Washington, D.C. – The U.S. Department of Energy (DOE) has joined with researchers to develop and test a new method for predicting the real-world fuel and electricity consumption of plug-in hybrid electric vehicles (PHEVs). 

The DOE National Renewable Energy Laboratory (NREL) is working with the Idaho National Laboratory and Argonne National Laboratory. After examining data, the NREL-developed method shows promise for reasonably predicting the PHEV’s average fuel and electricity use. 

Current rules for estimating miles per gallon on conventional vehicles do not work for PHEVs because they run on both electricity and gasoline. Testing is further complicated by the fact that these vehicles operate in two different modes, initially depleting energy from the large vehicle battery, and eventually sustaining the battery charge for longer-distance driving. Consensus is building on techniques to handle these complications, but a question remains of how to adjust raw certification cycle test results to best predict a PHEV’s average real-world energy use.

“Official fuel economy testing for all vehicles is conducted on chassis dynamometers, which are basically treadmills for cars and trucks,” said Jeff Gonder, NREL research engineer. “One subtlety of chassis dynamometer testing is that vehicle fuel economy measurements using decades-old standard speed profiles may be overly optimistic compared to today’s average on-road fuel use. Official methods exist to adjust the test cycle fuel economy of conventional vehicles to better estimate expected real-world fuel use, but a similar adjustment method has yet to be finalized for PHEVs.”

The Idaho National Laboratory (INL) monitors fleet fuel use of advanced technology cars as part of DOE’s Advanced Vehicle Testing Activity, and has accumulated more than a year’s worth of data on roughly 100 PHEVs of the same design. Because of limited purpose-built PHEV availability, these vehicles are production hybrid vehicles modified with aftermarket PHEV conversion kits. NREL applied its adjustment technique to dynamometer testing and compared the fuel economy predictions to on-road data from INL’s fleet evaluation. “After accounting for how frequently the PHEVs in the INL-monitored fleet actually plug in, we found excellent agreement between the adjusted test cycle predictions and the actual fleet fuel and electricity use.”

While the finding is promising, it will be important to repeat the analysis once dynamometer testing and substantial on-road fleet data becomes available for different PHEV designs, particularly those with greater electric driving capability. Fuel economy will vary greatly based on how the vehicle is driven, and the DOE said it will be important to educate PHEV drivers on how to obtain the best results.


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