1. Introduction
With the continuous advances in industrialization, the scale of production has become enormous, which has resulted in massive economic benefits, material development, and increased energy consumption. In recent years, with the growth of China’s economy, its energy supply has become increasingly tight, which has become a major problem affecting its sustained growth. The manufacturing industry is a pillar of the national economy, with the Chinese manufacturing industry accounting for over 57% of the total energy consumption in China, as shown in Figure 1 [1]. Consequently, the “Made in China 2025” industrial policy has made the “green manufacturing project” one of five major projects to be implemented, comprehensively promoting green manufacturing. The “14th Five Year Plan for Industrial Green Development” of the Ministry of Industry and Information Technology regards the “Industrial Energy Conservation and Energy Efficiency Improvement Project” as one of the eight major projects. Nonetheless, reducing energy consumption during the manufacturing process is a major challenge facing the manufacturing sector [2,3,4]. Therefore, to achieve sustainable economic development in China, energy conservation has become a critical issue, and the implementation of energy conservation has become a long-term strategic policy for national development.
Hydraulic systems have been widely used in construction machinery due to their advantages of large force transmission, simple structure, high accuracy, and rapid response, particularly in large machinery such as hydraulic presses, cranes, and machine tools [5]. However, because of the multiple energy transmission and conversion stages in hydraulic systems, their energy efficiency is low and their energy consumption is usually high [6]. For example, due to the mismatch between the output power and process demand power of hydraulic systems, the mechanical efficiency of the hydraulic press in the forming process is low (approximately 7%) [7,8]. Consequently, in engineering machinery, energy consumption analyses and system efficiency improvements in hydraulic systems have received widespread attention.
The application of hydraulic transmission technology is an important indicator of the industrial level of a country. The reform of hydraulic engineering education has attracted the attention of educators. Ettema et al. emphasized that applied research projects can strengthen the education of hydraulic engineering students. However, hydraulic engineering education should be consistent with the core missions of universities and promote basic research [9]. Vukasinovic et al. applied additive manufacturing technology in the reform of teaching about hydraulic components, and the results showed that using additive manufacturing technology to print hydraulic components makes it easier for students to master the basic knowledge of hydraulic components during the learning process [10]. Hu et al. pointed out that innovation in curriculum design at the system level is crucial for cultivating the sustainable development skills of students; they combined the results of education and the theory of five colors in psychology to reform the teaching of hydraulic engineering construction and management courses. The results showed that this model holds reference significance for strengthening higher education and cultivating abilities and moral standards [11]. Ruslan et al. discussed the application of an integrated project-based learning (IPBL) method to help students integrate knowledge gained from one course into another and demonstrated the effectiveness of the method through the production of hydraulic jacks [12]. Crookston et al. discussed the lessons learned and techniques for cultivating students and new civil engineers in the field of hydraulic engineering from both academic and industry perspectives. They also discussed the challenges faced by the current generation of young engineers in the constantly evolving classroom and workplace, which play important roles in promoting innovative teaching activities, cultivating creativity, and strengthening the bridge between academia and industry [13]. Chanson et al. pointed out that hydraulic engineers today face many new challenges and that the solutions rely on engineering innovation, excellent hydraulic research, and high-quality education at universities, as well as indispensable interactions between engineers, scientists, and water resource stakeholders [14].
The above research indicates that scholars have explored the teaching and practical reform of hydraulic engineering from different perspectives. However, advanced hydraulic energy-saving technology cannot be incorporated into current hydraulic transmission coursework, leading to a lack of understanding and mastery of recent research, both domestically and internationally. Consequently, the theoretical knowledge of students may be insufficient to support their future work prospects in industrial practice and production environments, ultimately leading to mismatches between the teaching of, research on, and practice of hydraulic energy-saving technology in the mechanical engineering field. Consequently, to address the energy-saving issues faced by the manufacturing industry, it has become essential to incorporate hydraulic energy-saving technology into the teaching and practice of mechanical engineering. The application of such technology could effectively improve the energy efficiency of engineering machinery to meet the energy conservation and emission reduction developmental needs in the manufacturing industry. Therefore, hydraulic energy-saving technology should be incorporated into the teaching of hydraulic transmissions at various universities—whether in theoretical teaching or engineering practice—to improve the cultivation of professional skills in students.
