In this study, the effects of changing the design of the air filter of an internal combustion diesel engine on engine performance and harmful exhaust emissions are experimentally investigated. In an internal combustion engine, a sufficient amount of clean air intake must be provided for complete combustion of the fuel. Air filters are used to provide this clean air. Otherwise, the particles in the air entering the cylinder pose a threat to the engine. Filtering unwanted particles in the air entering the combustion chamber is important to prevent cylinder wear and extend engine life. However, incorrect air filter design adversely affects engine performance, increases fuel consumption and harms the environment by increasing exhaust emissions. Correct air filter design also affects engine wear, operating parameters and operating cost. In the experimental study, the air filter of a diesel engine was designed and made operational, the performance and exhaust emission tests of the engine were carried out and the results obtained were presented in graphs. Depending on the design changes, an increase in engine power and improvements in fuel consumption have been achieved. In addition, when the exhaust emissions were analysed, it was observed that the formation of HC, CO, decreased by an average of 7%, while NOx compounds increased by approximately 2%.