Optical Diagnostics of Gasoline Compression Ignition : HCCI-PPC-Diffusion Combustion

Abstract: Access to clean and affordable energy is one of the cornerstones of the world's society. Since the introduction of the internal combustion engine, diesel engines have become widely used for transportation in the commercial sector. These engines are attractive as they have low fuel consumption, but are also associated with high emissions of air pollutants, such as NO$_X$ and soot. These emissions are directly toxic to human beings and some contribute strongly to the global warming.To face these issues, researchers have shifted focus to advanced combustion concepts, such as homogeneous charge compression ignition (HCCI) and partially premixed combustion (PPC). These concepts are two of many approaches known under the collective name of low temperature combustion (LTC). In conventional diesel combustion (CDC), fuel autoignites almost immediately and burns continuously as it is introduced in the combustion chamber. By contrast, LTC uses large amounts of exhaust gas recirculation (EGR), which extends the ignition delay and facilitates premixing of fuel and air before autoignition, thereby avoiding soot and NO$_X$ formation while achieving high efficiency. These concepts are limited to low load operation. To extend the load range, gasoline has proved attractive due to its high resistance to autoignition. In contrast to diesel, this feature allows LTC to be used at increased loads. Despite the benefits, LTC concepts are challenged by high UHC and CO emissions, especially at low loads. At high loads, high pressure rise rates due to long ignition delays become challenging. For this reason, gasoline LTC cannot be achieved over the full load range and consequently CDC-like combustion needs to be used at high load. Nevertheless, gasoline has proven beneficial at high loads as well, producing less soot than diesel combustion. Gasoline compression ignition exhibits both opportunities and challenges as an approach to achieve cleaner engines. This work addresses the underlying factors, using a newly built optical engine to visualise the combustion processes. The study covers the whole load range, linking the concepts of low to medium load LTC to high load, CDC-like gasoline combustion. The first part of the results presents a transition from HCCI to PPC, coupling the combustion characteristics to the level of premixing and the combustion chamber bulk temperature. The second part describes a likely cause of UHC and suggests a potential method to reduce the, using multiple injections. In this study, laser diagnostics are used to trace the fuel distribution. Second to last, an intermediate load step between PPC and high load is described, addressing the difficulties of high pressure rise rates by utilizing double injection strategies. The last part presents high load gasoline operation and the factors behind soot reduction in comparison to diesel combustion. These results provides a wide but collective baseline of the fundamentally different combustion modes in gasoline compression ignition, linked over the whole load range.