In a previous study, the authors compared the fuel-air mixing characteristics of gas jets and sprays in Diesel engine environments in the absence of combustion. A three-dimensional model for flows and sprays was used. It was shown that mixing was slower in gas jets relative to fast-evaporating sprays. In this study, which is an extension of the previous one, the direct-injection of gasesous methane, gaseous tetradecane and liquid tetradecane are studied using the same three-dimensional model. This study concentrates on combustion. It is shown that the fuel-air mixing rate and hence the burning rate are initially slower with gas injection. But subsequently, the relative burning rate of the two jets depends on the vaporization rate of the spray, on injection timing (through the chamber gas density), on ignition delay (through the degree of premixed combustion), on the enhancement of turbulence by combustion and on the shape of the combustion chamber that influences the availability of oxygen that the two jets need to entrain for combustion. It would appear that in direct injection of methane and liquid fuels, similar overall combustion rates can be achieved but the optimal configurations of the system made up of injection, in-cylinder flow and chamber geometry will be different for the two jets and fuels.