U Toronto team assess the climate trade-off between reduced CO2 and increased Black Carbon from GDI engines
The upside of gasoline direct injection (GDI) engines is widely seen as being improved fuel economy coupled with an increase in specific power (especially with turbocharging), enabling significant downsizing. The downside of GDI engines, however, is a substantial increase in emissions of particulate matter—a problem with which heretofore only diesels had to deal.
A significant fraction of the GDI PM2.5 is black carbon (BC)—a pollutant with large positive radiative forcing on the climate due to its ability to absorb incoming sunlight and reduce surface albedo on snow. In other words, while the use of GDI engines can reduce CO2, it also can increase BC—contributing to further warming. A new study by a team at the University of Toronto has made a preliminary assessment of the climate trade-off (i.e., CO2 vs. BC) to ensure that integration of GDI vehicles will result in a net reduction of CO2-equivalent emissions. Their paper appears in the ACS journal Environmental Science & Technology.
|GDI particulate emissions|
|GDI PM is primarily formed due to incomplete fuel volatilization causing fuel impingement on cylinder and piston surfaces, and incomplete fuel mixing with air resulting in pockets of fuel-rich combustion.|
|Less fuel-air mixing time is available with GDI engines compared to PFI engines, in which fuel mixes with the air intake upstream of the exhaust cylinder.|
|The extent of fuel-air mixing can be affected by injection system design. There are two major classes of injection systems for GDI engines: spray-guided and wall-guided, which differ in where the fuel is injected into the cylinder, the piston head design, and the fuel injection strategy.|
|Spray-guided systems direct some of the fuel toward the spark plug for ignition, while the remaining fuel is dispersed into the remainder of the cylinder. Wall-guided systems primarily rely on piston head geometry for fuel-air mixing; the fuel spray is injected near the wall toward the piston head where it is redirected toward the spark plug.|
|The authors note that there has been some movement toward spray-guided systems in commercial systems due to their potential to reduce the number of particles emitted and BC—likely due to improved fuel-air mixing with the spray-guided design.|
For the study, the Toronto team took BC emission rates they had determined in an earlier study, and compiled those with rates from the literature. From these emission rates, the authors constructed BC emissions scenarios to determine the fuel economy improvements needed to offset the increase in black carbon using both global warming potential (GWP) and global temperature potential (GTP) metrics.
GWP is a measure of the radiative forcing integrated over a given time horizon; GTP is an end-point metric of the temperature change at the end of a given time horizon.
The authors also explored the trade-offs between the fuel penalty of using a gasoline particulate filter and BC reduction efficiency. They used four emissions scenarios to represent the broad range of reported BC emissions, which were reported across variable ambient temperatures, vehicle operation, and fuel compositions.
They then converted the four BC emission rate scenarios into CO2-equivalent emissions using GWP data for a 20-year time horizon.
Subsequently, they calculated the required percent fuel economy improvement to offset the BC emissions by comparing the increase in CO2-equivalent emissions from BC (CO2 g-eq/ mile) to the reduction in CO2 emissions by reducing fuel consumption by a given percentage.
They assumed a baseline fuel economy of 21.4 mpg (10.99 l/100 km)—the average fuel economy of the in-use United States light-duty fleet in 2014. The cO2 emission factor was calculated assuming a fuel carbon content of 8887 g/gallon.
From this study, it was determined that a broad range of fuel economy improvements (0.04% to 26%) with GDI vehicles are required to offset the BC-induced warming. This large range was primarily due to the large uncertainty in the impacts of GDI engines on BC emissions. From the more complex GTP-based analysis with a sustained emission release scenario of 10 years, it was determined that replacing a fleet-average pre-2010 PFI vehicle with a fleet-average GDI vehicle will generally offer climate benefits within 20 years of putting the vehicle into use. As such, detrimental climate impacts due to the integration of the current generation GDI vehicles into the fleet will likely still be observed in the near-term, especially for new car purchase scenarios, where PFI vehicles outperform GDI vehicles on the basis of fuel economy and BC emissions.
Generally, installing a gasoline particulate filter with a < 1% fuel penalty would result in a net climate benefit for filtration efficiencies exceeding 80%. However, if BC emissions are sufficiently small … then installation of a GPF is never advantageous to the climate if there is any associated fuel penalty. Given that preliminary implementations of GPFs suggest that negligible fuel penalties are possible, we suggest a more holistic regulatory approach that also considers a reduction in BC as an offset to the demands for greater fuel efficiency by 2025. Furthermore, if GPFs can be implemented with a negligible fuel penalty, there may be considerable co-benefits to human health by a reduction in vehicle exhaust PM. This could also be achieved through modifications to GDI engine design that promote less PM and BC formation.
… Of greatest significance to BC emissions were fuel properties, in particular total aromatic content, which may vary substantially by geographic area. As such, it is recommended that additional GDI emissions studies in regions where total fuel aromatics are high, or studies of a broader range of fuel types are needed to better understand the potential range of BC emissions. Additionally, more studies of the impacts of spray-guided or advanced GDI engines on BC emissions are needed to verify if this engine technology results in lower BC emissions than the wall-guided GDI design. In this study, the BC emissions data from spray-guided GDI engines were too limited to develop an emissions scenario and our focus was on the climate impacts of GDI vehicles purchased now, at their current state of development over the near term. It is expected that as spray-guided injectors are increasingly used in GDI engines or as GDI engine technology develops, BC and PM emissions may be significantly reduced. As such, it is recommended that these metrics be revisited when more data is available from engines with advanced injection strategies.—Zimmerman
Naomi Zimmerman, Jonathan M. Wang, Cheol-Heon Jeong, James S. Wallace, and Greg J. Evans (2016) “Assessing the Climate Trade-Offs of Gasoline Direct Injection Engines” Environmental Science & Technology doi: 10.1021/acs.est.6b01800