Optical Diagnostics for Engine Efficiency in Gasoline Compression Ignition

Sammanfattning: The internal combustion engine has played a significant role in a vast number of applications from small cars to giant tankers, setting people free from geographical barriers and heavy labor. However, it has also generated many tons of harmful emissions and CO2that damage to humans and the environment. In response, many advanced combustion concepts have been proposed in recent decades, including homogeneous charge compression ignition (HCCI), reactivity controlled charge ignition (RCCI), and partially premixed combustion (PPC). PPC is considered a promising combustion method as it has low hydrocarbon and NOx emissions while maintaining high engine efficiency. This thesis explores the potential of PPC in terms of engine efficiency and CO2reduction. The research in this thesis explores how to understand the engine efficiency of PPC combustion and determine mechanisms behind efficiency improvements by using a heavy-duty optical engine with a primary reference fuel PRF87. Several strategies including double injection from HCCI to PPC modes, multiple injection strategies, and boundary conditions were used in the research. Double injection was used to determine what combinations of combustion modes from HCCI to PPC could achieve higher engine efficiency. Multipleinjection strategies (single injection, double injection, and triple injection) were then employed to determineways to further improve efficiency. Finally, boundary conditions like intake temperature and exhaust gasrecirculation (EGR) were tuned to achieve higher engine efficiencies in the optical engine. A high-speedcamera, Mie scattering, thermodynamic analysis, and computational fluid dynamics (CFD) simulation weresuccessively employed to explore the mechanisms behind phenomena.Engine efficiency trends in a transition from HCCI to PPC using double injections were studied in PaperI. The results show that jet-jet interaction plays both positive and negative roles in engine efficiency. The positive impact is that it can capture the too-lean fuel and air mixtures and promote combustion. However, if the well-mixed part of the first injection is sucked into the wake of a second injection and forms a more fuel-rich region, the jet-jet interaction results in high combustion loss. The thermodynamic analysis was conducted on three aspects of combustion: the combustion phase, heat transfer loss, and heat release (Papers II andIII). High efficiency can be achieved in several ways, early combustion phasing can take advantage of a higher effective expansion ratio, however, cases with a late combustion phase can also have high engine efficiency. The research shows that if the high-temperature regions in the combustion chamber are located in the piston bowl region, and with enough space to the piston and cylinder head surfaces the combustion becomes isolated and the heat transfer loss lowered. In triple injection, while the two first injections are kept constant, thein-cylinder temperature and mixing situation are the two key factors affecting the combustion of the third fuel injection. If the third fuel injection with late injection timing collides with the fuel from the second injection, the mixing process of the third injection fuel becomes worse. Therefore, jet-jet interaction should be avoided in such cases. The local in-cylinder temperature also affects the burn of the third injection fuel.