Spray and Combustion Visualization in an Optical HSDI Diesel Engine Operated in Low-Temperature Combustion Mode with Bio-diesel and Diesel Fuels

An optically accessible single-cylinder high-speed direct-injection (HSDI) Diesel engine equipped with a Bosch common rail injection system was used to study the spray and combustion processes for European low sulfur diesel, bio-diesel, and their blends at different blending ratio. Influences of injection timing and fuel type on liquid fuel evolution and combustion characteristics were investigated under similar loads. The in-cylinder pressure was measured and the heat release rate was calculated. High-speed Mie-scattering technique was employed to visualize the liquid distribution and evolution. High-speed combustion video was also captured for all the studied cases using the same frame rate. NOx emissions were measured in the exhaust pipe. The experimental results indicated that for all of the conditions the heat release rate was dominated by a premixed combustion pattern and the heat release rate peak became smaller with injection timing retardation for all test fuels. Bio-diesel fuels greatly affected the combustion process and emissions. The ignition and heat release rate peak occurred later with increasing bio-diesel content. Fuel impingement on the wall was observed for all of the conditions. The liquid penetration became longer and the fuel impingement was stronger with the increase of bio-diesel content in the fuel blends. For all the injection timings, lower soot luminosity was seen for bio-diesel blends than pure diesel fuel showing lower soot generation for bio-diesel fuels. However, the influence of bio-diesel blending ratio on the soot generation depended on specific injection strategy. The NOx emissions showed different trends. With conventional injection timing and late injection timing, the NO_x emissions consistently increased with the increase of bio-diesel content. But for an early injection strategy, there was a trade-off between ignition delay and oxygen content in affecting the NO_x emissions. For this early pre-TDC injection strategy, due to a higher in-cylinder temperature and a longer existence time of this high temperature, the NO_x emission was much higher than the other two injection timings. Retarded post-TDC injection timing resulted in simultaneous reduction in soot and NO_x emissions. By combining the injection timing optimization with bio-diesel fuel blending ratio, simultaneously reduction of soot and NO_x can be realized in practical engines.