Analytical studies of oxygen-enriched diesel engine combustion have indicated the various benefits as well as the need for using cheaper fuels with water addition. To verify analytical results, a series of single-cylinder diesel engine tests were conducted to investigate the concepts of oxygen enriched air (OEA) for combustion with water emulsified fuels. Cylinder pressure traces were obtained for inlet oxygen levels of 21% to 35% and fuel emulsions with water contents of 0% to 20%. Data for emulsified fuels included no. 2 and no. 4 diesel fuels. The excess oxygen for the tests was supplied from compressed bottled oxygen connected to the intake manifold. The cylinder pressure data was collected with an AVL pressure transducer and a personal computer-based data logging system. The crank angle was measured with an optical encoder. In each data run, 30 consecutive cycles were recorded and later averaged for analysis. The data analysis was done with a heat release analysis code written for a personal computer.The results indicate that water emulsified fuels consistently gave a slight improvement in thermal efficiency and a greater portion of the fuel energy was released in the early part of the combustion process compared with base fuels. OEA reduced the ignition delay and measurably changed the shape of the calculated heat release rate and cumulative heat release diagrams. Comparative cylinder gas temperatures, computed from measured cylinder pressures, are presented in an effort to explain changes observed in the emissions of nitrogen oxides (NOx), smoke, and particulate matters. The data indicates that smoke and particulate emissions decrease and NOx increases when intake O2 level is increased. A future test program to optimize the engine design and operating variables, including inlet oxygen level and water level in the fuel emulsion, would result in the realization of the full potential of these concepts.