The hot-test requires more parallel workstations, shown as H1 to H10 in Fig. 1, as it takes more time to perform the hot-test when compared to other workstations in the production line. After diesel engines are assembled on the main assembly line and go through a quick cold-test, a certain percentage of the engines need to go through a hot-test with changing loads. The engine hot-test production line is a part of the overall diesel engine assembly line of the plant. The layout of the production line is illustrated in Fig. 1. The engine hot-test production line of a diesel engine production plant located within the Yangtze River Delta in China is a typical example of HFSP.
The HFSP is commonly observed in chemical industries, transportation, manufacturing as well as healthcare. The hybrid flow-shop scheduling problem (HFSP), , is a widely studied variant of the classical flow-shop scheduling problem that considers the availability of parallel machines during certain stages, which can balance flexibility and cycle time among stages, as well as reduce bottleneck issues.
The effectiveness of the proposed DWSA is validated on three groups of instances and a real-world industrial case. Five mutation operators and a deduplication strategy are proposed to improve the population diversity. A new way of computing distances and a movement rule between individuals are designed. A hybrid initialization is used to ensure the quality of the initial population and diversity. The proposed algorithm adopts an encoding method based on the problem characteristic and a greedy delayed decoding strategy to avoid infeasible solutions. A discrete whale swarm algorithm (DWSA) is proposed to identify near-optimal solutions efficiently. Because of its NP-hardness and large scale, traditional optimization methods and heuristic rules cannot obtain satisfactory solutions. It extends the classical hybrid flow-shop scheduling problem by considering practical constraints on buffer area resources and alternative process routes. This paper studies a hybrid flow-shop scheduling problem with limited buffers and two process routes that comes from an engine hot-test production line in a diesel engine assembly plant.