Strategies to Improve Efficiency and Emissions in Spark Ignition Engines

Abstract

This dissertation investigates the knock mitigation strategies and their experimental validation for improving performance and emissions with three different engine parameters: (i)Fuels (oxygenated fuel gasoline blends and syngas addition); (ii)Mixture dilution (EGR and Lean dilution); (iii)Injection strategies (DI and PFI combined dual fuel injection and multiple injections). Besides, a newly discovered unique relation between knocking and particulate matter emissions is examined in the last part with several conceptual models for better understanding of this phenomenon. The first part of this dissertation is about the effects of three oxygenated fuels (2,5-dimethylfuran, ethanol, and isobutanol) blended in gasoline on engine combustion, knock, and particulate matter emissions. One of the most promising furan functional group fuels, 2,5-dimethylfuran, is experimentally compared with two common alcohol-type oxygenates, ethanol and iso-butanol. Three major parameters are varied for the examination of oxygenates: fuel type, blend ratio, and boost level. The results show that the 2,5-dimethylfuran blends have the ability to extend knock limits as much as ethanol, but with relatively higher particulate matter emissions. As a second fuel study, syngas (hydrogen and carbon monoxide) aided engine combustion is experimentally investigated under EGR diluted and lean conditions by focusing on knock propensity, thermal efficiency, and emissions. Knocking tendencies are analyzed, and the thermal efficiency and emissions difference are discussed as well. The results show that with increasing the syngas addition, knocking is strongly suppressed, and the effect is more beneficial with EGR dilution than with air dilution. For the study of fuel injection strategies, two concepts of injection strategies are introduced and experimentally investigated. The first injection strategy is combined direct and port fuel injection to extend knock and EGR dilution limits using gasoline and ethanol fuels. The results showed that dual injection was beneficial to shorten the burn duration and improve combustion stability, but dual injection is slightly more sensitive to knock than direct injection primarily due to increased unburned gas temperature. The particulate matter emissions from dual injection were slightly lower, and the gaseous emissions showed lower total hydrocarbons and similar nitrogen oxides compared with only using direct injection of E20 fuel. The second fuel injection strategy involves multiple direct injections, which inject fuel multiple times in a cycle. This study explores the effect of multiple injections on knock, engine performance, particulate matter, and gaseous emissions. Two aspects of multiple injection strategies were experimentally investigated: the number of injections and the timing of the injections. The results confirm that multiple injection maintains torque and combustion stability but increases knock limits and thermal efficiency due to improved heat release phasing. The gaseous pollutant emissions including nitrogen oxides and unburned hydrocarbons are significantly reduced. Particulate matter emissions are not directly related to the injection period, but the number of injections significantly reduces particulate matter emissions. Finally, a unique phenomenon of a relation between knock and particulate matter emissions is analyzed and explained with three conceptual models. In general, particulate matter emissions are rapidly increased after the onset of knock as the spark advances, and the increased amount of particulate matter emissions are proportionate to the knock intensity. This phenomenon is a novel observation, and there is very limited understanding of this phenomenon and the relationship. So, phenomenological analyses are introduced using several experimental data, and three theories are proposed to understand the phenomenon along with conceptual models.