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<p>Hydrogen-assisted combustion has emerged as a promising pathway for extending ultra-lean operating limits in spark-ignition engines; however, the physical mechanisms governing combustion stabilization under extreme dilution remain insufficiently understood. In particular, the existence of critical hydrogen concentrations separating partial combustion enhancement from fundamentally reorganized ultra-lean combustion behavior has not been systematically investigated. The present study experimentally examines hydrogen-assisted ultra-lean combustion in a dual-fuel spark-ignition engine over a wide range of air excess ratios (λ = 1.0–2.2) and hydrogen energy shares (0–40%HES). Combustion behavior was analyzed using a multi-level combustion-dynamics framework including cyclic variability, combustion phasing, pressure-rise evolution, heat-release topology, and thermodynamic response. The results reveal a pronounced threshold-type stabilization behavior rather than gradual linear improvement with increasing hydrogen fraction. A distinct transition region was identified between approximately 20% and 30%HES, separating dilution-sensitive combustion from a globally coherent ultra-lean combustion regime. Below the threshold, hydrogen enrichment primarily delayed instability onset but failed to suppress progressive deterioration of heat-release organization under increasing dilution. Above the threshold, however, the combustion process fundamentally reorganized into a dynamically coherent distributed combustion regime characterized by stabilized flame propagation, controlled instability growth, smooth heat-release evolution, and sustained thermodynamic effectiveness even at air excess ratios exceeding λ = 2.0. The findings demonstrate that stable ultra-lean combustion is governed more strongly by preservation of coherent heat-release organization than by maintenance of compact combustion duration alone. The identified threshold-controlled combustion transition therefore represents a physically meaningful restructuring of ultra-lean combustion dynamics rather than merely an extension of conventional lean-burn operation.</p>

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Threshold-Controlled Restructuring of Ultra-Lean Combustion Dynamics in a Hydrogen-Assisted Spark-Ignition Engine

  • 期刊论文

  • 出版模式：

    开放获取

  • 发表日期：

    2026年7月21日

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  • Michal Puškár,
  • Melichar Kopas

摘要

Hydrogen-assisted combustion has emerged as a promising pathway for extending ultra-lean operating limits in spark-ignition engines; however, the physical mechanisms governing combustion stabilization under extreme dilution remain insufficiently understood. In particular, the existence of critical hydrogen concentrations separating partial combustion enhancement from fundamentally reorganized ultra-lean combustion behavior has not been systematically investigated. The present study experimentally examines hydrogen-assisted ultra-lean combustion in a dual-fuel spark-ignition engine over a wide range of air excess ratios (λ = 1.0–2.2) and hydrogen energy shares (0–40%HES). Combustion behavior was analyzed using a multi-level combustion-dynamics framework including cyclic variability, combustion phasing, pressure-rise evolution, heat-release topology, and thermodynamic response. The results reveal a pronounced threshold-type stabilization behavior rather than gradual linear improvement with increasing hydrogen fraction. A distinct transition region was identified between approximately 20% and 30%HES, separating dilution-sensitive combustion from a globally coherent ultra-lean combustion regime. Below the threshold, hydrogen enrichment primarily delayed instability onset but failed to suppress progressive deterioration of heat-release organization under increasing dilution. Above the threshold, however, the combustion process fundamentally reorganized into a dynamically coherent distributed combustion regime characterized by stabilized flame propagation, controlled instability growth, smooth heat-release evolution, and sustained thermodynamic effectiveness even at air excess ratios exceeding λ = 2.0. The findings demonstrate that stable ultra-lean combustion is governed more strongly by preservation of coherent heat-release organization than by maintenance of compact combustion duration alone. The identified threshold-controlled combustion transition therefore represents a physically meaningful restructuring of ultra-lean combustion dynamics rather than merely an extension of conventional lean-burn operation.