Thermal characterization and structural optimization of a coolant preheating system for cold start of diesel–electric hybrid powertrains

Cold-start operation of diesel–electric hybrid powertrains under extremely cold conditions can lead to degraded engine performance, increased energy consumption, and reduced system reliability. However, the thermal behavior and structural optimization of coolant preheating systems under extremely low ambient temperatures remain insufficiently investigated. In particular, few studies have considered the coupled optimization of the target temperatures of the coolant and lubricating oil together with heat-transfer structures under an ambient temperature of 223.15 K. To address this issue, a one-dimensional engine preheating system model coupled with performance prediction and flow resistance models was developed to investigate the thermal behavior of coolant and lubricating oil during cold start. The starting-temperature boundary defined by the coolant and lubricating oil temperatures was determined and validated against vehicle test data. Furthermore, the effects of key heat-transfer structural parameters on the thermal response of the preheating system were systematically evaluated. The results indicate that the optimal target temperatures of the coolant and lubricating oil are 323.15 K and 274.36 K, respectively, while the corresponding optimal pipe diameter and heat-transfer area of the heat exchanger are 35 mm and 8.21 m², respectively. By optimizing the target temperatures and heat-transfer structure without increasing heating power, the required preheating time can be reduced by up to 21%. The proposed model and optimization framework provide quantitative design guidelines for coolant preheating systems and supports the development of more energy-efficient thermal management strategies for diesel–electric hybrid powertrains operating in extremely cold environments.