Abstract: Hydraulic torque converter is widely used in modern construction machinery transmission, which can not only transmit torque, but also change the torque. For modern large-scale construction machinery, its energy consumption is very large, but its efficiency is often low. So we always hope to improve the efficiency of construction machinery as much as possible. Therefore, it is particularly important to study the energy loss of hydraulic transmission. The loss of hydraulic torque converter in modern construction machinery is studied from the point of view of fluid mechanics.
Keywords: construction machinery hydraulic torque converter hydraulic loss mechanical loss volume loss
1 preface
In the transmission system of construction machinery, hydraulic mechanical transmission is generally adopted, which can meet the requirements of modern construction machinery with large traction, low speed, wide range of traction and driving speed, and free advance and retreat. The hydraulic torque converter installed in the hydraulic mechanical transmission has the functions of automatic torque change, speed change, vibration isolation, good starting performance and torque limit protection, which is more suitable for the needs of modern construction machinery.
In the hydraulic torque converter, the fluid flows around the annular channel composed of pump impeller, turbine and guide wheel, and energy is obtained from the pump impeller and transferred to the turbine. When the guide wheel is stationary, there is no energy exchange when the fluid passes through the guide wheel. However, the fluid flowing in the circulation circle is viscous, and there must be friction loss, which is directly related to its speed. The working channel is a non-prototype section with bending and diffusion, so its friction loss is much greater than that of the circular channel. In addition, under off-design conditions, impact losses will occur at the inlet of turbine and guide wheel. Therefore, the maximum efficiency of general hydraulic torque converter is 85%~92% [1]. For general construction machinery, the efficiency is generally low because of its heavy load, bad working conditions and serious wear of parts. Therefore, it is of great practical significance to study the energy loss of hydraulic torque converter.
2 working principle of hydraulic torque converter
The basic structure of hydraulic torque converter is shown in figure 1. It is mainly composed of three working wheels with curved blades, namely a rotatable pump wheel 4, a turbine wheel 3 and a fixed guide wheel 5. Each working wheel is usually made of high-strength light alloy by precision casting. Generally, the pump impeller 4 is integrated with the torque converter housing 2 and fixed on the connecting plate of the engine crankshaft 1 by bolts. The turbine 3 transmits power through a driven shaft 7. The guide wheel 5 is fixed on the fixed sleeve 6. After the torque converter is assembled, all working wheels form a circular cavity.
When the hydraulic torque converter works, the working fluid stored in the annular cavity not only circulates with the torque converter (i.e., moves in a implicative way), but also circulates in the direction shown by the arrow 1 (i.e., moves relatively). When the liquid leaves the impeller, it enters the turbine at a certain absolute speed and impacts the turbine blades, transferring the torque from the impeller to the turbine.
1. Engine crankshaft 2. Torque converter housing 3. turbine
4. Pump wheel 5. Guide wheel 6. Secure sleeve 7. driven wheel
Figure 1 Structure principle of hydraulic torque converter
3 Energy loss of hydraulic torque converter
To sum up, the process of hydraulic transmission is inevitably accompanied by energy loss. The energy loss of hydraulic torque converter is generally divided into three types: hydraulic loss, mechanical loss and volume loss.
3. 1 hydraulic loss
Hydraulic losses are divided into two categories: one is frictional resistance loss, and the other is local resistance loss.
1. Friction resistance loss
In the process of working liquid flowing in the circulation circle, there is a certain relative velocity between layers and between liquid and channel wall. Because of the viscosity of the liquid, there will be friction resistance, and the slow flow layer will hinder the fast flow layer. The energy lost per unit mass of liquid in order to overcome this resistance is called frictional resistance loss. In reference [2], the magnitude of frictional resistance loss is usually expressed as the percentage of velocity head v2/2g of liquid flow. In hydraulic transmission, the movement of liquid particles relative to the impeller is relative, so the friction resistance loss is expressed by the speed head of relative speed ω.
Where: L-the length of the runner, m; λ —— Friction resistance coefficient;
Rn refers to the hydraulic radius of the flow passage, and its value is equal to the ratio of the cross-sectional area of the flow passage to the wet perimeter, m.
