The engine parameters, air flow rate, fuel flow rate and emission parameters, unburned hydrocarbon, carbon monoxide, nitrogen oxide and smoke opacity are measured using the above instruments. Test conditions were designed to investigate the effect of oxygen concentration on engine performance and emission characteristics. Tests were carried out at different loadings starting from no load to the rated capacity of the engine with an incremental loading of 20%, at a constant speed of 1500 RPM. Consistency and repeatability of the engine operating conditions were ensured by first running it for approximately 10 minutes at 1500 rpm at 50% load until exhaust gas temperature reached 250 °C. Once these conditions are achieved, the test engine was brought to the required test condition and then allowed for at least two minutes before collecting the data. Four different levels of oxygen concentration, 21% (ambient air), 23%, 25% and 27% by volume, were used for the inlet air. The fuel injection timing and injection pressure were maintained at original setting while adding oxygen to the intake air.