The biogas diesel PPCCI (predominantly premixed charge compression ignition) mode where in biogas was inducted with air and diesel was injected into the cylinder using a common rail system was investigated. The injection timing of diesel which was in the range 55-70 degrees BTDC was found to be very influential as it affected combustion rate and phasing significantly. Though the best intake charge temperatures needed were in the range of 50-90 degrees C for best performance, operation even without charge heating was possible. For best performance about 80% of the total energy had to be supplied by biogas. At the best condition the thermal efficiency of the biogas diesel PPCCI mode was better than the biogas diesel DF (dual fuel) mode. HC (hydrocarbon) and NOx (nitric oxides) levels were in general significantly lower than the DF mode. The limited range of BMEPs (brake mean effective pressures) (2-4 bar) can be extended by combining the PPCCI mode with the normal diesel mode at low BMEPs and with the DF mode at high BMEPs. Thus PPCCI mode is an option for operating a diesel engine using biogas as the main fuel with low exhaust emissions. (C) 2015 Elsevier Ltd. All rights reserved.