Ignition in Pilot-Ignited Natural Gas Low Temperature Combustion: Multi-Zone Modeling and Experimental Results

In previous research conducted by the authors, the Advanced Low Pilot-Ignited Natural Gas (ALPING) combustion employing early injection of small (pilot) diesel sprays to ignite premixed natural gas-air mixtures was demonstrated to yield very low oxides of nitrogen (NO_x) emissions and fuel conversion efficiencies comparable to conventional diesel and dual fuel engines. In addition, it was observed that ignition of the diesel-air mixture in ALPING combustion had a profound influence on the ensuing natural gas combustion, engine performance and emissions. This paper discusses experimental and predicted ignition behavior for ALPING combustion in a single-cylinder engine at a medium load (BMEP = 6 bar), engine speed of 1700 rpm, and intake manifold temperature (T_in) of 75°C. Two ignition models were used to simulate diesel ignition under ALPING conditions: (a) Arrhenius-type ignition models, and (b) the Shell autoignition model. To the authors’ knowledge, the Shell model has previously not been implemented in a multi-zone phenomenological combustion simulation to simulate diesel ignition. The effects of pilot injection timing and T_in on ignition processes were analyzed from measured and predicted ignition delay trends. Experimental ignition delays showed a nonlinear trend (increasing from 11 to 51.5 degrees) in the 20°–60° BTDC injection timing range. Arrhenius-type ignition models were found to be inadequate and only yielded linear trends over the injection timing range. Even the inclusion of an equivalence ratio term in Arrhenius-type models did not render them satisfactory for the purpose of modeling ALPING ignition. The Shell model, on the other hand, predicted ignition better over the entire range of injection timings compared to the Arrhenius-type ignition delay models and also captured ignition delay trends at T_in = 95°C and T_in = 105°C. Parametric studies of the Shell model showed that the parameter A_p3, which affects chain propagation reactions, was important under medium load ALPING conditions. With all other model parameters remaining at their original values and only A_p3 modified to 8 × 10¹¹ (from its original value of 1 × 10¹³), the Shell model predictions closely matched experimental ignition delay trends at different injection timings and T_in.