A fuel conditioning system is applied to common rail direct injection or unit injector diesel engines. A liquid fuel is conditioned with gas for combustion in the combustion chambers. The system includes an elongate conditioning vessel, at least one fuel dispensing inlet, at least one carbon dioxide inlet port, and at least one conditioned fuel outlet port on the vessel. Carbon dioxide and diesel fuel are fed into the inlet portion of the vessel. The mixture travels a path of at least ten feet in the vessel to the outlet portion to cause the carbon dioxide gas to dissolve in the liquid fuel for forming a liquid/gas fuel solution. At least one high-pressure fuel pump feeds the liquid fuel/gas solution into fuel injectors.