In this study, we analyzed the current research status and latest progress in hydraulic energy-saving technology, explored the problems and challenges faced by hydraulic transmission technology, and investigated how to integrate hydraulic energy-saving technology into the current mechanical engineering training process from three perspectives—namely, teaching, research, and engineering practice. The selected case study emphasized the application of hydraulic energy-saving technology to a mechanical engineering major by incorporating it into the teaching process. Consequently, integrating hydraulic energy-saving technology into the hydraulics teaching process for mechanical engineering majors, clarifying the current development direction of hydraulic energy-saving technology, and applying it to engineering practice production could help satisfy the demand for talent cultivation in the sector in the future and positively impact the future direction of industrial production.
2. Methods: Objects and Framework
Driven by the rapid development of the economy, the construction machinery sector in China has entered a stage of rapid growth. However, the influence of environmental and related factors (such as mechanical and hydraulic energy conservation) means that many problems remain to be resolved to support energy conservation improvements and a more efficient engineering machinery sector in China.
Hydraulic systems are core components of power systems used in construction machinery. Consequently, to achieve energy conservation and emission reduction in the mechanical sector, the application of hydraulic energy-saving technology to the production and use of construction machinery is essential [15]. Increasing research and development of new environmentally friendly mechanical hydraulic energy-saving technologies not only promotes the improvement of their energy-saving effects and ensures the maximizing of mechanical engineering performance, but also helps build energy-saving and environmentally friendly societies. To adapt to the changes in the manufacturing industry and satisfy the needs of enterprises for practical hydraulic engineering competence, hydraulic energy-saving technology must be incorporated into the teaching, research, and practice of mechanical engineering. The framework of this study is illustrated in Figure 2.
Based on the latest research progress in hydraulic energy-saving technology, this paper elucidates the main research results associated with this technology from the perspectives of energy unit efficiency, control system efficiency, and energy recovery, thereby laying the foundation for integrating these research results into the teaching of mechanical courses. Based on the skill needs, training methods, and teaching reforms of mechanical engineering major in universities, this article proposes the integration of specific hydraulic energy-saving technologies into the classroom teaching curriculum. Based on the status of current research and continuing development of hydraulic energy-saving technology, we explored the problems and challenges faced by such technology from a teacher research perspective. Finally, from an engineering practice perspective and the integration of teaching and research perspective, this article discusses how to integrate hydraulic energy-saving technology into university teaching and how to research and implement the corresponding reforms.
3. Hydraulic Energy-Saving Technology
In recent years, the integration of hydraulic technology with computer control, sensing, and microelectronics technologies has been continually improving, with hydraulic systems developing toward high-pressure, integrated, digital, intelligent, high-precision, and reliable applications. Moreover, the energy consumption of hydraulic systems has increasingly attracted the attention of scholars, both domestically and internationally. Consequently, extensive research has been conducted on energy-saving technologies for hydraulic systems to improve the efficiency of energy units and control systems, as well as energy recovery.
Hydraulic systems are widely used in mechanical devices and engineering equipment; however, their energy efficiency is low. The main reason for this situation is that many energy-conversion links are present in such systems, and energy is lost when passing through each. Therefore, studying the theoretical model of energy units and improving the efficiency of unit energy conversion form the basis for improving the efficiency of energy systems. By studying the energy consumption characteristics of energy components (such as motors, pumps, hydraulic cylinders, and hydraulic system motors) and proposing corresponding energy-saving methods, the energy loss in hydraulic systems can be reduced. Gao et al. quantitatively evaluated the efficiency characteristics of the motor and pump in the power unit of a hydraulic system [16,17] and applied them to system efficiency improvement technology. Cheng et al. proposed a method of calculating the minimum cylinder length of a swash plate piston pump based on energy loss, which is a structural optimization design that ensures the highest efficiency of the piston pump [18]. Zhao et al. divided a hydraulic system into different energy units based on the energy-conversion characteristics of the hydraulic press. The energy flow between units is shown in Figure 3. The energy dissipation characteristics of each unit were quantitatively analyzed, and the results showed that the mismatch between the load characteristics and driving methods is the main reason for the low energy efficiency of the hydraulic system [7,19].
