The understanding of diesel sprays is very important to enable a better and cleaner marine engine design, but unfortunately little knowledge is openly available on marine engine fuel sprays. In this paper, evaporating sprays for medium speed marine engines were studied in a constant volume combustion chamber by performing optical measurements through Schlieren and Mie diagnostic techniques. The effects of ambient gas temperature and ambient gas density on vapor and liquid penetration were investigated by changing the target condition in the combustion chamber. A comparative study of two injectors with different nozzle diameters (0.38 mm and 0.44 mm) was also carried out at ambient density of 22.5 kg/m(3). Some empirical correlations of spray penetration have been modified to fit the spray measurement data. Due to the transient characteristics of the pump-line-nozzle injection system, a time-dependent injection pressure profile is suggested for calculation of spray penetration. The spray tip penetration at large distance under low density (7.6 and 15.2 kg/m(3)) conditions is expected to be proportional to t(2/3), which is supported by the model considering spray-induced gas turbulence effect. The t(1/2) law, where turbulence is not taken into account, is still valid under high density (22.5 kg/m(3)) conditions with higher engine load. The comparison of two models demonstrates that the effect of gas turbulence is influenced by the ambient gas density and engine load.