A non-evaporable spray model based on step response theory has been introduced, in which the diesel spray is treated as a critical-damped second order response system. The spray density and momentum have been modeled by step response functions, respectively. The new model fits experimental data quite well. Two key parameters, for density and momentum respectively, are introduced. These parameters are getting smaller when the injection pressure or ambient air density increases. (C) 2011 Elsevier Ltd. All rights reserved.