A kinetic study on the effects of methane substitution on the ignition of n-heptane/air mixture

Abstract

In the operation of natural gas engines using diesel pilot ignition (DPI), the ignition delay is longer than that in the operation of a comparable diesel engine. The cause of such effect is more complicated than reducing the concentration of oxidizer. In this research, the chemical kinetics of the ignition process is studied using a homogeneous model containing n-heptane, methane and air. The base fluid is the mixture of methane and air. The equivalence ratio of methane is varied to study the sensitivity of the ratio on the concentration of methane. The dependency on temperature is studied by varying the base fluid temperature. The base fluid is then mixed with the diesel surrogate fuel, n-heptane. The mole fraction of n-heptane, ZC7, covers a wide range to represent the local mixture in different locations of engine combustion chamber. To obtain a complete understanding of the ignition process, different ignition delay times are defined based on temperature increment, n-heptane conversion, and methane conversion. The effects of methane on the ignition reaction paths are analyzed for selected cases. The fundamental causes of the methane's effects are explained based on the reaction path analysis and sensitivity analysis.