To solve the above problems, the energy efficiency of equipment control systems can be improved from the perspective of energy matching. The main method is to adjust the output power of the system to match the required power. Energy-matching methods for adjusting the output power and demand power can be divided into two main categories. One type directly controls the executing components through system control methods (such as using a permanent magnet synchronous servo motor pump control system) without utilizing valve control devices. Alternatively, a load-sensing control strategy combining speed-controlled induction motors and quantitative pumps can be employed in hydraulic systems [20,21], and the minimum energy method can be used to control the drive redundancy of the equipment [22,23]. Peng et al. designed a closed-loop hydraulic control system with a servomotor directly driving a quantitative pump and proposed a pressure and speed control strategy based on fuzzy control, achieving precise control of the hydraulic system pressure and flow during the machining process [24]. Zheng et al. used an AC servo motor to drive a quantitative pump to form a new type of pump to control a servo hydraulic press, which has a good energy-saving effect, and studied its inherent energy-saving mechanism [25]. Huang et al. proposed a dual variable control method for variable displacement pumps driven by variable speed motors, which reduces the energy consumption of hydraulic forming equipment while improving the energy efficiency of the driving unit [26]. Li et al. proposed an energy-saving control method for shared drive systems in stamping production lines [8]. Xu et al. established a 10 MN hydraulic press energy consumption model, which reduced the energy consumption by approximately 50% by optimizing the system pressure [27].
Another method is to use a shared digital pump comprising multiple continuously working, differently sized quantitative pumps. The pumps can be arranged in parallel, as shown in Figure 4. The pump unit determines whether to contribute to the flow through the valve group control matrix, the collected flow being directly used to drive one or more hydraulic cylinders [28]. Mikko et al. applied digital valve control systems to control multi-chamber cylinders, improving the controllability of the system while reducing energy consumption [29]. Cristiano et al. used on–off valves in the control process of digital hydraulic system actuators, where large-flow-rate quantitative pumps or variable displacement pumps were decomposed into multiple small flow rate quantitative pumps, and the actuator speed, inlet and outlet pressure, flow rate of the quantitative pump, and system flow rate were analyzed. The results showed that the proposed hydraulic control system exhibited stable speed changes, good dynamic characteristics, low power consumption, and high energy storage capacity [30]. Based on this research, Li et al. proposed a digital control method for different actuators to share a hydraulic drive system. In this system, the same hydraulic drive system is shared at different times based on the load characteristics of each actuator [19,31]. This energy-matching method has considerable energy-saving potential but reduces the flexibility and production efficiency of the system.
Energy recovery is an important energy-saving technique in hydraulic systems. This method converts a portion of the potential energy into easily stored energy, releases it when needed, and uses energy regeneration methods, such as hydraulic accumulators to store kinetic energy or flywheels to store inertial energy. The potential energy generated by the vertically moving loads can also be reapplied to the driving system through recycling. Minav et al. studied a hydraulic lifting system using servo-driven motors and its energy recovery strategy. The hydraulic lifting system was directly controlled by a servo-driven motor and could achieve a maximum potential energy recovery of 66% at low power [32]. Tao et al. proposed optimizing the design of a permanent magnet synchronous generator in a hydraulic lifting potential energy recovery system to improve the energy recovery efficiency [33]. Yao et al. proposed an energy recovery method for a new type of pump accumulator composite power source combining a variable-frequency direct drive pump and accumulator for rapid prototyping hydraulic presses, which could reduce the total energy consumption by 65.3% compared to that of an electro-hydraulic proportional servo valve control system [34]. Yan et al. proposed a new type of flywheel energy storage system based on the control process of frequency converters, which adopts the methods of descending energy storage, deceleration energy storage, and forming release to reduce the installed power of the press and the energy consumption of the forming process [35]. Li et al. proposed a method of directly utilizing potential energy for load balancing, which reduces the energy loss during potential energy recovery and reuse [36]. Xu et al. designed a multi-accumulator hydraulic system combining pump control and valve control based on the load characteristics of the working process of the press, which could increase the average energy utilization rate by 20% [37].
