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. Sincethe introduction of the internal combustion engine, diesel engines have become widely usedfor transportation in the commercial sector. These engines are attractive as they have lowfuel consumption, but are also associated with high emissions of air pollutants, such asNOX and soot. These emissions are directly toxic to human beings and some contributestrongly to the global warming.To face these issues, researchers have shifted focus to advanced combustion concepts, suchas homogeneous charge compression ignition (HCCI) and partially premixed combustion(PPC). These concepts are two of many approaches known under the collective nameof low temperature combustion (LTC). In conventional diesel combustion (CDC), fuelautoignites almost immediately and burns continuously as it is introduced in the combustionchamber. 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 NOX formation while achieving high efficiency. Theseconcepts are limited to low load operation. To extend the load range, gasoline has provedattractive due to its high resistance to autoignition. In contrast to diesel, this feature allowsLTC to be used at increased loads. Despite the benefits, LTC concepts are challenged byhigh UHC and CO emissions, especially at low loads. At high loads, high pressure rise ratesdue to long ignition delays become challenging. For this reason, gasoline LTC cannot beachieved over the full load range and consequently CDC-like combustion needs to be usedat high load. Nevertheless, gasoline has proven beneficial at high loads as well, producingless soot than diesel combustion.Gasoline compression ignition exhibits both opportunities and challenges as an approachto achieve cleaner engines. This work addresses the underlying factors, using a newly builtoptical 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, couplingthe combustion characteristics to the level of premixing and the combustion chamber bulktemperature. The second part describes a likely cause of UHC and suggests a potentialmethod to reduce the, using multiple injections. In this study, laser diagnostics are used totrace the fuel distribution. Second to last, an intermediate load step between PPC and highload is described, addressing the difficulties of high pressure rise rates by utilizing double injectionstrategies. The last part presents high load gasoline operation and the factors behindsoot reduction in comparison to diesel combustion. These results provides a wide but collectivebaseline of the fundamentally different combustion modes in gasoline compressionignition, linked over the whole load range.