This study investigates the reduction of nitrogen oxide (NOx) emissions in diesel engines using computational simulations performed with Diesel-RK. Three in-cylinder control strategies were evaluated: exhaust gas recirculation (EGR), direct water injection, and variations in combustion chamber depth. Simulations were conducted on a six-cylinder, four-stroke diesel engine at engine speeds ranging from 1000 to 3000 rpm. The results show that increasing EGR lowers NOx emissions by reducing peak combustion temperatures through oxygen dilution and enhanced mixture heat capacity. Water injection was found to be the most effective strategy, reducing NOx to nearly negligible levels across all speeds by combining evaporative cooling, heat absorption, and oxygen displacement. Changes in combustion chamber depth had a moderate influence, with shallower chambers improving swirl and mixing, thereby lowering local temperature peaks and suppressing NOx formation. Overall, the findings highlight water injection as the most effective standalone strategy, while combining it with optimized chamber geometry and moderate EGR offers a promising integrated approach for NOx control in diesel engines.