Experimental Study on Structure Optimization and Dynamic Characteristics of Articulated Steering for Hydrogen Fuel Cell Engineering Vehicles

: The prominent problem of articulated steering structure of engineering vehicle is that there is pressure oscillation in the hydraulic system during steering, which seriously affects the performance of steering system. To solve this problem, the maximum stroke difference of left and right cylinders and the minimum maximum cylinder pressure are the optimization objectives, and the position of cylinder hinge point is the design variable. The multi-objective optimization design of articulated steering system is carried out by using the particle swarm optimization algorithm. After optimization, the maximum pressure of the steering system is reduced by 13.5%, and the oscillation amplitude is reduced by 16%, so the optimization effect is obvious. The dynamic characteristics of the hydraulic steering system under different loads, such as pressure and flow rate, are obtained through field steering tests of wheel loaders. The results show that the load has an important effect on the pressure response of the system, and the causes and influencing factors of pressure and flow fluctuation are determined. The relationship between mileage and hydrogen consumption is obtained, which provides data support for vehicle control strategy. The high-pressure overflow power consumption accounts for 60% of the total work, and the work lost on the steering gear reaches 36 kJ. The test results verify the rationality and correctness of the optimization method of steering mechanism and provide data support for the improvement in steering hydraulic system.


Introduction
Construction machinery has high fuel consumption and high emission density and causes serious environmental pollution.Accelerating the energy saving and emission reduction of construction machinery has become the industry's goal under the two-wheel drive of national energy transformation and upgrading and environmental protection.Hydrogenation only takes 2-3 min to maintain a constant power output and can be operated continuously with high intensity.If factors such as depreciation and recycling of hydrogen fuel cells are superimposed, the comprehensive use cost of hydrogen fuel cell loading vehicles is even lower than that of pure electric loading vehicles.Hydrogen fuel cells are expected to become one of the directions of power transformation in the field of construction machinery.The steering hydraulic system changes the direction of the loader through the steering mechanism to convert it on the flat and tortuous road.The operating characteristics of the steering hydraulic system directly affect the safety, efficiency, energy consumption, and operating comfort of the loader [1][2][3].In wheel loaders, the advantages of adopting this type of articulated front and rear frame connection are simplified steering mechanism layout, small steering radius, and good maneuverability during operation.Because the articulated frame can swing, the adaptability of the wheels to harsh ground conditions is greatly improved.The steering system of articulated vehicles is divided into two parts, in which the hydraulic system controls the movement of the steering cylinder, and the mechanical system mainly controls the steering mode.Full hydraulic steering is a core component of load-sensing full hydraulic steering system, in which the oil pump is the power source of the system.The valve core of the rotary valve is connected with the steering wheel, and the valve sleeve is rigidly connected with the cycloidal motor rotor to form a feedback loop, thus forming a closed-loop control system [4][5][6].However, the problem of pressure oscillation exists in the steering system of articulated engineering vehicles during the steering process, which causes pipeline damage and hydraulic damage.The transient high-pressure impact of oil pressure exists in the steering process for a long time, which seriously affects the performance stability and working reliability of the hydraulic steering system, causes vibration and noise phenomenon of the hydraulic steering system, cracks the pipeline, and damages the hydraulic components, so it is necessary to study the pressure oscillation [7].
Many studies have shown that the main cause of the pressure oscillation of the steering cylinder is the unsteady flow of hydraulic oil caused by the stroke difference of the left and right cylinders in the steering process.Therefore, to reduce the pressure pulsation of the steering cylinder, the stroke difference of the steering cylinder must be reduced.In addition, under the same steering conditions, the lower the pressure of the steering system, the smaller the adverse impact of pressure pulsation on the system, so the maximum steering cylinder pressure is required to be as small as possible [8].Through the simulation analysis method to find out the causes and influencing factors of this kind of oscillation phenomenon, it is a feasible research scheme to improve the design of the steering system, obtain better stability and handling performance, and extend the service life of the system and components [9].In recent years, many scholars have conducted a lot of research on the dynamic modeling of vehicle power steering systems by means of modeling design and simulation development [10][11][12][13].It is of great practical significance to optimize the articulated steering system according to the phenomenon of the hydraulic system's pressure oscillation to improve the stability of the steering system, reduce the probability of failure, and improve the life of the steering system [14].Moreover, previous experiments have found that the pressure oscillation of the steering system in the steering process is serious; especially, the pressure shock in the fast steering process is larger.Some researchers used professional hydraulic simulation software to establish a simulation model of the steering system and combined it with dynamics software to simulate the action of the tire and the ground during the steering process.The factors affecting the dynamic performance of the steering system are found out, which provides a good support for the improved design of the steering system [15][16][17][18].
The optimization of mechanical design has always been one of the important tasks in the engineering field.With the increasing complexity of modern engineering problems and the continuous improvement in design requirements, traditional design methods often cannot meet the needs of multiple conflicting objectives.In order to solve this problem, multi-objective optimization algorithms have become a hot research direction in the field of mechanical design.Multi-objective optimization algorithms aim to find a set of solutions that achieve a balance between multiple conflicting objectives.These goals can relate to various aspects of mechanical design, such as performance, weight, cost, reliability, etc.By optimizing these objectives, the mechanical system can be improved in many aspects, thus improving the overall design quality and competitiveness.Through the definition of design variables, objective functions, and constraints, we will explore how to use multi-objective optimization algorithms to find the best solution set in the design space of mechanical systems.At the same time, we will also consider the trade-offs and balances between different goals to find a set of Pareto optimal solutions that can still improve other goals when it is impossible to improve one goal.
The prominent problem of the steering system used by articulated engineering vehicles is that there is pressure oscillation in the steering process, which seriously affects the performance of the steering system.Many studies have shown that the main cause of the pressure oscillation of the steering cylinder is the unsteady flow of hydraulic oil caused by the stroke difference of the left and right cylinders in the steering process.Therefore, to reduce the pressure pulsation of the steering cylinder, the stroke difference of the steering cylinder must be reduced.In addition, under the same steering conditions, the lower the pressure of the steering system, the smaller the adverse impact of pressure pulsation on the system, so the maximum steering cylinder pressure is required to be as small as possible.Based on the above analysis, the articulated steering mechanism is optimized by using multi-objective particle swarm optimization (PSO) to minimize the maximum pressure of the steering cylinder and the moment arm difference of the left and right cylinders.Finally, the steering system of the loader is experimentally studied, and the factors affecting the dynamic characteristics of the system are further analyzed.

