Loader power-split transmission system based on a planetary gear set

Abstract

In hydraulic mechanical transmission loaders, a hydraulic torque converter can prevent an engine from stalling due to overloading of the loader during the spading process; however, the hydraulic torque converter also reduces the loader’s fuel economy because of its low transmission efficiency. To address this issue, the study designs an output-power-split transmission system that is applied to a hybrid loader. The designed transmission system removes the hydraulic torque converter in the power transmission system of a traditional loader and adopts a planetary gear set with a compact structure as the dynamic coupling element, thus allowing the output power of the loader to be split transmitted. During shoveling, the loader power-split transmission system based on a planetary gear set can prevent the motor from plugging and over-burning under conditions that ensure that the traction does not decrease. In addition, the transmission efficiency and loader fuel economy are higher in the proposed transmission system than in the power transmission system of a traditional loader. The test results show that the transmission efficiency of the designed system was 13.2% higher than that of the traditional hydraulic mechanical transmission loader.

Keywords

Hybrid power loader power-split transmission system planetary gear set

Introduction

In 2015, the number of construction machineries in China reached 6.908 million, and 1.744 million of those machineries were loaders.¹ Loaders are limited by their high fuel consumption and poor emissions resulting from the adverse working environment. Therefore, improving the energy efficiency of the engine has been a main focus of research on hybrid loaders. Since the first hybrid loader was developed in 2003, a large number of construction machinery makers (e.g. Kawasaki, Volvo, and Hitachi Construction Machinery) have conducted research on hybrid loaders, resulting in many achievements.^2–5 Hybrid power systems can be divided into three major categories according to the structure of the transmission system: series hybrid systems, parallel hybrid systems, and combined hybrid systems.⁶

In a serial hybrid system, the electromotor is connected to the controller via an electric circuit and drives the system operation directly. This connection type allows for a relatively simple structure of the series system;⁷ however, although the energy conservations and emission reductions of the vehicle are better when using a serial hybrid system than when using a parallel system or combined hybrid system, the cost of this system is higher because the energy is converted many times. In general, the engine and motor or the oil hydraulic pump driving the loader operate singly or in combination for the parallel hybrid loader. Because there are fewer energy conversions, the parallel hybrid loader has a higher energy utilization rate than the serial hybrid system in vehicles.^8–10 However, the regulating action of a parallel hybrid loader on the engine operating point is weaker. Parallel hybrid systems typically include hydraulic torque converters (HTCs), thereby limiting the energy conservation effect.^9,11–13 The combined hybrid system is connected to an engine, an alternator and an electromotor or hydraulic pump/motor through the power distribution unit and integrates the merits of the series and parallel systems. The engine is not affected by the vehicle driving cycle, allowing it to remain highly efficient for a long period.¹⁴ The construction machinery can perform the required task well under harsh conditions and realize energy conservation and emission reductions.

In 2008, Volvo launched a coaxial and parallel structure hybrid power loader (L220F Hybrid) that adopted a parallel structure of a hybrid oil and electric power system with a torque combination of a front-row transmission and single shaft.^15,16 In 2010, Liugong and Xugong Machinery Co., Ltd. launched hybrid wheel loaders CLG862-HYBRID and ZL50GS. These two models are parallel hybrid systems.^17,18 Caterpillar produces the hybrid wheel loader 966KXE. This loader uses advanced powertrain CVT and does not have a torque converter; furthermore, the hydraulic system of this technology can accomplish infinitely variable speeds; therefore, the transmission has the characteristics of infinitely variable speeds.¹⁹ In 2011, the Japanese company Kawasaki developed hybrid loader 65ZHybrid, in which the torque converter is replaced with a “HYTCs” system, which consists of planetary gears, motors, and alternators. The energy utilization rate is 35% higher than that of a traditional loader of the same level.¹⁸ In 2014, Hitachi Construction Machinery announced that they had developed a wheel loader equipped with a hybrid power system (ZW220HYB-5B). The electric alternator is driven by a diesel engine and generates electric power to drive the motor. The loader has no gearbox and adopts infinitely variable speeds; therefore, it has the advantage of no changes in the speed operation when the loader is digging or driving on a ramp, thereby decreasing the labor intensity of the drivers.⁵

The above analysis of the development history of the hybrid loader has shown that important advancements and prototypes have emerged in the hybrid loader technology field. At present, research on hybrid loaders is focused on parallel hybrid loaders and series hybrid loaders;^20–23 however, because these systems include an HTC, the engine does not always operate in the optimal operating condition, particularly under heavy loads, when the efficiency of the HTC is extremely low. With an increasing number of energy-saving schemes being proposed, the continuously variable transmission and planetary coupling structure are becoming the energy-saving schemes with the greatest potential. Both schemes improve the engine energy efficiency using hybrid power technology and improve the system drive efficiency by replacing the HTC with the new transmission scheme.

