Low-temperature combustion in a small-bore high-speed direct injection optically accessible diesel engine

Small-bore High Speed Direct-Injection (HSDI) diesel engines are becoming attractive candidates for light duty vehicles. However, exhaust emissions must be significantly reduced to meet the increasingly restrictive emission standards.; In the current study, the low temperature combustion mode with low soot and NOx emissions was investigated. Experimental investigations were carried out in an optically accessible diesel engine. The air-fuel mixing process, combustion, and late cycle soot were visualized through the application of advanced non-intrusive diagnostic techniques including Mie-scattering, Laser Induced Exciplex Fluorescence, natural flame luminosity imaging, and time-resolved 2-D light extinction method. The NOx emission was measured in the exhaust pipe. By controlling the in-cylinder air temperature at the start of the main injection, the low temperature combustion mode was achieved with low soot and low NOx emissions under an operating load up to 7 bar IMEP. The vapor fuel distribution images showed better mixing process for low temperature combustion and smoke-less flame was observed in the combustion images. The late cycle soot images further confirmed the low sooting feature. The emission reduction of the low temperature combustion mode is due to less soot formation during the early combustion stage and less diffusion flame region for NOx reduction.; The narrow angle direct injection technique was also studied. Spray images showed strong fuel impingement for narrow angle injectors. It was observed that fuel impingement before ignition benefited the air fuel mixing process. Combustion flame with ultra-low flame luminosity and homogeneous structure was observed under moderate load conditions. The late post-TDC injection strategy resulted in more preferable performance in terms of soot, NOx and fuel efficiency.; Experiments with renewable bio-diesel fuels were conducted, too. Less luminous flame was observed for bio-diesel fuel blends showing a lower soot formation rate. The flame luminosity decreased faster for bio-diesel fuel indicating a faster soot oxidation rate. The increased NOx emissions of bio-diesel fuels were greatly reduced by employing the low temperature combustion mode. Results showed that low temperature combustion with bio-diesel fuels could simultaneously reduce soot and NOx emissions.