Investigation of Flow Conditions and Tumble near the Spark Plug in a DI Optical Engine at Ignition 2018-01-0208
Tumble motion plays a significant role in modern spark-ignition engines in that it promotes mixing of air/fuel for homogeneous combustion and increases the flame propagation speed for higher thermal efficiency and lower combustion variability. Cycle-by-cycle variations in the flow near the spark plug introduce variability to the initial flame kernel development, stretching, and convection, and this variability is carried over to the entire combustion process. The design of current direct-injection spark-ignition engines aims to have a tumble flow in the vicinity of the spark plug at the time of ignition. This work investigates how the flow condition changes in the vicinity of the spark plug throughout the late compression stroke via high-speed imaging of a long ignition discharge arc channel and its stretching, and via flow field measurement by particle imaging velocimetry. It is observed that the flow motion near the spark plug varies significantly cycle to cycle and can change direction from the bulk tumble flow near the time of ignition, especially when the ignition timing is late in the cycle at low tumble conditions. At a higher tumble, the bulk flow motion is maintained past the early ignition timing; and at late ignition timing, only few cycles show changed flow direction near the spark plug with much lower probability than low tumble conditions. Analysis indicates that at low tumble conditions, the mean horizontal velocity near the spark plug changes from 3.93 m/s pointing to the exhaust side at 60°BTDC to 3.05 m/s pointing to the intake side at 20°BTDC at 1000 rpm; however it is well maintained at 10.63 m/s in average from 60°BTDC to 30°BTDC pointing to the exhaust side and the mean value decreases to 7.27 m/s at 20°BTDC with the maintained flow direction at high tumble conditions. Initial flame kernel convection and propagation were also investigated at the two studied tumble levels.