Book Description

One of the promising solutions to rising emission standards is the in-cylinder emission reduction, through low temperature combustion. Low temperature combustion defeats conventional soot-NOx trade off by simultaneous reduction of both emissions by controlling the in-cylinder temperature below the Soot and NOx forming temperature zones. The use of low temperature combustion strategy phases the combustion into the expansion stroke, making the entire combustion process highly sensitive to start of high temperature combustion. Early start of high temperature combustion results in the advancement of combustion, resulting in higher in-cylinder temperature and pressure promoting the formation of oxides of nitrogen. Delayed start of combustion results in the retardation of the high temperature combustion further into the expansion stroke the first stage combustion, in this case cool flame combustion, has an important role to play in the phasing of high temperature combustion, associated emissions and efficiency. The focus of this study is to investigate the difference in the cool flame combustion characteristics between petroleum diesel and soybean biodiesel, when operating in low temperature combustion mode. Previous studies have attributed the absence of the cool flame in biodiesel purely due to oxygen content of the biodiesel. Cycle-to-cycle variation, exhaust gas constituents, rail pressure and fuel penetration length were analyzed to determine the causes for difference in the cool flame characteristic between the two fuels. The result of the analysis was that cool flame combustion is present in all combustion processes and not a product of systematic error or due to the combustion of the partially combusted species in the recirculated exhaust gas. It does not entirely depend on the chemical composition of fuel and rather on the in-cylinder conditions in particular the ambient oxygen concentration. Lower ambient oxygen concentration causes the cool flame to advance with respect to the high temperature heat release, making it visible in the heat release profile. The appearance of the cool flame at increased rail pressure in biodiesel does not cause a change in the trend of ignition delay, unburned hydrocarbon or carbon monoxide with respect to rail pressure. It only results in the retardation of high temperature combustion, further into the expansion stroke. Low temperature combustion defeats conventional soot-NOx trade off by simultaneous reduction of both emissions by controlling the in-cylinder temperature below the Soot and NOx forming temperature zones. In this study, low temperature combustion is achieved with the use of high exhaust gas recirculation circulation and late injection timing, phasing the combustion in the expansion stroke. The use of low temperature combustion strategy phases the combustion into the expansion stroke, making the entire combustion process highly sensitive to start of high temperature combustion. Early start of high temperature combustion results in the advancement of combustion, resulting in higher in-cylinder temperature and pressure promoting the formation of oxides of nitrogen. Delayed start of combustion results in the retardation of the high temperature combustion further into the expansion stroke, increasing the concentration of unburned hydrocarbon in the exhaust. Hence the first stage combustion, in this case cool flame combustion, has an important role to play in the phasing of high temperature combustion, associated emissions and efficiency. The focus of this study is to investigate the difference in the cool flame combustion characteristics between petroleum diesel and soybean biodiesel, when operating in low temperature combustion mode. Previous studies have attributed the absence of the cool flame in biodiesel purely due to oxygen content of the biodiesel. Late injection timing along with EGR was used to achieve LTC combustion (verified by soot-NOx comparison with conventional combustion), to realize the difference in cool flame characteristics between the two fuels. Further, cycle-to-cycle variation, exhaust gas constituents, rail pressure and fuel penetration length were analyzed to determine the causes for difference in the cool flame characteristic between the two fuels. The result of the analysis was that cool flame combustion is present in all combustion processes and not a product of systematic error or due to the combustion of the partially combusted species in the recirculated exhaust gas. It does not entirely depend on the chemical composition of fuel and rather on the in-cylinder conditions in particular the ambient oxygen concentration. Lower ambient oxygen concentration causes the cool flame to advance with respect to the high temperature heat release, making it visible in the heat release profile. The appearance of the cool flame at increased rail pressure in biodiesel does not cause a change in the trend of ignition delay, unburned hydrocarbon or carbon monoxide with respect to rail pressure. It only results in the retardation of high temperature combustion, further into the expansion stroke. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/151940