TransForum Vol. 9, No. 2

PHEVs Need Further Research for Acceptable Payback

Fuel Consumption as a Function of Distance

In order to double the fuel displacement obtained with a 4kWh battery, the battery size had to be quadrupled to 16kWh.

Aymeric Rousseau and his team at Argonne studied the impact of real-world drive cycles on the fuel efficiency and costs of different plug-in hybrid electric vehicle (PHEV) configurations. They found that while different PHEV configurations all demonstrated great potential for replacing gasoline (with less gasoline consumed as more electricity was used), the benefit of adding a larger battery seemed to decrease with increasing battery pack size.

“In general, the larger the battery, the more fuel saved,” said Rousseau, principal investigator of the vehicle modeling and simulation group. “But with every increase in battery energy, there was no proportional decrease in fuel consumption.”

The study used Argonne’s Powertrain System Analysis Toolkit^© (PSAT) for test simulation. The battery power was sized to follow a specific all-electric mode driving cycle to meet the all-electric range (AER) requirements. Results showed that the fuel saved by going from 4 to 8 kilowatt-hours (kWh) was much greater than the fuel saved going from 12 to 16 kWh (see chart).

Battery energy use improved with the smaller batteries. For the 4kWh battery, 100 percent of the usable stored energy was consumed by the vehicle. While using the 8kWh battery, only 93 percent of the available energy was consumed. Some available energy remained for the larger batteries due to shorter trips. However, the vehicle continued to carry the battery energy that was not used, and the added weight of the larger battery impacted the vehicle’s fuel efficiency.

Drive Cycle Description and Analysis

Real-world drive cycles were measured by the U.S. Environmental Protection Agency in a 2005 study in Kansas City, Missouri, with more than 100 different drivers participating. PHEVs produced in 2001 and later were instrumented and their driving statistics were collected for a day. Vehicle speed was collected independently from the conventional vehicles on a second-by-second basis through an onboard diagnostic port and a global positioning system device.

Key Findings

Since drive cycles had different characteristics based on distances driven, the benefits of each vehicle configuration were dependent on how far the vehicle was driven. While electrical consumption was similar for short and long driving distances because they depended more heavily on the electrical power, the main differences occurred during medium distance trips. With medium trips it was more difficult to measure the benefits since there were low and high electrical energy driving demands.

Based on these characteristics and cost assumptions, the cost of PHEVs remains high. For the average driver, results of a cost-benefit analysis showed that assuming an electrical cost of $0.09/kWh and a fuel cost of $4/gallon, it would take 8 to 12.5 years to recover the additional cost of the PHEV and 7.5 years of driving an HEV to recover that vehicle’s additional cost, compared to a conventional vehicle. More research and development are needed to improve the cost efficiencies of batteries and fuel.

A portion of this broad study was presented at the 2009 Advanced Automotive Battery Conference (AABC) in Long Beach, California, on June 11, 2009.

This research was supported by the Department of Energy’s Vehicle Technologies Program under the direction of Lee Slezak and Phil Patterson.

October 2009