Recently,a downsized diesel engine have been developed for improvement of thermal efficiency.Injection pressure of the diesel engine tends to increase due to promotion of fuel atomization.Thus,diesel spray strongly impinges to a cavity wall in a combustion chamber as compared with a conventional diesel engine.There might be negative effect for heat loss in the engine due to the heat transfer between the impingement spray and the cavity wall.Therefore,it is necessary to understand the impingement phenomenon of the diesel spray for improvement of the thermal efficiency.In the past,there are some reports regarding fuel concentration in an impingement spray and establishment of the model for the spray.Moreover,flow characteristics for surrounding gas of an impingement diesel spray was investigated.In order to understand the impingement mechanism between the spray and the wall,velocity measurement in an impingement diesel spray was needed.Velocity information of the impingement spray might provide heat transfer mechanism between the spray and the wall.In this study,velocity field of the impingement diesel spray was measured with time-resolved PIV.Effect of ambient gas density and injection pressure on flow characteristics of the impingement diesel spray was investigated.For the mean velocity field,normalized velocity distributions of the impingement spray for various ambient gas density and injection pressure were evaluated.Distributions of the normalized velocity were almost the same regardless of ambient gas density and injection pressure.It was found that similarity of the velocity distribution in the impingement spray maintained even though the ambient gas density and injection pressure were changed.Moreover,the turbulent kinetic energy inside the impingement diesel spray decreased with an increase of ambient gas density.