<p>Internal combustion engines will continue to be the major power source for transportation. Spark ignition (SI) engine is still widely used for mobility due to its wide range of operating conditions. Engine key operating variables are primarily controlled by an already calibrated engine control unit (ECU). For air-fuel ratio accuracy control, it is quite necessary to investigate performance and emissions of SI engine near transition point which means air-fuel ratio was equal to 1, because engine operating condition transformed from oxygen-rich combustion to fuel-rich combustion at this point. Nowadays, a large number of researchers study the methods to control the sensitivity of the equivalence ratio sensor better. However, the effects of the control system largely depend on measurement system properties. Therefore, it is also necessary to improve the accuracy of the sensor utilized for optimizing control strategies. However, a trade-off relation exists: it costs much more money with a higher accuracy sensor, while a less precise sensor may result in larger variations of engine performance and emissions. Hence, it is important to choose a proper sensor to balance the cost and the accuracy. The objective of this work 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 employed in this study to analyze the engine responses at the optimum combustion phasing (based on maximum brake mean effective pressure) caused by a slight change of equivalence ratio under six different operation load conditions, which were composited of engine speed and intake pressure at high, medium and low level, respectively. The simulation model was based on SI Turbulent Flame combustion model which was a predictive model taking into account spark timing, flame and wall interaction and etc. The results demonstrated that variation of the fuel-air ratio had great influence on the emissions. The NOx and CO are sensitive to the air-fuel ratio. Compared with the variation of NOx and CO, the variation of the equivalence ratio had a much smaller difference on NOx. And the brake specific NOx inaccuracy was greater at lower intake pressure with the same fuel-air ratio accuracy. The results also indicated that the changes of emissions were smaller with a slight change of air-to-fuel ratio at a higher engine speed. As to the engine performance, it had a slight change with a little change of the fuel-air ratio. As a result, the rangeability of performance indicators were slight smaller than the variation of equivalence ratio. For a precise combustion control, the sensor accuracy is recommended to be higher than 1%.</p>

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

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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