Optical Study on the Effects of Wall and Intake Temperatures on Ammonia Combustion and Emissions

Abstract

Ammonia (NH3) as an alternative carbon-free fuel suffers from poor combustion and emission performance, limiting its large-scale commercial utilization in the automobile industry. It has been shown that ammonia combustion is sensitive to operating boundary conditions of actual engines. In this work, the effects of thermodynamic boundary conditions (i.e., wall and intake temperature) on ammonia combustion and emissions were investigated using an optical spark-ignition engine with a high compression ratio. Wall and intake temperatures were regulated by a coolant and an intake heater, respectively. The results show that raising the wall temperature can significantly improve combustion stability, combustion efficiency, and thermal efficiency, while intake temperatures can enhance ammonia combustion only when the wall temperature reaches beyond some thresholds. Combustion visualization shows that the intake and wall temperature affect different stages of the combustion process. Specifically, the intake temperature might predominantly influence the early stages of combustion, whereas the wall temperature might have a more significant impact on the entire combustion process. Elevating the intake temperature increases the flame propagation speed, while there are no obvious changes in terms of the wall temperature. Flame analysis shows that a high wall temperature corresponds to a lower flame stretch sensitivity, indicating reduced cyclic variations in flame propagation and alleviating the possibility of flame quenching. While a high intake temperature features a stronger flame response to turbulence at the early stage of flame development. Besides, NH3 and N2O emissions are significantly reduced due to the improvement of flame propagation instability and incomplete combustion based on the crevice mechanism, but there is an increase in NO and NO2 emissions. The chemical kinetic analysis indicates that increasing NO x emissions mainly comes from the temperature and pressure sensitivities of fuel-type NO x .