Transitioning between diesel only and diesel-piloted gaseous fuelled operating modes in a multimode i.c. engine

A method of transitioning between operating modes in a multimode engine including a diesel-only mode and a diesel piloted gas mode includes firstly terminating or initiating the supply of a gaseous fuel, depending on whether the system is transitioning to or from the pilot mode, and thereafter decreasing or increasing the diesel fuel supply quantity. Liquid fuel supply quantity is preferably altered in steps rather than discretely in order to avoid exceeding the lean limit of gas lambda. The number of steps and the percentage decrease or increase in each step preferably varies based at least in part on prevailing speed and load conditions. In fig.4, to transition from diesel-only to diesel piloted gas fuelling, firstly gas fuelling (curve 102) is increased immediately at time T1from zero to the quantity QGASDEMANDrequired for the prevailing conditions and, after a delay, diesel fuelling (curve 100) is reduced at time T3onwards from the initial level QDIESELDEMANDin steps 106 until reaching the pilot level QDIESELPILOTat time.