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Hydrogen is one of the most promising alternative fuels for the transportation industry. The use of hydrogen to enable lean burn in internal combustion engines is an attractive solution for reducing CO 2 emissions from two points of views: the substitution of carbon-based fuels and the increased thermal efficiency due to lean operation. Combining this strategy with the passive prechamber ignition system with gasoline/hydrogen blends is even more interesting. The main limitations of the passive pre-chamber concept in a high compression ratio spark-ignition engine were shown through engine experiments.A numerical study was then performed to evaluate the chance of extending the dilution limit by using hydrogen along with this technology. Results show how the use of hydrogen provides considerable benefits in the main chamber combustion process by enhancing the thermo-chemical properties of the mixture, increasing the flame speed, and improving the flame structure. Using an adequate gasoline-hydrogen blend proved to enable optimum burning rates at lean conditions, leading to a relevant thermal efficiency gain. K E Y W O R D Scomputational fluid dynamics, hydrogen combustion, passive pre-chamber, spark-ignition engine, ultra-lean combustion | INTRODUCTIONEvery year, the energy crisis associated with the world's demand of natural resources, used to power the industrial infrastructure of society, is becoming more urgent. 1 As human population grows, so does the consumption of energy coming from the limited fossil fuels that are available in the planet. Additionally, burning these fuels contributes to the production of greenhouse gases (GHG) like carbon dioxide (CO 2 ), hydro-carbon (HC), and other air-polluting emissions. These facts motivated several global treaties based on multiple research works, such as the Paris Agreement, 2 to aim for a decarbonization of the global energy network to improve sustainability and reduce pollution.List of Symbols/Abbreviations: SI, spark ignition; CI, compression ignition; TJI, turbulent jet ignition; HAJI, hydrogen assisted jet ignition; MC, main combustion chamber; PC, pre-chamber; ICE, internal combustion engine; H 2 ICE, hydrogen internal combustion engine; CFD, computational fluid dynamics; NO x , nitrogen oxides; H 2 , hydrogen; TWC, three-way catalyst; PFI, port fuel injection; DOHC, double over-head camshaft; CO 2 , carbon dioxide; λ, relative air-to-fuel ratio; RON95, 95 research octane number; CCV, cycle-to-cycle variability; TDC, top dead center; CAD, crank-angle degree; HRR, heat release rate; Δp, pressure difference between the pre-chamber and the main chamber; CA50, combustion phasing; IMEP, indicated mean effective pressure; σIMEP, variability of the indicated mean effective pressure; MAPO, maximum amplitude pressure oscillation; MBT, maximum break torque; ST, spark timing; URANS, unsteady Reynoldsaveraged Navier Stokes; ECFM, extended coherent flamelet model; TKE, turbulent kinetic energy; s L , laminar flame speed; l f , flame thickness; l t , turbulence length scale;...

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<div class="section abstract"><div class="htmlview paragraph">Pre-chamber spark ignition (PCSI) systems have been proven to improve combustion stability in highly-diluted and ultra-lean natural gas (NG) engine operation by providing spatially distributed ignition initiated by multiple turbulent flame-jets that lead to faster combustion compared to conventional spark ignition. This work investigates the physico-chemical processes that drive the ignition and subsequent combustion in the presence of combustion residuals (internal EGR) within the pre-chamber at varying EGR levels. The over-arching goal is to improve the dilution tolerance of PCSI systems for stoichiometric-operation of on-road heavy-duty natural gas engine. To this end, experiments were performed in a heavy-duty, optical, single-cylinder engine to explore the EGR dilution limits of a pre-chamber, spark-ignited, NG engine operated under stoichiometric conditions. A special skip-fire sequence is utilized to distinguish the effects of in-cylinder combustion residuals from external EGR. Optical diagnostics involving high-speed OH* chemiluminescence imaging and infrared (IR) imaging are acquired simultaneously to probe the ignition, development of pre-chamber jets and the subsequent combustion behavior of the main-chamber charge under various EGR dilution rates. The imaging results and the thermodynamic analysis show that the presence of combustion residuals within the pre-chamber lead to slower spark kernel development resulting in lower ΔP, retarded main-chamber ignition and subsequently slower main-chamber combustion. Cycle-to-cycle variations and tendency to misfire increased with increasing EGR dilution, with combustion residuals making this effect more pronounced. Advancing the spark timing, despite lowering ΔP (due to reduced fuel mass in the pre-chamber and increasing back pressure) can sufficiently compensate for reduced [O₂] at low to moderate EGR levels. Failed re-ignition of pre-chamber jets is the main cause of increased cycle-to-cycle fluctuations and misfire under the tested conditions.</div></div>