Theoretical And Experimental Study Of Waste Heat Recovery Of Engine Based On Rankine Cycle

For an internal combustion engine, only one third of the fuel energy is used effectively. The left is released through exhaust, cooling system and etc. Recovering the released energy using Rankine cycle can increase the brake power and reduce the fuel consumption and emission of CO2.The engine's energy distribution is analyzed from the aspects of energy balance, and the available energy of exhaust and cooling system is studied and compared.In this paper,total heat recovery efficiency is used to measure the performance of heat recovery system. The inlet temperature of expander, evaporating pressure and the cooling temperature's influences on the total heat recovery efficiency with different working fluids is analyzed. The results have shown that if the working fluid is water, the total heat recovery efficiency increases with the inlet temperature of expander increasing. The opposite happens if the working fluid is dry or isentropic fluids; there exists an optimum evaporating pressure to achieve the highest total heat recovery efficiency; as for working fluid, water has the highest total heat recovery efficiency when the exhaust temperature is high while organic fluids are advantageous when the exhaust temperature is low.The heater and cooler of heat recovery system are designed based on the working condition of a certain engine, and the calculation model of the designed heat exchangers is built to study influences of mass flow rate on the heat exchanger. The results have shown that with the mass flow rate of working fluid increasing, the total heat recovery efficiency increases first, then decreases. There exists an optimum mass flow rate of working fluid to achieve the highest total heat recovery efficiency.The calculation model of reciprocating piston expander is built by Matlab/Simulink software based on energy conservation equation, continuity equation, gas flowing equation, and heat transferring equation, to study the influences of different designed parameters and working parameters on the piston expander's power, efficiency and mass flow rate of working fluid, which provide directions for expander design. The results have shown that the increase of bore and stroke can lead to increased mass flow rate of working fluid while its influence on the achievable Carnot efficiency is very small; the mass flow rate of working fluid increases with the inlet pressure increasing while the achievable Carnot efficiency decreases with the inlet pressure increasing; the mass flow rate of working fluid decreases with the inlet temperature increasing and the achievable Carnot efficiency increases with the inlet temperature increasing. In the valve timing effects, intake valve's closing time has the greatest influence on the performance of the piston expander. The later intake valve's closing time is, the greater the mass flow rate of working fluid is and the smaller piston expander's achievable Carnot efficiency is. The intake valve's opening time is appropriate between -10~-20. The exhaust valve's opening time has small influence on the piston expander's performance. The exhaust valve's closing time is appropriate when it is close to the intake valve's opening time. The above conclusions have layed great theoratical groundation for the design of expander.Based on the general working condition of a certain engine, a simulated calculation of the heat recovery system is carried out, where the heat exchanger model and piston expander model is used, to optimize expander's designed parameters and choose an optimum working condition of working fluid. The influence of the working condition of engine on the heat recovery system is studied by the simulation. The results have shown that in the designed condition the power of the heat recovery system is the greatest. When the working condition of the engine deviates from the designed condition, the power of the heat recovery system decreased quickly, but the utilization rate of the exhaust available energy changes between the range of 20%~30%.The influence of the heat recovery system on the efficiency of engine is studied by GT-POWER software. The result has shown that the influence of the heat recovery system on the efficiency of the engine itself is very small. The total power of the engine system which is combined with the heat recovery system can increase by 5%~12%.A system has been established to experiment on recovering engine's exhaust energy and performance experiments have been done to check the model and conclusions. It has been found that there are problems with this system and how to improve it, which lay the groundwork for application of waste heat recovery system。...