Conditional Moment Closure Modelling for Dual-Fuel Combustion Engines with Pilot-Assisted Compression Ignition

Dual-fuel combustion is an attractive approach for utilizing alternative fuels such as natural gas in compression-ignition internal combustion engines. In this approach, pilot injection of a more reactive fuel provides a source of ignition for the premixed natural gas/air. The overall performance combines the high efficiency of a compression-ignition engine with the relatively low emissions associated with natural gas. However the combustion phenomena occurring in dual-fuel engines present a challenge for existing turbulent combustion models because, following ignition, flame propagates through a partially-reacted and inhomogeneous mixture of the two fuels. The objective of this study is to test a new modelling formulation that combines the ability of the Conditional Moment Closure (CMC) approach to describe autoignition of fuel sprays with the ability of the G-equation approach to describe the subsequent flame propagation. The effects of partially-ignited fuel on the flame propagation speed is taken into account by a new laminar flame speed model. This methodology can be used for the full range of fuel substitution from perfectly-premixed through to pure diesel operation. The hybrid modelling approach is used to simulate n-heptane pilot jet-ignited combustion of a premixed methane air charge in a rapid compression-expansion machine apparatus. The results show that the hybrid model adequately captures ignition and transition to premixed flame propagation, and the sensitivities of the predictions to the flame speed modelling and ignition criteria are explored.