Structure of power-split transmission system

A schematic diagram of the power-split transmission system of the loader based on a planetary gear set is shown in Figure 1; the system is composed of an engine, an input shaft, the first gear, the second gear, a planetary gear set, an output shaft, motor #1, motor #2, a power cell, an inverter, lockup devices, and a hydraulic working system. The planetary gear set includes a sun gear, front-row planetary gear, front-row gear ring, rear-row planetary gear, rear-row gear ring, and planetary carrier. The parameters of the front-row planetary gear are the same as those of the rear planetary gear; both are mounted on the same shaft, and the parameters of the front-row gear ring are also the same as those of the back-row gear ring. The engine, input shaft, first gear, and second gear are connected in turn. The second gear and front-row gear ring are connected coaxially; the planetary carrier and output shaft are connected coaxially; motor #1 is connected with the back-row gear ring; motor #2 is connected to the sun gear; and the power cell is connected to motor #1 and motor #2 through the inverter. The lockup devices include the first lockup device and second lockup device. The ends of the first and second lockup devices are connected to the input shaft and motor #2, respectively. The other ends of the two lockup devices are fixed connected to the frame.

Figure 1.

Schematic diagram of the power-split transmission system of the proposed hybrid power loader.

Operating characteristics of the power-split transmission system

According to the operation condition of the loader, the power-split mode is divided into two categories: low-speed power-split mode and high-speed power-split mode. The low-speed power-split mode is mainly used for low-speed and high-load conditions, such as shoveling and lifting. The power transmission route is shown in Figure 2.

Figure 2.

Power-split transmission line for the low-speed power mode.

In the low-speed power-split mode, the first lockup device is separated from the second lockup device. When the engine is operating, the power is transmitted to ride wheels via the input shaft, first gear, second gear, front-row gear ring, front-row planetary gear, planetary carrier and output shaft; motor #1 and motor #2 are working simultaneously, and the loader is placed in the continuously variable-speed state.

In the low-speed power-split mode, motor #2 transforms a portion of the power output from the engine into electrical energy using an alternator. The power cell discharges, with motor #2 simultaneously providing electrical energy to motor #1; motor #1 works as an electric motor and transforms electrical energy to mechanical energy. The power generated by motor #1 is eventually transmitted to the drive wheel to provide traction along with the engine via the rear-row gear ring, rear-row planetary gear, planetary carrier, and output shaft. A portion of the power of the engine is transmitted to the hydraulic work system through the input shaft.

The engine, sun gear, planetary carrier, front-row gear ring, and rear-row gear ring are abbreviated as ICE, S, C, R1, and R2, respectively.

In the low-speed power-split mode, the revolving speeds of the output shaft and planetary carrier are zero when the wheel of the loader is jammed. Thus, $n_{C}$ is zero

n_{C} = 0

(1)

The motion characteristic equation of the front-row planetary gear set is given as follows

n_{s} + α n_{R 1} = (α + 1) n_{c}

(2)

where $α = Z_{R 1} / Z_{S}$ . Combining equations (1) and (2) yields

n_{S} = - α n_{R 1}

(3)

Because the parameters of the front-row planetary gear and rear planetary gear, front-row gear ring, and back-row gear ring are identical, the relationship between them is as follows

n_{R 1} = n_{R 2}

(4)

In this mode, the engine is always in running. As shown in Figure 3, the rotation speed of the front-row gear ring is always proportional to the engine

n_{R 1} \propto n_{ICE} \neq 0

(5)

Figure 3.

Relationship between the different torques.

According to equations (3)–(5), the rotation speeds of the sun gear and rear-row gear ring are not zero in the low-speed power-split mode when the wheel of loader is jammed; that is, the two motors do not stop rotating.

