Extreme weather conditions in the Gulf, Qatar, have been taken as an example in this study, tend to be hot and humid mostly throughout the year, especially in the summer. Such weather negatively affects the performance and emissions of all combustion engines, in particular diesel engines. In this work, a modified air-conditioning system was designed for controlling the inlet air temperature and humidity of a naturally aspirated single-cylinder diesel engine. The study investigated the effect of running the engine at different controlled inlet air temperatures on the engine performance and emission characteristics. It was found that running the diesel engine at 20 °C inlet air temperature compared with 45 °C, the average ambient air temperature during summer in Qatar could increase the in-cylinder peak pressure by 10%, and the volumetric efficiency of the engine by 8.5%. Moreover, the air-to-fuel (A/F) ratio has increased by 27.5% with 20 °C compared with 45 °C, while a minor effect was observed on the specific fuel consumption. For the emissions, there was a considerable reduction rate in NOx emissions with about 83% at 20 °C as well as a 50% reduction in hydrocarbon (HC) emissions compared with 45 °C. Furthermore, the smoke emission has decreased by 40% at the engine full load. It was also proved that using the air-conditioning system of a vehicle to cool the intake air temperature is visible, as the net gained the power of the engine has increased by 14.5% when running at 20 °C compared with 45 °C.