In a biogas diesel predominantly premixed charge compression ignition (BDPPCCI) engine the effect of composition of biogas on combustion, performance and emissions was experimentally investigated. A twin cylinder automotive common rail engine with an open electronic control unit was run on one of its cylinders in this mode while the other cylinder was nonfunctional. Three biogas compositions with methane (CH4) proportions of 53-58% (as obtained from the plant), 67% and 22-25% were used at a constant engine speed of 1800 rpm. Stable engine operation in the BDPPCCI mode even with very low methane fractions was possible without adverse effects on efficiency and emissions. Reducing the methane proportion (i.e. high proportion of CO2) enabled the brake mean effective pressure (BMEP) to be extended from 4 bar to 5 bar with sufficient margin for start of injection (SOI). Lower CH4 (i.e. increased CO2 proportion) also allowed the use of retarded SOI for diesel which resulted in reduced smoke emissions. This not only improved the combustion phasing but also lowered the peak heat release rate leaving the thermal efficiency relatively unaffected. Results indicate that extremely low levels of NO and smoke can be reached in the BDPPCCI mode at the best efficiency operating condition if the biogas composition is altered based on the output. (C) 2017 Elsevier Ltd. All rights reserved.