Hydrogen internal combustion engines present a promising path towards carbon neutrality, yet their development is challenged by abnormal combustion phenomena like backfire and pre-ignition. These phenomena limit engine torque and reduce component reliability. This study is aimed to elucidate the mechanisms behind these phenomena in hydrogen internal combustion engines. We utilized a multi-cylinder engine with optical access for direct high-speed imaging of in-cylinder processes to visualize backfire and pre-ignition. Initial analysis, combining visualization data with one-dimensional (1D) simulations, indicated that high temperatures of the ground electrode of the spark plug could be a key trigger factor for abnormal combustion.
To investigate this hypothesis, the surface temperature of the ground electrode was measured under firing conditions using a two-color thermometry system. The measurements revealed that the electrode temperature exceeded the compressed gas temperature near Top Dead Center (TDC). This finding suggests the possibility of hot surface ignition initiated by heat transfer from the hot electrode surface to the hydrogen-air mixture prior to the spark event. To mitigate this, the ground electrode material was replaced with a material with higher thermal conductivity to improve heat dissipation. Subsequent tests on the multi-cylinder engine confirmed the effectiveness of this modification, and the spark plug with the high-conductivity ground electrode showed a significant reduction in pre-ignition frequency. These results establish that the ground electrode temperature is one of the factors contributing to abnormal combustion, particularly pre-ignition, in hydrogen engines. This study provides valuable insights into mitigating abnormal combustion in hydrogen internal combustion engines, advancing their development towards more reliable and efficient operation, and supporting the broader goal of carbon neutrality.