Energy-saving research of excavators is becoming one hot topic due to the increasing energy crisis and environmental deterioration recently. Hydraulic hybrid excavator based on common pressure rail (HHEC) provides an alternative with electric hybrid excavator because it has high power density and environment friendly and easy to modify based on the existing manufacture process. This paper is focused on the fuel consumption of HHEC and the actuator dynamic response to assure that the new system can save energy without sacrificing performance. Firstly, we introduce the basic principle of HHEC; then, the sizing process is presented; furthermore, the modeling period which combined mathematical analysis and experiment identification is listed. Finally, simulation results show that HHEC has a fast dynamic response which can be accepted in engineering and the fuel consumption can be reduced 21% to compare the original LS excavator and even 32% after adopting another smaller engine.
The demand for fuel efficient and low-emission hydraulic excavators has been improved due to the increasing energy crisis and environmental deterioration recently [
The simplified schematic of HHEC is shown in Figure
Simplified schematic of HHEC.
Structure of SHT.
It is noticed that this paper is to compare the fuel consumption between HHEC and the original systems during one digging cycle. The digging cycle includes an excavator digging a bucket of dirt, rotating and releasing the load onto a pile and then returning to its initial position. It can be found that the travel system is not involved in the cycle, so the sizing process and simulation will not include it. The design assumption is decided that the original hydraulic cylinders would maintain their original dimensions. This decision is useful for comparing the original system, and it will simplify the development of a prototype machine. In future work, the travel system can be integrated easily, so that we can use SHTs which are used for controlling the boom and stick to control travel motors because the travel system and other actuators are not usually used simultaneously [
The maximum pressure of the hydraulic accumulator should be chosen lower than the maximum system pressure which also should be safe for all components in system. So we choose the maximum system pressure according to the original system,
Volume selection of the hydraulic accumulator should follow the rule that it is enough to absorb the braking and gravitational energy. The swing and boom are the two biggest potential; hence, the maximum recovery energy can be calculated by
The volume of hydraulic accumulator is equal to
We define the relative volume which means that the three ports A, B, and T have their own volume with the rotation of the port plate. So the volumes are [
Equation (
Flow chart of sizing SHT.
Firstly, the transformer ratio
Curves of
The main pump is one critical component for keeping the pressure of the high pressure line quasiconstant. So the main pump should supply enough flow for system. The critical parameter for main pump is volume, which should meet the two requirements in the aspects of flow rate and power:
A whole model which intergraded with mechanical, hydraulic, and control systems is needed to simulate the actuators dynamic response and fuel consumption.
The 3D solid mechanical model is constructed by Solidworks and input Simulation X; then, the material and density are also input. Figure
Mechanical model of HEEC.
The dynamic response of engine is calculated by
The sum of torque load is equal to the sum of the main pump output torque and loss torque
The common control architecture in HHEC is using servo valves to control the position of the cylinder or motor. For the main pump and motor/pump, Figure
Displacement control mechanism of the main pump and motor/pump.
The dynamic characteristics of the servo valve are simplified as a 2nd-order transfer function, and the numbers are from catalog data. So the relation between output flow rate
The pressure build-up in the high pressure chamber is calculated using the following equation:
For the swing motor which drives the port plate, the pressure build-up equation is
In above equation,
A force balance is applied to the cylinder and results in the following:
The similar torque balance equation for the swing motor is calculated by
The main pump and motor/pump belong to the axial piston component and are very complicated to be described as mathematical equations. To make the model precise and easy to use, this paper adopts the method and date in [
Torque loss and volumetric loss under different conditions.
According to the above equation, we can calculate the real output flow and torque after inputing the three parameters: pump pressure, rotation speed, and displacement [
The displacement control system is introduced in Section
The block of the SHT is driven by the sum of toque which is generated by the three ports of the SHT. The sum of torque among three ports is as follows:
In the above equation, the torque of each port is followed:
Figure
Schematic of SHT control cylinder.
