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On May 16, 2024, the Science and Technology Festival of the Construction Machinery Industry (hereinafter referred to as the Science and Technology Festival) themed "High-end, Intelligent, Green" grandly opened in Hangzhou, Zhejiang Province. During the keynote speech session of the Science and Technology Festival, Professor Quan Long, Dean of the School of Mechanical Engineering at Taiyuan University of Technology, delivered a keynote speech entitled Exploration of Challenges and Development Paths of Construction Machinery Hydraulic Technology under the Electrification Trend.
In 2021, the dual carbon goals were written into the government work report at the Fourth Session of the 13th National People's Congress, which boosted the rapid development of electrification in the construction machinery industry. Within less than a year, many leading domestic construction machinery enterprises developed electric loaders, electric excavators and other electrified equipment. Some non-construction machinery enterprises also sought to seize the development opportunity of electrification to share the dividends of construction machinery electrification, such as Brayton. At the same time, foreign construction machinery enterprises also launched electric loaders and excavators, and new enterprises specializing in electrified construction equipment emerged, such as Hyliion in the United States.After nearly three years of development, what is the progress of electric construction machinery from equipment R&D to market promotion? According to statistics from the China Construction Machinery Industry Association, the sales volume of electric loaders exceeded 3,000 units in 2023. As early as more than a decade ago, Hitachi Construction Machinery, Komatsu and other enterprises launched hybrid loaders and excavators, but they failed to gain market recognition for a long time. The remarkable achievements we have made in such a short period indicate significant technological innovations. Especially in 2024, the electric loader industry has achieved even greater development; statistics show that the sales volume alone reached more than 600 units in March.However, while electric loaders have been rapidly promoted and applied, the popularization of electric excavators has not been satisfactory. What are the underlying reasons? The following analysis from the perspective of energy flow is intended to provide insights for subsequent technological R&D.
The energy transmission path of traditional loaders is from diesel engine to hydraulic torque converter, then to reducer. One branch drives the hydraulic pump to power the working system, and the other drives the traveling system. Affected by the efficiency of the internal combustion engine, the overall energy efficiency of the machine is relatively low. After replacing the internal combustion engine with an electric motor, although the energy efficiency of the hydraulic system is still low, its impact on the overall system efficiency is limited due to the short working time and small power proportion. After the hydraulic torque converter is removed from the traveling system, energy transmission mainly relies on the electric motor and reducer, which account for a large proportion of working time and power, with an energy efficiency of about 70%. This not only saves energy and reduces carbon emissions, but also enables the recovery of battery costs in a short time, making electric loaders widely recognized by the market.For excavators, the traditional energy flow is that the internal combustion engine drives the hydraulic pump, which then drives the working mechanism through the valve control system. Affected by the internal combustion engine, the overall energy efficiency of the machine is less than 15%, and the energy efficiency of the hydraulic system is below 30%. After electrification, the energy efficiency of the power source has increased to 90%, but affected by the hydraulic system, the overall energy efficiency of the machine is still only about 30%, resulting in 70% of the battery energy being wasted. Although the battery capacity has been greatly increased, the low efficiency leads to insufficient working time to meet user demands. In addition, the battery cost accounts for about 40% of the total machine cost, resulting in a long investment payback period, which makes it difficult for electric excavators to be promoted and applied as rapidly as electric loaders.It can be concluded that the low energy efficiency of the hydraulic system of electric loaders is a key point to be improved in the next step, but the product is mainly used for traveling, so it has quickly gained market recognition. For construction machinery mainly used for operational tasks, such as excavators, the core problem is how to improve the energy efficiency of the hydraulic system, which is a challenge facing the entire industry.
In 2015, Volvo Construction Equipment launched an excavator driven by all-electric actuators, introducing electric cylinders, which were previously mainly used in indoor equipment, into the field of construction machinery. Subsequently, Yanmar and Komatsu of Japan launched small unmanned pure electric excavators, but the practicability and reliability of unmanned control technology in medium and large excavators remain to be verified. Some domestic enterprises have also launched medium-sized excavators and loaders with electric cylinders as actuators and battery and liquefied natural gas as power sources, adopting the range-extended mode to increase working time.Meanwhile, Moog of the United States has cooperated with Bobcat and Komatsu to launch wheel and skid-steer loaders driven by electric cylinders. A German research institution compared the characteristics of a six-degree-of-freedom platform driven by electric mechanical actuators (EMA) and electro-hydrostatic actuators (EHA) respectively. The maximum energy efficiency of the two schemes is basically equivalent, with EMA at 74% and EHA at 71%. However, the maximum output force of EHA is more than three times that of EMA, indicating that hydraulic cylinders still have advantages such as high power density, large output force, and resistance to impact and vibration even with the development of electric cylinders.
