Natural gas (NG) is an attractive fuel for heavy-duty internal combustion engines because of its potential for reduced CO₂, particulate, and NO_X emissions and lower cost of ownership. Pilot-ignited direct-injected NG (PIDING) combustion uses a small pilot injection of diesel to ignite a main direct injection of NG. Recent studies have demonstrated that increased NG premixing is a viable strategy to increase PIDING indicated efficiency and further reduce particulate and CO emissions while maintaining low CH₄ emissions. However, it is unclear how the combustion strategies relate to one another, or where they fit within the continuum of NG stratification. The objective of this work is to present a systematic evaluation of pilot combustion, NG combustion, and emissions behavior of stratified-premixed PIDING combustion modes that span from fully-premixed to non-premixed conditions. A sweep of the relative injection timing, $\mathrm{RIT}$, of NG and pilot diesel was performed in a heavy-duty PIDING engine with $P_{\mathrm{inj}}$ = 140–220 bar, ${\varphi }_g$ = 0.47–0.71, and a constant NG energy fraction of 94%. Apparent heat release rate and emissions analyses identified interactions between the pilot fuel and NG, and qualitatively characterized the impact of NG stratification on combustion and emissions. Changes in the $\mathrm{RIT}$ resulted in six distinct PIDING combustion regimes, for all considered injection pressures and equivalence ratios: (i) RIT-insensitive premixed, (ii) stratified-premixed (early-cycle injection), (iii) NG jet impingement transition, (iv) stratified-premixed (late-cycle injection), (v) variable premixed fraction, and (vi) minimally-premixed. Parametric definitions for the bounds of each regime of combustion were valid for the wide range of $P_{\mathrm{inj}}$ and ${\varphi }_g$ investigated, and are expected to be relevant for other PIDING engines, as previously identified regimes agree with those identified here. This conceptual framework encompasses and validates the findings of previous stratified PIDING investigations, including optimal ranges of operation that provide significantly increased efficiency and lower emissions of incomplete combustion products.

天然气 (NG) 是一种对重型内燃机很有吸引力的燃料，因为它具有减少 CO ₂、微粒和 NO _X排放以及降低拥有成本的潜力。引燃直喷 NG (PIDING) 燃烧使用少量柴油引燃来点燃 NG 的主直喷。最近的研究表明，增加 NG 预混合是提高 PIDING 指示效率并进一步减少颗粒物和 CO 排放同时保持低 CH _{4的可行策略}排放量。然而，尚不清楚燃烧策略如何相互关联，或者它们在 NG 分层连续体中的位置。这项工作的目的是对从完全预混到非预混条件的分层预混 PIDING 燃烧模式的引燃、NG 燃烧和排放行为进行系统评估。扫描相对喷射时间，$\mathrm{RIT}$, NG 和试点柴油是在重型 PIDING 发动机中进行的$P_{\mathrm{注入}}$ = 140–220 巴，${\phi }_G$ = 0.47–0.71，NG 能量分数恒定为 94%。表观热释放率和排放分析确定了引燃燃料和天然气之间的相互作用，并定性描述了天然气分层对燃烧和排放的影响。的变化$\mathrm{RIT}$对于所有考虑的喷射压力和当量比，产生了六种不同的 PIDING 燃烧状态：(i) RIT 不敏感预混，(ii) 分层预混（早期循环喷射），(iii) NG 射流冲击过渡，(iv) 分层-预混（后期循环注入），（v）可变预混分数，和（vi）最低限度预混。每种燃烧状态边界的参数定义适用于广泛的范围$P_{\mathrm{注入}}$和${\phi }_G$调查，并预计与其他 PIDING 发动机相关，因为先前确定的制度与此处确定的制度一致。该概念框架包含并验证了先前分层 PIDING 调查的结果，包括可显着提高效率和降低不完全燃烧产物排放的最佳运行范围。