The present study deals with the simulation of a Diesel engine fuelled by natural gas/diesel in dual fuel mode to optimize the engine behaviour in terms of performance and emissions. In dual fuel mode, the natural gas is introduced into the engine’s intake system. Near the end of the compression stroke, diesel fuel is injected and ignites, causing the natural gas to burn. The engine itself is virtually unaltered, but for the addition of a gas injection system. The CO2 emissions are considerably reduced because of the lower carbon content of the fuel. Furthermore, potential advantages of dual-fuel engines include diesel-like efficiency and brake mean effective pressure with much lower emissions of oxides of nitrogen and particulate matter. In previous papers, the authors have presented some CFD results obtained by two 3D codes by varying the diesel/NG ratio and the diesel pilot injection timing at different loads. The calculations have been referred to a light duty direct injection diesel engine, of which some experimental data were available, obtained both in full diesel and Dual Fuel operating conditions. These data have allowed to realize a fitting of the models. The phenomena involved in the cylinder are very complex and the numerical results obtained demonstrate a strong dependence on the boundary conditions imposed at the cylinder control system, provided by experimental data. Therefore, a comprehensive simulation of all engine should be necessary, by testing numerous operating conditions. In fact, the reduced experimental test cases available do not allow an overall view of the engine behaviour in the different operating conditions and cannot provide appreciable inlet conditions in cylinder for 3D combustion calculations. At the same time, the 3D results can define some inputs (turbulence, combustion law, etc.) for the one-dimensional simulation of the entire system fora preliminary calibration of some engine parameters. In particular, the calculations have been made by using an advanced 1D engine cycle simulation software enable to carry out performance simulations based on virtually any intake, combustion, exhaust system and turbocharger design, at different operating conditions, by varying large number of parameters. Thecode is based on one-dimensional flow through ducts and zero-dimensional in-cylinder calculation. Detailed modelling of individual components is included to specify the phenomena in the singular components.