Seite - 8 - in Hybrid Electric Vehicles

electric power could be decoupled, and the system has a high operating flexibility enabling three modes of operation: purely combustion; purely electric and hybrid. Usually, a PHEVs are managed in purely electric mode at low speeds, until the battery charge state reaches a predetermined low threshold, typically 30%. 2.3. Combination of parallel and series HEVs In the series-parallel hybrid configuration can be highlighted two main power paths. In mechanical power path, the energy generated by the combustion engine is directly trans- mitted to the wheels, while the electric path the energy generated by the thermal engine is converted first into electrical energy by means of the generator and then again converted to mechanical energy delivered at the wheels. It is possible therefore to have mixed architectures denominated “power splits” in which the installed power is divided by means of mechanical couplers. Combination of parallel and series hybrid configurations is further divided into sub- categories based on how the power is distributed [37]. PHEVs are even more suitable topolo- gies than HEVs for reducing fuel consumption because, unlike HEVs, they may be charged from external electric power sources [38]. In all the configurations, regenerative braking can be used to charge the battery [36]. Moreover to make recharging of batteries easier, some configu- rations are equipped with an on-board charger and defined Plug-in electric vehicle (PEV) [39]. 3. Sub-system components of hybrid vehicles 3.1. Electric motors The energy efficiency of a vehicle power train depends on, among other features, the size of its components. The optimization problem of sizing the electric motor, engine, and battery pack must consider both performance and cost specifications [40, 41]. Among electric motors, although the permanent magnet synchronous motor is considered as the benchmark, other types of motors are being explored for use in HEVs. Currently, there is some concern on the supply and cost of rare-earth permanent magnets. Considerable research efforts have been made to find alternative electric motor solutions with the lowest possible use of these materials [42, 43]. For instance, some automotive applications use induction motors or switched reluctance motors [34]. Figure 4 shows the most conceiv- able electric motor scenario in forthcoming years. Compared with hydraulics, electric drives provide better controllability and dynamic response and require less maintenance. Similarly to electric power, hydraulic power can be distributed quite easily on the implement; however, hydraulics suffers from poor efficiency in part-load operating conditions [44]. The specific electric drives for agricultural tractors are listed in Refs. [45, 46]. 3.2. Continuous variable transmission (CVT) Working vehicles drive at low speed, and the energy consumed in accelerating and climb- ing slopes should be partially recovered at decelerating and descending slopes. Compared with urban and on-road vehicles, construction and agricultural are used in a lower range of Hybrid Electric Vehicles8