Numerical simulation and experimental study on the n-heptane HCCI combustion with port injection of reaction additive

The control of ignition timing in the homogenous charge compression ignition (HCCI) of n-heptane by port injection of reaction inhibitors were studied in a single cylinder engine. Four suppression additive including methanol, ethanol, iso-propanol, and methyl tert-butyl ether (MTBE) were used in the experiments. The inhibition effectiveness on HCCI combustion with various additives was compared under the same n-heptane equivalence ratio and total fuel equivalence ratio. The experimental results found that the suppression effectiveness increased in the order: MTBE<iso­-propanol<<ethanol<methanol. But, ethanol is the best additive with the considering of the operating ranges, thermal efficiency, and emissions. For ethanol/n-heptane HCCI combustion, partial combustion may be observed when the mole ratio of ethanol to total fuel larger than 0.20, misfire occurred when mole ratio of ethanol to total fuel larger than 0.25. Moreover, CO emission is strongly dependant on the maximum combustion temperature, while HC emissions are mainly determined by the mole ratio of ethanol to total fuel. To obtain chemical mechanistic information relevant to the ignition behavior, chemical analysis with detailed chemical kinetics were conducted. The simulation results also confirmed the retarding of the ignition timing by ethanol addition. In addition, it can be found from the simulation that, HCHO, CO, and C₂H₅OH could not be oxidized completely and maintain a high level if the partial combustion or misfire occurred (for example at leaner fuel/air mixture).