Study quantifies impact of fuel composition on PM emissions from gasoline direct injection engines
14 December 2015
Researchers from Tsinghua University and Peking University have investigated the effects of fuel properties on particulate emissions gasoline direct injection engines (GDI). The study results, reported in the journal Fuel, demonstrated that the fuel composition has a significant on particulate emissions from GDI engines.
Although turbocharged GDI engines offer the attractive combination of both increased fuel efficiency and performance due to their higher volumetric efficiencies at high load, they also tend to produce more PM than PFI engines, with PM mass levels exceeding those of diesels equipped with diesel particulate filters, as well as conventional port-fuel injected vehicles. (Storey et al., 2014).
A recent open access study published in Nature’s Scientific Reports (Zhang & Cao, 2015) found that only 25 out of 190 cities in China could meet the National Ambient Air Quality Standards of China; the population-weighted mean of PM2.5 in Chinese cities is 61 μg/m3—about 3 times as high as global population-weighted mean, highlighting a high health risk.
According to data released by the Beijing Environmental Protection Bureau (EPB) in 2014, motor vehicles were responsible for 31% of local PM2.5 emissions in 2014.
In the introduction to their paper, the Tsinghua/Peking team noted that in GDI operation, gasoline is injected into the cylinder during the intake stroke, leaving insufficient time for the gasoline to evaporate and to mix with air before ignition. This inhomogeneity unavoidably leads to diffusion flame, in which soot-like particulates are formed.
It is known that vehicular particulate emissions can be affected by the fuel properties, such as aromatics, olefin, sulfur content, volatility and oxygenate. … The objective of this research work is to investigate different fuel compositions impacts on primary particulate emissions, VOCs and conventional gaseous emissions from gasoline powered vehicles (including both PFI and DIG). Since it is known that secondary particulate emission is a significant contributor to air quality, this research work also takes this into consideration.
—Yinhui et al.
In the new paper, the research team blended six test fuels with different aromatics, olefin, sulfur, Methyl-cyclopentadienyl Manganese Tricarbonyl (MMT) and ethanol content to use with a GDI engine (certified to China Phase 4, equivalent to Euro 4) produced by a Chinese OEM to examine the influences on primary particulate emission including mass; number; size distribution; compounds including Polycyclic Aromatic Hydrocarbons (PAHs); and the toxicity of PAHs emissions.
Among their findings:
Fuel composition—especially aromatics content—had a significant impact on PM emissions. Higher aromatics in gasoline resulted in much higher PM (mass), PN (particle number) and PAHs emissions, with higher toxicity to human health. The researchers concluded that reducing aromatics content is an important means to reduce primary particulate emissions and improve air quality.
Reducing olefin content resulted in reduced PM and PN emissions especially under high-load operation, but did not improve PAHs emissions levels by much. It did, however, contribute to the reduction of toxicity of PAHs.
E10 showed limited improvement on PM emissions compared with the effect of reducing aromatics and olefin content. Additionally, E10 increased PN emissions under low-load conditions.
Typical China Phase V gasoline did not definitely reduce vehicle emissions versus the typical Phase IV gasoline with higher sulfur (<50 ppm) and manganese (< 8 mpg Mn/l) in the China market. Reductions in sulfur below 50 ppm resulted in very limited PM emissions improvement—far less than the increase in emissions caused by increased aromatics content.
The GDI tailpipe particulates consist mainly of EC (elemental carbon), OM (organic matter) and small amounts of inorganic ions. The mass percentage of EC in the total tailpipe PM increased as load increased.
Three-way catalysts have a significant impact on PM, helping to reduce OM greatly (67–85%), resulting in an increase of mass percentage of EC in the the total PM of post-TWC versus pre-TWC configurations.
Resources
Wang Yinhui, Zheng Rong, Qin Yanhong, Peng Jianfei, Li Mengren, Lei Jianrong, Wu Yusheng, Hu Min, Shuai Shijin (2015) “The impact of fuel compositions on the particulate emissions of direct injection gasoline engine,” Fuel, Volume 166, Pages 543-552doi: 10.1016/j.fuel.2015.11.019
Storey, J., Lewis, S., Szybist, J., Thomas, J. et al. (2014) “Novel Characterization of GDI Engine Exhaust for Gasoline and Mid-Level Gasoline-Alcohol Blends,” SAE Int. J. Fuels Lubr. 7(2):571-579 doi: 10.4271/2014-01-1606
Yan-Lin Zhang & Fang Cao (2015) “Fine particulate matter (PM2.5) in China at a city level” Scientific Reports 5, Article number: 14884 doi: 10.1038/srep14884
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