The diesel engines are widely used in industries like road transport, railway, agriculture, military, mining, construction, maritime and fixed electricity production for provide the high torque, durability and low fuel combustion. The reduction of oil reserves and strict emission regulations have led many researchers to work on alternative, clean and renewable sources in an effort to reduce greenhouse gases for emissions and to increase the efficiency of internal combustion engines. Among these, biodiesel is one of alternative fuels, and can be used without any changes in the engine. For a better performance of the engine, alternative methods are used by adding different additive substances to the diesel fuel. Because both features of injection systems and quality of injected fuel significantly affect the performance of diesel engine. In this study; refined rapeseed oil was produced from biodiesel by transesterification method, certain n-heptane was added to the produced biodiesel and the engine tests were performed. As a result of these conducted experiments; while maximum torque increase on the fuel KYME-H4 was 3.25%, the maximum power increase was 2.48%. Maximum gas pressure and maximum heat release for all fuel are formed in almost the same crank angle. In the study, 4% n-heptane mixture has given the best results in terms of engine performance and combustion characteristics.