In this study, in order to convert a 2.4 L reciprocating gasoline engine into a hydrogen engine an experimental device for supplying hydrogen fuel was installed. Additionally, an injector that is capable of supplying the hydrogen fuel was installed. The basic combustion characteristics, including torque, were investigated by driving the engine with a universal engine control unit. To achieve stable combustion and maximize output, the intake and exhaust valve opening times were changed and the excess air ratio of the mixture was controlled. The changes in the torque, excess air ratio, hydrogen fuel, and intake airflow rate, were compared under low engine speed and high load (wide open throttle) operating conditions without throttling. As the intake valve opening time advanced at a certain excess air ratio, the intake air amount and torque increased. When the opening time of the exhaust valve was retarded, the intake airflow rate and torque decreased. The torque and thermal efficiency decreased when the opening time of the intake and exhaust valve advanced excessively. The change of the mixture condition’s excess air ratio did not influence the tendency of the torque variation when the exhaust valve opening time and torque increased, and when the mixture became richer and the intake valve opening time was fixed. Under a condition that was more retarded than the 332 CAD condition, the torque decreased by about 2 Nm with the 5 CAD of intake valve opening time retards. The maximum torque of 138.1 Nm was obtained at an optimized intake and the exhaust valve opening time was 327 crank angle degree (CAD) and 161 CAD, respectively, when the excess air ratio was 1.14 and the backfire was suppressed. Backfire occurred because of the temperature increase in the combustion chamber rather than because of the change in the fuel distribution under the rich mixture condition, where the other combustion control factors were constantly fixed from a three-dimensional (3D) computational fluid dynamics (CFD) code simulation.

The plate-fin heat exchanger was designed for the liquid cooling thermal management system of the hybrid electric propulsion system for an electric vertical take-off and landing (eVTOL) vehicle. The offset-strip fin design was applied, and the performance of the heat exchanger was evaluated, particularly with respect to the inclination of the airflow entering the heat exchanger. The estimated performance during the design phase matched well with the experimental results. The inclination of the heat exchanger had a minimal effect on thermal performance, with a slight increase in performance as the inclination increased. However, the pressure difference along the airflow was affected, likely increasing as the inclination increased. The sensitivity of various parameters on coolant temperature was also investigated. The air inlet temperature had a significant effect on coolant temperature, followed by the coolant flow rate. Therefore, when designing the thermal management system, careful consideration should be given to the ambient air temperature and coolant flow rate.

This study presents a hybrid CFD–ε–NTU–neural network framework for evaluating the thermal performance of compact heat exchangers installed in confined nacelle environments under severe space and weight constraints. Airflow was simulated using steady Reynolds-Averaged Navier–Stokes (RANS) equations with the shear stress transport (SST) k–ω turbulence model, while heat-exchanger performance was evaluated using the effectiveness–number of transfer units (ε–NTU) method. To provide a continuous functional interface between the analytical model and the CFD solver, a neural network was employed as a representation of ε–NTU-based thermal performance. The proposed framework was validated against wind tunnel experiments, showing good agreement with measured data, with a maximum deviation of approximately 2.2 °C in coolant outlet temperature. Using the validated framework, a multi-objective redesign of a compact heat exchanger was performed under practical installation constraints. The selected design achieved a 28.4% reduction in heat exchanger mass while maintaining cooling performance comparable to the baseline design. Installation performance was evaluated numerically in a modified nacelle configuration, where the redesigned heat exchanger increased the air mass flow rate by approximately 13% under identical fan operating conditions because of its reduced air-side flow resistance. The proposed framework provides a practical and efficient approach for system-level design and installation analysis of compact heat exchangers for aircraft thermal management systems.