New renewable fuels have been developed for diesel engines, contributing to the reduction of carbon emissions and to fuel security. However, the combustion characteristics of these fuels and emissions still remain unclear. A study to investigate diesel engine performance integrated with biogas and hydrogen has been carried out. Biogas is principally a mixture of methane (CH4) and carbon dioxide (CO2) along with other trace gases. In this study the simulated gaseous biogas (60% CH4 and 40% CO2 vol.) and hydrogen (2% vol.) fuels were fed into the engine intake manifold and diesel fuel was injected into cylinder as a pilot ignition fuel. The effects of biogas and hydrogen showed reduced PM compared with diesel combustion. In addition, up to 39 and 33% reduction in total particulate mass and smoke, respectively, was seen. However, there was a slight increase in particle number when gaseous fuel addition was used, where the particulate size distributions have moved towards the nucleation region thus benefitting the after-treatment systems. Reduced thermal efficiency was observed for the gaseous fuel addition.