NOx Reduction Strategies for Compression Ignition Engines
Open Access
- Author:
- Chapman, Elana M.
- Graduate Program:
- Fuel Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 21, 2008
- Committee Members:
- Andre Louis Boehman, Committee Chair/Co-Chair
Alan W Scaroni, Committee Member
Angela Lueking, Committee Member
Thomas Litzinger, Committee Member
Daniel Connell Haworth, Committee Member - Keywords:
- NOx
oxides of nitrogen
compression ignition engines
biodiesel
DME
dimethyl ether
diesel
alternative fuels - Abstract:
- The scope of this investigation is to explore strategies to reduce NOx emissions from compression ignition engines. Two methods are presented in this collection of studies: 1) NOx reduction accomplished through a change in fuel formulation, specifically through a change in the saturated fuel carbon chains of biodiesel; and 2) NOx reduction accomplished through a mixed mode combustion process utilizing a fumigated fuel and a pilot injection of diesel fuel. In the first study, a light duty diesel engine was used to investigate the change in saturation of a biodiesel fuel and its impact on NOx emissions. Previous studies have shown that a reduction in the iodine value of a biodiesel fuel produces a reduction in NOx emissions. The iodine value of the fuel is reduced through the saturation of the C18 molecules via hydrogenation of biodiesel fuel. Experiments were performed at several speeds and loads without exhaust gas recirculation (EGR), and a NOx reduction with the hydrogenated diesel fuel was observed. For all the modes studied, the NOx emission was higher for the biodiesel and lower for the hydrogenated biodiesel in comparison to the ultra low sulfur diesel (ULSD) fuel. Results from the calculation of the adiabatic flame temperature shows that the results could be explained by the difference in adiabatic flame temperature of the fuel, thus influencing the prompt NOx contribution in addition to the thermal contribution. Since the adiabatic flame temperatures are similar for the hydrogenated biodiesel and the ULSD, yet the NOx reduction with the hydrogenated biodiesel is much lower than the USLD levels, another explanation for the reduction is suggested: the additional prompt NOx contribution from the change in fuel chemistry. The second study investigated the NOx reductions which could be achieved with a mixed mode combustion process utilizing a fumigated fuel and a pilot injection of diesel fuel. In this research, the fumigated fuel was dimethyl ether (DME) and DME/Methane blends, while the pilot injection fuel was ULSD. Several sets of experiments were performed to study the ignition of the fumigated fuel, and its impact on the NOx emissions. In the first set of experiments, the DME concentration was spanned over a range of 15 to 44% energy equivalent of the total fuel requirement. An approximately 20% reduction in NOx emissions was observed up to 35% DME energy equivalent. As the energy equivalent increased above 35%, the NOx emissions began to increase with the increase in the peak of the high temperature heat release (HTHR). While the NOx emissions decreased, there was also a significant shift in the NO to NO2 conversion for all DME fumigation test conditions in comparison to the baseline diesel cases. For 25% DME energy equivalent, the injection timing of the pilot diesel was retarded and a reduction in the NOx emissions was observed. The low temperature heat release (LTHR) and the HTHR remained constant in magnitude and timing while the injection timing of the pilot diesel was retarded. The peak pressure for the premixed and diffusion portions merged, with increasing premixed DME combustion. With retarded injection timing, NOx reduction occurred as a result of the decrease in the bulk cylinder temperature and in the combustion duration before cylinder quenching from the exhaust stroke. In the second set of experiments, the intake air temperature was increased to study the impact on NOx and the mixed mode combustion process. While the amount of DME residual in the exhaust decreased along with the total hydrocarbon and CO emissions, the NOx emissions increased with increasing bulk cylinder temperature. For the speed and load used in this experiment, there was enough fuel and compression to combust most of the fuel, yet not enough to complete the combustion of the unburned hydrocarbons and CO. While air heating shifted the stoichiometry of the fuel and air mixture by reducing the density of air, the heating led to increased NOx with reduction in the NO to NO2 conversion. This may indicate that the system was above the low temperature range for this conversion to occur. In the third set of experiments, a small amount of Methane was introduced into the system to study the impact on the cetane number of the fumigated fuel. On a brake specific power basis, the Methane addition reduced the NOx emissions more than with only DME, however the NO to NO2 conversion was lower. NOx emissions were further reduced by retarding the injection timing, but increased with increasing intake air temperature. Through the use of the intake air heating, it was observed that the ignition of the DME/Methane blend was advanced with a smaller LTHR and a higher HTHR. While NOx emissions increased with the increase in bulk cylinder temperature, only the NO emissions increased while NO2 remained constant. Gaseous emissions analysis showed that the heating caused greater conversion of the Methane and DME during combustion.