A thermodynamic cycle simulation model is described for a dual fuel open combustion chamber compression ignition engine. The model has been formulated for the compression, combustion, expansion, exhaust and intake processes. The combustion model is simulated by estimating the heat release rates due to the injected dissel fuel and the secondary fuel (biogas) supplied through the intake manifold. The injected fuel's heat release rate is computed from the preparation and burning rates, uhile that of the biogas is determined from the entrainmBnt of air and biogas mixture into the fuel spray. A finite difference scheme is employed to solve the gas exchange process for both the exhaust and intake systems. The overall comparison shous that the model satisfactorily predicted the experimental data.