## 5 Conclusions  
In this article, the characteristics of the hydrogen fuel cell, power cell, and ultracapacitor are analyzed and studied, and a simple model is established to verify that a single energy source cannot meet the characteristics of high and fast-changing power demand during tractor operation. Thus, a hybrid tractor energy system with a hydrogen fuel cell, power cell, and ultracapacitor is designed to provide a theoretical basis for the design of the composite energy tractor.  
A multi-layer control-based energy management strategy based on haar wavelet transform and logic rules is designed using the integrated design characteristics of the composite energy sources. The output power of each energy source is optimized with the goal of efficiency optimization and smoothing the output power of the PEMFC system with the tractor power performance as the constraint. The optimization realizes the multilayer decoupling of high-frequency signal, low-frequency signal, sub-high frequency signal, and steady-state signal of tractor load demand power signal, and the decoupled power signal is reasonably allocated to each energy source.  
Using model-in-the-loop experiments for verification, the average output power fluctuation per second of the PEMFC system with multilayer control-based EMS based on the haar-wavelet rules decreased by 11.01%, 3.56%, and the maximum output power fluctuation per second decreased by 3.29 kW, 2.15 kW, respectively, during the experimental cycle compared with the single-layer control strategies such as fuzzy control and power-following control. In addition, the operating point of Pemfc is closer to its operating point at maximum efficiency, and the PEMFC system efficiency is effectively improved the operating point of the PEMFC is closer to its operating point at maximum efficiency, and the efficiency of the PEMFC system is effectively improved, so the equivalent hydrogen consumption of the whole tractor is reduced by 16.1% and 6.8%, respectively. The results show that the multilayer controlled energy management strategy designed in this article can make full use of the advantages of the composite energy source on the basis of meeting the load power demand of the whole vehicle, so that the fuel cell can output power in an efficient and smooth working condition, thus improving the economy of the whole vehicle and extending its service life, which is more in line with the green energy-saving policy.  
This article focuses on an optimal design approach for the EMS of hybrid tractors considering efficiency optimization and output power smoothing. Nowadays, more complex and multi-objective objectives are being developed; however, the operation of a PEMFC system is a complex process and more variable factors need to be considered in practical engineering, such as temperature, humidity, emissions, and overload operation. In addition, this paper simply uses some existing findings to show that smooth power output can extend the life of the PEMFC, without quantifying how it affects system degradation in detail. However, due to space limitations, these studies will be presented in subsequent research articles.  
Conceptualization, W.X., L.X. and M.L.; methodology, W.X. and L.X.; software, W.X. and M.L.; validation, W.X., L.X. and M.L.; formal analysis, L.X. and M.L.; investigation, S.Z.; resources, W.X. and S.Z.; writing--original draft preparation, W.X., L.X. and M.L.; writing--review and editing, W.X., L.X., M.L. and S.Z.; visualization, W.X.; supervision, L.X., M.L. and S.Z.; project administration, W.X., L.X., M.L. and S.Z.; funding acquisition, L.X. and M.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Major Science and Technology Project of Henan Province (221100240400), Key Scientific Research Project of Colleges and Universities in Henan Province (no. 20A460013), and the National Key Research and Development Program of China (no. 2017 YFD070020402).  
Not applicable.  
The authors declare no conflict of interest.