The paper focuses on the physics of sprays using large eddy simulation (LES) and Lagrangian particle tracking (LPT). The LES/LPT was compared to previously unpublished experimental fuel spray data in two ambient gas densities, 39 and 115 kg/m(3). The higher density case corresponds to a near-future engine environment with maximum cylinder pressure of the order of 300 bar, whereas the lower density case resembles typical present-day engine conditions. The accuracy of the results was quantified by calculating the resolved part of the turbulent kinetic energy and using a LES quality index analysis. The sprays produced by the LES/LPT had many similarities with the experimental sprays. On a global scale, spray penetration, spray opening angle, and spray dispersion were found to be well captured by the LES/LPT. The results indicated that the effect of subgrid scales on particle dispersion was small and hence no explicit particle dispersion model was required. Similarities were also found locally as LES/LPT produced small-scale flow structures indicated by the Q-criterion, preferential concentrations, and voids free of droplets. Finally, we propose a new gas phase mixing indicator in order to quantify turbulent mixing. Results from the novel mixing indicator suggest that for a given spray penetration, the higher ambient gas density spray yields an increased mixing rate.