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Fuel efficiency alone does not bring climate benefits for eco-engines

Gasoline direct injection engines use less fuel, but produce more black carbon

by Melissae Fellet
July 15, 2016

Schematic of GDI engine and standard gasoline engine.
Credit: Environ. Sci. Technol.
In a gasoline direct injection engine (left), an injector sprays a mist of fuel (yellow) into a cylinder, where it mixes with cool air (blue) entering the cylinder. After the mixture combusts, hot exhaust (orange) leaves through the valve on the right. In a standard engine (right), the fuel is mixed with the air before it enters the cylinder.

Car engines that rely on gasoline direct injection (GDI) are more efficient than traditional engines, and are set to make up more than half of the U.S. passenger fleet by 2020. Yet despite their fuel savings, switching to these engines could actually accelerate climate change, according to an analysis of their exhaust emissions (Environ. Sci. Technol. 2016, DOI: 10.1021/acs.est.6b01800). Today’s GDI engines release more heat-absorbing black carbon particles in their exhaust than standard engines, and the study found that they would need a further fuel efficiency boost of as much as 14% if they are to become less of a climate burden than conventional engines.

GDI engines mimic some of the features of diesel engines that give them superior fuel efficiency. Fuel injected directly into the cylinders can burn in the presence of extra oxygen, creating a mixture of gases that more efficiently pushes the piston in the cylinder as the mixture expands. But sometimes, injected fuel sticks to the cylinder walls and burns incompletely, producing sooty black carbon. This can form up to 30% of the particulate exhaust from GDI engines, and contributes to atmospheric and surface global warming. What’s more, current GDI engines are often tuned to run without extra oxygen—because this is more compatible with standard catalytic converters that control other types of emissions. But running this way limits fuel efficiency gains.

Naomi Zimmerman, now at Carnegie Mellon University, and her colleagues gathered information, from her own experiments and other published studies, about the black carbon emission rates of the most common GDI engine design and of standard gasoline engines. She was surprised at the wide range of results: GDI black carbon emissions were 0.11 to 9.9 mg/km greater than from traditional gasoline engines. The variation, she thinks, comes from factors like outside temperature and fuel composition.

To evaluate the tradeoff between increased black carbon and decreased carbon dioxide from GDI engines, the researchers used accepted models to calculate the warming potential of black carbon relative to carbon dioxide. They determined that GDI vehicles on average offer a gain in fuel efficiency of about 1% over the average for 2010 traditional engine vehicles. This gain is near the break even point where mean increase in black carbon emissions outweighs fuel economy improvements, Zimmerman says. For the highest black carbon emission scenario, a GDI engine might need as much as 14% better fuel efficiency to break even on climate change impact. When purchasing a new car, she recommends buying one with a traditional gasoline engine instead of a current GDI engine because for an equivalent car, the traditional engine’s lower black carbon emissions will offset the fuel economy difference.

However, automakers are developing ways to reduce particulate emissions from GDI engines. Mercedes-Benz is the first automaker to gradually install gasoline particulate filters in the exhausts of new vehicles with GDI engines. But the filters are not being widely adopted because they could create backpressure that decreases fuel efficiency. The researchers estimate that for most emissions scenarios, a gasoline particulate filter would have to remove 80% of particles with a less than 1% fuel efficiency penalty to offer climate benefits.

Ray Minjares of the International Council on Clean Transportation notes that more stringent emissions standards are coming in the U.S. and Europe, so particle emissions from GDI engines will have to drop, whether through particle filters, design changes, or other means. Zimmerman agrees, but in the meantime, she says, “A lot of vehicles are going out there without any particulate matter reduction strategies.”


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