An internal combustion engine apparatus, comprising a combustion chamber, an intake valve for admitting an intake charge into said combustion chamber, a first fuel injector for injecting a first fuel directly into said combustion chamber, a second fuel injector for injecting a second fuel into said intake charge upstream of said intake valve, and a computer operatively connected with said first fuel injector and said second fuel injector to actuate injection of fuel respectively therefrom, wherein said first fuel is a gaseous fuel and said second fuel is a liquid fuel, and wherein said computer is programmed to - command a gaseous-to-liquid fuel ratio as a function of inputs comprising at least one of gaseous fuel storage pressure, gaseous fuel injection pressure, gaseous fuel mass, engine speed, engine torque, inlet air temperature, inlet air humidity, intake air manifold pressure, intake air mass, knock detection, operating history, torque command, and emissions, - select a gaseous-fuel operating mode when a quantity of gaseous fuel in a storage vessel is above an upper threshold, select a dual-fuel operating mode when the quantity of gaseous fuel is below the upper threshold and above a lower threshold and select a liquid-fuel operating mode when the quantity of gaseous fuel is below the lower threshold, - selectively actuate the first fuel injector to inject the gaseous fuel during the gaseous-fuel operating mode, the gaseous-to-liquid fuel ratio being mathematically undefined during the gaseous-fuel operating mode, - selectively actuate the second fuel injector to inject the liquid fuel during the liquid-fuel operating mode, the gaseous-to-liquid fuel ratio being zero during the liquid-fuel operating mode, and - selectively actuate said first and second fuel injectors (170; 350) during said dual-fuel operating mode, said gaseous-to-liquid fuel ratio being mathematically defined and having a value greater than zero during said dual-fuel operating mode. - command a gaseous-to-liquid fuel ratio as a function of inputs comprising at least one of gaseous fuel storage pressure, gaseous fuel injection pressure, gaseous fuel mass, engine speed, engine torque, inlet air temperature, inlet air humidity, intake air manifold pressure, intake air mass, knock detection, operating history, torque command, and emissions, - select a gaseous-fuel operating mode when a quantity of gaseous fuel in a storage vessel is above an upper threshold, select a dual-fuel operating mode when the quantity of gaseous fuel is below the upper threshold and above a lower threshold and select a liquid-fuel operating mode when the quantity of gaseous fuel is below the lower threshold, - selectively actuate the first fuel injector to inject the gaseous fuel during the gaseous-fuel operating mode, the gaseous-to-liquid fuel ratio being mathematically undefined during the gaseous-fuel operating mode, - selectively actuate the second fuel injector to inject the liquid fuel during the liquid-fuel operating mode, the gaseous-to-liquid fuel ratio being zero during the liquid-fuel operating mode, and - selectively actuate said first and second fuel injectors (170; 350) during said dual-fuel operating mode, said gaseous-to-liquid fuel ratio being mathematically defined and having a value greater than zero during said dual-fuel operating mode.