Multi-stage heat release is an important feature of hydrocarbon auto-ignition that influences engine operation. This work presents findings of previously unreported three-stage heat release in the auto-ignition of n-heptane/air mixtures at lean equivalence ratios and high pressures. Detailed homogenous gas-phase chemical kinetic simulations were utilized to identify conditions where two-stage and three-stage heat release exist. Temperature and heat release profiles of lean n-heptane/air auto-ignition display three distinct stages of heat release, which is notably different than two-stage heat release typically reported for stoichiometric fuel/air mixtures. Concentration profiles of key radicals (HO₂ and OH) and intermediate/product species (CO and CO₂) also display unique behavior in the lean auto-ignition case. Rapid compression machine measurements were performed at a lean equivalence ratio to confirm the existence of three-stage heat release in experiments. Laser diagnostic measurements of CO concentrations in the RCM indicate similar concentration-time profiles as those predicted by kinetic modeling. Computational singular perturbation was then used to identify key reactions and species contributing to explosive time scales at various points of the three-stage ignition process. Comparisons with two-stage ignition at stoichiometric conditions indicate that thermal runaway at the second stage of heat release is inhibited under lean conditions. H + O₂ chain branching and CO oxidation reactions drive high-temperature heat release under stoichiometric conditions, but these reactions are suppressed by H, OH, and HO₂ radical termination reactions at lean conditions, leading to a distinct third stage of heat release.

多级放热是碳氢化合物自燃的重要特征，会影响发动机的运行。这项工作提出了在稀薄当量比和高压下正庚烷/空气混合物自燃过程中未报告的三阶段放热的发现。详细的均相气相化学动力学模拟用于确定存在两级和三级放热的条件。稀正庚烷/空气自动点火的温度和放热曲线显示出三个不同的放热阶段，这与化学计量燃料/空气混合物通常报告的两阶段放热显着不同。关键自由基（HO ₂和OH）和中间/产物种类（CO和CO ₂）的浓度曲线）还可以在稀薄自动点火情况下显示出独特的行为。在稀薄当量比下进行快速压缩机测量，以确认实验中存在三级放热。RCM中CO浓度的激光诊断测量表明，与动力学模型预测的浓度-时间曲线相似。然后，使用计算奇异摄动来识别在三阶段点火过程的各个点处导致爆炸时间尺度的关键反应和物质。在化学计量条件下与两阶段点火的比较表明，在稀薄条件下，第二阶段放热的热失控受到抑制。H + O ₂链支化反应和CO氧化反应在化学计量条件下驱动高温放热，但是在稀薄条件下，这些反应会受到H，OH和HO ₂自由基终止反应的抑制，从而导致明显的第三阶段放热。