Impacts of spark discharge energy scheduling on flame kernel formation under flow conditions 2019-01-0727
Lean burn or diluted combustion are effective in improving the thermal efficiency of spark ignition engines. However, the flame propagation speed is often reduced under such conditions, which may negatively affects the stability of engine operation. Under these conditions, charge motions are intentionally enhanced to promote the flame propagation speed. The spark ignition and flame kernel formation process under flow conditions are significantly different from the cases under quiescent conditions. In this research, a constant volume combustion chamber is used to investigate the performance of lean or diluted combustion using different spark ignition strategies under flow conditions. These spark strategies include different current levels, which vary from 40 mA to over 400 mA, and discharge duration from 0.4 ms to as long as 13 ms. Extreme cases of “micro-explosion” with current level over 800 A and duration of about 10 µs are also reported. The effects of different spark strategies on the flame propagation process vary with the flow velocity. Under quiescent or low flow conditions, the extremely high current level - in the order of hundreds of amps, results in the “explosive” effect and thus larger flame kernel. When the flow velocity is increased, the spark plasma is stretched by the flow, resulting in larger contact area between the combustible mixture and the spark plasma. In addition, the gas flow brings more fresh mixture to the vicinity of the spark gap, which effectively increases the flame kernel area. Under these conditions, faster flame propagation is observed with sufficiently long discharge duration (about 2 ms) and moderately high discharge current (about 300 mA).