NOₓ measurement and characterization in a gaseous fueled high-pressure direct-injection engine

Internal combustion engines (ICE) produce emissions that are harmful to the environment and human health. Strict governmental regulations put in place to reduce these harmful emissions have driven engine advancements such as high-pressure direct-injection (HPDI) of natural gas (NG) technology developed by Westport Fuel Systems (WFS). Because NG has a lower flame temperature than diesel, nitrogen oxides NO and NO₂ (NOₓ), can be slightly reduced; nevertheless, they are still a problematic harmful emission in HPDI engines. The effects of exhaust gas recirculation (EGR), known to reduce in-cylinder temperatures and thus NOₓ emissions in diesel compression ignition (CI) engines, is not as well understood in HPDI engines. The intent of this research is to develop a better understanding of the sensitivity of NOₓ to the specific effects of EGR (in-cylinder temperature, oxygen concentration, and combustion duration) in HPDI engines. This was accomplished by identifying the limits of EGR as a NOₓ reduction strategy in HPDI engines using a dry EGR system on a single cylinder research engine (SCRE). A baseline engine operating condition was developed to maintain a constant engine load of 12 bar gross indicated mean effective pressure (GIMEP), constant combustion phasing, and constant engine speed throughout an EGR sweep. To better understand the role oxygen concentration plays in NOₓ reduction, two equivalence ratios (φ) were tested and held constant throughout the EGR sweep: 0.6 and 0.7. The maximum EGR rate tested was ∼50% for each φ. Combustion instability (measured by the coefficient of variability (COV) of peak cylinder pressure (PCP) and GIMEP) increased by 2 and 3% at maximum EGR for φ = 0.6 and 0.7, respectively. NOx emissions were reduced ∼80% up to 25% EGR. However, NOₓ sensitivity to the effects of EGR diminish significantly at rates above 35%. The inverse is also true for particulate matter (PM) and methane in that these emissions significantly increase at EGR rates above 35%. Lastly, a kinetics analysis of the effects of EGR in a simplified HPDI model was developed to identify that NOx is twice as sensitive to temperature changes as to changes in oxygen concentration changes.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat