Investigation of cluster nozzle concepts for direct injection diesel engines

摘要

For last several decades, there has been a lot of research about DI (Direct Injection) diesel engines because they have a clear advantage in terms of CO2 (Carbon Dioxide) emissions compared to gasoline engines. Furthermore, because of its ability to operate at low equivalence ratios, the diesel engine produces lower CO (Carbon Monoxide) and HC (Hydro-Carbon) emissions. Unfortunately, the diesel engine suffers from relatively high NOx (Nitrogen Oxides) and PM (Particulate Matter) emissions. In diesel engine research, keeping low fuel consumption in all operating conditions and reduction of PM and NOx emissions, are both critically important problems which need to be addressed. For this reason, research about diesel engines is being actively pursued in recent times. PM and NOx could be considerably reduced using low temperature combustion under part load conditions. But, further investigation is also necessary for fuel consumption and emission reduction under high load conditions. Reduction of orifice diameter, multiple injections and high pressure injection are avenues of rigorous research with the injection systems evolving rapidly. Fuel consumption and emission of PM are improved by using high pressure injection due to better air-fuel mixing while there are still problems with NOx under all operating conditions and insufficient reduction in PM emissions under high load conditions. Diesel engine noise and up to some extent, PM and NOx emissions are reduced by the use of multiple injections. However, the reduction achieved by multiple injections is not enough to satisfy the regulations on emission which are becoming more and more stringent. Reduction in diameter of orifices has some positive effects, such as significant reduction of PM and improvement in air-fuel mixing through better atomization and evaporation. On the other hand, it also has a negative effect under high load conditions, which is the reduced spray tip penetration with high temperature and pressure in combustion chamber. Based on prior knowledge available on cluster nozzles, several cluster nozzles were designed for the present work. They were comprehensively investigated by varying various parameters. Cluster nozzles consist of many groups (pairs in this case) of small orifices, and are aimed to achieve improved emission by low temperature combustion under part load conditions and lower emission levels and fuel consumption levels by high pressure injection under high load conditions. The nozzles were designed with different numbers of hole-pairs and different geometric configurations. Among the parameters varied during the engine experiments were swirl, injection pressure and piston geometry. The study helped in improving the understanding about characteristics of in-cylinder combustion phenomena, performance and emissions were also improved as a consequence. Results from the cluster nozzles were compared with those for a conventional nozzle, henceforth mentioned as the reference nozzle, to check the improvement. The cluster nozzles, having small orifice sizes, were operated with low temperature combustion under part load conditions. Emissions and fuel consumption were clearly improved together for this condition. Also at medium and high loads, it was possible to achieve lower PM emission level and fuel consumption by using proper combination of nozzle type, in-cylinder geometry and high injection pressure. Optimized nozzle design and in-cylinder geometry made small improvements under high speed full load condition which requires rapid combustion speed. As a conclusion of this work, it could be said that though there are some difficulties in the application, especially at high load and high speed condition, the cluster nozzle concept can be implemented as a reliable solution for the current problem of achieving better combustion and emission reduction.