In this study, the performance and emissions characteristics of an uncooled, thermally insulated diesel that utilized an optimized injector-tip configuration are examined. When the uncooled engine was compared to a conventional water-cooled engine at the same brake power and airflow, the uncooled engine had equal or superior fuel consumption, significantly higher nitric oxide emissions, and significantly lower smoke and particulate emissions. The dramatic reduction in smoke emitted by the uncooled engine was not observed in studies reported earlier. This smoke reduction is attributed to the high gas temperatures and increased rates of air-fuel mixing that augmented the rate of oxidation of the soot particles when the injector tip was optimized for the uncooled engine and airflow was adjusted to match that of the cooled engine. Heat-release analyses showed that the uncooled engine had less premixed combustion and significantly shorter combustion duration than the water-cooled engine. Finally, evidence is presented which may contradict a previously published hypothesis of Woschni et al. on the efficiency limitation of uncooled engines.