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UBC Theses and Dissertations

Performance of a partially stratified-charge gasoline engine Brown, Greg

Abstract

Stratified-charge lean burn engines have shown promise in their ability to reduce certain emissions and improve fuel economy while still providing acceptable driveability and performance. A local charge stratification process, the Partially Stratified-Charge (PSC) concept, has been developed at the University of British Columbia in an attempt to further improve lean burn operation. In the PSC engine, a lean homogeneous mixture is inducted into the combustion chamber and is compressed during the compression stroke. Just prior to the spark, a small amount of fuel is injected into the combustion chamber in the vicinity of the spark plug. This produces a rich pocket that can be ignited more easily and facilitates the combustion of the remaining lean homogeneous mixture. The main objective of the research presented here was to implement a PSC system on a gasoline engine and examine the performance of the PSC system relative to its homogeneous fuelled counterpart. Two different PSC systems were tested. The first system was based on a natural gas PSC system developed during previous work while the second system was a new gasoline PSC system developed specifically for this research. Analysis of the performance and emissions results for the natural gas PSC studies showed significant advantages over conventional lean burn and stoichiometric homogeneous-charge operation. The most notable improvement was up to a 15% extension in the relative air-to-fuel ratio (λ). Through this extension of the lean limit, the useable range of brake mean effective pressure was expanded, by up to 20%, when using PSC. A reduction in nitrogen oxide emissions and an increase in total unburned hydrocarbons also accompanied this extension of the lean limit. In-cylinder pressure data analysis demonstrated significantly higher peak in-cylinder pressures and shorter ignition delays with PSC. Throttled tests with the natural gas PSC system revealed the potential to reduce nitrogen oxide emissions, carbon monoxide emissions and brake specific fuel consumption over stoichiometric homogeneous charge operation. Improvements in PSC volumetric efficiency of up to 15% and 5% were realised compared to stoichiometric homogeneous and conventional lean burn operation, respectively. Extensive experiments were also undertaken with a gasoline PSC system, however, performance improvements, though expected, were not observed. Examinations of the gasoline PSC system suggested that there were some challenges in achieving local charge stratification in the vicinity of the ignition source. These results implied that further optimisation, and perhaps a redesign of the gasoline PSC system, would be required to fully realise the performance benefits demonstrated with the natural gas PSC system.

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