The above results represent the latest developments in hydraulic energy-saving technologies. Several mature energy-saving technologies have been used in engineering machinery and industrial production, whereas other research proposals have provided a theoretical foundation and direction for researchers. However, these achievements are rarely incorporated into the teaching of hydraulic transmission courses, which therefore cannot sufficiently train mechanical engineering professionals. Consequently, the methods of integrating hydraulic energy-saving technology into current teaching, research, and engineering practice more effectively must be explored.
4. Integration of Hydraulic Energy-Saving Technology into Mechanical Engineering Education and Research
Based on the latest advanced hydraulic energy-saving technology research mentioned above, as well as future research trends, hydraulic energy-saving technology must be incorporated into the teaching and research process to cultivate professionals who can satisfy the current engineering machinery development needs. Teachers should actively apply for research projects in the field of hydraulic energy conservation at all levels, support undergraduate training in research projects, and promote professional and project-based teaching, all with the aim of achieving the integration of hydraulic energy conservation theory and methods into specific engineering disciplines, professional teaching, and practical innovation. The specific details are outlined here.
Integrating hydraulic energy-saving technology into hydraulic theory and practice courses.
Here, students first need to clarify the energy loss characteristics of each energy unit in a hydraulic system and integrate hydraulic energy-saving technology from a component selection perspective. They are then required to master the design of various energy-saving hydraulic circuits, including quantitative pumps and variable frequency speed control motor electro-hydraulic systems, load sensitive control systems, and secondary regulation hydrostatic transmission systems. Finally, students are required to understand the application of other energy-saving technologies, such as installing energy recovery devices, adopting new power systems, and using advanced computer technology to improve control systems.
Consequently, practical coursework is an effective means through which students and teachers can better understand and master hydraulic energy-saving technology, ensuring better integration of hydraulic energy-saving technology into the mechanical engineering teaching and research process, over and above the integration of hydraulic energy-saving technology into the classroom. By designing experimental courses related to hydraulic energy-saving technology and building corresponding experimental platforms, students can further consolidate their theoretical knowledge. Additionally, teachers can better integrate hydraulic energy-saving technology research results into the teaching process, and students can be encouraged to apply their knowledge during enterprise training to solve the current energy-saving and emission-reduction problems that enterprises may be facing, as shown in Figure 5.
Building a platform for school–enterprise cooperation, achieving course integration, and building a new cooperative talent cultivation model.
Utilization efficiency can be improved by integrating the educational resources of colleges and universities with the industrial resources of enterprises. In the teaching process, the school sets the hydraulic transmission class for the enterprise, or the enterprise provides practical skills guidance for students so that they can experience the business environment and gain operational experience before graduation. Enterprises regard students as their own employees, and unified enterprise management and assessment processes are conducted with them—that is, the training of high-quality, skilled talent to meet the needs of the enterprise can be considered to be the educational goal. Accordingly, reaching a consensus with the school to create an appropriate teaching mode for their joint training requirements is essential. Through the construction of a school–enterprise cooperative platform, the application of hydraulic energy-saving technology in teaching, research, and engineering practice can be further promoted by the organic combination of classroom teaching and practical training operations, cultivating a comprehensive teaching mode suitable for different industries and enterprises.
Promoting teaching through research.
Hydraulic energy-saving technology research can be introduced into the teaching content and course design, such that classroom teaching and practical content can be improved. Teachers can actively apply for research projects related to hydraulic energy-saving technology and allow students to participate in the completion of research content derived from relevant research projects, improving their cognitive abilities with respect to hydraulic energy-saving technology, and further promoting such research progress.
Teacher cultivation.
The teachers of hydraulic transmission courses should visit hydraulic-related enterprises to conduct in-depth research, master the latest hydraulic energy-saving technologies, engage in related research, and achieve specific research outcomes. Moreover, experts in the field can be invited to conduct academic lectures to improve the quality of teaching.
Competition to promote learning.
Various hydraulic transmission technology innovation competitions at all levels can be conducted to improve the cognitive abilities of students related to hydraulic energy-saving technologies. Students can be encouraged to participate actively in various energy-saving and emission-reduction competitions and to strengthen their understanding of green manufacturing and related energy-saving concepts.
