Numerical Research on the Effect of Operating Parameters on Combustion and Performance Characteristics of Cyclohexane/N-Heptane HCCI

Abstract

A numerical simulation method was used in this study to explore the effects of equivalence ratio, intake air temperature, intake air pressure, compression ratio, and engine speed on the combustion and performance characteristics of cyclohexane/n-heptane dual-fuel HCCI (homogeneously charged compression ignition) using a zero-dimensional single-zone combustion model. In this paper, the ignition delay time and laminar flame speed were used to verify the selected chemical reaction kinetics mechanism, and the effect of each engine parameter variation on the combustion performance of cyclohexane/n-heptane dual-fuel HCCI was numerically simulated. The results demonstrate that the advanced combustion phase occurred with an increase in equivalence ratio, intake air temperature, intake pressure, and compression ratio as well as a decrease in engine speed. The work indicated increases on average by about 27% for each 0.2 increment in equivalence ratio. The excessive rise in intake air temperature leads to higher pressure rise rate and lower indicated work. For every 10 kPa improvement in intake pressure, the starting point of combustion is advanced by about 1.3°CA, and the peak pressure in the cylinder is improved by about 11 bar. The appropriate improvement of the compression ratio increases the heat release rate and the indicated work. The effect of engine speed is not so significant. The peak in-cylinder pressure rises by only about 1.22 bar for each 100 rpm increment in engine speed, and the indicated work and indicated mean effective pressure improve only about 3%. Ultimately, it is found that the introduction of cyclohexane makes the low-temperature heat release region in HCCI combustion of cyclohexane/n-heptane weaker than that of pure heptane.