| Increasingly stringent emissions statutes and the need for energy security and self-sufficiency are primary driving factors to consider alternatives to fossil fuels. In this regard pilot-ignited natural gas engines are clean, efficient, economical, and thereby, viable alternatives to diesel engines.; Experiments conducted on a single cylinder compression ignited engine over a wide range of injection timings (15°-70° BTDC), revealed the presence of a unique lean-burn, low-temperature combustion regime, at advanced injection timings (45°-70° BTDC), that resulted in very low oxides of nitrogen (NOx) (less than 0.2 g/kWh) emissions, negligible particulate emissions (PM), and diesel-engine equivalent thermal efficiencies (40 percent at full load). This combustion strategy was termed the Advanced injection Low Pilot Ignited Natural Gas (ALPING) combustion. However, the low NOx emissions were accompanied by increased hydrocarbon emissions (HC) (greater than 70 g/kWh) and cyclic combustion variations (greater than 10 percent), especially at low load operation.; Research described in this dissertation focuses on examining the cause-and-effect relationships underlying the high HC emissions and cyclic combustion fluctuations at advanced injection timings (50°-70° BTDC), and proposing strategies to mitigate the same. Ignition and combustion phasing are identified as important parameters that dictate the extent of cyclic combustion fluctuations and HC emissions in pilot ignited natural gas engines.; To reduce cyclic combustion fluctuations and HC emissions at low loads, thermal management strategies including intake charge temperature control and fuel management strategies including pilot fuel quantity control were investigated. It was found that increasing intake charge temperature resulted in the best improvements in performance, stability and emissions at low load operation.; Other strategies investigated to reduce low load combustion, stability and HC emissions involved the use of hot exhaust gas recirculation (EGR). Addition of hot EGR is believed to lead to one or more of the following effects on the intake mixture including oxygen depletion, increased temperatures, dilution, and active recycling of unburned hydrocarbons to effect re-burn in subsequent cycles. It is reported that hot EGR addition results in a significantly improved low load performance, stability and emissions. |
