Potential trade-off between fuel economy and particulate emissions
Dr. Neil Canter, Contributing Editor | TLT Tech Beat October 2016
Wider use of GDI engines to improve fuel economy may also increase black carbon emissions.
KEY CONCEPTS
• Use of GDI engines is increasing as a way to improve fuel economy while maintaining performance in passenger cars.
• A new study shows that a trade-off exists in GDI engines between average fuel economy improvements and black carbon emissions.
• Two approaches under study to reduce black carbon emissions is the use of spray-guided systems in GDI engines and gasoline particulate filters.
THE IMPENDING NEW PASSENGER CAR MOTOR OIL SPECIFICATION GF-6, due to launch in 2018, requires engine oil lubricants to help provide better fuel economy. The U.S. government has a corporate average fuel economy goal of 54.5 miles per gallon by 2025, which is a considerable increase from the estimated 2017 figure of 38 miles per gallon.
Besides improving fuel economy, another challenge facing the automotive industry has been to reduce emissions. This concern has been prevalent with heavy-duty vehicles. As noted in a previous TLT article, heavy-duty vehicles may represent only 2% of the vehicles operating in the U.S., but they contribute 30% of the (NO)x and 50%-60% of the particulate emissions (1).
In the previous TLT article, a new process for measuring heavy-duty vehicle emissions known as the On-Road Heavy Duty Vehicle Emissions Monitoring System (OHMS) was described. Testing done in two locations in the state of California showed that heavy-duty emissions are declining especially for more recent models. In particular, heavy-duty vehicles built since 2011 declined more than a factor of 30 as compared to heavy-duty vehicles built before 2007 due to the former having diesel particulate filters.
The regulatory driver to improve fuel economy has led the automotive industry to introduce gasoline direct injection engines (GDI) in place of traditional port fuel injection (PFI) engines. This step when combined with turbocharging also enables power to be increased.
One concern that has surfaced is whether the increasing use of GDI engines also lead to an increase in emissions. A particular issue is black carbon emissions.
Dr. Naomi Zimmerman, postdoctoral research associate at Carnegie Mellon University-Center for Atmospheric Particle Studies, says, “GDI particle emissions contain a mixture of elemental carbon (or black carbon) and organic carbon species. The ratio of elemental carbon to organic carbon species in both gas direct injection and port fuel injection engines ranges from 0.5 to 30, based on a review of past literature.”
Black carbon emissions have a negative impact on global warming. Zimmerman explains, “Black carbon is a strong absorber of sunlight, and it can also reduce surface reflectivity when deposited on snow and ice. It is well established that an increase in black carbon emissions is linked to accelerated snow and ice melting in colder climates.”
In past work done at the University of Toronto, Zimmerman noted that black carbon emissions were higher in real world studies of vehicles operating with GDI as compared to PFI engines. She says, “The source of the higher black carbon emissions in GDI engines is attributed to the method used to inject fuel into the combustion chamber. The wall-guided GDI approach (see Figure 3B) leads to less time for the fuel to mix with air and fuel can coat cylinder walls, which upon ignition produces a higher concentration of carbon black due to incomplete combustion in fuel-rich regions.”
Figure 3. Studies show that there may be a trade-off between fuel economy and carbon black emissions in wall-guided GDI engines (B). One possible approach to reduce carbon black emissions is to use spray-guided GDI engines (A). Both GDI engine types display better fuel economy than PFI engines (C). (Figure courtesy of the University of Toronto and Reprinted with permission from Zimmerman, N., Wang, J., Jeong, C., Wallace, J. and Evans, G. (2016), “Assessing the Climate Trade-Offs of Gasoline Direct Injection Engines,” Environmental Science & Technology, 50 (15), pp. 8385-8392, Copyright 2016, American Chemical Society.)
Based on this information, Zimmerman and her past colleagues at the University of Toronto, Greg Evans, professor in the department of chemical engineering and applied chemistry and director of the Southern Ontario Centre for Atmospheric Aerosol Research and James Wallace, professor of mechanical engineering and director of the Engine Research and Development Laboratory compiled black carbon emissions coming from vehicles operating with GDI and PFI engines and determined if the increase in fuel economy realized from GDI engines may also lead to an increase in black carbon emissions.
BREAK EVEN POINTS
Zimmerman and her colleagues analyzed data from published work and past studies to determine that GDI engines emit black carbon emissions at a rate ranging from 0.18-15.9 milligrams per mile driven higher than PFI engines. This result led the researchers to examine what rate of fuel economy improvement will be needed to offset the potential increase in global warming induced by black carbon emissions.
Zimmerman says, “We randomly selected GDI and PFI black carbon emission rates from available data and calculated the difference 100,000 times using a technique called Monte Carlo simulations. This led to the production of low, mean, high and extreme emissions scenarios. Our results indicate that average fuel economy improvements between 0.14% and 14% are needed to offset black carbon emissions from vehicles operating with GDI engines.”
Zimmerman notes that this wide range of break-even points is due to a high degree of uncertainty in this study. Several approaches are under evaluation to determine how to reduce black carbon emissions in GDI engines.
Zimmerman says, “One idea is to utilize spray-guided systems in GDI engines (see Figure 3A) to better mix the fuel and air to generate more efficient combustion. A second idea is to determine the benefit of using gasoline particulate filters in much the same manner as they are used in diesel engines.”
Two reasons why gasoline particulate filters are not favored is their added cost and a small fuel economy reduction that Zimmerman estimates can range from negligible to 3.0%. The type of fuel also is a factor in affecting black carbon emissions. Zimmerman says, “Past studies have shown that the aromatic content of the fuel can have a significant influence.”
As for what role the automotive engine oil plays in impacting carbon black emissions, Zimmerman is uncertain. She says, “In all probability, the engine oil is a factor in influencing emissions, but there are no published studies on North American GDI vehicles to determine the importance of the lubricant.”
Zimmerman believes that further work is needed to evaluate different types of fuels and more real-world monitoring should also be done in various locations to develop more data.
Additional information can be found in a recent article (2) or by contacting Zimmerman at nzimmerm@andrew.cmu.edu.
REFERENCES
1. Canter, N. (2015), “New method for measuring heavy-duty vehicle emissions,” TLT, 71 (4), pp. 12-13.
2. Zimmerman, N., Wang, J., Jeong, C., Wallace, J. and Evans, G. (2016), “Assessing the climate trade-offs of gasoline direct injection engines,” Environmental Science & Technology, 50 (15), pp. 8385-8392.
Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.