Mixing Enhancement by a Bump Ring in a Combustion Chamber for Compound Combustion

Engine experiments have shown that simultaneous reductions of NOx and soot emissions can be achieved by the so called BUMP (Bump-up mixing process) combustion chamber. In order to understand the underlying mechanism of emission reduction, a STAR-CD based multi-dimensional combustion modeling was carried out for a heavy-duty diesel engine with the BUMP combustion chamber. The results from an impingement gas jet experiment were also presented and compared with computer modeling. The results showed that complex air motion with high turbulence was obtained by adoption of the bump ring. The fuel/air mixing rate was promoted greatly. Therefore, for the BUMP combustion chamber, much fuel fell in the optimum equivalence ratio range than that of the baseline chamber. The computations also indicated that the whole combustion process in the BUMP combustion chamber may be classified as premixed combustion and typical spray diffusion combustion, and the premixed combustion may be divided into three stages due to stratification in temperatures of corresponding stratified fuel. The lean mixture above criteria temperature (800K ) auto-ignited first and triggered the combustion of mixture of around stoichiometric equivalence ratio, and then the mixture in fuel/air equivalence range of 1.25 to 1.67 burnt, which was originally with the lowest temperature in the stratification. The mechanism for simultaneous reduction of soot and NO emissions with the BUMP combustion chamber was attributed to the increase in fuel proportion in the range of optimum equivalence ratio and lean mixture.