The present work evaluates the effects of using blends of diesel fuel with cottonseed or sunflower oils and their (methyl ester) bio-diesel in proportions of 10% and 20% (by vol.), on the combustion and emissions behavior of a fully instrumented, six-cylinder, turbocharged and after-cooled, heavy-duty, direct injection (DI), 'Mercedes-Benz' diesel engine. Combustion chamber and fuel injection pressure diagrams are obtained at two speeds and three loads. A heat release analysis of the experimentally obtained cylinder pressure diagrams is developed and used. Plots of histories in the combustion chamber of the heat release rate and temperatures, and the variation of interesting quantities such as maximum cylinder pressures and their rates, maximum cylinder temperatures and ignition delays reveal some interesting features, which shed light into the combustion mechanism and emissions formation when using these bio-fuels. The analysis results, together with the differing physical and chemical properties of these bio-fuels against those for the diesel fuel, which constitutes the 'baseline' fuel, aid the correct interpretation of the basic regulated emissions of smoke and nitrogen oxides measured at the engine exhaust. (C) 2012 Elsevier Ltd. All rights reserved.