Experimental data are presented to show how diesel combustion systems respond to increase of fuel injection rate. Concepts of a fuel spray entrainment parameter, a maximum useful injection rate, and a condition termed ‘impingement’ are introduced to correlate and interpret widely differing responses. Best possible smoke and BSFC values in swirl type direct injection engines are obtained for injection rates 15% to 33% higher than normal values, but in practice lower rates must be used to satisfy emissions and other requirements. Engines with a high swirl rate and impingement give a superior ‘retardability’ for normal injection rates. Computer model calculations also show that there is a maximum useful injection rate and explain the relative fuel economies for different diesel combustion systems.