Automobiles emit many pollutants. These pollutants become a big threat to our environment. This study examines the influence of oxygen enriched hydrogen (OEH) gas as a combustion catalyst in a DI diesel engine operating with varying injection times of a diesel fuel. For this study, OEH gas was produced by the process of electro-chemical dissociation of water. The OEH gas at 4.6 litres per minute (lpm) was aspirated into an engine cylinder along with intake air at varied injection times of a diesel fuel. Three injection times were selected. The first was the standard injection time of 23°BTDC (before top dead centre) recommended by the engine manufacturer, the second one was the retarded injection time of 19°BTDC and the third one was the advanced injection time of 27°BTDC. When the OEH gas was inducted at 100% rated load of the engine at the standard injection time, the brake thermal efficiency increased by 16.45% and the emissions of oxides of nitrogen (NOX) increased by 16.9%. All other engine-out emissions, such as carbon monoxide (CO), unburned hydrocarbon (UBHC) and smoke, were reduced by 15.38%, 19.7% and 28.57%, respectively, compared to diesel combustion. At the advanced injection time, the brake thermal efficiency and the NOX emission were increased by 19.03% and 21.42%, respectively. Other engine-out emissions, such as CO, UBHC and smoke, were reduced by 11.53%, 22.72% and 30.95%, respectively. However, at the retarded injection time, the brake thermal efficiency increased by 12.21% and engine-out emissions, such as CO, UBHC, NOX and smoke, were reduced by 7.69%, 12.12%, 9.04% and 19.04%, respectively. From the data, it is evident that the diesel engine can be operated efficiently using the OEH gas as a combustion catalyst with the optimized injection timing of a diesel fuel. © The Royal Society of Chemistry 2015.