A three-zone heat-release rate model for dual-fuel combustion

Dual-fuel engines are modified compression ignition engines where the primary fuel source is provided by a gaseous fuel, and ignition is provided by a reduced quantity of diesel: the pilot. The generally accepted description of the subsequent combustion process is that initially, around half of the pilot will burn and entrain some gaseous fuel in an overall rich process. Then the remaining pilot fuel burns with an increasing amount of the primary fuel until the final stage where a flame propagation process engulfs the remaining gaseous fuel. In this paper a three-zone model will be derived, benchmarked against data obtained for diesel combustion, and then applied to experimental data from a dual-fuel test program. The three-zone model will also be used to investigate the assumptions made regarding the mechanism of dual-fuel combustion proceeding in three-stages. It will be demonstrated that evidence was scant in support of the accepted description of three stages of dual-fuel combustion in a direct injection engine. The conclusion of this work is that that the dual-fuel combustion may be better considered as a diesel combustion process, where the gaseous fuel modifies the reaction zone surrounding each igniting droplet of the pilot fuel.