Development of Exhaust Heat Recovery System Using Highly Heat-Conductive SiC Honeycomb 2018-01-0048

Reducing the fuel consumption of powertrains in internal combustion engines is still a major objective from an environmental viewpoint. Internal combustion engines waste a huge part of the fuel energy as heat in the exhaust line. Currently, exhaust heat recovery (EHR) systems are attracting attention as an effective means of reducing fuel consumption by collecting heat from waste exhaust gas and using it for rapid warming up of the engine and cabin heating [1, 2, 3, 4]. The benefits of the EHR system are affected by a trade-off between the efficacy of the recovered useful thermal energy and the adverse effect of the additional weight (heat mass) of the system [5]. Conventional EHR systems have a complex heat exchanger structure and a structure in which a bypass pipe and heat exchanger are connected in parallel, giving them a large size and heavy weight. We have developed a new-concept silicon carbide (SiC) heat exchanger with a dense SiC honeycomb. A feature of the SiC heat exchanger is its SiC honeycomb having a thermal conductivity seven times higher than that of stainless steel and a large contact area with the exhaust gas. We propose two different prototypes (coaxial-bypass and no-bypass systems) of the new-concept heat exchanger. This new system is highly compact and lightweight, and has superior mountability in cars to a conventional EHR system, Furthermore, we present the results of a comparative vehicle test with the two different prototypes of the new-concept SiC heat exchanger and a conventional EHR system with a metal heat exchanger. We carried out the vehicle testing using the Worldwide Harmonized Light-Duty Test Cycle (WLTC) at 10 °C (with cabin heating) and 25 °C. The coaxial-bypass prototype achieved high fuel economy performance owing to the earlier coolant warming than the conventional system under both test conditions. The no-bypass prototype showed the feasibility of no-bypass (continuous heat recovery) operation because of not only improved fuel economy but also a low pressure drop and suppressed heat recovery at a high thermal load. In other words, the results of the no-bypass prototype showed the possibility of a highly compact and lightweight EHR system without a bypass structure for passenger cars. The SiC EHR system with a highly heat-conductive SiC honeycomb is compact and expected to improve the fuel economy performance of vehicles.