并联液压混合动力装载机的系统配置和能源控制策略的研究.docx

Research on the system configuration and energy control strategy for parallel hydraulic hybrid loader
Sun Hui, ,
Jing Junqing
Jiangsu Xuzhou Construction Machinery Research Institute, Jiangsu, China
Accepted 30 October 2009. Available online 22 November 2009.
http://dx.doi.org/10.1016/j.autcon2009.10.006, How to Cite or Link Using DOI
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Abstract
Aimed at the frequent starts/stops operation characteristics of the loader, an energy saving scheme with parallel hydraulic hybrid system is proposed to regenerate and reuse the braking energy normally lost in a conventional loader. Hydraulic hybrid system is designed and sized to capture braking energy from normal–moderate braking operating modes while ensures hybrid system working in higher efficiency region. According to the higher power density characteristic of hydraulic hybrid system, the regenerative braking strategy and energy reuse strategy are designed to coordinate all the powertrain components in an optimal manner while satisfying performance constraints of loader. Energy controller using logic threshold approach is built up to control the dynamic transitions among the various operation modes. Simulation and experimental results show the parallel hydraulic hybrid loader (PHHL) effectively recovered and reused the braking energy, improved the working performance of loader and effectively reduced the fuel consumption.
Keywords
Loader;
Hydraulic hybrid system;
Braking energy regeneration
Energy management;
Hydraulic pump/motor

1. Introduction
With the rising concern in a global scale environmental issue, energy saving in automobiles is a very important subject [1] and [2]. Off-road vehicles have been paid much attention in the field of energy saving and environment protection, for the reason of their large application quantities, high fuel consumption and bad emissions [3]. Loader has the characteristics of frequent starts/stops and larger vehicle weight which generated significant amounts of braking energy. Usually, this part of energy is wasted by the frictional braking system which caused the energy loss and heat generation in the system. Therefore, the braking energy recovery and reuse is a promising way to improve fuel economy and working performance of off-road vehicles [4] and [5]. There is a wealth of literature focused on hybrid electric engineering vehicles, but the publications devoted to hydraulic propulsion options are relatively scarce [6] and [7].
A parallel hydraulic hybrid loader is one that contains two sources of power, with one source being an internal combustion engine. The other power source is hydraulic pump/motor and accumulators. One feature of PHHL is the ability to recover energy normally lost during braking and store the energy in a hydraulic accumulator. The stored energy is used to provide the total command power during loader launching or provide auxiliary traction power during shoveling operation mode, which avoids the engine working in the regions of low efficiency and significant emissions. In this paper, an energy saving scheme with parallel hydraulic hybrid system is proposed to capture the braking energy normally lost to friction brakes. Hydraulic hybrid system is designed and sized to capture braking energy from normal-moderate braking operating modes while ensures hybrid system working in higher efficiency region. According to the advantage of high power density for hydraulic accumulator and the operation characteristics of loader (stop-and-go duty cycles), the regenerative braking strategy and energy reuse strategy are designed to coordinate all the powertrain components in an optimal manner while satisfying performance constraints. Energy control strategy is built up around the concept of multi-level hierarchic control system. PHHL simulation model and the control system simulation model based on the proposed energy control strategy are developed in Matlab/Simulink environment. Experimental results and simulation results demonstrate that PHHL with proposed control strategy effectively improves the fuel economy and working performance.
2. Configuration of the PHHL
2.1. Operating principle of PHHL
Fig. 1 presents the configuration of proposed PHHL, which consists primarily of an engine, hydraulic torque converter, boom tanks, dump tanks, a high pressure accumulator, a hydraulic reservoir, a variable displacement hydraulic pump, and a variable displacement hydraulic pump/motor unit, clutch, transmission, torque coupler and differential. The hydraulic pump/motor is coupled to the propeller shaft via a torque coupler. Engine is the power source of the loader, one part of the engine power is used to drive the loader through hydraulic torque converter and transmission, the remainder part of engine power is used to realize the shifting and loading functions by means of hydraulic cylinders. Hydraulic regenerative system, which consists of a hydraulic pump/motor, hydraulic accumulator, hydraulic reservoir and hydraulic relief valve, et al. drives the loader together with the engine. During deceleration, the hydraulic pump/motor decelerates the loader while operating as a pump to capture the energy normally lost to friction brakes in a conventional loader. Also, when the vehicle brake is applied, the hydraulic pump/motor uses the braking energy to charge the hydraulic fluid from a low pressure hydraulic accumulator into a high-pressure accumulator, increasing the pressure of the nitrogen gas in the high pressure accumulator. The high pressure hydraulic fluid is used by the hydraulic pump/motor unit to generate torque during the next acceleration [8] and [9]. Hydraulic pump/motor is designed and sized to capture braking energy from normal, moderate braking events and is supplemented by friction brakes for aggressive braking. While shoveling and digging, the hydraulic pump/motor works in motor mode to provide auxiliary traction power for the loader which ensures the engine working in better fuel economy region and reducing the overflow losses of hydraulic system.

