An optically accessible D] Diesel engine has been constructed tostudy combustion, emission, spray, and flow field phenomena. The goal of the present investigation is tostudy the effect that intake charge temperature variation at constant density has on combustion, emissions, and spray vaporization in both quiescent and swirling environments. The results indicate that raising intake temperatures decreased the ignition delay, peak rate of premixed burning, and premixed fraction. Increasing intake temperature increased the peak rate of diffusion burning in the quiescent environment, but mixing effects balanced temperature effects in the swirl environment and peak diffusion burning remained constant. In general, NOx increased with increasing temperature and amount of diffusion burning, but lower temperature data suggests that premixed and diffusion burning are both contributing to NO production. Combustion photography shows that first visible light occurs very close tostart of combustion and typically before peak premixed burning which indicates that visible light is emitted in the premixed stage of combustion. Under non firing conditions, shadowgraph photography indicates that increasing intake temperature had little effect on fuel plume penetration tothe walls. however, swirl had a dramatic effect on tangential fuel distribution.