Effects of temperature and pressure fluctuations on exergy loss characteristics of hydrogen auto-ignition processes

Abstract

Intake temperature and pressure fluctuations prior to main ignition are one of the reasons affecting the efficiency of internal combustion engines. In this study, the effects of temperature and pressure fluctuations with varied amplitudes and frequencies on the exergy loss of hydrogen auto-ignition processes were numerically investigated in an adiabatic constant-volume system at engine-relevant conditions. The results revealed that the increase in temperature fluctuation amplitudes primarily decreases the exergy loss due to chemical reactions, and the exergy loss due to incomplete combustion remains unchanged. Specifically, the total exergy loss is reduced by approximately 1.5% with a temperature fluctuation amplitude of 100 K at a frequency of 2 ms−1. Furthermore, with the same temperature fluctuation amplitude, increasing the frequency of temperature fluctuation from 2 ms−1 to 7 ms−1 leads to a 0.7% increase in the total exergy loss. On the other hand, the effects of pressure fluctuation on the exergy loss of hydrogen auto-ignition processes are negligible compared with those of temperature fluctuation. Through chemical kinetic analysis, it is found that temperature fluctuation promotes the consumption pathway of O2 to generate OH, rather than the collision with H to produce HO2. Consequently, the reduced mole fraction of HO2 inhibits the related HO2-consumption reactions, including HO2+HOH + OH and HO2+HO2H2O2+O2. Moreover, with temperature fluctuation, due to the lower fraction of third-body collision molecules, H2O, the reactivity of the third-body reaction, H + O2+M=HO2+M, is decreased. The reduced reaction rates of these reactions lead to decreased total exergy loss, indicating that the fuel energy conversion process may benefit from temperature fluctuation.