Dimensionless Quantities in Pre-chamber Turbulent Jet Ignition of Premixed Methane/Air

Stringent oxides of nitrogen (NO_x) and particulate matter (PM) emission standards and regulations, increased consumer concern about greenhouse gas (GHG) emissions and cost-effective competition with battery-electric vehicles (BEV) have forced the automotive engine manufacturers to search for novel and innovative strategies. A viable approach for clean, high-efficiency operation for modern spark-ignited (SI) gasoline or natural gas engines is downsized boosted ultra-lean operation. Ultra-lean ignition process, however, gives rise to some serious challenges. Ignition becomes increasingly difficult with the leaner mixture. Moreover, engine operation becomes susceptible to misfires, cycle-to-cycle variability, reduced efficiency, increase unburned hydrocarbon (UHC) emission, etc. Turbulent jet ignition (TJI)—where pre-chamber combustion generated turbulent jet consisting of hot combustion products and active radicals ignite a lean mixture—is a robust yet straightforward ignition strategy that has potential to solve challenges encountered at lean-burn operation. The current study explores the fundamentals as well as the scalability aspect of the TJI strategy. Even though the TJI system is easy to implement, the fluid mechanics and chemical kinetics processes involved in TJI are incredibly complicated due to the complex coupling between turbulence and chemistry. Ignition and flame propagation behavior of TJI were studied in custom-built, optically accessible, constant-volume vessel for lean (ϕ= 0.55 - 0.95 ) methane/air using simultaneous high-speed schlieren and OH* chemiluminescence imaging. Detailed characteristics of two distinct types of ignition mechanisms—jet ignition and flame ignition—and their dependence on the critical thermophysical and geometric parameters were discussed. The impact of four significant parameters—initial pressure, nozzle diameter, pre-chamber equivalence ratio and pre-chamber volume—was carefully examined. An in-depth discussion of various competing pre-chamber processes on TJI emphasized the necessity of non-dimensionalization. The non-dimensionalization of governing equations for compressible reacting flow produced four dimensionless numbers associated with TJI—Reynolds, Schmidt, Prandtl and Damköhler number. Assuming the transport coefficients are independent of temperature and kinematic transport coefficients are directly proportional to the temperature yielded unity Schmidt and Prandtl number. The remainder two non-dimensional numbers—Reynolds and Damköhler number, regulated the TJI mechanisms of premixed methane/air. Correlations were developed for TJI using Reynolds and Damköhler number. Lastly, the crucial factors and challenges in developing a scalable TJI system were addressed. These results provide useful insights about the TJI fundamentals and serve as a guideline for future pre-chamber design and optimization.