PHEV testing is further complicated by the fact that these vehicles operate in two different modes based on the distance they are driven (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 first two complications, but one question that remains is 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. 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.

—NREL Research Engineer Jeff Gonder

In an effort to objectively predict on-road energy use of a PHEV, NREL developed and is proposing a method to adjust the standard test cycle results from each mode—charge depleting (CD) and charge sustaining (CS)—of PHEV operation. The adjusted values are then combined into a single fuel economy prediction.

The Blended Method...assumes that the increase in gasoline use during CD mode is the same as the increase calculated for CS mode. This works well for blended PHEVs that have lower electric power capabilities for CD mode, and would thus require additional engine power (blended with the electrical power output) for more aggressive driving. The downside of this method is that PHEVs with high electric power capabilities may not need help from the engine and therefore would not use more gasoline in CD mode but would simply deplete their battery energy over a shorter distance. It is also possible that a blended PHEV would actually increase its depletion distance in the event that the vehicle controller commanded the added engine output in CD mode to be high power (to achieve high engine efficiency) and thus prolonged its battery depletion.

Even so, such tradeoffs between CD fuel consumption and depletion distance should somewhat balance out through UF [utility factor] application. For instance, though this Blended Method for applying adjustments may penalize the high electric power PHEV with some excess CD fuel use, the method assumes a longer CD distance than the vehicle actually achieves. This gives it an inflated UF weighting for CD fuel displacement (relative to its CS fuel use). These two factors may roughly balance each other out when calculating the total combined consumption. A similar balance (in the reverse direction) could work out for the longer depletion distance blended PHEV.

—Gonder et al.

NREL has applied this technique to its PHEV analysis for several years, but until recently has not been able to validate it against data on a large number of PHEVs operating on the road. Partnering with the two other DOE laboratories provided that opportunity.

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). ANL had collected dynamometer test data on the same type of vehicle to evaluate PHEV test procedures over the standard certification cycle speed profiles.

This analysis provided a great example of collaboration among three DOE labs. NREL applied the adjustment technique we developed to PHEV data from ANL dynamometer testing and compared the fuel economy predictions to on-road data from INL’s large fleet evaluation effort. After accounting for how frequently the PHEV’s 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.

—Jeff Gonder

For the Hymotion Prius PHEVs, NREL’s method estimated fuel consumption of 4.2 L/100 km (55 mpg) and electricity consumption of 5.5 kWh/100 km (89 Wh/mi). This represents an average decrease in gasoline fuel consumption of roughly 20% relative to the comparable HEV Prius window sticker value.

The validation has so far only been possible on a single aftermarket conversion PHEV design—a Prius converted with a Hymotion pack. 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, NREL notes.

While this initial comparison is promising, further evaluation of the proposed adjustment method will be required on other vehicles and PHEV platforms. In particular, it will be important to evaluate how well the method predicts in-use performance for non-blended PHEVs that possess high electric power capability. As discussed, the proposed method utilizes a simplifying assumption that the PHEV will deplete its stored battery energy over the same distance in real-world cycles as in the dynamometer tests, which may not be the case in reality. This paper described how the UF application may help offset such differences, but further evaluation will need to confirm how well this works out.

—Gonder et al.

In the meantime, NREL plans to extend the analysis by simulating “virtual” fleets with a variety of PHEV powertrains operating on GPS driving profiles obtained from conventional vehicle travel surveys.

While this process seeks to predict the average on-road fuel and electricity use from a large number of PHEVs, fuel economy will vary greatly based on how the vehicle is driven and it will be important to educate PHEV drivers on how to obtain the best results, NREL notes.

The results of this research were presented at the recent 5^th IEEE Vehicle Power and Propulsion Conference (VPPC’09) in Dearborn, Mich. Gonder published an earlier paper discussing broader PHEV fuel economy reporting issues and serves on the SAE subcommittee (chaired by ANL) that will shortly be issuing an official recommended practice for measuring PHEV fuel economy.

Resources

• J. Gonder, A. Brooker, R. Carlson and J. Smart. Deriving In-Use PHEV Fuel Economy Predictions from Standardized Test Cycle Results (Conference Paper NREL/CP-540-46251, August 2009)