when going from 8 to 16 kWh, with only a 10% increase from 12 to 16 kWh. Looked at another way, 4 kWh of battery energy provides 50% of the fuel displacement gains achieved with a 16 kWh battery.

The Argonne team found similar electrical consumption across the PHEV options for short distances. The largest discrepancies are found with medium distances of 15-25 miles; these drive cycles are characterized by both low and large power demands. While the split 8 kWh option will have an engine on event, the series configurations will continue to operate in all electric mode.

Cost benefit analysis. Based on the results, Argonne also modeled out the cost-benefit of the different configurations. Some of those findings included:

• Assuming an electrical cost of $0.09/kWh and a fuel cost of $4/gallon, the HEV breaks even at 7.5 years, while the PHEVs range from 8 to 12.5 years.

• A longer daily drive distance can significantly reduce payback time. Based on the assumption considered, one should drive at least 30 miles per day to have an acceptable payback (4-6 yeas for small energy batteries and 6-8 years for larger battery energies).

• HEVs are more cost-effective than the split 4 kWh for driving longer than 30 miles, but the order is reversed for shorter distances.

• When driving long distances (> 40 miles), both series configurations achieve similar payback since the additional battery cost is offset by fuel efficiency benefits.

• Compared to the HEV powertrain, payback is close to 8 years for low energy batteries and 11 years for larger batteries.

• An increase in fuel price from $4 to $5/gallon decreases the payback time by one year on average.

The group is looking at additional drive cycles, Rousseau said.

Based on the assumptions considered, for the mid-term, the cost of PHEVs remains high, requiring further research and development for batteries and electric vehicles.

—Aymeric Rousseau

Resources

• Aymeric Rousseau et al. (2009) Impact of Real World Drive Cycles On PHEV Fuel Efficiency and Cost for Different Powertrain and Battery Characteristics (presented at AABC 2009)