Development of Advanced Non-bypass Exhaust Heat Recovery System Using Highly Heat-conductive SiC Honeycomb 2019-01-0153
Owing to the high efficiency of the internal combustion engine and the assistance of the electric motor in a hybrid car, the thermal energy in the vehicle is insufficient at the time of starting the engine or using the cabin heater. An exhaust heat recovery (EHR) system can reuse the thermal energy in the exhaust gas to warm up the engine coolant, and it can improve fuel economy and in-vehicle comfort, especially in winter. Many current products, in which a complicated metal heat exchanger and a bypass pipe are arranged in parallel, are still large and heavy making them difficult to mount in compact cars. At WCX’ 18, the authors reported two EHR systems, a non-bypass system and a coaxial bypass system, using a highly heat-conductive SiC honeycomb, and the results of vehicle tests with these prototypes. In particular, the non-bypass system is compact and lightweight compared with current products on the market. However, because of the continuous recovery of heat during driving, it did not have sufficient heat recovery performance to prevent the vehicle from overheating at a high engine load. In this paper, an advanced non-bypass EHR system using a highly heat-conductive SiC honeycomb is proposed. This advanced system has a double-layered coolant passage structure for automatically limiting the amount of recovered heat at a high coolant temperature or high engine load instead of a bypass valve function. The sub-passage in the double-layered coolant passage can act as a heat conduction layer or a heat rejection layer depending on the coolant temperature and engine load. This paper describes the basic concept of the advanced non-bypass EHR system and vehicle test 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.