Multi-Variable Sensitivity Analysis and Ranking of Control Factors Impact in a Stoichiometric Micro-Pilot Natural Gas Engine at Medium Loads

A diesel piloted natural gas engine's performance varies depending on operating conditions and has performed best under medium to high loads. It can often equal or better the fuel conversion efficiency of a diesel-only engine in this operating range. This paper presents a study performed on a multi-cylinder Cummins ISB 6.7L diesel engine converted to run stoichiometric natural gas/diesel micro-pilot combustion with a maximum diesel contribution of 10%. This study systematically quantifies and ranks the sensitivity of control factors on combustion and performance while operating at medium loads. The effects of combustion control parameters, including the pilot start of injection, pilot injection pressure, pilot injection quantity, exhaust gas recirculation, and global equivalence ratio, were tested using a design of experiments orthogonal matrix approach. Specific outcomes from this research lead to fundamental and essential new knowledge in identifying the dominant factors for optimizing engine performance (i.e., thermal efficiency, combustion stability, combustion duration). The results provide a path forward for developing a high-efficiency engine and an optimized fuel and air handling system by ranking different controlling parameters for each performance metric studied. It was observed that exhaust gas recirculation and diesel pilot start of injection are the most influential parameters controlling medium load performance. In contrast, intake air temperature and pilot injection pressure have the least impact on the condition studied.