Energy saving and emission reduction has become a world theme. The wheel loader has a harsh working environment, complicated working conditions, and drastic changes in load, which cannot give full play to the performance of the engine. Therefore, the variable speed transmission of the vehicle plays a vital role in improving the efficiency and performance of the vehicle. The input coupling general characteristic equation of the wheel loader hydraulic-mechanical continuously variable transmission is established, and the reasonable structure form of the working condition connection is determined. A general efficiency equation with a reasonable structure for efficiency analysis is established, and a joint verification method of simulation and experiment is proposed to make the efficiency analysis result more accurate. Through this method, the researchers analyzed the transmission performance of the transmission more accurately, and provided important guidance and verification for the early design of HMCVT. Finally, a reasonably optimized product is obtained, which reduces the design cost and cycle.
Related papers were published on "Applied Sciences" with the title "Characteristic Analysis and Co-Validation of Hydro-Mechanical Continuously Variable Transmission Based on the Wheel Loader".
As an off-highway vehicle, wheel loader has complex working conditions and drastic load changes. Therefore, the automobile transmission plays a vital role in improving the efficiency and performance of the automobile. The traditional transmission of a wheel loader is a hydraulic torque converter mechanical transmission. Therefore, many scholars have conducted research on the traditional hydraulic torque converter transmission. However, the efficiency of the torque converter has a greater impact on the efficiency of the transmission system, and no matter what structure is adopted, it will not lead to a significant increase in efficiency. Therefore, the Hydromechanical Continuously Variable Transmission (HMCVT) has received extensive attention. Hydro-mechanical continuously variable transmission (HMCVT) is a power split transmission. The composition of HMCVT is shown in Figure 1. The input power is divided into mechanical transmission and static pressure continuously variable transmission through the power shunt device, and the power of the two channels is combined through the power connection device (PJD). High-efficiency power transmission is realized through mechanical transmission, and continuously variable transmission is realized through hydrostatic transmission.
Figure 1. The composition of the Hydromechanical Continuously Variable Transmission (HMCVT).
Characteristic analysis of wheel loader
The wheel loader is driven by the power of the engine, which is transmitted to the wheels through the transmission and driven by the driving force on the ground, as shown in Figure 2.
Figure 2. Schematic diagram of the force of the loader.
The driving resistance mainly refers to the rolling resistance Ff and the air resistance Fω. If the loader is driven uphill, it must overcome the ramp resistance Fi, which is caused by its own gravity; if the loader accelerates, it must also overcome the inertial force Fa. The driving force FK of the wheel loader can be calculated as follows:
In this working condition, the loader often accelerates and decelerates throughout the working cycle. Therefore, in view of the working conditions of the loader, the researchers focused on the force characteristics of the loader under acceleration. As shown in Figure 2, the loader accelerates on a level road.
Speed characteristics
HMCVT divides power into two paths, one is a fixed transmission ratio (mechanical path), and the other includes a static pressure continuously variable transmission. The two are reconnected to the output shaft. The continuously variable transmission relies on a planetary gear train (PG) to realize the function of power splitting or coupling. There are two different types of HMCVT, as shown in Figure 3. As shown in Figure 3a, it is an input coupling type, PG is used as a power junction device (PJD), as shown in Figure 3b, it is an output coupling type, and PG is used as a power tapping device (PSD).
Figure 3. Types of HMCVT. (a) Input coupling type; (b) Output coupling type.
Mechanical transmissions are often shifted by clutches or brakes. Therefore, when the transmission ratio of the water pressure CVT changes in different directions, the range of the transmission ratio of the HMCVT will be expanded, so the ratio of the power flow of the water pressure CVT can be kept as low as possible in each speed range, realizing an efficient multi-range HMCVT With the ever-changing spread. Multi-range HMCVT mostly uses this input coupling method, because it can achieve high efficiency operating points for all mechanical power flows. Dana Rexroth (HVT) R2 and R3 use the concept of three ranges: HVT has a static pressure drive range and two input coupled power split ranges.
The basic components of multi-range HMCVT have three working states:
1. H range, in which the transmission has only hydrostatic power.
2. HM range, forward transmission of PG.
3. The range of HM, and the reverse transmission of PG.
Speed characteristics
From Figure 4, we can get:
Figure 4. Input coupling type of HMCVT.
in conclusion
In order to better study the characteristics of HMCVT applied to wheel loader, the researchers established the input coupling characteristic equation of HMCVT based on the characteristics of wheel loader, and analyzed the power flow transmission mode of HMCVT. Then, the factors affecting the hydrostatic power ratio are studied and analyzed, mainly from the connection between working conditions and the planetary arrangement structure.
In the early stage of the design, the researchers proposed a joint verification method of simulation and experiment to make the efficiency calculation results more accurate. Taking a wheel loader as an example, this method can be used for efficiency analysis under different working conditions. The results show that when the pressure is 50% of the maximum pressure and the pump displacement reaches the maximum displacement, the hydrostatic CVT efficiency is the highest, and the HMCVT efficiency is also the highest, which is more accurate than the theoretical calculation result. It provides important guidance and verification for the early design of HMCVT, and avoids the irrationality of the structure after the prototype is manufactured. Finally, the design cost and design cycle are saved.
