A study on autoignition characteristics of H2-O2 mixtures with diluents of Ar/N2 in rapid compression machine for argon power cycle engines

Abstract

The autoignition features of H2-O2 are crucial for Argon Power Cycle (APC) engines to understand the abnormal combustion in spark-ignition mode and to realize stable ignition in compression-ignition mode. A well-validated H2-O2 mechanism is a cornerstone to all hydrocarbon reaction kinetics emphasizing its importance in fundamental research. Here measurements of the ignition delay times of H2-O2 mixtures have been conducted in a rapid compression machine at 1.0, 3.0, and 5.0 MPa, 903 ~ 1042 K, with excess oxygen ratio of 1.0, 2.0, and 4.0, dilution ratio of 70%, 80%, and 90%, with argon ratio in the diluent of 50%, 60%, 70%, and 75%. 394 data points and 85 data sets were consequently obtained. A comprehensive comparison between 17 selected literature mechanisms was then performed. Results show that the ignition delay time of H2 is very sensitive to the temperature, especially near the ignition limit. The argon ratio in the diluent has a remarkable effect on the ignition delay. High pressure accelerates the autoignition, while both a high dilution ratio and a high excess oxygen ratio inhibit the autoignition. This research work suggests that a combination of highly diluted and ultra-lean combustion may inhibit knock in APC engines. H + O2 (+M) = HO2 (+M) and H + O2 = O + OH are the most sensitive reactions at 1.0 MPa over 950 K, while 2OH (+M) = H2O2 (+M) and H + H2O2 = H2 + HO2 are the most sensitive reactions at both 3.0 and 5.0 MPa.