Examination of the effects of turbulence flow on discharge behavior in SI engine using high-speed PIV and discharge images
Ultra-lean combustion has the potential both to increase the thermal efficiency and to reduce NOx emissions in SI engine. However, the ultra-lean mixture condition, combined with strong flow for enhancing the combustion speed, causes poor ignition and thus misfire problems. Therefore, it is important to clarify the relationship between the flow and the spark discharge behavior, such as discharge elongation and re-discharge. This study discusses about the performance of discharge in a SI engine specially designed for ultra-lean operation. In particular, this study focuses on the influences of turbulence characteristics on discharge behavior. The flow measurement near the ignition plug and photographing the discharge were conducted simultaneously in an optical engine. The flow was measured by high-resolution and high-speed PIV system. The discharge behavior was photographed with multiple high-speed cameras. The cycles in which the discharge extended on the velocity distribution plane were extracted, and the relationship between the flow and the discharge behavior was discussed only in those cycles. The velocity distribution showed that a high shear rate was generated near the ignition plug by the flow passing between electrodes and the surrounding flow. In addition, it was clarified that the turbulence characteristic value near the spark plug changes with each crank angle. The velocity distribution and the discharge motion obtained by the simultaneous measurement revealed that the repeated elongation and shortening of discharge strongly related to the turbulent flow characteristics. These findings may help to understand the spark discharge phenomena and to optimize the discharge for ultra-lean SI engines.