Book Description

Abstract : A significant challenge facing spark-ignited (SI) engines to achieve higher efficiency via highly diluted combustion is the quickly increased combustion instability as the dilution level increases. The flow motion variation has been identified as a dominant factor that introduces combustion variability at highly diluted conditions. However, the detailed mechanism of how the variation in flow motion impacts the flame kernel development and introduces combustion instability is only partially understood. This research investigated the impacts of the in-cylinder flow on variability of the ignition and early flame kernel development in a single cylinder direct-injection spark-ignition (DI-SI) optically accessible engine and accessory test benches. Firstly, different types of spark plug electrode designs were studied on a spark plug flow fixture bench. The results showed that the impacts of incoming flow on the ignition output vary as the incoming flow direction is changed. The ignition energy drops and discharge duration prolongs as the incoming flow is blocked by any part of the electrodes. Secondly, the flow motion near the spark plug region was investigated in an optical engine through high-speed imaging of the spark discharge arc stretching and flow field measurement via particle imaging velocimetry. It was observed that at motored conditions there is a consistent trend that the flow can change direction from the bulk motion as the piston approaches the top dead center (TDC). The transition of flow direction near the spark plug is a source of variability in initial flame kernel convection and propagation. It was found that by increasing the tumble motion, the transition window can be retarded later in the cycle. Lastly, the impacts of the unstable incylinder flow motion on the flame kernel initiation and development was studied at both diluted and stoichiometric conditions. The 2D and 1D probability distribution functions of the flame kernel indicated that the reversed flow direction near the spark plug at ignition timing results in smaller and less stretched flame kernels, which are a source of combustion variability. As the tumble level is increased, the window where the transition of flow direction occurs near the spark plug also changes. This research indicates that, at the studied loads and speeds, higher tumble level helps maintain a consistent flow motion near the spark plug at the ignition timing, which results in faster growing flame kernels.