Effects of Hydrogen Injection Strategies on Combustion Characteristics in a Jet-Ignition Natural Gas Engine

To alleviate the challenges of ignition difficulty， slow flame propagation， and knock tendency in large-bore natural gas engines operating under lean-burn conditions， the effects of hydrogen-assisted turbulent jet ignition on reliable ignition and significantly acceleration of flame development were studied by computational fluid dynamics. The effects of single-injection strategies and double-injection strategies on jet characteristics and combustion performance were investigated at a fixed hydrogen energy fraction of 9%， with injection timings set at 330° and 90° crankshaft angle before top dead center （BTDC） of the pison compression stroke. The results show that the pressure difference between the pre-chamber and the main chamber increases initially and then decreases during the jet-ignition process. The single early-injection strategy exhibits the highest jet-flame velocity and temperature. Under the double-injection strategy， local flame quenching occurs near the orifices， and ignition is governed by the jet-flame mechanism. The jet flame develops a hydrogen-wrapped structure and exhibits combustion stratification characterized by locally fuel-rich zones and globally lean-burn regions. Compared with the baseline engine， the double-injection strategy increases the indicated thermal efficiency to 44.02%. As the mass of hydrogen introduced during the late injection （90° BTDC） increases， more unburned hydrogen is transported directly into the main chamber for combustion， resulting in an extended jet-ignition duration. At similar indicated thermal efficiency， the NO_x specific emissions decrease by 24.26%.