Abstract

Dual-fuel engines are modified compression ignition engines, where the primary source of fuel is a gaseous fuel, and ignition is provided by a ‘pilot’ injection of a reduced quantity of diesel. The generally accepted understanding of the dual-fuel engine describes its combustion process as proceeding in three stages. Initially, around half of the pilot will burn and entrain some gaseous fuel into an overall fuel-rich process. Subsequently, the remaining pilot fuel burns and entrains an increasing amount of the primary fuel into its reaction zone. In the final stage, a flame propagation process engulfs the remaining gaseous fuel.

In this article, a three-zone model for the analysis of heat-release rate during the dual-fuel combustion process will be derived. This model will be tested against data obtained for diesel combustion and then applied to experimental data from a dual-fuel test program. It will be shown that there is little evidence to support the generally accepted description of the dual-fuel combustion process in a direct injection engine. The conclusion of this work is that 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.