More on the Latest GDI Pollution Study

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Feb. 27, 2020—A Feb. 11 news story here at NOLN highlighted a new study that suggests that the carbon buildup in direct injection engines might not be confined to within.

According to the study from the University of Georgia College of Engineering, GDIs might be discharging black carbon aerosols out of the tailpipe at a higher rate than a port fuel injected vehicle. The localized effects, particularly in urban areas, might have more social effects than the long-term efficiency gains (and, thus, carbon dioxide emission reductions) might not justify these short-term effects.

“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 more quickly,” according to a press release from UGA on the research.

A link to the study can be found here.

NOLN caught up with Rawad Saleh, an assistant professor specializing in aerosols and air quality, to hear more about his work as the study’s principal investigator.


Would pollution from black carbon aerosols be considered “smog” to the layperson?

Black carbon is a constituent of smog, but does not make all of it. Black carbon is part of the bigger family of “particulate matter” (PM), which is simply defined as small particles flying in air. 

PM includes many other components in addition to black carbon. PM is an important constituent of smog, and can contain—in addition to black carbon—thousands of different organic species that are either produced directly from combustion or form due to chemical reactions in the atmosphere.


What is it about GDI engines that causes them to produce more black carbon aerosol than port fuel injected vehicles?

For the same reason why diesel engines also produce a lot of black carbon. When the fuel is injected directly in the cylinder, a portion of the fuel droplets combust before it is well-mixed with the oxygen in the air. Black carbon is a product of this fuel-rich (or oxygen-depleted) combustion, which can be referred to as “incomplete combustion.” PFI engines mix the fuel and air before combustion in the cylinder, so the occurrence of these fuel-rich zones is minimized in PFI compared to GDI.


Would GDI's pairing with, for example, a battery-hybrid powertrain setup help to mitigate the effects in your research?

To a certain extent. However, a better alternative would still be pairing the hybrid system with a PFI engine with low emission standards (e.g. SULEV [super ultra-low emission vehicle]). While GDI is more fuel-efficient than PFI, the social cost of the increase in black carbon (i.e. PM) emissions outweighs the benefits.

Using a gasoline particulate filter (GPF) might help, but the fuel efficiency penalty associated with GPFs is still an open question.


How does the black carbon aerosol emissions from GDI engines compare to that of diesel engines?

GDI engines produce significantly less black carbon (per unit fuel burned or per unit mile driven) than diesel engines. I don’t have exact numbers.

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