Improving the thermal efficiency of spark ignition (SI) engines is strongly required due to its widespread use but considerably less efficiency than that of compression ignition (CI) engines. 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 increase with the level of air dilution, and become unacceptable. For improving the thermal efficiency by extending the lean-stability limit, this study examines the effects of spark discharge energy and in-cylinder turbulence level on lean limits and cycle-to-cycle variations of combustion for SI engine operation. The spark discharge energy was increased by a high-energy inductive ignition system using ten spark coils and the in-cylinder turbulence level was enhanced by a custom adapter installed in the intake port.

The results show that increased spark discharge energy by ten spark coils is effective at shifting the lean-stability limit to leaner operation, compared to that of a single spark coil. With shift of the lean-stability limit, significant improvement of thermal efficiency is observed, relative to that of stoichiometric operation. Furthermore, a combination of increased spark discharge energy and enhanced in-cylinder turbulence level makes it possible to allow stable operation at more extended lean-stability limit. This is mainly attributed to shortening the durations of spark timing-to-CA5 and CA10-to-CA90 by both increased spark discharge energy and enhanced in-cylinder turbulence level. However, the cycle-to-cycle variations of SI combustion increase with increasing excess-air ratio even for operation by ten spark coils with the intake port adapter. Finally, the relationship between the spark discharge energy and the SI combustion is examined and compared for ultra-lean operation without and with the intake port adapter. Although indicated thermal efficiency is improved by increased spark discharge energy, the variations of the spark discharge energy do not relate to the variations of the combustion, since total spark discharge energy does not affect both durations of the spark timing-to-CA5 and the CA10-to-CA90, and eventually the heat-release efficiency.

由于其广泛使用，因此迫切需要提高火花点火（SI）发动机的热效率，但其效率远低于压缩点火（CI）发动机。尽管相对于传统的化学计量SI发动机，稀薄SI发动机的运行可以显着提高热效率，但燃烧的周期变化随空气稀释水平的增加而增加，并且变得不可接受。为了通过扩展稀薄稳定性极限来提高热效率，本研究研究了火花放电能量和缸内湍流水平对稀薄极限和SI发动机运行的燃烧周期变化的影响。

结果表明，与单个火花塞相比，增加十个火花塞的火花放电能量可以有效地将稀薄稳定性极限转换为稀薄运行。随着稀薄稳定性极限的改变，相对于化学计量操作，观察到热效率的显着改善。此外，增加的火花放电能量和增强的缸内湍流度的组合使得可以在更扩展的稀燃稳定性极限下实现稳定运行。这主要归因于通过增加火花放电能量和增强缸内湍流水平来缩短火花正时至CA5和CA10至CA90的持续时间。然而，即使使用进气口适配器通过十个点火线圈运行，SI燃烧的循环变化也会随着过量空气比率的增加而增加。最后，检查了火花放电能量与SI燃烧之间的关系，并比较了无进气道适配器和有进气道适配器的超稀薄运行情况。尽管通过增加火花放电能量可以提高指示的热效率，但是火花放电能量的变化与燃烧的变化无关，因为总的火花放电能量不会同时影响到到达CA5的火花持续时间和到达CA10的持续时间。到CA90，最终达到放热效率。