Pathways for Low Emissions Utilizing Spray Targeted Reactivity Stratification (STARS) in High Efficiency Natural Gas Dual Fuel Combustion

Abstract

Internal combustion engines are a key aspect of society, and their continued use poses challenges from an environmental standpoint since they emit pollutant and greenhouse gas emissions. This dissertation focuses on experimental analysis of dual-fuel low temperature combustion (LTC), which can be used as a strategy to reduce engine-out emissions and increase engine efficiencies. Dual fuel LTC uses two different fuels, a high reactivity fuel (HRF) and a low reactivity fuel (LRF). The HRF has a higher cetane number than the LRF, which allows for easier auto-ignition in compression ignition engines. Dual fuel engines also utilize high air to fuel ratios to achieve LTC. This, combined with early injection timings of the HRF, helps to reduce oxides of nitrogen (NOx) emissions. At low load conditions, this is a problem since higher cycle-to-cycle variations can increase pollutants such as unburned hydrocarbons (UHC) and carbon monoxide (CO). To combat this, a firm understanding of dual fuel LTC is required, as well as a strategy for reducing the cycle-to-cycle variations. The first part of this work further identifies a combustion heat release ‘transformation region' across different HRF injection timings wherein in-cylinder conditions arise that are conducive for ultra-low NOx emissions. This phenomenon occurs for different IC engine platforms and different fueling combinations. An experimental analysis, 0D chemical kinetic analysis, and 3D computation fluid dynamic (CFD) analysis were combined to elucidate the underlying causes for this phenomenon. The local stratification level of the fuel/air mixture was identified as the likely cause of combustion heat release transformation with changing HRF injection timing. The second part of the present work builds upon the findings of the first part by utilizing local stratification to mitigate cycle-to-cycle variations that are present at low loads. A framework of experiments was formulated for both a low engine load of 5 bar gross indicated mean effective pressure (IMEPg) and a high load of 15 bar IMEPg, wherein an injection strategy concept termed Spray TArgeted Reactivity Stratification (STARS) was utilized using both diesel and Polyoxymethelene-dimethyl-ether (POMDME) as HRFs. A steep decrease in UHC and CO emissions (> 80% reductions) as well as improved engine operation stability were demonstrated using both HRFs with dual fuel LTC at 5 bar IMEPg. Further, potential for emissions mitigation and efficiency improvement are discussed, as well as differences in the experimental results shown between the differing HRFs.