Stratified hydrogen combustion with various mixing processes

Hydrogen is recognized as a key alternative fuel for mitigating greenhouse-gas emissions owing to its high fuel efficiency and carbon-free combustion. In the stratified charge combustion (SCC) mode, ensuring optimal air-fuel mixing in the combustion chamber is crucial because the local equivalence ratio has a dominant influence on combustion characteristics. Therefore, this study aims to build a detailed understanding of stratified hydrogen combustion under various local equivalence ratios. Laser-induced breakdown spectroscopy (LIBS) was used to measure the local equivalence ratios in hydrogen jets at different mixture-formation times (MFTs) and laser-ignition points (LIPs). The results showed that shorter MFTs induced highly stratified mixtures with elevated local equivalence ratios exceeding 2.0, enhancing the laminar flame speed and maximizing the conversion of chemical energy into pressure gain, resulting in a representative total heat release over three times higher compared to longer MFTs. Furthermore, ignition near the injector tip produced leaner mixtures with equivalence ratios around 0.3, whereas downstream LIPs generated peak local equivalence ratios around 2.0, facilitating rapid flame propagation and increased heat release by 25 %. • Local equivalence ratio critically influences hydrogen combustion characteristics. • Short MFT enhances stratification, boosting flame speed and pressure conversion. • Optimal LIP location maximizes hydrogen stratification and combustion efficiency. • Schlieren and LIBS effectively analyze hydrogen SCC and local equivalence ratios. • Efficient SCC requires optimizing local equivalence ratio and stratification levels.