Purdue University in the United States compared the energy characteristics of a 5t hydraulic excavator driven by a centralized motor driving multiple variable displacement hydraulic pumps through a transfer case and a distributed motor driving fixed displacement hydraulic pumps respectively. The centralized motor scheme can be applied to engine systems. Compared with excavators with load-sensing hydraulic systems driven by internal combustion engines, the installed power of the centralized motor scheme is basically the same, but the energy efficiency is increased by more than 50%. The fully distributed scheme can also improve energy efficiency by more than 50%, but the installed power is many times higher than that of traditional equipment driven by internal combustion engines; research data shows it is 8 times higher (possibly due to different conditions). In general, the distributed scheme requires a large installed power.INNAS of the Netherlands and BUCHER of Germany have launched a new type of hydraulic pump - the floating cup pump, with an overall efficiency of 94% to 97%, which is a revolutionary change. BUCHER has applied this pump and its system to some boom-type construction machinery to drive heavy-duty lifting mechanisms. Compared with the load-sensing system, its driving power and energy consumption are only about 1/5 of the original model, and the pump has a very high starting efficiency, representing a huge improvement in the hydraulic field.Another progress is the digital variable displacement pump, which changes the output flow not by adjusting the swing angle, but by changing the oil suction and discharge volume of each plunger. Such products adopt a modular design in structure, where the number of plungers of a single pump can be adjusted, and multiple pumps can be flexibly combined. The adoption of valve distribution also greatly improves the pump efficiency compared with existing plunger pumps. This design concept has been extensively tested and researched on loaders and excavators, and the energy efficiency can be increased by 50% to 60%.Volvo has cooperated with Nolhed of Finland to launch an actuator driven by a combination of multi-chamber digital cylinders and digital valves. The digital valve replaces a proportional valve with four on-off valves and adopts independent valve port control. Through the cooperation of the cylinder and the valve, the output force of the cylinder has dozens of combinations from positive to negative, which can eliminate the throttling loss caused by load differences during the composite drive of multiple actuators. By recovering kinetic and potential energy and adopting schemes such as a constant pressure network, the system energy efficiency has been increased from less than 20% in the past to more than 60%. This is also a widely recognized scheme in the industry, and Volvo has applied such technology to excavators and loaders.
A significant improvement in the energy efficiency of the hydraulic system can solve the aforementioned problem of the promotion and application of electrified construction machinery being restricted by the low energy efficiency of the hydraulic system.All the solutions mentioned above are research achievements of foreign enterprises, and some technologies are not open to China. For example, high-efficiency floating cup pumps are currently being launched in Germany and the United States, and other technical paths are also protected by intellectual property rights. Therefore, as a major manufacturing country of construction machinery, China must develop independent high-efficiency drive and transmission technology solutions to meet the challenges brought by electrification.The following is a brief introduction to the research work we are carrying out. Simply put, it is the integration of electric mechanical actuators (EMA) and electro-hydrostatic actuators (EHA). The power density of the electric cylinder can be improved by changing the area of the hydraulic cylinder and the system pressure. After integration, the actuator has the advantages of both EMA and EHA while overcoming their respective shortcomings. In essence, a small-power electric cylinder, equivalent to a proportional valve or servo valve but without throttling loss, is used to replace the servo proportional valve of the existing system to control the movement of the actuator with high energy efficiency and performance.In the research, we first conducted a comprehensive analysis and test of the actuator scheme on the established test bench. After the test results verified the effectiveness of the scheme, we installed the actuator on a 6t hydraulic excavator in the laboratory, replacing the original stick hydraulic cylinder and boom hydraulic cylinder for verification, and also verified its impact and vibration resistance characteristics. By integrating the hydraulic cylinder and the electric cylinder, the new actuator inherits the advantages of the hydraulic cylinder in impact and vibration resistance. Tests show that compared with the original load-sensing system, the energy consumption of the stick system is reduced by 44%, and that of the boom system by 70%. The overall energy efficiency of a complete digging and loading cycle is increased from 25% of the original system to 57%. Further integration of slewing and bucket technology can further improve the overall energy efficiency of the machine.
- 1. The energy efficiency of the transmission system is the key factor restricting the promotion and application of electrified construction machinery mainly for operational tasks, which has a great impact on the purchase cost, working time and ease of use. Substantially improving the energy efficiency of the drive and transmission system is the best way to solve this problem.
- 2. Electro-mechanical transmission has the advantages of high energy efficiency and good control characteristics, and is regarded as a transmission method expected to replace hydraulic technology. However, it transmits power rigidly, and the transmission force is limited by the volume and weight of the motor. At present, it has certain advantages in small construction machinery, and its adaptability and reliability in high-power equipment and harsh working conditions with large impact and vibration still need to be further verified.
- 3. The technologies of digital displacement pumps, floating cup pumps and digital hydraulic cylinders developed abroad are relatively mature, which can greatly improve the energy efficiency of hydraulic systems and have been applied and verified in electrified construction machinery. These technologies will form new bottleneck technologies restricting China's development of electrified construction machinery.
- 4. To achieve overtaking on curves with the help of electrification technology, China must develop independent high-efficiency transmission technologies to solve the problem of low energy efficiency of hydraulic systems. The integrated drive mode of electro-hydraulic dual power combines the advantages of both electric and hydraulic transmission methods, featuring high power density, high energy efficiency and high performance, opening up a brand-new technical path that needs to be further verified in medium and large equipment for its effectiveness.
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Posted on 2026-2-4 19:07:57