The mixing of fuel with air in a diesel engine strongly dictates the specific fuel consumption and exhaust smoke. Many experimental studies reported the optimum swirl for a given diesel engine at a given operating condition. However, the attempts to correlate the relative penetration, or cross wind velocity or the ratio of total momenta of fuel and air, or the angular speed of sprays at a characteristic time resulted in only partial success in the past. The present work introduces the concept of useful air. The ratio of momentum of the useful air to the total momentum of injected fuel near TDC at the end of ignition delay period is found to bear a universal relationship with the indicated efficiency and dry soot emissions in case of combustion chambers supported by air swirl. The concept is enhanced when the injected quantity is small, by considering spray detachment from the nozzle tip and the swirling cloud of fuel vapour near the walls of combustion chamber. The correlation was validated for different combustion chamber designs of engine bores from 75mm to 110mm, running at rated speeds from 1200rpm to 2500rpm. The proposed correlation of momentum ratio has universal application for different fueling, operating speeds, combustion chamber shapes and bores when the combustion is intensely supported by air swirl.