Development of Advanced Non-Bypass Exhaust Heat Recovery System Using Highly Heat-Conductive SiC Honeycomb

An exhaust heat recovery (EHR) system is an effective and attractive means of improving fuel economy and in-vehicle comfort, especially of hybrid cars in winter. However, many conventional bypass systems, which have a bypass pipe and bypass valve with a thermal actuator, are still large and heavy, and it is necessary not only to effectively improve the heat recovery but also to minimize the size and weight of EHR systems. Sakuma et al. reported new-concept heat exchangers and EHR systems using a highly heat-conductive SiC honeycomb, including a non-bypass system. However, since this non-bypass system always recovers heat from the exhaust gas, its heat recovery performance was set so as not to exceed the cooling capability of the radiator at a high engine load to prevent overheating of the vehicle. Therefore, it is necessary to both reduce the recovered heat at a high engine load or high coolant temperature and improve the recovered heat at a low engine load or low coolant temperature for the non-bypass system. This paper proposes an advanced non-bypass EHR system with a double-layered coolant passage structure for automatically limiting the amount of recovered heat depending on the coolant temperature or engine load instead of a bypass valve mechanism. This advanced system can realize both effective heat recovery in an engine warm-up phase and heat rejection at a high engine load or high coolant temperature. This paper explains the basic concept of the advanced non-bypass EHR system and analyzes vehicle results obtained with this prototype, which showed high heat recovery performance in the cold-start phase and high heat rejection performance at a high engine load.