## 4 Current Research Status of Energy Management Techniques
### Real-Time Optimal Control Strategy  
Real-time optimal control strategies are the closest to objective reality but are also the most difficult to apply in practice as they are more demanding in terms of computing power. However, there is still much research that has been conducted on real-time optimal control strategies. [PERSON] et al. [77] presented a predictive management model to optimize energy utilization in a passenger vehicle. In addition, later, a comparative investigation of these strategies was conducted to assist in the choice of which strategy to apply [128]. However, it showed that there is no one energy management method that is most suitable for all scenarios but rather the characteristics of the object and scenarios must be taken into consideration practically so that the most appropriate management strategy is purposefully designed. To employ an online control strategy, [PERSON] et al. [129] further offered a hierarchical MPC scheme to enhance the energy economy and application of various scenarios. The results indicated that the proposed approach enhances performance, on average, by 7.3% and 5.9% in the simulation and experiment, respectively. For hydraulic electric hybrid vehicles, [PERSON] et al. [130] reported a real-time global energy control method to optimize energy utilization. Through simulation, the results proved that the suggested strategy is feasible and has an energy saving of 32.14%. As mentioned above, a control method generally does not meet the problem requirements of the target object. Therefore, the synthesis of methods becomes the best way to address energy management and optimization problems. [PERSON] and [PERSON] et al. conducted an in-depth study of smart energy control for hydraulic electric vehicles. Firstly, the same authors in Ref. [89] provided a novel intelligent controller (IH-HCS = ISSMC + IPDOC + LFPIDC) for the same hydraulic electric vehicle as previously mentioned to enhance the overall energy efficiency and minimize the total energy use, as shown in Figure 22a. The new results highlighted that the IHHCS control strategy makes the optimal energy consumption of 3732 kJ, while the SF is 3835 kJ and the OFLC strategy is 3800 kJ. In a word, compared to OFLC and SF, this paper presented a strategy that significantly optimizes fuel economy, enhances energy efficiency and is adaptable to a wider range of scenarios. Secondly, a hybrid hydraulic-electric intelligent vehicle with a reliable and robust energy control strategy (IRHHCS = ISSMBMC + REMS + SRFTC) was designed to minimize fuel consumption and extend battery lifespan [40], as shown in Figure 22b. The simulation findings demonstrated that the IRHHCS algorithm optimized the lowest energy consumption of 43,223 kJ, while the HCSF was 51,809 kJ and the FBS strategy was 47,115 kJ. Moreover, the proposed control strategy was proven to be feasible when current and/or voltage sensors are deficient. In the third Ref. [90], the hybrid electric urban bus was modified by introducing hydraulics to control the tri-hybrid powertrain smoothly and efficiently; the other components were the same as the previous structure, as shown in Figure 23. There are four working modes: the electric mode, the hydraulic via ICE mode and degraded mode, the hydraulic via accumulator mode and the hybrid mode, which is somewhat complex to control. Therefore, the author designed an EMS that managed to overcome the uncertainties during the driving modes controlled. Figure 22: IHHCS and IRHHCS of hydraulic-electric buses [40,89]: **(a)**IHHCS; **(b)**IRHHCS. Finally, the author succeeded to report that both the abrupt transitions during mode switching and hybrid operation phases are better thanks to the optimization of the hybrid torque split by the optimal control algorithm. Figure 23: Proposed control strategy and results of the hybrid hydraulic–electric system [90]: (**a**) control strategy for BUSINOVA bus; (**b**) comparison of three strategies under UDC; (**c**) comparison of three strategies under FTP-75.