A Numerical Study Into the Importance of Equivalence Ratio Measurement Accuracy for Spark Ignition Engines

Abstract The internal combustion engines will continue to be the major power source for transportation. Spark ignition (SI) engines are still widely used for mobility due to their wide range of operating conditions. The engine key operating variables are primarily controlled by an already calibrated engine control unit (ECU). However, a less precise sensor may lead to larger variations in engine performance and emissions. The goal of this study was to investigate the importance of an air-fuel ratio sensor sensitivity during various engine operations. A one-dimensional (1D) computational fluid dynamics (CFD) model was utilized in this study to analyze the engine responses caused by a slight change of equivalence ratio at various engine speeds and loads. The results indicated that variations in the fuel-air ratio had a significant effect on emissions. Carbon monoxide was most sensitive to the air-fuel ratio, followed by nitrogen oxides, while unburned hydrocarbons were not sensitive to it. For engine performance, it varied slightly with small changes in the fuel-to-air ratio. The results also demonstrated that at higher engine speeds, slight changes in the air-fuel ratio resulted in smaller changes in emissions. Overall, for precise combustion control, it is recommended that the accuracy of the sensor is higher than 1% at least for the engine investigated in this study.