Whether your new fuel-efficient engine actually emits fewer harmful emissions depends on where you live and what you’re putting in the tank, according to new research from the University of Toronto (UT) Faculty of Applied Science and Engineering.

New cars aim to deliver high performance with maximum fuel efficiency, making them easier on both the environment and the wallet. To do this, auto manufacturers are adopting a smaller, more fuel-efficient engine called gasoline direct injection (GDI). Between model years 2009 and 2015, the percentage of new vehicles sold with GDI engines jumped from 5% to 46%.

But research led by Greg Evans, professor in the Department of Chemical Engineering and Applied Chemistry, shows that GDI isn’t necessarily a greener choice. Although GDI engines emit lower levels of CO2, they emit more black carbon—the climate-warming particles commonly known as soot—and toxic volatile organic compounds, such as benzene and toluene.

Dr. Cheol-Heon Jeong (l) and Professor Greg Evans measured emissions from GDI engines and evaluated climate trade-offs. It turns out greater efficiency doesn’t always mean "greener." Image credit: Tyler Irving. “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,” says Evans. “If a vehicle emits a small amount of soot, it can completely negate the lower amount of CO2 that it’s emitting.”

When Evans and UT colleagues studied the chemical composition of emissions from GDI engines, they found that GDI emissions ranked in the 73rd percentile of all vehicles studied for black carbon and between the 80th and 90th percentiles for volatile organic compounds. When they then examined the climate trade-offs between reductions in CO2 emissions and increases in black carbon to determine whether this new engine type confers a net climate benefit, they found that whether GDI engines are the "greener" choice depends on several factors, including fuel composition, temperature and lifetime of the vehicle.

“We found that in some cases, you need up to a 20% improvement in fuel economy in order to offset black carbon emissions,” says fellow researcher Dr. Naomi 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 is harder to achieve in Canada.”

Evans and Zimmerman also compared conventional gasoline engine types, called port fuel injection (PFI), with GDI engines. They found that replacing an older PFI engine from 2005 with a new GDI engine from 2015 yielded an 11% improvement in fuel economy, but swapping a 2010 PFI engine with a 2015 GDI engine resulted in only a 1% improvement in fuel economy.

“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,” says Evans. “As engine designs improve, the balance [among] all these factors could change again.”