Lean combustion of stratified hydrogen in a constant volume chamber

•Hydrogen stratified charge combustion was realized in a constant volume chamber with 10 MPa direct injection.•The effects of the mixture formation time and ambient density on the hydrogen energy conversion process was investigated.•The local equivalence ratio of hydrogen charge was the main parameter for efficient energy extraction by combustion.•Spray morphology plays a major role in flame development and rise time.•The visible-spectrum emissions were measured during the hydrogen-rich combustion. To mitigate carbon emissions from the transportation sector, the application of hydrogen as a fuel for internal combustion engines was investigated. Owing to the merits of employing hydrogen in lean-burn spark-ignition engines, hydrogen stratified charge combustion (SCC) was tested in a constant-volume chamber. The mixture formation time (MFT) and ambient density were modulated to analyze their effects on the hydrogen SCC. Additionally, optical diagnostics were conducted to understand the behavior of the hydrogen flame. Short and long MFTs resulted in high and low burning velocities, respectively, based on the degree of fuel stratification. When the ambient density was high, nonlinear combustion characteristics were detected owing to the collapsed structure of the hydrogen jet. The measurement of a red-colored visible emission from the hydrogen SCC could be evidence of a hydrogen-rich combustion area. The local equivalence ratio and morphology of the hydrogen jet were the key parameters for the hydrogen SCC. By understanding the effects of MFT and ambient density on hydrogen SCC, high-efficient and clean transportation may be achievable with optimized injection and ignition timings.