5. Case Study
5.1. Practical Teaching Reform Cases
5.1.1. Experimental Methods
The practice of teaching hydraulic transmission has been reformed to integrate hydraulic energy-saving technologies more effectively into the teaching and research structure of mechanical engineering majors. Frequency-conversion energy-saving technology has been applied in hydraulic transmission experiment courses to train students on the concepts of energy saving, emission reduction, and sustainable development in the use of construction machinery. Based on research results using the energy-matching method to study the hydraulic system driving unit of a variable pump driven by a variable frequency motor, experimental content related to hydraulic energy-saving technology could be designed.
An energy-saving hydraulic test bench with motor–pump matching was constructed for testing (the control principle of which is shown in Figure 6). During the experimental process, the energy-saving effect of the optimal motor speed and pump discharge under the set working conditions was verified by adjusting and controlling the parameters of the frequency converter and variable piston pump. This approach enables students to master the application method of frequency-conversion energy-saving technology in hydraulic systems.
The experimental method involved performing experimental tests under a variety of working conditions on a test bench, the main steps of which were as follows: set the load power (one of the various working conditions); set the output flow of the hydraulic system to 20 L/min; adjust the motor speed from 500 to 1500 r/min, controlling the interval of each gear to 100 r/min; maintain the output flow of the hydraulic system at 20 L/min by adjusting the plunger pump displacement (after each adjustment, the efficiency of the driving unit of the hydraulic system can be calculated and recorded); and analyze the characteristics of the driving unit efficiency changes with motor speed under different working conditions.
5.1.2. Experimental Results and Analysis
The experimental test results for the optimal speed and efficiency under five different working conditions are listed in Table 1. The relationship between motor speed and the overall efficiency of the drive unit under each working condition is shown in Figure 7a. The relationship between the output efficiency and motor speed of the drive unit under a load power of 2 kW is used as an example, as shown in Figure 7b.
The results show that within the speed range of the hydraulic pump, under a light load and when the load power remains constant, the efficiency gradually increases before decreasing with changes in the motor speed. Therefore, an optimal speed exists under the same load. Under different load conditions, the optimal speed exhibits a gradual increase with increasing load.
In a hydraulic system, under certain operating conditions, the efficiency influencing factors of the two main components of the driving unit, the motor and the pump, are different, and the same influencing factor also has different effects on them. This situation leads to the problem of low pump efficiency in situations in which the motor efficiency is high, which does not have a substantial effect on improving the efficiency of energy conversion in the hydraulic system drive unit. Therefore, the optimal matching method between the drive unit motor and pump under a single operating condition is considered to obtain the maximum pump energy output and the minimum motor energy input of the hydraulic system. In summary, under the driving mode of a variable pump using a variable frequency motor, an optimal motor speed and a corresponding optimal pump displacement exist under the condition of a constant load; thus, the overall energy efficiency of the hydraulic system drive unit is highest under this load, proving the effectiveness of the motor–pump matching energy-saving method.
5.2. Discussion
5.2.1. Discussion of the Experimental Results
The above matching energy-saving method was applied to a hydraulic system with variable load conditions, and the application principle was designed as shown in Figure 8.
Under certain working conditions, when the system is stable, the system flow and pressure can be measured using flow and pressure sensors, respectively, with the collected data input into the controller. The controller can then automatically determine the optimal motor speed and corresponding pump displacement under the working conditions. The inverter control signal and variable pump mechanism control signal can then be fed back, and the hydraulic system can be driven in a more energy efficient manner. The implementation of the above practices has significant implications for innovative student practices related to energy saving and emission reduction.
5.2.2. Discussion of Practical Teaching Reform
The implementation of the above experimental courses is based on the progress in hydraulic energy-saving technology research by teachers. During practical training, theoretical hydraulic transmission content can be applied by students, and the energy efficiency of traditional hydraulic transmission can be compared with that of hydraulic systems using energy-matching methods. Moreover, students can understand the effective application of hydraulic energy-saving technologies in engineering practice.
