Injection rate shape has a great influence on the spray evolution, and consequently on the performance and emission characteristics of compression ignition engines. In this study, effect of different ramp injection rate shapes on the performance and exhaust emissions of a natural gas/diesel reactivity controlled compression ignition (RCCI) engine was investigated. 64 numerical experiments were performed to study effect of two characteristic parameters of the ramp injection rate shape, including ramp duration and ramp injection rate on the engine gross indicated efficiency (GIE) and emissions formation. It was realized that for a constant value of the ramp duration, the engine gross indicated efficiency and emissions are lower, and CO and unburned hydrocarbons (UHC) emissions are higher for medium values of the ramp injection rate. Moreover, for a constant value of the ramp injection rate, the engine gross indicated efficiency and emissions are higher, and CO and unburned hydrocarbons emissions are lower for medium values of the ramp duration. The optimum values of the ramp duration and ramp injection rate were determined and it was revealed that the optimum ramp injection rate shape can improve the engine gross indicated efficiency by 54.78%.
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