# A Two-Echelon Electric Vehicle Routing Problem with Time Windows and Battery Swapping Stations
## 2 Literature Review
### Related Research on the 2E-VRP  
The two-echelon vehicle routing problem (2E-VRP) can be regarded as a special case of the two-echelon location routing problem (2E-LRP), where the locations of the depots and satellites are usually given. [PERSON] et al. [26] gave the review of the 2E-VRP and the 2E-LRP. The routing problem was first applied to the two-echelon transportation system by [PERSON] and [PERSON] [27]. After that, the definition of a 2E-VRP was given by [PERSON] et al. [28]. The 2E-VRP transportation system was first proposed in [PERSON] et al. [29]. According to their research, it can be concluded that a mathematical model of the 2E-VRP based on the flow of the demand of each arc was first introduced. Constraints such as time windows of customers and synchronization were considered in the 2E-VRP in [PERSON] et al. [30]. [PERSON] et al. [31] considered the real-time transshipment capacity variation of the 2E-VRP, and in their work, a mathematical model was given. Some exact algorithms were adopted to solve the 2E-VRP, and they can be found in [PERSON] et al. [32], [PERSON] et al. [33], and [PERSON] et al. [34]. However, most of the 2E-VRPs were solved by heuristics. [PERSON] et al. [35] introduced a multi-start heuristic that solved the two-echelon transportation subproblems iteratively. [PERSON] et al. [36] proposed the large neighborhood search (LNS) combined with local search to solve the 2E-VRP and only applying ICVs both in two echelons. An adaptive large neighborhood search (ALNS) algorithm with kinds of destroy and repair operations was proposed by [PERSON] et al. [37] to solve the problem. [PERSON] et al. [38] combined the VNS algorithm and the integer programming algorithm to solve the two-echelon capacitated vehicle routing problem with environmental considerations (2E-CVRP-E). [PERSON] et al. [39] presented a new two-echelon routing problem, which considered a synchronized truck-drone operation by allowing several drones to fly from a truck. [PERSON] et al. [40] presented and discussed research on the last-mile delivery.  
The EVRP and the two-echelon network issues have drawn more and more public attention in academic research and practical application in the literature. In order to determine the urban distribution strategy under the limit of battery range, [PERSON] et al. [41] first studied the 2E-VRP in the context of EV application. [PERSON] et al. [25] used EVs in the second-echelon transportation, and they observed that the detour miles due to recharging decrease as a function of the charging stations' density; however, they did not consider the time windows of customers and the unilateral time windows of satellites. Considering that most distribution companies follow the customer-oriented service standard nowadays, how to satisfy the requests of customers, such as time windows, is more important. For the two-echelon transportation system, studying unilateral time windows of satellites can avoid extra wait at satellites for vehicles. [PERSON] et al. [42] proposed the mathematical model of the 2E-VRP in the context of ICV and EV application, which considered the time windows.  
Based on the survey, it is not difficult to find that only a limited number of papers with a focus on 2E-VRP involving EVs, where the research on the mixed fleet and integrated optimization for two echelons are rare. Motivated by the state of the art of the investigations and the managerial requirements from the real applications, this paper aims at providing a new solution to this problem.