Gasoline direct-injection (GDI) engines have higher fuel economy compared to the more widely used port fuel injection (PFI) engines. Although real-world fuel economy improvements from GDI technology alone are close to 1.5%, they can reach 8% by downsizing and turbocharging the engine, which can be achieved on GDI engines without loss of power compared to PFI engines. As a result, the market share of GDI-equipped vehicles has increased dramatically over the past decade and is expected to reach 50% of new gasoline vehicles sold in 2016. Widespread adoption of new engine technologies raises concerns about changes in emissions and their effects on air quality and the climate.

Recent studies have compared emissions of PFI and GDI vehicles, including particle number and mass, gaseous pollutants, and nonmethane organic gas (NMOG) composition for a limited number of compounds. However, many of these studies only tested very small fleets (including single vehicles), making it difficult to draw conclusions about the effects of widespread adoption of GDI vehicles on the aggregate emissions from the entire vehicle fleet because of the vehicle-to-vehicle variability in tailpipe emissions. There is substantial variability in vehicle-to-vehicle emissions due to differences in engine design (PFI, spray-guided GDI, wall-guided GDI, etc.), engine calibration (spark timing, valve timing, etc.), emission control technologies, and vehicle age and maintenance history.

… The EPA GHG program is aimed at reducing tailpipe CO₂ emissions. The increased fuel economy of GDI engines means lower CO₂ emissions per mile; however, higher BC [black carbon] emissions (the most-potent absorptive agent of anthropogenic PM) could potentially offset any climate benefits of reduced CO₂ emissions.… In this study, we present a comprehensive database of emissions from a fleet of GDI- and PFI-equipped light-duty gasoline vehicles tested on a chassis dynamometer over the cold-start unified cycle (UC). Measurements include gas- and particle-phase emissions, particle number, particle size distributions, and speciated NMOG emissions. We use the data to quantify the effects of engine technology, emission standards, and cold-start on emissions. We estimate ozone and SOA formation potential. Finally, we analyze the potential climate effects of switching a PFI to a GDI fleet.

—Saliba et al.

For the study, the team collected data from 82 light-duty gasoline vehicles spanning a wide range of model years (1988−2014); vehicle types (passenger cars and light-duty trucks); engine technologies (GDI and PFI); emission certification standards (Tier1 to SULEV), and manufacturers. All the vehicles were tested using commercial gasoline that met the summertime California fuel standards.

Measured PM mass (shown in box-whiskers), median EC (black diamond data points), and median primary organic aerosol (POA) (green circle data points) emission factors (mg/mi) for different vehicle classes. Horizontal lines indicate PM emission standards: current LEV2 regulation of 10 mg/mi; Tier3 and LEV3 regulations of 3 mg/mi; and LEV3 regulations of 1 mg/mi (starting 2025). Boxes represent the 25th to 75th percentiles, the horizontal line indicates the median, and the whiskers cover 99.3% of the data. Credit: ACS, Saliba et al. Click to enlarge.

Among the findings from the study:

• For vehicles certified to the same emissions standard, there is no statistical difference of regulated gas-phase pollutant emissions between PFIs and GDIs. However, GDIs had, on average, a factor of 2 higher particulate matter (PM) mass emissions than PFIs due to higher elemental carbon (EC) emissions.

• SULEV-certified GDIs have a factor of 2 lower PM mass emissions than GDIs certified as ultralow-emission vehicles (3.0 ± 1.1 versus 6.3 ± 1.1 mg/mi), suggesting improvements in engine design and calibration.

• Comprehensive organic speciation revealed no statistically significant differences in the composition of the volatile organic compounds emissions between PFI and GDIs, including benzene, toluene, ethylbenzene, and xylenes (BTEX). Therefore, the secondary organic aerosol and ozone formation potential of the exhaust does not depend on engine technology.

• Cold-start contributes a larger fraction of the total unified cycle emissions for vehicles meeting more-stringent emission standards. Organic gas emissions were the most sensitive to cold-start compared to the other pollutants tested. There were no statistically significant differences in the effects of cold-start on GDIs and PFIs.

For our fleet, increases in the fuel economy of 1.6% (0.5−2.4%; 95% confidence interval) are sufficient to offset warming due to increased BC emissions from GDIs. This is much lower than the measured 14.5% increase in fuel economy between PFIs and GDIs. Therefore, our data suggest that there will be a net climate benefit associated with switching from PFIs to GDIs, similar to previous results. However, the increased BC emissions from GDIs reduces their potential climate benefits by 10−20%. This reduction is likely larger in the real world because our increase in fuel economy (14.5%) between GDIs and PFIs is larger than that reported for on-road measurements.

—Saliba et al.

Resources

• Georges Saliba, Rawad Saleh, Yunliang Zhao, Albert A. Presto, Andrew T. Lambe, Bruce Frodin, Satya Sardar, Hector Maldonado, Christine Maddox, Andrew A. May, Greg T. Drozd, Allen H. Goldstein, Lynn M. Russell, Fabian Hagen, and Allen L. Robinson (2017) “Comparison of Gasoline Direct-Injection (GDI) and Port Fuel Injection (PFI) Vehicle Emissions: Emission Certification Standards, Cold-Start, Secondary Organic Aerosol Formation Potential, and Potential Climate Impacts” Environmental Science & Technology doi: 10.1021/acs.est.6b06509