Articulated Hydraulic Steering System
The front frame and rear frame of a wheel loader with an articulated full hydraulic steering mechanism are connected by hinged pins.The front and rear frames can be rotated around the pin shaft to obtain the relative rotation of the front and rear frames of the loader, as shown in Figure 1a.Due to the small steering radius of articulated steering, the steering distance and time are shortened, so that the loader can operate in a narrow site.The basic principle of articulated steering structure is to make the front and rear frames rotate relatively through the expansion of the oil cylinder, as shown in Figure 1b.The steering resistance of the loader is overcome by lengthening the rod chamber and shortening the rod-less chamber, so that the hinge point of the front frame and the rear frame of the loader can be rotated as the rotating center.During the steering process, the steering pump outlet oil enters the steering gear through the priority valve port.The steering device changes the direction of the hydraulic oil flow, making the hydraulic oil enter the left of the large chamber of the cylinder and the right of the small chamber of the cylinder, so as to obtain the left and right steering of the loader.When turning left to the large cavity of the cylinder and turning right to the small cavity of the cylinder, the force generated by the hydraulic oil in the left cylinder pushes the piston rod to move in the extended direction.The left turn cylinder piston rod is further and further away from the bottom of the cylinder, and the hydraulic force is F1.At the same time, the hydraulic oil of the right steering cylinder pushes the piston rod to move in the direction of the shortening of the piston rod, and the hydraulic force is F2.
Through the analysis of the articulated steering system, it can be seen that the maximum stroke difference and the maximum pressure of the left and right steering cylinders are related to the position of the articulated point of the steering cylinder.The coordinates of points A, B, C, and D of the left and right cylinders' articulated points in the cylinder articulated steering mechanism are x A y A , x B −y B , −x A y A , −x B −y B , respectively, as shown in Figure 2. The left and right steering cylinders are arranged symmetrically, where o is the hinged point of the front and rear frames, L 1 +L 2 is the wheelbase, L 1 is the distance between the hinged point and the front axle of the loader, and L 2 is the distance between the hinged point and the rear axle of the loader.When the loader travels in a straight line, the steering angle relative to the hinged front frame and the rear frame is defined as γ.Throughout the structural analysis of the steering system, the origin of the cartesian axes is O. Through the analysis of the articulated steering system, it can be seen that the maximum stroke difference and the maximum pressure of the left and right steering cylinders are related to the position of the articulated point of the steering cylinder.The coordinates of points A, B, C, and D of the left and right cylinders' articulated points in the cylinder articulated steering mechanism are ( ) x y , ( ) , respectively, as shown in Figure 2. The left and right steering cylinders are arranged symmetrically, where o is the hinged point of the front and rear frames, L is the distance between the hinged point and the front axle of the loader, and 2 L is the distance between the hinged point and the rear axle of the loader.
When the loader travels in a straight line, the steering angle relative to the hinged front frame and the rear frame is defined as γ .Throughout the structural analysis of the steering system, the origin of the cartesian axes is O.The distance from point A to point O can be obtained according to the following geometric relationship: The distance from point A to point O can be obtained according to the following geometric relationship: Distance from point B to point O is as follows: After the variable orifice on the steering gear is opened, the hydraulic oil at the outlet of the steering pump enters the steering hydraulic system through the priority valve port when the steering gear is working.The opening degree of the variable throttle on the steering gear increases when the steering wheel speed is increased.The orifice flow equation is as follows: where Q is the flow through the orifice; C d is the flow coefficient; A is the flow area of the valve port; P 1 and P 2 are the pressure before and after throttling, respectively; and ρ is the density of the oil.

