Potentiality for Optimizing Operational Performance and Thermal Management of Diesel Truck Engine Rankine Cycle by Recovering Heat in EGR Cooler 2010-01-0315

Further reduction of brake specific fuel consumption (bsfc) in heavy-duty diesel engines, which are used for vehicle applications, is of utmost importance due to high fuel prices, global warming issue (CO₂ emissions) and continuously stringent environmental regulations. Specifically, the necessity for further reduction of specific diesel oil consumption and increase of vehicle mileage, respectively, is more pronounced in large haul diesel trucks due to technical, environmental and economical reasons. Heavy-duty (HD) direction injection (DI) diesel engines are used in these vehicles, which indicate a rather high power output in the range of 200-400 kW. During recent years, various measures have been proposed from engine manufacturers and researchers for improving combustion process and through that, increasing the fuel economy of diesel engines. However, the implementation of all these measures showed that the brake specific consumption of HDDI diesel engines cannot be significantly reduced, unless new ideas or techniques are employed. A promising technique is the exhaust heat utilization from diesel truck engines for power production since approximately 30-40% of fuel supplied energy is rejected to the ambient. In the past, various attempts have been made to utilize exhaust heat from diesel engines using bottoming cycles. However, no serious long-term investigation has been conducted at that time since the fuel cost has not yet become an issue of primary concern. In addition, there is a lack of information concerning the potential ways of improving exhaust gas heat recovery from heavy-duty diesel engines. For this reason, a theoretical study is conducted herein using a newly developed simulation model to examine the potentiality of improving the overall efficiency of a heavy-duty truck diesel engine using a steam Rankine bottoming cycle. The diesel engine is equipped with an exhaust gas recirculation (EGR) system. Hence, rejected heat is also recovered from the exhaust gas stream when it flows through the EGR cooler. Specifically, heat rejected from EGR cooler is utilized to superheat and/or partially evaporate the working media of the bottoming cycle. A thermodynamic simulation model of the Rankine cycle has been developed for calculating the performance parameters of the bottoming cycle with and without EGR cooler. The proposed bottoming cycle model takes into account the effect of the pressure drop in the exhaust gas heat exchanger on the variation of engine back-pressure. Volume and weight constraints of exhaust gas and EGR heat exchangers are also taken into consideration by the thermodynamic model. A parametric study is performed using the developed model to examine the effects of varying high pressure of the steam Rankine cycle on the thermal performance parameters of the installation when either recovering or not heat from EGR cooler. Predictions were generated for several operational parameters of the heat recovery installation such as the relative bsfc improvement, the cycle power output and the steam temperature and mass flow rate at 1700 rpm engine speed and at various engine loads. The criteria for selecting the optimum high pressure value of the Rankine cycle are two-fold: The first is the maximization of cycle power output and the second is the minimization of the total amount of heat rejected to the ambient. The analysis of the preliminary results indicate that the steam Rankine cycle may operate efficiently at higher values of vaporization pressure reducing at the same time the heat rejected to the engine radiator due to EGR cooler heat recovery.