Advanced diesel combustion of high cetane number fuels and the impacts on the combustion process

Advanced diesel combustion is of great interest due to the promise of simultaneously reducing emissions of nitrogen oxides (NOX) and particulate matter (PM), while maintaining or improving efficiency. However, the extended ignition delay along with the combustion of a partially premixed charge results in excessive emissions of total hydrocarbons (THC) and carbon monoxide (CO) from incomplete combustion. In this study, a light-duty turbodiesel engine was operated in an advanced diesel combustion mode, specifically high efficiency clean combustion (HECC), using three different fuels including a conventional ultra-low sulfur diesel fuel (diesel), a synthetic fuel produced in a high temperature Fischer-Tropsch (HTFT) process, and a synthetic fuel produced in a low temperature Fischer-Tropsch (LTFT) process. Start of injection (SOI) timing was swept from -8° ATDC to 0° ATDC to find the optimized injection timing for each fuel. The HTFT fuel, which had a derived cetane number (DCN) of 51, was found to decrease THC and CO emission by 32% and 31%, respectively, compared to the diesel fuel, which had a DCN of 45. The higher ignition quality of the HTFT fuel was found to reduce emissions from incomplete combustion by presumably consuming more of the fuel charge before it reached a region of the cylinder where it was too lean to effectively burn. However, with the HTFT fuel, NOX and PM emissions increased relative to the diesel baseline due to a higher peak heat release rate, presumably caused by 2% less EGR during the HTFT fuel's operation. In contrast, the LTFT fuel with a DCN of 81 enabled an 80% reduction in THC emissions and a 74% reduction in CO emissions compared to the diesel fuel. The LTFT fuel, though having a very short ignition delay, did not increase NOX and PM emissions apparently due to the fuel burning in a shorter, less intense premixed combustion phase followed by a prominent mixing-controlled combustion phase. This study revealed that a high ignition quality (DCN 81) fuel is well suited for operation under a high EGR advanced diesel combustion mode and led to reductions in all primary pollutant emissions.;The higher ignition quality fuels were shown to have leaner critical phi, but a question remained regarding fuel compositional effects on critical phi. Blends of n-dodecane/toluene and n-dodecane/isooctane were prepared to have the same DCN as n-heptane. Critical phi measurements of the two blends and n-heptane revealed that critical phi could vary between fuels with the same DCN. The n-dodecane/toluene blend was found to have a leaner critical phi than the n-dodecane/iso-octane blend under low compression ratios (CRs) or simulated EGR. It was concluded that the 11% greater n-paraffin content of the n-dodecane/toluene blend compared to the n-dodecane / iso-octane, resulted in more LTHR and a leaner critical phi. The critical phi of the low cetane number FACE fuels were also determined. A linear correlation with an R2 coefficient of 0.949 was observed between critical phi and n-paraffin mass content of the low CN FACE fuels. This result corroborated with the conclusion of the DCN parity blends, that the critical phi of a fuel is governed by the fraction of reactive components (n-paraffins), which increases low temperature heat release (LTHR). The critical phi measurements of the low cetane number FACE fuels were compared to CO and THC emissions from a light-duty turbodiesel engine operating in advanced combustion mode with the low cetane number (CN) FACE fuels. Low CO and THC emission produced from early SOI timings are suggestive of a correlation with lean critical phi measurements. It was concluded that the relationship between low CO and THC emissions and a lean critical phi is only present when early SOI timing produces an overly-lean fuel-air charge. It was conjectured that the contribution of CO and THC emissions from overly-rich regions becomes larger, and a fuel with a leaner critical phi would not reduce these emissions. These results suggest that a fuel can be blended to have a low ignition quality, which is desired for high efficiency advanced combustion operations and with high n-paraffin content to reduce CO and THC emission. (Abstract shortened by UMI.).