Optimization Model
Because the relative speed of the front and rear frames is constantly changing with the change in the angle, the relative speed of the front and rear frames is not only related to the size of the steering angle, but also related to the rotation direction.The relative speed of front and rear frames is the smallest in the medium condition, and the relative speed of front and rear frames is the largest in the extreme steering condition.In the process of turning from one limit position to another, the speed is greater at the limit position at the beginning of the turn than at the limit position at the end of the turn.The maximum relative speed of the front and rear frames appears at the steering limit position and the beginning of steering.The pressure fluctuation in the internal hydraulic oil is relatively large due to the time difference between the response time and the movement speed of the two cylinders.Therefore, the unsteady flow of hydraulic oil caused by the stroke difference between the two cylinders during the steering process is the main cause of pressure oscillation.The full hydraulic steering system of articulated rubber wheel vehicle not only has a serious pressure oscillation in the steering process, but also the speed and load have an important effect on the pressure response of the system.Therefore, particle swarm optimization is used to optimize the articulated steering mechanism to minimize the maximum pressure of the steering cylinder and the minimum difference between the right and left cylinders.

Optimization Objectives
In order to make the amplitude and pressure of the pressure oscillation of the full hydraulic steering system as small as possible, the maximum stroke difference and maximum pressure of the left and right steering cylinders should be as small as possible during the steering process.Because there are two optimization objectives with different dimensions, the concept of relative deviation in ideal point method is used to consider the influence of two factors on the system equally.The objective functions are determined as follows: where f 1 (x) is the stroke difference of the left and right steering cylinders (The maximum stroke difference between the elongation of the rod cavity and the shortening of the rodfree cavity can be referred to in Formulas ( 5) and ( 6) for specific meaning.);f * 1 (x) is the minimum maximum stroke difference between left and right steering cylinders in the feasible region; f 2 (x) is the maximum pressure of the steering cylinder; and f * 2 (x) is the minimum maximum pressure of the steering cylinder within the feasible region.

Design Variables
Since the coordinates of points A, B, C, and D at the left and right cylinder hinge points are x A y A , x B −y B , −x A y A , and −x B −y B , respectively, the design variables,x A , y A , x B , and y B , are determined.The maximum length of the cylinder in the limit position is as follows: where γ max is the maximum transfer of the cylinder limit.The minimum length is as follows: where γ min is the minimum limit angle of the cylinder.

Constraints
The maximum length of the cylinder is set as L max and the minimum length as L min .Considering the layout requirements of the car body and the cylinder structure, the boundary constraint equation is established as follows: where X A2 is the horizontal coordinate of point A 2 .In order to avoid dead center and ensure steering efficiency, the ultimate rotation angle of the cylinder has the following constraints: In order to ensure normal steering, the steering structure should meet the following triangular condition: In order to ensure that the movement of the cylinder meets the stability requirements, the expansion ratio of the cylinder should meet the following requirements under the premise of ensuring the full utilization of the cylinder length: During the steering process, the structural limitations of the cylinder are as follows: where L i is the minimum structural size of the cylinder; L j is the maximum stroke of the cylinder.The basic particle swarm optimization algorithm is used to solve unconstrained optimization problems, but it needs to solve engineering optimization problems with nonlinear constraints.Therefore, the constrained optimization problems are transformed into unconstrained optimization problems by constructing fitness functions with penalty functions, as follows: min f (x) The penalty function defined by the inequality set constraint is as follows: where F(x) is the objective function; m is the number of constraint functions; and M is the penalty factor.

