Are lower levels of CO2 but higher black carbon emissions worth the climate trade-off of fuel-efficient engines?

The adoption of fuel efficient engines like the spark ignition gasoline direct injection (GDI) engine has gained popularity recently because of increasing regulations on CO2 emissions from fuel combustion. In 2011, the United States Corporate Average Fuel Economy standards tightened to reduce CO2 emissions. Between 2009 and 2015 sales of new vehicles with GDI engines have increased from five to 46 per cent in the United States, as an alternative to the traditional port fuel injection (PFI). It’s projected that by 2020 over 50 per cent of vehicles will be equipped with GDI engines.

New research from the University of Toronto’s Faculty of Applied Science & Engineering show how GDI engines may emit lower levels of CO2 but more black carbon, a climate-warming pollutant.

The study is published in the recent issue of Environmental Science & Technology by lead author Dr. Naomi Zimmerman and senior authors Dr. Greg Evans, professor in the Department of Chemical Engineering & Applied Chemistry and director of the Southern Ontario Centre for Atmospheric Aerosol Research, and Dr. Jim Wallace, professor in the Department of Mechanical and Industrial Engineering and director of the Engine Research & Development Lab.

Black carbon makes up a significant part of GDI engine exhaust. The particulate matter found in GDI exhaust forms from incomplete fuel mixing with air. There is less fuel-air mixing time in GDI engines as compared to PFI engines.

In this study, researchers evaluated different black carbon emission scenarios to determine the fuel economy improvements needed to offset the increase in black carbon emissions. They found that fuel economy improvements from 0.04 to 26 per cent are needed with GDI vehicles to offset the black carbon induced warming. This large range of fuel economy occurred because of the large uncertainty in the impacts of GDI engines on black carbon emissions.

The climate benefits of replacing PFI-engine equipped vehicles with GDI-engine equipped vehicles were investigated based on the global temperature potential (GTP) analysis. The researchers determined that replacing a pre-2010 PFI vehicle with a GDI vehicle would offer climate benefits within 20 years of using the GDI vehicle. But for new car purchases there’d be detrimental climate impacts to integrate current generation GDI vehicles since PFI vehicles outperform GDI vehicles in terms of fuel economy and black carbon emissions.

Previous studies have investigated the installation of gasoline particulate filters (GPFs) to reduce black carbon and total particle emissions from GDI vehicles. In this study, researchers report that installing gasoline particulate filters would have a net climate benefit, so long as the fuel penalty is less than one per cent and the filtration efficiency is greater than 80 per cent. They also suggest that modifications to the GDI engine design that promote less particulate matter and black carbon formation could help implement gasoline particulate filters with negligible fuel penalty.

Another factor influencing the black carbon emissions of GDI engines is the fuel properties, specifically the total aromatic content, which can vary with season and geographic area. For example California has specific regulations on the allowable aromatic content of gasoline. But such regulations do not exist federally in the United States or in Canada.

Looking ahead, the researchers expected that black carbon and particulate matter emissions will be significantly reduced as GDI engine technology develops and spray-guided injectors are used in these engines to improve the fuel-air mixing.


Assessing the Climate Trade-Offs of Gasoline Direct Injection Engines
Naomi Zimmerman, Jonathan M. Wang, Cheol-Heon Jeong, James S. Wallace, and Greg J. Evans Environmental Science & Technology 2016 50 (15), 8385-8392
DOI: 10.1021/acs.est.6b01800