Dual-fuel (DF) engines, in which premixed natural gas and air in an open-type combustion chamber is ignited by diesel-fuel pilot sprays, have been more popular for marine use than pre-chamber spark ignition (PCSI) engines because of their superior durability. However, control of ignition and combustion in DF engines is more difficult than in PCSI engines. In this context, this study focuses on the ignition stability of n-heptane pilot-fuel jets injected into a compressed premixed charge of natural gas and air at low-load conditions. To aid understanding of the experimental data, chemical-kinetics simulations were carried out in a simplified engine-environment that provided insight into the chemical effects of methane (CH₄) on pilot-fuel ignition. The simulations reveal that CH₄ has an effect on both stages of n-heptane autoignition: the small, first-stage, cool-flame-type, low-temperature ignition (LTI) and the larger, second-stage, high-temperature ignition (HTI). As the ratio of pilot-fuel to CH₄ entrained into the spray decreases, the initial oxidization of CH₄ consumes the OH radicals produced by pilot-fuel decomposition during LTI, thereby inhibiting its progression to HTI. Using imaging diagnostics, the spatial and temporal progression of LTI and HTI in DF combustion are measured in a heavy-duty optical engine, and the imaging data are analyzed to understand the cause of severe fluctuations in ignition timing and combustion completeness at low-load conditions. Images of cool-flame and hydroxyl radical (OH*) chemiluminescence serve as indicators of LTI and HTI, respectively. The cycle-to-cycle and spatial variation in ignition extracted from the imaging data are used as key metrics of comparison. The imaging data indicate that the local concentration of the pilot-fuel and the richness of the surrounding natural-gas air mixture are important for LTI and HTI, but in different ways. In particular, higher injection pressures and shorter injection durations increase the mixing rate, leading to lower concentrations of pilot-fuel more quickly, which can inhibit HTI even as LTI remains relatively robust. Decreasing the injection pressure from 80 MPa to 40 MPa and increasing the injection duration from 500 µs to 760 µs maintained constant pilot-fuel mass, while promoting robust transition from LTI to HTI by effectively slowing the mixing rate. This allows enough residence time for the OH radicals, produced by the two-stage ignition chemistry of the pilot-fuel, to accelerate the transition from LTI to HTI before being consumed by CH₄ oxidation. Thus from a practical perspective, for a premixed natural gas fuel–air equivalence-ratio, it is possible to improve the “stability” of the combustion process by solely manipulating the pilot-fuel injection parameters while maintaining constant mass of injected pilot-fuel. This allows for tailoring mixing trajectories to offset changes in fuel ignition chemistry, so as to promote a robust transition from LTI to HTI by changing the balance between the local concentration of the pilot-fuel and richness of the premixed natural gas and air. This could prove to be a valuable tool for combustion design to improve fuel efficiency or reduce noise or perhaps even reduce heat-transfer losses by locating early combustion away from in-cylinder walls.

双燃料（DF）发动机在开放式燃烧​​室中将天然气和空气预混合，并由柴油燃料引燃剂点燃，因此在船用领域比预燃室火花点火（PCSI）发动机更受欢迎。其卓越的耐用性。但是，与PCSI发动机相比，DF发动机的点火和燃烧控制更加困难。在这种情况下，本研究着重于在低负荷条件下注入压缩的天然气和空气预混合装料中的正庚烷引燃燃料喷嘴的点火稳定性。为了帮助理解实验数据，在简化的发动机环境中进行了化学动力学模拟，从而深入了解了甲烷（CH ₄）对引燃燃料的化学作用。模拟表明CH ₄对正庚烷自燃的两个阶段都有影响：较小的第一阶段，冷火焰类型的低温点火（LTI）和较大的第二阶段，高温点火（HTI）。随着引燃燃料与喷雾中夹带的CH ₄的比例降低，CH ₄的初始氧化消耗了LTI期间由引燃燃料分解产生的OH自由基，从而抑制了其发展为HTI。使用成像诊断程序，可在重型光学引擎中测量DF燃烧中LTI和HTI的时空变化，并分析成像数据以了解低负荷条件下点火正时和燃烧完整性严重波动的原因。冷焰和羟基自由基（OH *）化学发光的图像分别用作LTI和HTI的指示剂。从成像数据中提取的点火的逐周期和空间变化被用作比较的关键指标。成像数据表明，对于LTI和HTI，引燃燃料的局部浓度和周围天然气混合气的浓郁度很重要，但方式不同。特别是，较高的喷射压力和较短的喷射持续时间可提高混合速率，从而更快地降低引燃燃料的浓度，即使LTI保持相对稳定，也可以抑制HTI。将喷射压力从80 MPa降低到40 MPa，将喷射持续时间从500 µs增加到760 µs，可以保持恒定的引燃燃料质量，同时通过有效地降低混合速率来促进从LTI到HTI的稳定过渡。这为飞行员燃料的两阶段点火化学产生的OH自由基留有足够的停留时间，以加速从LTI到HTI的过渡，然后被CH消耗掉。将喷射压力从80 MPa降低到40 MPa，将喷射持续时间从500 µs增加到760 µs，可以保持恒定的引燃燃料质量，同时通过有效地降低混合速率来促进从LTI到HTI的稳定过渡。这为飞行员燃料的两阶段点火化学产生的OH自由基留有足够的停留时间，以加速从LTI到HTI的过渡，然后被CH消耗掉。将喷射压力从80 MPa降低到40 MPa，将喷射持续时间从500 µs增加到760 µs，可以保持恒定的引燃燃料质量，同时通过有效地降低混合速率来促进从LTI到HTI的稳定过渡。这为飞行员燃料的两阶段点火化学产生的OH自由基留有足够的停留时间，以加速从LTI到HTI的过渡，然后被CH消耗掉。₄氧化。因此，从实际的角度来看，对于预混天然气燃料-空气当量比，可以通过仅操纵引燃燃料喷射参数，同时保持恒定的引燃燃料质量来改善燃烧过程的“稳定性”。这允许定制混合轨迹以抵消燃料着火化学的变化，从而通过改变引燃燃料的局部浓度与预混天然气和空气的浓度之间的平衡来促进从LTI到HTI的稳健过渡。这可能被证明是用于燃烧设计的有价值的工具，可通过将早期燃烧定位在缸内壁之外来提高燃油效率或降低噪音，甚至降低传热损失。