Investigating the Potential of an Integrated Coolant Waste Heat Recovery System in an HD Engine Using PPC Operation

With the increasing focus on reducing emissions and making fuel efficient vehicles within the automotive industry over the past few years, new methods are constantly being investigated to improve the efficiency of the powertrain. One such method is recovering waste heat from the exhaust gases as well as the coolant using a thermodynamic cycle such as a Rankine cycle. However, most studies looking into low temperature or coolant heat recovery investigate the use of a separate secondary cycle for the recovery of waste heat itself. This has the disadvantage of having the working fluid at a lower temperature than the coolant which reduces the recovery efficiency. This paper investigates the potential of an integrated Rankine cycle waste heat recovery system where the coolant also acts as the refrigerant and is integrated with the exhaust gas recirculation waste heat recovery. The refrigerant/coolant used for this study is ethanol, while being used in two modes for low temperature/coolant recovery: using the engine as the preheater and using it as an evaporator. Using a combination of GT Power and Matlab, a Scania D13 engine was simulated in partially premixed combustion operation with a waste heat recovery system. For the engine load-speed range, the coolant flow rate, pressure ratio and superheat were swept for determining the optimal values for maximizing output power. It was seen that while using the engine both as a preheater and as an evaporator the recoverable power increased in comparison to using only the exhaust gas recirculation heat for recovery. When using the engine for preheating, the recoverable power increased marginally with an indicated efficiency gain of less than 0.5 percentage points whereas when using the engine for the evaporation of the coolant, the indicated efficiency showed gains of up to 1.7 percentage points in comparison to using EGR-only heat recovery with a total gain in indicated efficiency of up to 5.5 percentage points. This larger gain in recoverable power while using the engine as an evaporator in comparison to as a preheater is due to the location of the pinch point in analyzing the heat exchange process. The system behavior was also studied with regards to the pressure ratio, the mass flow rate of coolant and the superheat. It was generally observed that at higher loads and speeds these parameters increased as more waste heat was available for recovery for the system.