Fig. 1. Configuration of parallel hydraulic hybrid loader.
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Loader has the characteristics of frequent starts/stops and larger weight, which ensures regenerating and reusing significant amounts of braking energy in the hybrid configurations. Electric hybrid technology has the advantage of high energy density which is well suited for light vehicles to improve fuel economy and emissions [10]. But a large challenge is the relatively low power density of electrical storage media. Due to their high internal resistance, both fuel cells as well as batteries have a low power density and are only marginally suitable for recovering brake energy [11] and [12]. Unlike electric batteries, hydraulic accumulators have the ability of accepting exceptionally high rates of charging and discharging. A combination of high efficiency and high charging/discharging rates enables effective regeneration and re-use of braking energy in heavy engineering vehicles. In general, the major advantages of the proposed hydraulic hybrid drive over other solutions are the higher efficiency, higher power performance and relatively minor modifications to the drive train, thereby making it possible to retrofit existing loader with hydraulic hybrid system.
2.2. Sizing of hydraulic hybrid system
Hydraulic hybrid system is designed and sized to regenerate the total braking energy from normal braking mode of full load loader.
2.2.1. Hydraulic accumulator
The hydro-pneumatic accumulator contains the hydraulic fluid and inert gas such as Nitrogen, separated by a bladder is used for energy storage in PHHL. As the pump transfers the hydraulic fluid into the accumulator, the pressure of the gas sealed inside of it increases, thus storing energy. When discharging, fluid flows out through the motor and into the reservoir. Hydraulic accumulator is designed to have adequate capacity for storing the braking energy of loader. The volume of hydraulic accumulator may be expressed as:
(1)

where pmin is the minimum working pressure of hydraulic accumulator, V1 is the gas volume corresponding the minimum working pressure, n is poly-index of gas, v is the working speed of the loader, m is the loader mass.
Minimum working pressure of hydraulic accumulator is determined by Eq. 2).
(2)

where i0 is the final ratio of the load, iP/M is the ratio of torque coupler, r is the wheel radius, VP/M is the maximum pump/motor displacement, dv/dt is the target deceleration.
Maximum working pressure of accumulator cannot exceed the allowed maximum pressure of hydraulic accumulator which is expressed as follow.
(3)
pmax≤pacc,max.
2.2.2. Hydraulic pump/motor
The assistant power source in PHHL is an axial piston pump/motor with variable displacement, whose displacement is adjusted via inclination of the swash plate to absorb or to produce desired torque [6]. While launching, hydraulic pump/motor provides the total propulsion power avoiding engine operating in the regions of low efficiency and significant emissions. During braking, hydraulic pump/motor is adjusted via inclination of the swash plate to produce desired braking torque. Accordingly, hydraulic pump/motor displacement is determined by the following equations.
(4)
min(VP/M)=max(VP/M1,VP/M2)
(5)

(6)

