The effect of oxygen enriched air was tested for a diesel-biogas dual fuel engine. The operation and perforrnance characteristics, such as thermal efficiency, pollutant emissions and combustion parameters were determined. Experiments have been carried out with a stationary compression ignition (CI) engine coupled with a generator in dual mode using a typical biogas composition of 60 vol. %CH4 and 40% vol. %CO2. For every engine load evaluated, the oxygen concentration in the intake air engine was varied from 21% to 27% O-2 v/v. Ignition delay time and methane emissions were decreased when using oxygen enriched air with respect to normal air (21%O-2), whereas the thermal efficiency was increased. (c) 2012 Elsevier Ltd. All rights reserved.