Optimization Of Efficiency Of A Combined Organic Rankine Cycle Used For Multiple Grades Waste Heat Recovery Of IC Engines

The thermal efficiency of diesel engine is generally30%~45%, and that ofgasoline engine is20%~30%.There are about55%~65%of energy discharged into theenvironment in the form of cooling water and exhaust waste heat, which causesenergy waste and the serious pollution to the environment. If this part of wastedenergy could be recovered effectively, the output power of the internal combustionengine would increase without any increase in fuel consumption.There are multiple grades of internal combustion engine waste heat, EGR(exhaust gas recirculation) waste heat for the high grade heat (over500℃), theexhaust waste heat for the medium grade waste heat (200~500℃), charged air andcooling water waste heat for the low grade waste heat (under200℃). Each part ofwaste heat has large temperature difference. Because the high and low grade wasteheat restrain each other, using the traditional organic Rankine cycle (ORC)to recovermultiple grades waste heat is unable to fully recover each part of waste heat. Based onthis, this paper puts forward a two-stage combined ORC system which can utilize themultiple grades waste heat of internal combustion engines comprehensively andefficiently. When the high and low temperature stage all use supercritical cycle,irreversible loss can be reduced effectively. Based on the experimental data of a6cylinders4strokes diesel engine and the first law and second law of thermodynamicsmodel, this paper builds the simulation model of the system and carries on calculationand analysis. The paper investigates the optimization of the system, and gets the bestsystem layout. This paper also makes working fluid selection, and analyzes the effectof high and low temperature stage parameters on system performances and efficiency.It is concluded that the comparison results of high temperature stage working fluid isbenzene> cyclohexane> toluene> MM (siloxane)> n-decane> n-dodecane, and lowtemperature stage working fluid is R32> R143a> R125> R218.Through theoretical analysis, when high temperature stage selects benzene asworking fluid, the evaporating pressure is6MPa, condensing pressure is0.2MPa,low temperature stage chooses R32as working fluid, the evaporation pressure is6.2MPa, the condensation temperature is35℃, the net power can reach40.08kW,thermal efficiency11.57%, total irreversible loss4.157kW, irreversible efficiency 90.6%, and the engine efficiency is increased to46.7%on the basis of the originalefficiency40%.In conclusion, a two-stage combined ORC system used for multiple grades wasteheat recovery of internal combustion engines is presented and analyzed in this paper,and the performance prediction on this combined system has been made. These workwill lay a theoretical basis for the future experimental study and application of thiswaste heat recovery system. Research shows that recovery and conversion of enginewaste heat based on this combined ORC system is an effective means to save energyconsumption of engines.