Particulate Matter Emissions from Partially Premixed Combustion with Diesel, Gasoline and Ethanol

Abstract

To achieve cleaner combustion and higher efficiency in compression ignition (CI) engines, many new combustion strategies have been developed. Among these new concepts, partially premixed combustion (PPC) attracts a lot of attention, because of its possibility to achieve simultaneously low soot and NOx. Compared to homogeneous charge compression ignition (HCCI) combustion, charge stratification in PPC can lead to increased soot emissions. This thesis deals with questions related to soot emissions in PPC. The main focus is to gain information and better understanding of soot particle characteristics with diesel, gasoline and ethanol fuels with varied in-cylinder emission control parameters.By means of injection timing, it is possible to have combustion from HCCI into PPC mode with the assistance of intake temperature. PPC shows benefits of higher engine efficiency and lower UHC and CO emissions over HCCI. However, it can also face the challenges of higher soot emissions. The study carried out with altered dilutions and different kinds of fuels illustrates that NOx emissions can be suppressed by increasing exhaust gas recirculation (EGR) or reducing intake pressure, but at the expense of an increase in soot emissions with diesel and gasoline fuels. The significant soot increase and largely reduced engine efficiency in stoichiometric operations also indicated low possibility for clean PPC with simple three-way catalyst with these fuels. On the contrary, ethanol emitted close to zero level soot emissions regardless of variations in engine operating parameters. This has made it an attractive fuel for PPC study.To be compliant with future stringent exhaust gas legislations for CI engines, soot exhaust after-treatment system may need together with new fuel strategies in PPC operations. Hence, information of the corresponding soot particle characteristics, including particle number and size, is necessary. Ethanol, high-octane and low-octane gasoline were used to perform PPC soot emissions investigations, with diesel fuel as a comparison. In-cylinder emission control parameters, such as injection timing, intake temperature, EGR and injection pressure were selected and tested to find their effects on soot emissions. Retarding injection timing can increase fuel stratification, which resulted in increased soot emissions of larger particle size and higher number density. Other engine parameters showed two quite different trends for fossil fuels and ethanol fuel respectively. When EGR increased, first soot mass emissions increased with higher particle number and larger size. Upon higher EGR, soot mass decreased with smaller particles and lower particle number concentration. Increasing intake temperature or reducing injection pressure can promote soot production with larger particle size and higher particle number concentration. Compared to diesel, gasoline showed great improvements in emission levels due to lower particle number emissions and smaller particle sizes, particularly with high octane gasoline fuel. On the other hand, ethanol produced ultra-low soot mass emissions and number emissions in all condition. Consequently it requires less engine efficiency compromises to comply with the legislation standards.In the meantime, the exhaust after-treatment system can be simplified. Very slight soot emission change in response to variations in engine conditions also increases the robustness. In addition to the findings in the exhaust, an in-cylinder soot particle analysis was done via in-cylinder gas fast sampling technique and on-line aerosol instruments. It has revealed that during combustion, EGR reduced both soot formation and soot oxidation, but the more reduced soot oxidation was the main reason for increased soot mass emissions in diesel PPC. Comparison of soot processes with gasoline and diesel indicated that, very low soot formation was the main reason for lower exhaust soot emissions in gasoline PPC. Much larger particles were formed in diesel PPC