Fuel-Efficient Engines at an Environmental Cost
Tanya Trofimencoff posted on August 03, 2016 |
Direct injection engines emit lower levels of CO2 but also produce more climate-warming black carbon...
A prototype GDI engine. (Image courtesy of Gonek.)
A prototype GDI engine. (Image courtesy of Gonek.)

Automobile manufacturers are faced with the constant challenge to improve fuel efficiency while minimizing their environmental impact. To deliver high performance with maximum fuel efficiency, automotive engineers created gasoline direct injection (GDI) engines. Between 2009 and 2015, the percentage of new vehicles sold with GDI engines jumped from 5 to 46 percent.

After taking a closer look, however, engineers at the University of Toronto discovered that GDI engines emit more black carbon, which is the climate-warming soot, and toxic volatile organic compounds (VOCs) like benzene and toluene. In other words, the benefit of emitting less carbon dioxide into the atmosphere may not balance the negative effects of black carbon and VOCs.

"The whole motivation for creating these engines in the first place was fuel efficiency. But what we haven't considered are the other climate-related emissions," said Greg Evans, a professor in the University of Toronto’s department of chemical engineering, "If a vehicle emits a small amount of soot, it can completely negate the lower amount of CO2 that it's emitting."


Studying GDI Emissions

Evans, Dr. Naomi Zimmerman (ChemE PhD) and Professor Jim Wallace (MIE) studied the chemical composition of emissions from GDI engines and found that GDI emissions ranked in the 73rd percentile of all vehicles studied for black carbon, and in the 80th to 90th percentile for VOCs. Ultimately, whether or not the GDI engines have the potential to be environmentally friendly depends on fuel composition, temperature and vehicle lifetimes.

Senior Research Associate Cheol-Heon Jeong and Professor Greg Evans measured emissions from GDI engines and evaluated climate tradeoffs of the more efficient engine type. (Image courtesy of the American Association for the Advancement of Science.)
Senior Research Associate Cheol-Heon Jeong and Professor Greg Evans measured emissions from GDI engines and evaluated climate tradeoffs of the more efficient engine type. (Image courtesy of the American Association for the Advancement of Science.)

"We found that in some cases, you need up to a 20 percent improvement in fuel economy in order to offset black carbon emissions," noted Zimmerman. "Offsetting the black carbon might be realistic in a place like California, where fuel composition is more strictly regulated and seasonal temperatures fluctuate less, but it’s harder to achieve in Canada."

Evans and Zimmerman also compared conventional gasoline engine types, also known as port fuel injection (PFI) engines, with GDI engines. Replacing a PFI engine from 2005 with a GDI engine from 2015 showed an 11 percent improvement in fuel economy. However, changing a 2010 PFI with a 2015 GDI engine saw only 1 percent improvement.


Should GDI Be Discontinued?

Currently, GDI does not seem to provide adequate fuel economy to justify replacing PFI engines today. The researchers would have to demonstrate significant improvements in harmful effects on the environment to make GDI engines a more viable consumption option. Since the technology is effective for fuel efficiency, however, the researchers are motivated to address these environmental issues. "

All the complex interactions show that because of the well-intentioned desire to mitigate climate warming, the technology is changing quickly and we are not properly considering all the trade-offs and side effects," said Evans. "As engine designs improve, the balance between all these factors could change again."

How can we deal with extant carbon emissions? This geothermal power is turning CO2 to stone.

Recommended For You