On the Application of Joule-Cycle-Based Waste Heat Recovery to Heavy-Duty Vehicles 2024-01-2589
Internal combustion engines are becoming ever more efficient as mankind seeks to mitigate the effects of climate change while still maintaining the benefits that a mechanized society has brought to the global economy. As peak values, mass production spark-ignition engines can now achieve approximately 40% brake thermal efficiency and heavy-duty truck compression-ignition engines can approach 50%. While commendable, the unfortunate truth is that the remainder gets emitted as waste heat and is sent to the atmosphere to no useful purpose. Clearly, if one could recover some of this waste heat for beneficial use then this is likely to become important as new means of mitigating fossil CO2 emissions are demanded. A previous study by the authors has identified that the closed Joule cycle (or complications of it beginning to approximate the closed Ericsson cycle) could reasonably be developed to provide a practical means of recovering exhaust heat when applied to a large ship engine. In that previous work there was a sensitivity shown between overall pressure ratio and the ratio of specific heats of the gas being used as the working fluid and, providing those variables were appropriately chosen, relatively high efficiencies and specific work outputs appeared to be achievable. While marine engines might seem to be ideal applications for this technology, in no small part due to the effectively infinite and relatively low-temperature sink available at the bottom of the cycle, their low exhaust temperatures (arising from their inherently high efficiencies) and the existing placement of scrubbers and economizers in the exhaust gas run makes the practical application of waste heat recovery (WHR) more difficult on them; nevertheless, using real exhaust gas compositions, the previous work clearly showed some significant potential in that arena, even if the exact level of upper temperature available in the cycle is still unknown.
Given the early indications that Joule-cycle based WHR could work in already-efficient marine applications, this paper investigates the practicality of such methods of recovering exhaust heat in another sector – heavy-duty road transport. In this application, the challenge of a more difficult rejection of heat to the atmosphere on the cold side of the cycle is offset by a hotter exhaust gas temperature. Versus light-duty applications, long-distance transport can offer the chance for more continuous operation with fewer transients to reduce average efficiency, plus a direct economic payback in the form of lower operating costs.
To investigate this opportunity modelling was performed using data in the literature already published for a diesel-engined truck which was then input to one of the Joule-cycle-based WHR models already developed for the initial marine-based project. These results show that this WHR concept could usefully be applied to truck use. An open Joule cycle system is then proposed and this too is investigated; here an increased benefit was predicted because, unlike for the closed Joule cycle approach, the working fluid flow rate in the system can be varied over a wider range, and the final heat exchange is avoided, giving a reduced lower cycle temperature.
