Effects of spark plug type and ignition energy on combustion performance in an optical SI engine fueled with methane

Highlights: • Lean combustion is investigated in an optical engine fueled with methane. • There are close correlations between spark plug types and ignition energy with combustion stability. • Multi-claw spark plugs behave the optimal performance. • High ignition energy has a greater influence on combustion instability. • Combustion improvement is contributed by faster and lower variation of early flame. -- Abstract: Increasing thermal efficiency and reducing cycle-to-cycle variations are the main challenges for modern natural gas engines. In this study, the role of different spark-ignition methods in improving engine performance was investigated in an optical engine fueled with methane, and cycle-to-cycle variations and flame evolutions were addressed under lean combustion conditions. The experimental results show that there are close correlations between spark plug types and ignition energy with combustion stability: under medium ignition energy conditions, multi-claw spark plugs behave the optimal performance in combustion stability and thermal efficiency, whereas different spark plug types show almost identical capability in combustion performance under both low and high ignition energy conditions. Meanwhile, compared with spark plug types, high ignition energy has a greater influence on combustion stability improvement. Optical flame images show that the improved combustion stability is contributed by faster flame development. To identify the fundamental reasons, an empirical criterion of mass fraction burned was adopted to quantify early flame evolutions. It shows that multi-claw spark plug and high ignition energy mainly promote initial flame propagation, manifesting lower cycle-to-cycle variations in this combustion stage. The results shall give insights into the application of multi-claw spark plug and high ignition energy on the improvement of natural gas engine performance while controlling cyclic variation.