Waste heat recovery (WHR) in internal combustion engines (ICEs) is very interesting opportunity for reducing fuel consumption and CO₂ emissions. Among the different heat sources within an ICE, exhaust gases are certainly the most suitable for potential recovery.

The most promising technology is represented by power units based on organic Rankine cycles (ORCs). Unfortunately, their actual efficiency is far from that obtainable using only thermodynamic evaluations: low efficiencies of small-scale machines, strong off-design conditions, and backpressure effect are the main reasons. To improve the conversion efficiency, this paper presents a combined solution, coupling two thermodynamic cycles: Joule-Brayton and Rankine-Hirn ones. The first (top cycle) considers supercritical CO₂ as the working fluid and the second considers an organic fluid (R1233zDe, bottom cycle). The combined recovery unit inherently introduces further complexity, but realizes an overall net efficiency 3–4% higher than that of a single ORC-based recovery unit.

The hot source is represented by the exhaust gas of an IVECO F1C reciprocating engine, considering twelve experimental operating conditions that fully represent its overall behaviour. The combined unit was modelled via a software platform in which the main components of the ORC unit were experimentally validated. The best thermodynamic choices of the top and bottom cycles, as well as their mutual interference, were identified under the condition of maximum power recoverable; this implies an overall optimization of the combined unit considered as an integrated system. Moreover, the components must be handled when off-design conditions (produced by the unavoidable variations of the hot source) occur: insufficient heat transferred to the two working fluids, produced by an over- or under-designed heat exchanger, would prevent the proper operation of the two units, thereby reducing the final mechanical power recovered. To address this critical issue, the three main heat exchangers were designed and sized for a suitable ICE working point, and their behaviour verified to guarantee a suitable maximum temperature of the supercritical CO₂ and full vaporisation of the organic fluid. These two conditions assure the operability of the combined recovery system in a wide range of engine working points and a maximum recovered mechanical power up to 9% with respect to the engine brake one.

内燃机（ICE）中的废热回收（WHR）是减少燃料消耗和CO ₂排放的非常有趣的机会。在ICE中不同的热源中，废气无疑是最适合潜在回收的废气。

最有前途的技术以基于有机朗肯循环（ORC）的功率单元为代表。不幸的是，它们的实际效率远非仅使用热力学评估所能获得的：小型机器的低效率，强大的非设计条件以及背压效应是主要原因。为了提高转化效率，本文提出了一种结合了两个热力学循环的组合解决方案：焦耳-布雷顿循环和兰金-希恩循环。第一个（顶部循环）将超临界CO ₂视为工作流体，第二个（顶部循环）将有机流体（R1233zDe，底部循环）视为工作流体。组合的恢复单元必然会带来更多的复杂性，但与单个基于ORC的恢复单元相比，其整体净效率要高出3-4％。

考虑到十二种实验运行条件，热源由依维柯F1C往复式发动机的废气代表，完全代表其整体性能。通过软件平台对组合单元进行建模，在该平台上对ORC单元的主要组件进行了实验验证。在最大可恢复功率的条件下，确定了顶部和底部循环的最佳热力学选择及其相互干扰。这意味着对作为一个集成系统的组合单元进行整体优化。此外，必须在出现非设计条件（由热源不可避免的变化产生）时处理这些组件：过度设计的热交换器或设计不足的热交换器产生的热量传递到两种工作流体的热量不足，会妨碍两个单元的正常运行，从而降低最终回收的机械功率。为了解决这个关键问题，设计了三个主要热交换器并确定了其尺寸，以适合ICE工作点，并对其性能进行了验证，以确保超临界CO的合适最高温度。₂，有机液体完全汽化。这两个条件确保了组合回收系统在各种发动机工作点上的可操作性，并且相对于发动机制动器之一，最大回收机械功率高达9％。