For the arm cylinder, the flow rate goes into the bore side of cylinder, and moves out of the rod side of cylinder are calculated by
For the flow rate of the SHT, the flow goes into the A port and out from the B port of SHT which follows the equations below:
After combining the above equations, we can get the pressure build-up equation for the B port of the SHT:
In the HHEC, the rotation load is driven by the motor/pump, and the dynamic equation is
The PHP contains a main pump, hydraulic accumulator, and the actuators. The pressure of the PHP is calculated by the following equation, and it is noticed that again travel motors are omitted in the part:
Also, in the above equation,
The controller design includes two aspects: the first one is in order to accomplish the speed control by regulating the swash plate angle and the port plate angle of the SHT, and the other is by controlling both engine speed and the displacement of the main pump to reduce the fuel consumption. However, recalling the object of the paper is to study the fuel consumption of the HHEC under the condition which maintained the performance. Hence, to make the comparison fairly, we still adopt the previous constant speed control for the engine which is used universally among construction machinery. It means that the speed governor will regulate the throttle when load changes so as to keep the engine speed constant. Therefore, the controller design for the second part will focus on the displacement control of the main pump.
Relationship between pressure and setting displacement of main pump.
Maximum output torque of engine.
In the above equation,
The final command value of the displacement of the main pump is to compare two rules and choose the smaller one. The rules are integrated in the module “main pump displacement control”, and the whole control flow chat is plotted in Figure
Control principle of HEEC.
The whole simulation model is constructed according to the equations above and built by the Simulink software. In addition, the mechanical model which is designed by software simulation X is integrated, and Figure
Top module of the whole simulation model.
The speed response of each actuator is shown in Figure
Speed response of each actuator.
Pressure change of PHP.
We can find that the pressure of the PHP ranges from 26 to 27 MPa (106 Pa), and recall that the setting pressure of main pump is 26 MPa. It means that the PHPE is a quasi-constant pressure system which is beneficial to enhance the control performance of the actuators. There are two main reasons for the quasi-constant system pressure. The first one is for the high power of the engine which can keep the displacement of the pump large enough to the supply flow rate. The other reason is because of the accumulator. It can increase the capacity of the PHP so as to balance the flow rate. Actually, the second reason is also influenced by the first since it will also reduce the engine power, but the original engine power seems higher. Of course, this kind of sizing and control strategy is not optimal. So the long-term future objective of this project is to reduce the fuel consumption by reducing the engine power. Then the engine and accumulator can work together; for example, the main pump and accumulator will supply the flow rate together when the system requirement is large; for example otherwise, the redundant flow rate can be stored in the accumulator. For this setting, the smaller engine can work around the minimum fuel consumption area.
According to Figure
Fuel consumption values of different system settings.
System type | Engine rated power (kW) | Fuel consumption (g) | Fuel saving ratio (compared with the LS system) |
---|---|---|---|
LS system | 35 | 45 | — |
HEEC | 35 | 35.6 | 21% |
HEEC | 25 | 20.5 | 32% |
Engine operating point during cycle.
It needs to be noticed that in the whole cycle, there are no actuator movement commands during the first 6 s, and the main engine power is used to charge the accumulator. So, with the more cycles’ work, the average fuel consumption can be reduced further. Hence, it can be expected that the average fuel consumption can be reduced further after running more cycles. Of course, the engine operating points do not lie in the optimal fuel consumption area which should be located along with the line tangent of equal power contour line and equal fuel consumption contour line. So the future objective of this project is to finish the optimal control by controlling engine speed and displacement of the main pump simultaneously.
This paper focuses on the fuel consumption of the hydraulic hybrid excavator based on CPR under the precondition of maintaining the control performance. After introducing the principle of CPR and the hydraulic transformer which we have designed, the sizing procedure is finished. Then, a comprehensive excavator model including mechanical, hydraulic, and control systems has been developed by using Simulink. Simulation results show that the new speed control system has a good servo capability. However, it is also necessary to study some advanced control method to enhance the actuator control performance. Moreover, the fuel consumption is reduced 21% compared with the original system, and the result also can be even reduced 32% after adopting one smaller engine. The reasons for improving the fuel consumption are due to the elimination of metering losses and the ability to recover the added power. It is also necessary to notice that the source for reduced fuel consumption is from the operating point of the engine, so the future research will also consider control of the engine’s operating point so as to minimize fuel consumption.
The authors acknowledge the contribution of National Natural Science Foundation of China (50875054, 51275123) and Open fund of State Key Laboratory of Fluid Power Transmission and Control, Zhejiang University (GZKF-2008003).