Investigation of Premixed Lean Diesel Combustion with Ultra High Pressure Injection

Abstract

Aggravation of combustion efficiency cause by increase of un-burnt fuel is the major problem in case of premixed compression ignition combustion with early timing injection. In this study, formation of lean air-fuel mixture by near TDC injection timing was investigated. As a natural consequence of this approach, permissible mixing time fall short. Therefore ultra high pressure injection system combined with micro hole nozzle was used in this work for increase of turbulent mixing rate. In this study, commercial Diesel fuel of Japan (JIS#2) was used as the test fuel. Cetane number of this fuel is 62. Some of previous research work shows that required injection timing for lean mixture formation to reduce NOx emission with high cetane number fuel was -90 deg.ATDC or more advanced point. On the other hand, injection timing of indicating extremely low NOx emission level is -38 deg.ATDC in our test result. Therefore increasing of turbulent mixing rate by ultra high pressure injection combined with micro hole nozzle is very effective to reduce necessary mixing time for lean mixture formation. In addition, fuel injection can carry out under higher ambient density and temperature condition than the conditions of previous work. Therefore rapid vaporization and momentum exchange bring rapid air-fuel mixing and wall wetting reduction. In this result, HC and CO emissions were also reduced. Characteristics of mixture formation process in ultra high pressure injection were also investigated by using KIVA-II code. This simulation results indicated that increase of turbulent energy was very effective for improvement of leanness and homogeneity of mixture. In this research work, influence of total kinetic energy of in-cylinder gas for mixture formation process was also investigated. And this result indicated the possibility of further improvement about necessary mixing time reduction for lean mixture formation.