Adding any inert gas as diluent to the inlet air reduces oxides of nitrogen (NOx) emissions. In this study, carbon dioxide (CO2) was used as diluent and introduced to the intake manifold of a diesel engine at a ratio of 2%, 4% and 6% respectively. The investigation was conducted on a four stroke, four-cylinder, indirect injection (IDI), turbocharged diesel engine and was concerned with the effect of using diluting CO2 in the intake manifold and injection pressure on engine torque, power, brake mean effective pressure, specific fuel consumption, carbon monoxide, smoke and NOx emissions. The tests have demonstrated that NOx is reduced by the introduction of CO2 in the inlet charge. The results are given in the graph. (c) 2004 Elsevier Ltd. All rights reserved.