While significant sooty black deposits—and even, occasionally, visible puffs coming from the tailpipe—used to be a sign your gasoline car needed a tune-up, they’re a normal fact of life with many newer cars that are supposedly cleaner-burning. 

What’s the disconnect? While the modern gasoline direct-injection technologies being phased in help add to fuel-efficiency and reduce carbon-dioxide emissions, they produce more black carbon aerosols.

And as new modeling suggests, they're potentially not a good thing for keeping the atmosphere from warming. 

A study announced last week from the University of Georgia College of Engineering, published in December in the journal Environmental Science and Technology, makes a prediction: that the black carbon emissions from direct-injection vehicles will add to atmospheric warming—and more importantly, that the potential damage from that will outweigh the reduction of carbon dioxide achieved through direct injection. 

Rawad Saleh, the study’s principal investigator, says that this study is the first to actually model the effects of the increase in black carbon. 

The study notes that “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.” 

Smog over Los Angeles, courtesy Flickr user steven-buss

Smog over Los Angeles, courtesy Flickr user steven-buss

Furthermore, they predict that it might take decades before the CO2 benefits outweigh the costs—which include local-health impacts. By then the fleet will be mostly electric, according to most predictions. 

According to the 2019 Automotive Trends Report, from the EPA, 51% of the U.S. fleet had direct-injection engines. The number climbed as high as 100% for Mercedes-Benz, and as low as 2% percent (by volume) for Toyota; but EPA estimates project that 98% of gasoline engines in new cars will have direct injection by 2025. 

Green Car Reports has over the years checked with several automakers over questions about the large amounts of observable soot coming from GDI engines, and the general response is that this is normal, and the costs outweigh the benefits. While we know there’s a flip side to this argument—such that suppliers and the automotive industry are currently refining the technology, and that it opens up more possibilities like dynamic cylinder deactivation—these results suggest otherwise. 

A number of automakers, including Volkswagen and Mercedes-Benz, are countering this tendency by introducing gasoline particulate filters (GPF)—but as the researchers point out, that can reduce fuel-efficiency and carbon-dioxide gains.