In this work, the first and second laws of thermodynamics are employed to evaluate exergetic parameters of a diesel engine at different speed and load ranges. Various blends of waste cooking oil biodiesel with diesel fuel were tested in a direct injection diesel engine at different speeds (1300, 1600, and 2000 rpm) and loads (25%, 50%, and 75% of the rated load). Combustion reactions were determined using emission data and then exergetic parameters of the engine such as fuel exergy, heat loss exergy, exhaust loss exergy, and exergy destruction were calculated and analyzed. Results showed that fuel and heat losses as well as exhaust exergies increase by increasing engine speed and load. Exergy destruction result indicated an opposite trend. It was found that the exergy destruction is the major part of exergy losses by about 45–50% for fuel blends. The maximum exergetic efficiency was found 38.77% at engine speed of 1300 rpm and engine load of 75% rated load.

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