Improving the thermal efficiency of spark ignition (SI) engine is strongly required due to its widespread use but considerably less efficiency than that of compression ignition (CI) engine. Although lean SI engine operation can offer substantial improvements of the thermal efficiency relative to that of traditional stoichiometric SI operation, the cycle-to-cycle variations of combustion increases with the level of air dilution, and becomes unacceptable. To improve the stability of lean operation, this study examines the effects of spark discharge pattern and tumble level on cycle-to-cycle variations of combustion at lean limits. The spark discharge pattern was altered by a custom inductive ignition system using ten spark coils and the tumble level was increased by a custom adapter installed in the intake port (tumble adapter). The results show that increased discharge energy by ten spark coils (without discharge offset) extends the lean limits with an improvement of indicated thermal efficiency (th), relative to that of stoichiometric operation. Furthermore, a combination of increased discharge energy and higher tumble level by tumble adapter makes it possible to allow stable operation at more extended lean limit, which is mainly attributed to shortening the duration of ST-to-CA5 and CA10-to-CA90. To examine the effect of spark discharge pattern on cycle-to-cycle variations of combustion at lean limits, the spark discharge pattern was altered for optimized combination of discharge energy and duration by adjusting the time offset between five discharges. The discharge offset of 0.25ms enables most stable ultra-lean operation, with COV of IMEPn less than 5% up to =2.0.
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