Gaseous fuels have important emission and resource benefits but are not generally suitable for compression ignition (diesel) CI engines because of their low cetane numbers. However, the dual-fuel (DF) concept solves this problem. At the university of New South Wales (UNSW), fundamental studies are being followed with the development of a DF combustion bomb and a rapid compression machine. A three-dimensional (3D) finite volume code has been developed to describe their constant volume, DF combustion. The recent focus of the work simulates factors of importance in engines such as the effect of swirl, the duration of the ignition delay, and the gaseous fuel characteristics on the DF combustion process.