Because the pump impeller, turbine and guide wheel all have friction phenomenon in the transmission process, the sum of friction loss should be the sum of the three, namely:
∑hm=hmB+hmT+h mD (2)
2. Local resistance loss
(1) Impact loss
Generally speaking, the liquid flow at the inlet of the impeller is different from the inlet direction of the blade bone line. This will lead to eddy current loss and additional friction loss caused by channel contraction in the escape zone. The angle Δ β c between the inlet relative velocity ω0 and the bone line is the angle of attack, as shown in Figure 2. Δ β c is positive or negative. When ω0 flows to the blade working face, δ β C is positive; When ω0 flows to the back of the blade, δ β c is negative. The pressure on the working face of the blade is high, but the pressure on the back is low.
A pump impeller inlet angle of attack b turbine inlet angle of attack
Figure 2 Inlet angle of attack
When the relative velocity Ω 0 deviates from the blade bone line, it often forms a shedding area on the blade surface, which makes the flow channel contract in the shedding area. Impact loss is related to impact loss speed and impact loss coefficient. The speed of impact loss is shown in Figure 3.
Figure 3 Impact loss velocity
Where: HC-impact loss energy head, m;
φc- impact loss coefficient;
Ω c-impact loss speed, m/s
Similarly, the pump impeller, turbine and guide wheel also have impact losses, so the impact losses in the pump impeller are as follows:
∑hc=hcB+hcT+h cD (3)
(2) Loss of sudden expansion and sudden contraction
The flow section of the vaneless area before the impeller inlet is larger than that after the impeller inlet. The flow cross section of the impeller outlet is smaller than that of the bladeless area behind the outlet. There is a sudden contraction loss at the inlet of the impeller and a sudden expansion loss at the outlet. This is caused by the extrusion of the blade. These losses are calculated according to the formula in reference [3]:
Where: htk—- sudden expansion of unit energy loss, m;
HTS-sudden decrease of unit energy loss, m;
ξts- loss coefficient of sudden decrease, = 0.4 ~ 0.5;
VM3-axial speed at the outlet of impeller, m/s;
Vm0-axial speed before impeller inlet, m/s
Therefore, the total expansion and contraction energy losses are:
∑ht=htK+htS (6)
(3) Diffusion loss
For hydraulic transmission, there are diffusion tube flow channels, such as the flow channel in the pump impeller, the first half of the flow channel in the turbine and the first half of the guide wheel of the integrated hydraulic torque converter. The loss of the diffuser is calculated as follows:
Where: VM 1- axial velocity of initial section of diffuser;
VM2-refers to the axial velocity at the end of the diffuser;
φk—— diffusion loss coefficient.
From the above, the total hydraulic loss is:
∑h=∑hm+∑hc+∑ht +∑hk (8)
3.2 Mechanical loss
When power is transmitted through hydraulic transmission, mechanical losses include bearing and seal losses of pump shaft, friction losses of pump disc-friction losses between the outer surface of pump wheel and liquid, and friction losses of turbine disc-friction losses between the outer surface of turbine and liquid. All these mechanical losses will consume the energy of the power machine and affect the efficiency of hydraulic transmission.
By improving the matching accuracy and reasonably selecting lubricating oil and sealing materials, the loss of bearings and seals can be controlled below 65438 0% under rated working conditions [4]. Mechanical friction loss is mainly the disc friction loss of rotating parts such as pump impeller and turbine. When the relative speed is high, the friction loss of the disk is large. In addition, not all disk friction consumes electricity, so it must be analyzed in detail.
3.3 Volume loss
Because most of the liquid at the outlet of the pump wheel flows into the turbine, this part of the liquid flows into the guide wheel from the turbine and then returns to the pump wheel, which plays a role in transmitting force. There is a relatively small annular gap between the pump impeller inlet and the inner ring of the stator outlet, and there is the same gap between the turbine outlet and the inner ring of the stator inlet. This gap makes the impellers not contact with each other, so there is no mechanical friction between the impellers. However, the pressure at both ends of this annular gap is not equal, and a part of liquid will flow from the high cavity to the low cavity through these gaps. The pressure at the outlet of the pump impeller is higher than that at the inlet of the pump impeller and also higher than that at the outlet of the turbine impeller, so the liquid flows from the outlet of the pump impeller through the annular seal and flows around the inner ring of the pump impeller to the inlet of the pump impeller. The research of water pump shows that when the specific speed is 100~200, the volume loss ratio is less than 1.5%[4]. Compared with the hydraulic loss, it is much smaller, so this term can also be ignored in the calculation, that is, ηv≈ 1.
