An experimental study was conducted to evaluate the use of gaseous fuels as alternative fuels for diesel engines without extensive modification. Experiments were conducted on a two-cylinder MAN DO22M direct injection diesel engine with various proportions of aspirated propane, using the liquid diesel fuel, which is injected through the conventional fuel injection equipment, as ignition source.During dual fueling, up to 90% of the diesel fuel energy input was replaced with propane gas while the engine thermal conversion efficiency was maintained at similar levels to those obtained with straight diesel fueling. However, at full load and high degree of propane use, the thermal efficiency obtained was slightly lower to that for pure diesel operation. At constant speed the maximum engine power output was increased about 20% by dual fueling. The CO concentrations were considerably higher at all loads using dual fuels; this effect is more pronounced for loads less than 50% of maximum rated power; for higher loads the value of the CO emissions are considerably lower. In contrast the NOx levels were always decreased with the introduction of the gaseous fuel.