Experimental investigations were carried out on a single cylinder diesel engine under dual fuel mode (biodiesel-simulated biogas). The pongamia pinaata biodiesel (B100) was used as pilot fuel whereas biogas composition was simulated by blending of CO2 (30%, 40% and 50% by weight) in natural gas and was used as main fuel. Natural gas and CO2 were injected into the engine's intake manifold using two gas injectors with individual control mechanism. The brake thermal efficiency (BTE) of the engine decreases with increase in CO2 content in natural gas. NOx and smoke emissions decreased significantly with the biogas-fueled engine whereas HC and CO emissions increased marginally. The combustion characteristics including heat release rate and cumulative heat release decreased with biogas. A notable point emerged from this study is the effect of CO2 in natural gas on the performance, combustion and emissions characteristics of the engine are less as compared to the effect of its energy share and load. It is well established through this study that the raw biogas could be used in a biodiesel-fueled diesel engine under dual fuel mode. However, increase in CO and HC emissions needs to be reduced using suitable techniques. (C)2017 Elsevier Ltd. All rights reserved.