Through the relationship between transmission lines shown in Figure 2, the relationship between the torques of the sun gear, planetary carrier, front-row gear ring, and back-row gear ring is obtained, as shown in Figure 3. In Figure 3,T_S and $n_{S}$ represent the torque and speed of the sun gear, respectively; T_R₁ and $n_{R 1}$ represent the torque and speed of the front-row gear ring, respectively; and T_R₂ and n_R₂ represent the torque and speed of the back-row gear ring, respectively.

The relationship between the different torques is presented in Figure 3. The relationships between the torques of sun gear 4, planetary carrier 11, front-row gear ring 3, and rear-row gear ring 7 are as follows

T_{C} = T_{R 1} - T_{R 2} + T_{s}

(6)

T_{R 1} - T_{R 2} = α T_{s}

(7)

where $T_{R 1}$ is supplied by the engine, $T_{R 2}$ is supplied by motor #1, and $T_{S}$ is supplied by motor #2.

Equation (8) can be obtained from equations (6) and (7)

T_{C} = (α + 1) T_{S}

(8)

In the low-speed power-split mode, the planetary carrier can supply high traction torque.

When the loader is shoveling, one can ensure that the traction is not decreased and the motor does not plug and over-burn.

The high-speed power-split mode is mainly suitable for the loader running at high speeds during converse site processing. The power transmission route is shown in Figure 4.

Figure 4.

Power-split transmission line of the loader operating at high speed.

In the high-speed power-split mode, motor #1 functions as an alternator that converts a portion of the power output by the engine into electrical energy and supplies it to motor #2. Motor #2 functions in the form of an electric motor, which converts electrical energy into mechanical energy, and the generated power is transmitted to drive wheels before finally passing through the sun gear, front-row planetary gear, planetary carrier, and output shaft, thereby providing traction with the engine together.

Experiments on the transmission efficiency

The real loader experiment

Taking the ZL50 wheel loader as an example and stonework as the job object, the transmission efficiency of the loader is tested.

Through comparing various schemes and the possibility of installing sensors, the experimental scheme for testing the transmission shaft is finally determined according to the research content. The scheme is as follows:^24–29

Arrange the torque and revolving speed transducer in the front axle shaft and measure its torque and revolving speed;

Arrange the torque and revolving speed transducer in the rear axle shaft and measure its torque and revolving speed;

Arrange the torque and revolving speed sensor in the output shaft to measure its torque and revolving speed. The sensors layout scheme is shown in Figure 5.

Figure 5.

Transmission efficiency test scheme and sensor layout.

We adopt the work mode of the V-style six stages and allow the loader to move 50 decaliters; then, we record the torque and revolving speed of the front axle shaft, the rear axle shaft, and the output shaft of the engine. Figure 6 presents the test value of the experiment, where channels 1 and 2 are the revolving speed of the front axle left shaft and that of the right shaft, respectively; channels 3 and 4 are the revolving speed of the rear axle left shaft and that of the right shaft, respectively; channel 5 is the revolving speed of the input shaft of the transmission system; channels 6 and 7 are the torque of the front axle left shaft and that of the front axle right shaft, respectively; channels 8 and 9 are the torque of the front axle left shaft and that of the front axle right shaft, respectively; and channel 10 is the torque of the input shaft of the transmission system. Using vib.sys and ncode software to process the tested signals and selecting 50 test working cycles, continuous time-domain signals are acquired by filtering and eliminating any abnormal peak values.

Figure 6.

Test signals of the loader.

The input power of the loader can be calculated from formula (9) as follows

p_{e} = \frac{T_{e} n_{e}}{9550}

(9)

where $P_{e}$ is the input power of the loader (kw), $T_{e}$ is the torque of the input shaft (N m), and $n_{e}$ is the revolving speed of the input shaft (r/min).

The input power of the loader can be calculated according to formula (9), and the torque and speed of the input shaft tested by the NO. 5 channel and NO. 10 channel are given in Figure 7.

Figure 7.

Input power and output power of the transmission system.

Similarly, the output power values of the front axle shaft and the rear axle shaft can be calculated. Then, the output power of the transmission system is acquired by adding the two figures together, as shown in Figure 7.

The transmission efficiency of the loaders is obtained according to formula (10)

η = \frac{p_{e 1}}{p_{e 2}}

(10)

where P_e₁ is the average output power of the transmission system, P_e₂ is the average input power of the transmission system, and $η$ is the average transmission efficiency.

