In this study, the effects on the performance and emissions of a diesel engine operated with excess air by increasing the intake air pressure were investigated. For this purpose, the performance and emissions of a single-cylinder, compression-ignition, naturally aspirated engine at constant speed (2800 rpm) and different loads (25%, 50%, and 100%) were experimentally investigated first fed by natural suction pressure (NA), and then by feeding it with additional intake air pressure of 5, 7.5, 10, 12.5, 15, 17.5 and 20 kPa. As a result of our experiments it was found that by increasing the intake air pressure at certain ratios, brake specific fuel consumption(bsfc)decreased by 18%, CO emission by 78%, HC emission by 70% and soot emission by 63%, while NOx emission is increased by 5%, and exhaust gas temperature decreased.