Combustion stabilities and cycle-by-cycle variations of n-heptane homogeneous charge compression ignition combustion

In this paper, the combustion stabilities and cycle-by-cycle variations of n-heptane homogeneous charge compression ignition (HCCI) combustion were investigated. In-cylinder pressures of 100 combustion cycles under typical operation conditions including partial burn (8.33 mg/cycle), normal combustion (12.6 mg/cycle), and knock combustion (16.7 mg/cycle) were recorded. Consequently, cycle-by-cycle variations of combustion parameters and performance parameters were analyzed. Furthermore, the interdependency between the combustion parameters and/or performance parameters was evaluated. The results reveals that, during the low-temperature reaction (LTR) region, the coefficient of variations (COV) of the ignition timing, initial temperature, and crank angle corresponding to the peak value of the heat release rate (HRR) are less than 3%. The cycle-by-cycle variations of the second stage combustion are strongly affected by the fuel equivalence ratio, and COVs improved substantially with the increase of the fuel concentrations. Meanwhile, both of the peak values of the HRR during the LTR and high-temperature reaction (HTR) show obviously cycle-to-cycle variations, and COVs increase with the fuel supply rate. Regarding the cycle-by-cycle variations of the performance parameters, COVs of the maximum in-cylinder pressure and pressure rise rate are very small except for the knock combustion. On the contrary, cycle-to-cycle variations of indicated mean effective pressure (IMEP) and indicated thermal efficiency are very small except for the partial combustion, and variations improve with the increase of the fuel supply rate.