A numerical study on the use of biogas and diesel in a dual-fueled directly-injected engine has been conducted. The objective of this study is to determine the effect of using biogas on engine performance, combustion, and emissions. The main fuel is biogas which is premixed with air in order to form a homogeneous mixture. The mixture is then compressed and ignited by injecting diesel fuel before TDC. The pilot fuel is expected to lead to multiple ignition points in the cylinder in order to achieve uniform combustion in the cylinder. The expected benefits are lower nitrogen oxides and soot compared to pure diesel combustion. Numerical simulations using CFD software were used to simulate fuel-air mixture, compression, fuel injection, combustion, and emissions. Different quantities of biogas and diesel were investigated to determine the optimum mixture ratio. Since biogas, which is natural gas produced from human waste, contains large quantities of carbon dioxide, the effect of carbon dioxide content in the fuel was investigated. The results of this study agree very well with results from other studies found in the literature.