Design And Operation Research On Engine Waste Heat Recovery Systems In Heavy-duty Truck Application

The energy saving and emission reduction of the internal combustion engine(ICE),the main equipment for power supply,is quite significant in dealing with the energy issues and environmental problems.The waste heat recovery(WHR)systems based on Rankine cycle has been considered as one of the key technologies beyond in-cylinder approaches to improve the efficiency of ICE,which extract energy from engine waste heat and convert it to useful power.When applying WHR systems to heavy-duty trucks application,the intrinsic characteristics of heavy-duty trucks,shown as the limited integration space and highly transient working conditions,have posed great challenges to the commercialization of such technology.Current work focuses on the design and operation of engine WHR systems for heavy-duty trucks.Faced with the integration characteristics of heavy-duty trucks,the size of the components in WHR systems are strictly restricted,especially of the heat exchangers.The pressure drops in the heat exchangers would influence the system performance,the heat exchanger design and the working fluid selection.Therefore,an improved thermo-economic analysis approach was proposed for such space-constrained application,considering the interaction of the heat exchanger pressure drops and system performance to realize a simultaneous optimization of heat exchanger design and cycle parameters.The newly proposed approach effectively helps avoiding over-estimation of system thermo-economic performance and sub-optimal results of heat exchanger design and working fluid selection.Besides,the comparison of CO₂transcritical power cycle(CTPC)systems and organic Rankine cycle systems reveals that the CTPC systems are less sensitive to the integration characteristics of heavy-duty trucks than ORC systems.Regarding the highly transient working condition characteristics of heavy-duty trucks,a novel design framework was proposed considering off-design performance under possible heat source variations at the design stage.A comprehensive component-to-system model was developed coupling the system performance at the design condition and off-design conditions,with the objective of minimizing the levelized cost of electricity over its whole lifetime.The case studies of typical engines demonstrated that the optimal design point was moved from the maximum condition to a part-load condition,indicating downsizing the equipment and WHR systems is not only much more favorable in terms of economic performance but also more robust to the possible fluctuations of the heat sources.After designing the CTPC systems,their overall operation performance was investigated,i.e.,the performance under engine mapping conditions and the dynamic performance when engine condition changes.Firstly,a dynamic model was developed to predict the optimal performance of CTPC systems under engine mapping conditions.Results indicated the systems designed at a part-load condition can match the engine condition to as high as 78.28%.Across the whole engine conditions,there is a saturation area and an infeasible area for the CTPC systems.The optimal performance can be achieved by adjusting the mass flow rates of the working fluid and the exhaust involved in heat transfer.Then dynamic tests were conducted to examine the dynamic performance of CTPC systems,which was evaluated by dynamic characteristics.The comparison results of different configurations demonstrated the preheated and recuperative CTPC(PR-CTPC)system has the fastest response speed.Analysis of the operating parameters indicated the CTPC systems are robust when facing narrow fluctuations of engine conditions,i.e.,maintaining the ability to hold the temperatures and pressures and output power when engine condition changes,while swift when required to make adjustment to deal with the fluctuations of heat sources.Finally,an integrated simulation model of CTPC systems and heavy-duty trucks was developed.A multi-mode operating strategy was proposed for the integrated system based on heat source characteristics.Thus a hierarchical and module control structure was built with mode recognition in the top layer and the PID control in the bottom layer.Within which,the performance of the integrated systems over a driving cycle was predicted and the possible aspects for performance improvement were analyzed.Results showed the PR-CTPC system is the most promising,allowing a 2.3%improvement in engine brake thermal efficiency over the whole driving cycle by utilizing 48.9%of the exhaust and 72.8%of the coolant energy,even when the pump and turbine efficiencies are as low as 50%.Increasing the power mode percentage and enhancing pump and turbine performance are the most effective approach to improving system performance.By increasing pump and turbine efficiencies to 70%,the contribution of the PR-CTPC system can be 4.2%.