Efficiency analysis
When the pump impeller speed n 1 is constant, the impact loss mainly depends on the turbine impeller speed n2. The efficiency ηPTD of torque converter should be the ratio of output power to input power, that is:
Obviously, when n2=0, η PTD = 0; When n2=n20, because M2=0, ηPTD=0. The relationship curve between efficiency ηPTD and n2 is shown in Figure 4.
Fig. 4 Efficiency curve of hydraulic torque converter
When the hydraulic torque converter is used, there is no special requirement for the speed ratio of the design working condition if the working condition changes greatly. Because the highest efficiency of the hydraulic torque converter is only 85%~92%, when the starting torque coefficient K0 is required to be large, the speed ratio corresponding to the highest efficiency is generally less than 0.6, and when ITB >:(iTB)K = 1, its efficiency will drop rapidly. In order to have higher efficiency under the condition of high speed ratio, integrated hydraulic torque converter or locking hydraulic torque converter can be adopted.
(1) integrated hydraulic torque converter
Features: the guide wheel is installed on the fixed guide wheel seat through a one-way clutch, and the pump wheel and turbine are symmetrically arranged in the structural arrangement.
When ITB；; 1)，M D =-MT-MB & gt； 0, at this time, the one-way clutch does not rotate under the wedging force, so the guide wheel is fixed, which is the working state of the torque converter. When iTB & gt(i TB)K= 1, MD
Fig. 5 Structure diagram and characteristics of integrated hydraulic torque converter
(2) Lock the hydraulic torque converter
The turbine is connected with the pump impeller through the locking clutch M. As can be seen from the characteristic curve (as shown in Figure 6), the locking hydraulic torque converter is used in ITB >: when (iTB)K= 1, it is more efficient than the integrated hydraulic torque converter, but due to the blast loss, although the pump impeller is rigidly connected with the turbine, the efficiency cannot reach 100%. Moreover, when the pump wheel and turbine are arranged asymmetrically, there will be fluid flowing in the circulation circle and some energy will be consumed.
Fig. 6 Structure diagram and original characteristics of single-stage locking torque converter
In addition, in order to ensure that the hydraulic transmission vehicle can start the engine reliably only by using the engine or trailer, in addition to the locking hydraulic torque converter, the following methods can be adopted: ① the hydraulic torque converter with auxiliary radial blades in the inner ring; ② Install a hydraulic retarder as an auxiliary braking device.
4 hydraulic loss characteristics of construction machinery
Although the mechanism of friction resistance loss of torque converter is simple, it is difficult to obtain mathematical model and realize quantitative analysis [6]. Usually, the rotating speed of construction machinery is relatively low, and the friction resistance loss is relatively small, which has little effect on work efficiency. Moreover, for the airtight hydraulic torque converter, because the relative flow is constant, that is, it does not change too much with the working conditions, the friction resistance loss is also relatively constant. As mentioned above, the general volume loss can also be ignored. Therefore, the impact loss of hydraulic torque converter is the main factor affecting the efficiency of construction machinery.
For a specific impeller, its impact loss is determined by the formula (12). Its mathematical model is:
Where: I '- transmission ratio at the highest efficiency.
I > It can be seen that when i≤iDH, the total impact loss of the torque converter is a parabola with the ordinate i=i' as symmetry, and when I >:iDH, it is approximately constant, as shown in Figure 7. When i=i' and ∑hc=0, it shows that there is no impact loss when the pump impeller speed is close to the turbine speed; When i=0, the impact loss is the largest, which is consistent with the working conditions of construction machinery.
Fig. 7 Hydraulic loss curve
5 conclusion
Through the above analysis of the energy loss of hydraulic torque converter, it can be concluded that the main factor causing the energy loss of hydraulic torque converter is the impact loss, and its characteristics are analyzed. It is also pointed out that when the required starting torque coefficient K 0 is large, its efficiency is generally small. In order to have higher efficiency at high speed ratio, comprehensive torque converter or locking torque converter can be used to improve its power. The research on energy loss of hydraulic torque converter is of guiding significance to designers and manufacturers of hydraulic transmission of construction machinery.