In this study, waste cooking oil from restaurants was used to produce a renewable and sustainable biodiesel through transesterification process. The Treated Waste Cooking Oil (TWCO) fuel produced has shown very promising chemical and physical properties; most notably; cetane number (similar to 49) and sulphur content (8 mg/kg). The combustion of conventional diesel and Treated Waste Cooking Oil (TWCO)-diesel blend (50/50 by volume; shown as TD50) was examined at different engine conditions. The combustion of TD50 resulted in a considerable reduction in the smoke opacity and unburnt hydrocarbons associated with an increase in the CO2 and NOx emissions due to unintentional advance of fuel injection timing, caused by the higher bulk modulus of TD50 fuel. Results indicated an increase in brake specific fuel consumption with simultaneous reduction in the engine thermal efficiency compared to conventional diesel. The lower smoke opacity in the case of TD50 fuel assists NOx reductions by exploiting the higher EGR tolerance of TD50 fuel. Thus, a more encouraging NOx-smoke tradeoff may be selected to reduce both NOx and smoke. The effect of EGR in NOx reduction was more noticeable with 1050 than conventional diesel fuel. (C) 2011 Elsevier B.V. All rights reserved.