2.2.3. Ratio design of torque coupler
Hydraulic hybrid system is coupled to the propeller shaft via a torque coupler. Therefore, the torque coupler should have appropriate drive ratio to keep the hydraulic pump/motor working in the high efficiency region during braking and driving, which is expressed as Eq. (7).
(7)

where ne_P/M is the rational speed corresponding the highest efficiency of hydraulic pump/motor.
2.3. Optimization for key components sizes
The designed parameters of hydraulic hybrid system have remarkable influence on the working performance and fuel saving capacity. Therefore, an optimization process is needed to find the best design parameters for maximum fuel economy while satisfying the loader performance constraints. Then the optimization framework is setup using the adaptive simulated annealing genetic algorithm (ASAGA), which is designed to take advantages of genetic algorithm and simulated annealing algorithm for distinguishing the optimal values [13]. The optimization results led to the following set of design parameters as shown in Table 1. The influence of iP/M on the loader is shown in Fig. 2. Simulation results show the braking energy regenerate capacity of PHHL improves with the increase of iP/M value, but the working efficiency of pump/motor decreases correspondingly. When the value of iP/M approaches the optimal value (for example, 30), with the continuous increase of iP/M, the braking energy regenerative capacity is almost unchanged, but the hydraulic pump/motor working efficiency decreases remarkably, and over-speed phenomenon of hydraulic pump/motor often occurs.
Table 1. Basic components size and optimization results of PHHL.

Parameter Name
Values
Loader parameters
Wheel diameter (m)
0.75
Engine power/revolution (rpm)
154 kW/2200
Specification load (kg)
5000
Total vehicle mass (kg)
17,000
Max speed (km/h)
35
Gear ratio
2.547, 0.683, R1.864
Main gear ratio
22.85
Hydraulic accumulator
Volume (L)
30
Max pressure (MPa)
26
Min pressure (MPa)
15
Torque coupler
Transmission ratio
3.0
Hydraulic pump/motor
Type
A4VG90
Full-size table

Fig. 2. Influence of iP/M on the PHHL.
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3. Energy control strategy of PHHL
The primary task of the energy control strategy is to ensure safety operation regardless of the driver demand and loader states, but the ultimate goal is to maximize the fuel economy and improve the working performance of PHHL. Once the system configuration, components and operation cycle are fixed, fuel economy of PHHL depends mainly on the energy control strategy which consists of the regenerative braking strategy and energy reuse strategy [6] and [14].
3.1. Hydraulic regenerative braking strategy
Unlike the traditional loader, the braking force of PHHL is composed of the hydraulic regenerative braking force, traditional frictional braking force and engine anti-trailer force.
3.1.1. Engine anti-trailer force
The anti-trailer braking force of engine is expressed as the following equation.
(8)

where Ie is engine moment of inertia, z is the braking intensity, ηr is the transmission efficiency of the engine, ig is the transmission gear ratio, g is the gravitational acceleration.
Considering the influence of engine anti-trailer braking, the relations of front wheel braking force Ff1 and the rear wheel braking force Ff2 are as follows [15]:
(9)

where βb is braking force distribution coefficient.
While slight braking, the engine anti-trailer braking is not used and the total braking force is provided by the hydraulic pump/motor to recover the braking energy as much as possible.
3.1.2. Hydraulic regenerative braking force
Regenerative braking force generated by hydraulic pump/motor may be expressed as:
(10)

Then the braking intensity generated by hydraulic pump/motor may be expressed as follow.
(11)

In the case of operation conditions, the travel speed of loader is relatively lower and the braking intensity is relatively lower (z ≤ 0.2). Therefore, hydraulic pump/motor should be used to capture braking energy for normal and moderate brake as much as possible.
3.2. Distribution rules for the regenerative braking force
For PHHL, the braking force distribution is different from the traditional loader because the hydraulic pump/motor can regenerate most of the braking energy [8]. The braking force distribution strategy of PHHL is shown in Fig. 3. When braking intensity z ≤ 0.2 (low-mild braking), which generally appears in the normal operating conditions of the loader, shown as OA in Fig. 3. Hydraulic regenerative braking system provides the total braking force. If braking intensity 0.2 < z < 0.5, the combined braking mode is used, that is hydraulic regenerative braking system cooperated with friction braking system to provide the total braking force. Braking force difference of the required braking force and maximum regenerative braking force provided by hydraulic pump/motor is provided by the frictional braking system. If braking density z ≥ 0.5, the total braking force is provided by frictional braking system to ensure the safety of emergency braking.

Fig. 3. Braking force distribution curves of PHHL.
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In the cases of full load and no-load conditions, the weight of loader varies greatly which causes significant amounts varies of the braking energy in hybrid configurations. Therefore, the changes of PHHL's load should be taken into account during the designing of regenerative braking strategy. Usually, the hydraulic pump/motor is sized to rege