According to the input and output power of the transmission system shown in Figure 7, the calculated average efficiency of the transmission system of the ZL50 loader is 63.5%.

Bench test for the hybrid loader system

The hybrid loader experiment is performed on the test bench; the bench is mainly composed of the power system, transmission system, hydraulic system, and control system.³⁰ A schematic diagram of the test bench is shown in Figure 8.

Figure 8.

Schematic diagram of the test bench.

The power system is composed of the crude oil engine, alternator, battery, and electromotor. The alternator drives the hydraulic system, and the residual energy is converted to electricity and stored in the battery to drive the electromotor work. The information acquired from the sensors is input to the controllers and processed by a microprocessor to control the engine and electromotor.

The load of the loader consists of the transmission system load and hydraulic system load; in the test bench, the loader hydraulic system is simulated by the proportion overflow valve, and the running system is simulated by the hydraulic dynamometer.

The drive system load is mainly composed of the gearbox and dynamometer. The load is driven by the direct current electromotor and is used to simulate the loader operation. The direct current electromotor has an automatic variable-speed function and can reverse by itself; thus, the back gear is not necessary for the gearbox, and its gear change is relatively fixed. The measurement devices include a controller, hydraulic dynamometer, and force-measured device; the system load energy is absorbed by the loss of the hydraulic dynamometer. The output power is calculated using the acquired output speed and torque.

The working load of the hydraulic system is simulated by a pilot-operated electricity proportion overflow valve.³¹ To control the working pressure of the overflow valve, the tested working pressure spectrum data of the hydraulic system is loaded into the overflow valve, and the loading spectrum digital signals are transformed into voltage signals through the serial port. The overflow valve is controlled by the pulse width modulation (PWM), and the control pressure overflow valve is regulated by changing the duty ratio of the PWM waveform. According to test data used to calibrate the single chip micyoco (SCM), the SCM computes and outputs the duty ratio of the PWM based on the calibrated data after the data overflow valve controller receives the pressure control signal, and then, the drive proportional overflow valve achieves pressure control.

The exit pressure loading spectrum and transmission shaft loading spectrum of the transmission pump, working pump, and steering pump for a real loader test are shown in Figure 5. The field-tested working pressure spectrum data of the hydraulic system are loaded into the overflow valve to simulate the hydraulic system load. The load spectrum tested in the real loader test of the transmission shaft is loaded through the power-measured devices controllers, and the hydraulic dynamometer takes advantage of a vortex to produce braking torque to simulate the transmission system load. After recording the experimental transmission efficiency of the real loader and bench test, the fuel economies of the two components are calculated for the two types of transmission; the calculated results are shown in Figure 9.

Figure 9.

Transmission efficiency obtained using different drive systems.

As shown in Figure 9, the transmission efficiency of the loader power-split transmission system based on a planetary gear set was 76.7%, which was higher than that of the hydraulic mechanical transmission loader (63.5%). Therefore, the transmission efficiency of the loader power-split transmission system based on a planetary gear set was 13.2% higher than that of the hydraulic mechanical transmission loader.

Conclusion

This article designed a loader power-split transmission system based on a planetary gear set. The transmission system adopts a planetary gear set with a compact structure as the dynamic coupling element and enables the output power of loader to be split transmitted. In addition, the HTC is removed in the transmission system, thereby effectively increasing both the transmission efficiency and loader fuel economy.

The power-split mode of the system can be divided into two categories according to the operation condition of the loader: low-speed power-split mode and high-speed power-split mode. The low-speed power-split mode is mainly used for low-speed and high-load conditions, such as shoveling and lifting, whereas the high-speed power-split mode is mainly suitable for the loader running at high speed during converse site processing. When shoveling, the transmission system can prevent the motor from plugging and over-burning under the condition that traction is not reduced.

Two experiments, namely, a real loader experiment and a bench test, were performed to investigate the energy savings of the hybrid loader system. Comparing the experimental results revealed that the transmission efficiency was 13.2% higher for the hybrid loader system than for the hydraulic mechanical transmission loader.

Footnotes

Handling Editor: Yangmin Li

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was financially supported by the National Natural Science Foundation of China (Grant No. 51205151) and a prospective industry-university-research cooperation research project in Jiangsu province of China (Grant No. BY2016061-01).

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