Investigation of Particulate Matter Emissions from Gasoline Direct Injection Engines: Effects of Lean Combustion Strategies, Fuel Properties, and Lube Oil Additives

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Investigation of Particulate Matter Emissions from Gasoline Direct Injection Engines: Effects of Lean Combustion Strategies, Fuel Properties, and Lube Oil Additives

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2020-10

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Gasoline direct injection (GDI) engines have been widely adopted in passenger vehicles as a replacement for less efficient port-fuel injection (PFI) engines. However, GDI engines emit particulate matter (PM) in much higher concentrations than PFI engines. Human exposure to PM is harmful, causing increased morbidity and mortality. Additionally, PM from internal combustion engines (ICEs) is highly carbonaceous, and due to its high absorption of electromagnetic radiation, contributes to atmospheric warming and climate change. For these reasons, PM emissions from GDI engines are regulated, and the regulatory limits are becoming increasingly stringent. Regulatory limits on PM are an ongoing challenge for GDI engine development, especially in Europe and much of Asia where a particle number limit is enforced. This body of work experimentally investigates PM emissions from GDI engines, PM mitigation strategies, and a novel measurement method that can be used to meet the challenge of verifying reductions in engine emitted PM.Lean-burn GDI engine operation can further improve thermal efficiency. However, lean-burn fuel injection strategies often lead to higher PM emissions. In the first part of this work, PM emissions were experimentally studied from different GDI combustion modes to identify synergies for simultaneous PM and greenhouse gas (GHG) reduction. Lean stratified injection modes were studied and found to benefit from marginally higher thermal efficiencies than standard GDI combustion modes (stoichiometric) but they produced order/s of magnitude higher PM emissions. It was determined that the use of these modes would only be acceptable with proper implementation of a gasoline particulate filter (GPF). Lean homogeneous modes were studied and showed slightly higher thermal efficiencies, and in some cases, lower PM emissions than the stoichiometric combustion modes. This highlighted a key area of interest for reducing both PM and GHG emissions. A concerning finding was that there was a high fraction of solid particles in the 10 nm range emitted from GDI engines. This size range is effectively unchecked by current regulations and poses the highest risk to human health. The work provides further evidence for advocating to include 10 nm particles in regulatory number limits. The second part of this work focused on the effects of fuel properties on PM emissions from GDI engines. Fuel physicochemical properties play a role in particle formation because soot, the main constituent of engine PM, is formed through incomplete combustion of hydrocarbons. Fuels were studied over three sets of experiments: steady state engine operation, regulatory driving transients, and a particle density investigation. It was found that with clean fuel injector tips and under steady state stoichiometric operation, fuel properties did not have a significant impact on PM emissions because the charge mixture preparation was ideal for complete combustion. However, with under nonideal mixture conditions, such as transients or stratified injection strategies, high aromatics and low volatility fuels produced much higher concentrations of PM. Interestingly, it was shown that fuel ethanol concentration has opposing effects on PM emissions. The high heat of vaporization slows fuel evaporation resulting in incomplete combustion and particle formation, but the effect of diluting the fuel aromatics reduces the chemical pathways to soot. A novel “Virtual Drivetrain” software program was developed to test the engine dynamometer as if it were in a vehicle performing and regulatory driving cycle. Through testing various fuels, it was shown that the state of soot loading on the GPF was more influential on tailpipe PM emissions than the fuel used. This demonstrated the optimizing and controlling the state of loading on the GPF. Because the amount of soot loaded on a GPF influences filtration efficiency, monitoring the state of loading is important for controlling PM emissions. To do this, understanding of soot oxidation rates is critical. Offline thermogravimetric (TGA) methods, the commonly used approach for measuring oxidation rate in the literature, may not accurately represent oxidation rates on a filter where the macroscopic morphology of the soot, the oxidizer flow process, and the potential for catalytic effects from a filter washcoating all likely play a role in the oxidation process. To resolve this, a novel, in-situ soot oxidative reactivity measurement method was developed. The novel method was demonstrated with a catalytic GPF using oils with additives that would produce soot of different reactivity. It was shown that calcium oil additives increase soot reactivity, while zinc dialkyldithiophosphate (ZDDP) oil additives inhibit oxidation and reduce oxidation rates. These findings and magnitude of the calculated oxidation rates agree with literature. However, calculated oxidation rates were higher for much lower temperatures than reported in literature using TGA indicating a catalytic effect from the GPF wash-coating. The developed method can provide more in-use relevant soot oxidation rates than TGA, and will be useful to researchers, engine developers, and fuel and lubricant scientists. The last part of this work considered dicarboxylic acid emissions from both GDI and diesel engines equipped with aftertreatment systems (ATS). Although not necessarily emitted as particulate matter from engine, dicarboxylic acids participate in secondary organic aerosol formation. Thus, answering whether modern ATS eliminate dicarboxylic acids is important because it has not yet been studied. It was found that dicarboxylic acids were emitted from both GDI and diesel engines, and their emission was very sensitive to engine conditions. The ATS largely eliminated dicarboxylic acids, but a significant fraction penetrated the emission control devices under certain conditions. Further study involving assessment of the relative contribution of these acids to atmospheric aerosols is required to understand if improvements in catalysts are warranted to mitigate these acid emissions.

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University of Minnesota Ph.D. dissertation. October 2020. Major: Mechanical Engineering. Advisor: William Northrop. 1 computer file (PDF); xviii, 147 pages.

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Bock, Noah. (2020). Investigation of Particulate Matter Emissions from Gasoline Direct Injection Engines: Effects of Lean Combustion Strategies, Fuel Properties, and Lube Oil Additives. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/225912.

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