Investigations of diesel and natural gas injection interaction on combustion characteristics of a high-pressure direct-injection dual-fuel engine based on large eddy simulation

HPDI (high-pressure direct-injection) with pilot ignition is modern technology developed for heavy-duty natural gas engines. The dynamics of coherent flow structures due to diesel and natural gas jet play a significant role on ignition characteristics. In this study, a large eddy simulation (LES) framework coupled with chemistry solver is conducted for three-dimensional modelling of the thermal process of a HPDI engine. By integrating the Dynamic Mode Decomposition (DMD) algorithm, the break-up and attenuation process of unstable flow structures accompanied by different scale vortex formation and dissipation is able to be effectively demonstrated from fuel jet. The prime in-cylinder flow field structures from natural gas injection to its ignition is characterized by the vortex entrainment phenomenon resulting from the impingement between the natural gas jet and active products from diesel combustion. This phenomenon leads to enhanced heat transfer and exchange of active radicals by which the ignition of the natural gas is therefore facilitated, especially when angle β (the intersection angle between diesel and nature gas jet) is decreased. Moreover, the present study extends the ability of reaction-rate based global pathway analysis to evaluate the reactivity of OH additions to CH4/air mixture. In summary, the interactive dual fuel turbulent combustion process of the HPDI engine is theoretically elucidated, wherein the synergetic kinetics of vortex entrainment-mixing and chemical reaction facilitate the ignition of low reactivity natural gas.