Gasoline Partially Premixed Combustion - An Advanced Internal Combustion Engine Concept Aimed to High Efficiency, Low Emissions and Low Acoustic Noise in the Whole Load Range
Sammanfattning: Environmental concerns such as global warming, the surge of crude oil prices and more stringent emission regulations are demanding the development of internal combustion engines that are highly efficient and low polluting. With classical internal combustion engine concepts such as diesel compression ignition and gasoline spark ignition engines, a trade-off exists between local and global emissions i.e. NOx, soot, CO and HC vs. CO2. For instance in the middle of the 1990’s when local emission regulations became tighter, the average brake efficiency, in heavy duty engine applications, dropped from 44.5 to 41.5%. The introduction of HCCI combustion, Homogenous Charge Compression Ignition, seemed to be the solution to the trade-off between local and global emissions. This concept was achieving ultra low NOx and soot, simultaneously with high efficiency. More than a decade of research has demonstrated that the viability of HCCI applications are limited to low load operations. Excessive acoustic noise, lack of direct control of the combustion and too diluted air-fuel mixture requirement make this concept not feasible to be applied to the whole load-speed engine range. The objective of this thesis was to develop a new combustion concept capable of running over the whole load range and achieve very high efficiency, low emissions, and acceptable acoustic noise without any drawback in combustion control. The target was achieved by burning gasoline in partially premixed combustion, PPC, mode. PPC is a mix of classical diesel combustion process where fuel is injected while combustion is occurring and HCCI using a fully premixed charge. If standard diesel combustion is black and HCCI is white, PPC is some shade of gray. If it is light gray much of the fuel is mixed with air before combustion, if it is dark gray combustion is more stratified and resembling diesel diffusion combustion. Advanced running conditions were developed in order to achieve high efficiency, low emissions and low acoustic noise in the whole load range; simply injecting gasoline in a diesel engine is not enough. The PPC concept was studied and developed using two heavy duty single cylinder engines, a Scania D12 and a Scania D13. When the concept was fully developed indicated efficiencies between 52 and 55% were reached from idle to 26 bar gross IMEP. This was accomplished keeping NOx below 0.40 g/kWh simultaneously with soot values not higher than 0.30 FSN and a maximum relative pressure rise rate below 8 bar/CAD. Gasoline PPC was also briefly studied in a light duty VOLVO D5 engine.
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