The gasoline direct injection (GDI) engine is one of the most prominent technologies car manufacturers adopted to achieve the fuel economy and carbon dioxide emission goals established in 2012 by the US Environmental Protection Agency. The market share of GDI-equipped vehicles increased from 2.3% in model year 2008 to 51% in model year 2018. The EPA projects 93% of vehicles in the U.S. will be equipped with GDI engines by 2025.

However, while this technology boosts fuel efficiency and reduces CO₂ emissions, GDI engines also produce more black carbon aerosols than traditional port fuel injection (PFI) engines.

Unlike the conventional port fuel injection (PFI) engines which mix fuel and air prior to injection into the engine cylinders, the GDI technology involves spraying the fuel directly into the cylinders, allowing for higher compression ratios. As a result, GDI engines achieve higher combustion efficiencies compared to their PFI counterparts, leading to the enhanced fuel economy and consequently, reduced CO₂ emissions by up to 14%.

However, similar to diesel engines, the direct injection of fuel in GDI engines creates fuel-rich pockets near the injection zone, and the combustion conditions in these pockets are conducive to formation of carbonaceous particulate matter (PM), especially black carbon (BC). Consequently, GDI engines emit larger amounts of BC compared to PFI engines, as has been confirmed by several laboratory studies.

—Neyestani et al.

A strong absorber of solar radiation, black carbon exhibits significant climate warming properties.

In a study published in the ACS journal Environmental Science and Technology, a team of researchers at UGA predicts the increase in black carbon emissions from GDI-powered vehicles will fuel climate warming in urban areas of the US that will significantly exceed the cooling associated with a reduction in CO₂.

… the increase in BC associated with the shift would lead to an annual average positive radiative effect over the U.S. of approximately +0.075 W/m², with values as large as +0.45 W/m² over urban regions. On the other hand, the reduction in CO₂ emissions associated with the enhanced fuel economy of GDI vehicles would yield a globally uniform negative radiative effect, estimated to be −0.013 W/m² over a 20 year time horizon. Therefore, the climate burden of the increase in BC emissions dominates over the US, especially over source regions.

—Neyestani et al.

Credit: ACS, Neyestani et al.

In addition, they believe the shift will nearly double the premature mortality rate associated with vehicle emissions, from 855 deaths annually to 1,599. The researchers estimate the annual social cost of these premature deaths at $5.95 billion.

Even though emissions from gasoline vehicles constitute a small fraction of the black carbon in the atmosphere, the vehicle emissions are concentrated in regions with high population densities, which magnifies their effect.

—Rawad Saleh, an assistant professor in UGA’s School of Environmental, Civil, Agricultural and Mechanical Engineering and the study’s principal investigator

The increase of black carbon is an unintended consequence of the shift to GDI-equipped vehicles that some scientists suspected was based on experimental data, according to Saleh. He says the UGA study is the first to place these experimental findings in a complex modeling framework to investigate the trade-off between CO₂ reduction and an increase in black carbon.

While previous research has reported the shift to GDI engines will result in net benefits for the global climate, the UGA researchers say these benefits are rather small and can only be realized on timescales of decades. Meanwhile, the negative impact of black carbon can be felt instantaneously.

Our research shows the climate trade-off is much different on the regional scale, especially in areas with high vehicle densities. In these regions, the climate burden induced by the increase in black carbon dominates over the climate benefits of the reduction in CO₂. The study concludes the social cost associated with the acute localized climate burden and public health impacts induced by GDI vehicles largely overweigh their marginal global climate benefits.

—Rawad Saleh

While a quantitative economic analysis that weighs the social costs associated with these impacts against the benefits associated with the reduction in CO₂ emissions is beyond the scope of this work, our results provide strong evidence that such analysis is necessary for developing robust policy vis-à-vis the position of GDI in the landscape of future vehicle technologies.

We note that the results obtained in this study do not account for the potential of incorporating gasoline particulate filters (GPFs) for reducing BC emissions from GDI vehicles. GPFs impose a penalty on fuel economy and are subject to technical challenges that need to be resolved before reliable practical implementation. Our results suggest that the development and implementation of GPFs is crucial for GDI engines to be a viable solution for enhanced fuel economy without compromising air quality.

—Neyestani et al.

Resources

• Rawad Saleh

Resources

• Soroush E. Neyestani, Stacy Walters, Gabriele Pfister, Gabriel J. Kooperman, and Rawad Saleh (2020) “Direct Radiative Effect and Public Health Implications of Aerosol Emissions Associated with Shifting to Gasoline Direct Injection (GDI) Technologies in Light-Duty Vehicles in the United States” Environmental Science & Technology 54 (2), 687-696 doi: 10.1021/acs.est.9b04115