A novel intra-cycle waste heat recovery (ICWHR) methodology, applied to an internal combustion engine is presented in this study. Through a split type thermodynamic cycle design, quasi-isothermal compression of the charge air and isobaric combustion of the air/fuel mixture can be performed separately in two chambers. Within such a design, the exhaust heat can be recovered to the intake air flow between the compression chamber and combustion chamber. Consequently, the recovered energy can be re-utilized in the combustor directly, and an intra-cycle waste heat recovery process can be achieved. To investigate the fundamental aspects of this new methodology, a comparative study between the conventional Rankine based WHR and the new ICWHR was undertaken. Both theoretical and numerical analysis were applied to evaluate the performance characteristics of these two technologies. The ICWHR cycle differs from the Rankine cycle in that an energy conversion subsystem is not necessary since the recovered energy is sent back to the combustion chamber directly, and then the system efficiency is improved significantly. Furthermore, the theoretical results indicate that the full cycle efficiency of ICWHR system is determined by the regeneration effectiveness, the compression ratio and the fuel equivalence ratio, then the limitations of Rankine cycle, such as working fluid selection and system parameter calibration can be avoided mechanically. Finally, through a one dimensional system model, analysis of optimal operation range, system efficiency and the heat transfer behaviours of ICWHR system are discussed in this paper and comparisons made with a Rankine cycle WHR system

Ammonia (NH₃) is gaining recognition as a viable “green” transportation fuel due to its zero-carbon characteristic, its high energy density and its widespread availability. However, NH₃ has a high auto-ignition temperature, resulting in potential emissions of NO_x and unburned NH₃. Addressing combustion challenges requires innovative solutions, such as the application of combustion promoters to enhance NH₃ combustibility. This review article focuses on the compatibility of NH₃ as a fuel for spark-ignition (SI) engines, examining its combustion under various modes including pure NH₃ combustion, gasoline blends, NH₃/hydrogen (H₂) blends, and NH₃/natural gas blends in single or dual-fuel configurations. The formation of nitrogen oxides (NO_x) and slip-NH₃ is explored to understand emissions species such as NO and N₂O. Additionally, the article highlights the limitations of NH₃ as a fuel for SI combustion. The comprehensive discussion provided in this review aims to fill a critical gap in the literature regarding NH₃’s feasibility as a zero-carbon fuel for SI engines, particularly in the maritime sector. By offering insights into NH₃ combustion characteristics and emissions profiles, the article seeks to provide a roadmap for leveraging NH₃ as a suitable non-carbon fuel to decarbonize the marine sector and advance global sustainability goals.