The purpose of this investigation is to reduce the exhaust emissions in a diesel engine without any penalty in the combustion performance using a dual-fuel combustion strategy. The in-cylinder direct injection for diesel and the port injection for gasoline and biogas were applied in a single cylinder diesel engine. The diesel used as the in-cylinder injection source was injected at a very early injection timing (before top dead center (BTDC) 40 degrees), and the biogas and gasoline were injected around the top dead center (TDC). Based on the experimental results, it was revealed that the increase of the port injection ratio caused the increase of IMEP in dual-fuel combustion with very early in-cylinder injection timing. In particular, the high rate of port injection can achieve the level of the single combustion of the conventional diesel (e.g., BTDC 5 degrees injection timing). The increase of the port injection ratio caused the increase of the ignition delay. In addition, the increasing width of the ignition delay in diesel-biogas DFC is higher than diesel-gasoline DFC. The NOx and soot emission can be simultaneously reduced by the application of dual-fuel combustion as well as the reduction of the total particle number. The HC and CO emissions in DFC are higher than the conventional single diesel combustion but it is lower than early injection diesel combustion. (C) 2016 Elsevier Ltd. All rights reserved.