There is very little quantitative data on the fuel and emissions benefits of HEVs during real-world driving. Quantitative data on actual, real-world, in-use HEV carbon dioxide (CO₂) emissions and fuel consumption are essential to validate projections on transportation energy use and for accurate regulatory mobile source emission models such as EPA’s Motor Vehicle Emission Simulator (MOVES). Prior studies are not well-suited to forecast the magnitude of current-technology HEVs on transportation energy use because the data were from laboratory studies of one or two early model HEVs (i.e., Toyota Prius, Honda Insight) compared to conventional vehicles (CVs) of dissimilar make/model and vehicle size.

… We compare real-world tailpipe emissions data for one HEV to the same manufacturer and model year CV counterpart to quantify the CO₂ emissions and fuel consumption benefits of the HEV during real-world driving across all northern Vermont seasons on terrain that includes steep and rolling hills. Steep positive road grade and cold ambient temperatures both adversely affect HEV operation. Our results with one vehicle pair demonstrate that similar studies can be used to develop robust models of all types of HEV platforms under actual driving conditions and improve quantitative estimates of the future fleet contributions to the US CO₂ emissions inventory and petroleum consumption. Studies such as these are critical as HEVs and other electrified vehicles (i.e., PHEVs and BEVs) comprise a larger portion of the on-road fleet.

—Holmén and Sentoff (2015)

Advances in portable emission measurement systems (PEMS), on-board diagnostics (OBD) and GPS logging technology have enabled the collection of time-resolved vehicle data and tailpipe exhaust emissions under real-world operating conditions for a large proportion of the vehicle fleet. Techniques to reliably link measured vehicle location to road grade are still needed, however, for robust fuel consumption and tailpipe emissions estimates using a Vehicle Specific Power (VSP) framework, the authors said.

(VSP, proposed by José Luis Jiménez-Palacios in his doctoral dissertation at MIT in 1999, has been since widely used to quantify the impact of vehicle operating conditions on emission and energy consumption estimates that are dependent upon speed, roadway grade and acceleration or deceleration on the basis of the second-by-second vehicle operation. VSP has been incorporated into a number of emission models including EPA’s MOVES.)

For the study, the team used its total on-board tailpipe emissions measurement system (TOTEMS) instrumentation suite to collect 1-Hz vehicle performance and emissions data from two 2010 Toyota Camry vehicles—one conventional, one hybrid—over an 18-month sampling period. A single driver (24-year-old female) operated both vehicles along the same figure-eight shaped route, comprising 25 km (15.5 miles) “outbound” driving through downtown Burlington, Vermont (“city”) and interstate highway I-89 to Richmond, VT (“highway”) and returning via 26.5 km (16.5 miles) of rural and suburban arterial roads (“inbound” section, all arterial) driving.

Road grades varied from −13.2 to 11.5% with the majority of travel between ±4% grade.

The recorded 1 Hz data for vehicle speed (v), acceleration (a), road grade (grade) were combined with vehicle constants—coefficient of drag (CD), vehicle cross-sectional area (A), and vehicle mass (m), a rotating mass factor (ε), coefficient of rolling resistance (CR)—as well as air density (ρa)—computed from measured air temperature, to quantify instantaneous VSP to enable comparisons between the conventional and hybrid vehicles.

With real-world conditions included in the VSP-derived fuel consumption rates, the researchers found that fuel use computed for the conventional vehicle on the regulatory FTP cycle was 29% higher than the EPA sticker value predicted for city driving and 96% higher under highway driving.

Fuel consumption benefit factor by VSP bins (1 kW/ton resolution) (note log scale on y-axis). Credit: ACS, Holmén & Sentoff. Click to enlarge.

For the HEV, the EPA city cycle fuel consumption estimate agreed within 2% of the real-world prediction, but HEV real-world fuel use was 66% higher than the EPA estimate under highway driving. (Steep positive road grade and cold ambient temperatures both adversely affect HEV operation.)

Our real-world VSP-specific FCR measurements suggest computed FTP (city) cycle CV fuel use would be nearly 2 times higher than for the HEV and 1.3 times higher than the HEV for HWFET (highway) driving. These ratios exceed the factors that EPA fuel economy values would predict for these vehicle types. Thus, inclusion of real-world road grade and ambient conditions in our VSP and emissions measurements, compared to controlled laboratory tests, show fuel consumption benefits for the HEV increased 18% and 33% (highway and city, respectively) compared to estimates based on EPA adjusted fuel economy “sticker” values. Thus, the real-world fuel economy benefit of adoption of HEVs may be better than for estimates based on EPA “sticker” fuel economy alone.

—Holmén and Sentoff (2015)

Resources

• Britt A. Holmén and Karen M. Sentoff (2015) “Hybrid-Electric Passenger Car Carbon Dioxide and Fuel Consumption Benefits Based on Real-World Driving” Environmental Science & Technology doi: 10.1021/acs.est.5b01203

• Zhai, H.; Frey, H. C.; Rouphail N. M. (2011) “Development of a modal emissions model for a hybrid electric vehicle,” Transp. Res. Part Transp. Environ. 16 (6), 444−450 doi: 10.1016/j.trd.2011.05.001

• Jimenez, J. L. (1999) “Understanding and Quantifying Motor Vehicle Emissions with Vehicle Specific Power and TILDAS Remote Sensing,” Doctoral Dissertation, Massachusetts Institute of Technology