Effects of various discharge strategies on ignition and combustion of lean natural gas mixture under the static and turbulent conditions

Diluted combustion can promote thermal efficiency and decrease NOx emissions for natural gas engines, but also presents high cycle-by-cycle variations. Enhancing the in-cylinder flow field is beneficial for the combustion stability, but brings challenges for the stable ignition. Therefore, advanced ignition strategies are critical for extending the diluted limit. In this work, by using the constant volume combustion vessel, rapid compression and expansion machine, and multi-cylinder natural gas engine, the lean methane/air or natural gas/air mixtures under the static and turbulent conditions are established to compare various discharge strategies. The comparisons include: (1) different discharge energies, (2) power and duration enhancements with similar discharge energies, (3) the continuous discharge and multi-strike. For the static condition, once the flame kernel can be successfully established, enhancing discharge power or duration has few effects on ignition. The continuous discharge and multi-strike show no clear differences. At the lean limit, the multi-strike shows better performance than the continuous discharge. For the turbulent condition, increasing discharge energy can promote the initial flame propagation. When the flow velocity around the spark plug increases to some extent, enhancing the discharge duration shows better performance than enhancing the discharge power. Moreover, the continuous discharge is better for the ignition processes than the multi-strike. For the multi-cylinder engine tests, compared to the single strike of one coil, the continuous discharge and multi-strike extend the lean limit from the excess air ratio of 1.51 to 1.61. The multi-strike is more beneficial for the combustion phase stability than the continuous discharge.