An experimental programme examining performance and emissions from spark- and compression-ignition engines, running on a variety of bio-fuels, including simulated bio-gas and commercial seed oil is presented. Both engines were single-cylinder laboratory-type engines of comparable power output having variable speed and load capability, the spark-ignition engine additionally having variable compression ratio. For bio-gas, containing carbon dioxide, emissions of oxides of nitrogen were reduced relative to natural gas, while unburnt hydrocarbons were increased. Brake power and specific fuel consumption changed little and carbon monoxide was predominantly affected by air:fuel ratio. Equivalent effects were demonstrated with nitrogen replacing carbon dioxide in the simulated bio-gas and similar trends were evident as compression ratio was increased. Seed-oil bio-fuel gave similar performance to diesel fuel without major disadvantages, other than an increased specific fuel consumption. Tests with cetane and rape-seed methyl ester bio-diesel are also presented for comparison. Specific fuel consumption was about the same and specific NOx emissions were lower with bio-fuel than results from the spark-ignition engine tests running on biogas. (c) 2006 Elsevier Ltd. All rights reserved.