The implementation of practical courses will not only enable students to keep up with hydraulic energy-saving technology developments and master advanced hydraulic transmission technology but will also integrate the concepts of energy saving and emission reduction into the learning and training process of traditional hydraulic transmission content. Consequently, students will be able to clearly understand the energy efficiency testing and analysis methods under different hydraulic transmission power source transmission modes, enriching the practical training content. By mastering the matching energy-saving method (in the context of the variable pump speed regulation mode driven by a variable frequency motor), students can establish an experimental basis for the development of energy-saving technology for construction machinery, laying a foundation for them to engage in production practice or related research.
6. Conclusions
This study explored the application of hydraulic energy-saving technology in the teaching, research, and practice of mechanical engineering, integrating energy-saving concepts into the talent cultivation process. The main research achievements were clarified from energy unit efficiency, control system efficiency, and energy recovery perspectives; the problems and challenges faced when hydraulic energy-saving technology is integrated into the teaching and research of mechanical engineering, based on the talent needs, training methods, and teaching reforms of the mechanical engineering major in universities, were elucidated; and the integration of specific hydraulic energy-saving technologies into the teaching and research process was discussed. Based on the latest hydraulic energy conservation research by the authors, the practical teaching of hydraulic transmission was reformed in the case study, which enabled students to master the latest hydraulic energy conservation technology and its applications in the practical learning process. Integrating hydraulic energy-saving technology into the teaching of and research on mechanical engineering will not only enrich mechanical engineering curricula but will also provide research direction and methodology support for university teachers, providing new ideas for cultivating mechanical engineering talent with energy-saving and emission-reduction awareness and knowledge.
The application of hydraulic energy-saving technology into the construction machinery and machinery industries is an important means of achieving energy conservation and emission reduction, which can provide technical support for the transformation and upgrading of the machinery sector and is an important means of its green development. This process includes clarifying the latest advanced hydraulic energy-saving technology research progress, ensuring that research results can meet the developmental needs of the sector. Moreover, employees in the mechanical engineering sector must master the latest hydraulic energy-saving technology and apply it in practice, which requires universities to integrate hydraulic energy-saving technology into theoretical and practical coursework in relevant fields, so that future employees have the ability to apply this technology. Finally, to adapt to the greening of the manufacturing sector, enterprises need to implement corresponding reforms and apply advanced hydraulic energy-saving technology actively to their production activities. Consequently, in the cultivation of mechanical engineering professionals, whether in theoretical teaching or engineering practice, universities should integrate hydraulic energy-saving technology into the development of student knowledge and cultivation of professional skills.
Conceptualization, M.G. and Q.W.; methodology, X.S.; investigation, Q.L.; data curation, L.Z.; writing—original draft preparation, M.G.; writing—review and editing, Q.W.; funding acquisition, M.G., Q.W. and X.S. All authors have read and agreed to the published version of the manuscript.
Not applicable.
Not applicable.
Data are contained within the article.
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 5. Integration points of hydraulic energy-saving technology theory and methodology.
Enlarge this image.Figure 7. Relationship between the overall efficiency of the driving unit and motor speed under various operating conditions.
Driving unit efficiency changes with the motor speed under different working conditions.
| Motor Speed (r/min) | Overall Energy Efficiency of the Drive Unit (%) | ||||
|---|---|---|---|---|---|
| Load Power 1.0 kW | Load Power 2.0 kW | Load Power 3.0 kW | Load Power 4.0 kW | Load Power 5.0 kW | |
| 600 | 66.23 | 78.12 | - | - | - |
| 700 | 68.49 | 83.23 | 81.07 | 74.49 | 72.86 |
| 800 | 68.97 | 86.00 | 83.06 | 76.63 | 75.33 |
| 900 | 70.21 | 86.84 | 86.51 | 84.57 | 82.72 |
| 1000 | 69.32 | 87.90 | 88.65 | 90.50 | 91.05 |
| 1100 | 67.57 | 86.77 | 88.97 | 92.81 | 93.48 |
| 1200 | 64.94 | 84.89 | 87.41 | 92.38 | 94.46 |
| 1300 | 62.89 | 83.22 | 86.81 | 92.17 | 95.20 |
| 1400 | 61.35 | 76.74 | 85.03 | 92.59 | 94.78 |
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