Development of a numerical model for ignition phenomena in a micro pilot ignited dual fuel engine with external mixture formation

In this paper, a numerical investigation of the ignition process of dual fuel engines is presented. Optical measurements revealed that a homogeneous natural gas charge ignited by a small diesel pilot comprises the combustion phenomena of compression ignition of the pilot fuel as well as premixed flame propagation. The 3-Zones Extended Coherent Flame Model (ECFM3Z) was selected, since it can treat auto-ignition, premixed flame propagation and diffusion flame aspects. Usually combustion models in multi-dimensional computational fluid dynamics (CFD) software packages are designed to handle only one reactive species representing the fuel concentration. In the context of the ECFM3Z model the concept of a multi-component fuel is applied to dual fuel operation. Since the available ignition models were not able to accurately describe the ignition characteristics of the investigated setup, a new dual fuel auto-ignition model was developed. Ignition delay times of the fuel blend are tabulated using a detailed reaction mechanism for n-heptane. Thereby, the local progress of pre-ignition reactions in the CFD simulation can be calculated. The ignition model is validated against experiments conducted with a periodically chargeable constant volume combustion chamber. The proposed model is capable to reproduce the ignition delay as well as the location of the flame kernels. The CFD simulations show the effect of temperature stratification and variations in the injection pressure on the apparent ignition delay of the micro pilot.