Waste Heat Recovery Simulation Model based on the Rankine Cycle for Passenger Cars applications

Abstract

The demand for more energy efficient passenger cars is steadily increasing due to increasing fuel prices, future emission regulations and awareness of the effects that the emissions of carbon dioxide has on the climate. As a large portion of the fuel energy in internal combustion engines is lost as heat, ways of utilizing more of this waste heat has gained interest in recent years. The most promising method to recover waste heat for automotive applications is with assistance of the Rankine cycle. The exhaust temperature and flow of exhaust gases are highly variable and hence the modeling of a Rankine based waste heat recovery system is of transient nature. To accurately predict the heat energy that is possible to utilize careful modeling of the heat exchanging devices is of utmost importance. Especially the prediction of evaporation in the boiler is important since steam engines requires saturated steam conditions to work efficiently. Direct transmission of mechanical energy to the powertrain demands that the expander speed and motor engine speed is matched, which creates a dynamic system where the pump pressure has to be controlled. This thesis has investigated the possibilities to mathematically model a waste heat recovery system for passenger cars with the help of the model based software SIMULINK and the fluid library CoolProp. The results shows that it is beneficial to recover waste heat through a Rankine system. The waste heat efficiency achieved were found to be 2.85% for city driving, 5.00% for country roads and 5.02% for free ways. These results were achieved for a gear ratio of 1.5:1, which was found to be the best ratio.