Effect of discharge current boost on ignition and combustion under cross flow conditions

Abstract

Abstract A strong electric spark can effectively create a high-quality ignition and hence, a stable initial flame kernel for spark ignition engines under severe operating conditions, such as ultra-lean combustion and excessive exhaust gas recirculation (EGR) dilution. Cyclic variation of the ignition process is influenced by the complex in-cylinder flow environment and the stochastic the plasma state, which can result in significant combustion fluctuations, eventually deteriorating the engine performance. In this study, with a customized, unique spark ignition system, the effect of discharge current setting on ignition and combustion is thoroughly studied under cross-flow conditions similar to a spark ignition direct injection (SIDI) engine. The discharge voltage and current, as well as high-speed images of plasma channel were recorded simultaneously for the spark discharge event. A boosted discharge voltage yields a higher discharge current, a prolonged spark channel length and an enlarged plasma area, and therefore, can resist high-velocity cross-flows. Results of the lean combustion investigation in a constant volume combustion chamber demonstrate that the increased average discharge current can effectively improve the initial flame kernel size by generating a larger plasma size, thereby accelerating the early flame propagation in presence of a cross-flow. But an insufficient average discharge current leads to a misfire. Combustion in an optical engine further validated the effectiveness of increasing average discharge current in enhancing combustion stability. Even though the actual in-cylinder flow near the top dead center (TDC) is highly transient, strong and complex, with high in-cylinder pressure, increasing average discharge current can still improve the consistency and stability of ignition. The study indicates that the discharge current of the electric spark can be an effective criterion for evaluating the ignition performance.