Optimization Method and Process
Compared with other optimization algorithms, PSO can efficiently deal with multiobjective optimization problems in the optimization process of steering mechanism [19,20].The optimization of steering mechanism has the characteristics of being nonlinear, multivariable, and multi-objective.Particle swarm optimization has better convergence and is easy to find the global optimal solution, which is more suitable for dealing with complex nonlinear problems.In the optimization process of particle swarm optimization algorithm, the speed and position of particles are constantly updated to lead the whole population to the optimal solution distribution area, so as to achieve the purpose of optimization.Because the multi-objective particle swarm optimization algorithm has the characteristics of simple parameter setting, fast convergence speed, and ability to provide multiple solutions, it effectively compensates for the shortcomings of limitations of optimization methods in traditional design optimization methods based on experience [21].In the calculation of particle swarm optimization algorithm, each particle has a corresponding target fitness value, and each particle has its own flight speed, and searches the solution space with the optimal position particle.The optimal solution found by each particle in the course of flight is called the individual optimal, and the optimal solution of all particles in the group is called the group optimal.In the process of optimization, each particle ensures that the group finally obtains the optimal goal by seeking the optimal position of the individual and the optimal position of the group [22].
The particle swarm optimization algorithm solves the problem by moving through the search space according to a simple formula of particle velocity and position.Each particle's movement is influenced by its known local optimal position but is also guided to the known best position in the search space.As more and more local optimal locations are discovered, eventually, the particle will move to the global optimal location, as shown in Figure 3.The optimization process of PSO is as follows [23,24]: (1) Initialize the particle swarm, randomly generate the positions and velocities of all particles, and determine that the optimal position and the optimal position of the particles searched so far are the optimal positions searched so far in the whole particle swarm; (2) For each particle, its adaptation value is compared with the adaptation value of the optimal position experienced by the particle.If it is better, it is taken as the current optimal position of the particle; (3) For each particle, its adaptation value is compared with the adaptive value of the optimal position experienced by the whole particle swarm.If it is better, it is taken as the optimal position of the current particle swarm; (4) Update the velocity and position of particles; (5) If the termination condition (usually the preset number of iterations and the lower limit of the fitness value) is not met, then, return to step (2); otherwise, exit the algorithm and obtain the optimal solution.

Optimization Results
According to the above constraints, considering the convergence speed and other factors, the population size and the number of iterations are set to 100 and 300, respectively.Taking the maximum number of iterations as the program termination condition, the structure optimization calculation of the steering mechanism was carried out, and the convergence trend process is shown in Figure 4.The inertial weight factor, which directly affects the particle velocity, is an important factor in the algorithm and should be gradually reduced with the increase in the number of iterations.The inertia weight is constructed as a function of the number of iterations by using linear descending weight method.Using the MATLAB2018b software as algorithm platform, the particle swarm optimization algorithm is written, and the global optimal solution of the independent variable is obtained through the algorithm.According to the fitness curve of particles, it can be seen that the algorithm convergence speed is fast, indicating that the parameter setting in the algorithm is reasonable, as shown in Figure 4.The calculation results show that the obtained candidate solutions are abundant, evenly distributed, and stable, and the algorithm can achieve the expected results in the execution of the design and optimization tasks.

Optimization Results
According to the above constraints, considering the convergence speed and other factors, the population size and the number of iterations are set to 100 and 300, respectively.Taking the maximum number of iterations as the program termination condition, the structure optimization calculation of the steering mechanism was carried out, and the convergence trend process is shown in Figure 4.The inertial weight factor, which directly affects the particle velocity, is an important factor in the algorithm and should be gradually reduced with the increase in the number of iterations.The inertia weight is constructed as a function of the number of iterations by using linear descending weight method.Using the MATLAB2018b software as algorithm platform, the particle swarm optimization algorithm is written, and the global optimal solution of the independent variable is obtained through the algorithm.According to the fitness curve of particles, it can be seen that the algorithm convergence speed is fast, indicating that the parameter setting in the algorithm is reasonable, as shown in Figure 4.The calculation results show that the obtained candidate solutions are abundant, evenly distributed, and stable, and the algorithm can achieve the expected results in the execution of the design and optimization tasks.It can be found in Table 1 that the optimization results can reduce the maximum pressure of the steering cylinder by 13.5% and the maximum stroke difference between the left and right cylinders by 16% under the steady state condition, and the optimization effect is obvious.Maximum stroke of cylinder (m) 0.75 0.63

Experimental Study of Steering Hydraulic System
The steering hydraulic system of the loader is a coaxial flow amplification hydraulic steering system, which can improve the working efficiency of the wheel loader and greatly reduce the energy consumption.The pressure signal is transmitted to the pressure sensor during the steering process, and the signal is converted by the data acquisition system.The pressure sensor needs to be installed, respectively, in the steering pump outlet position, the working pump outlet position, the left of the hydraulic cylinder large cavity oil circuit, and the right of the hydraulic cylinder large cavity oil circuit.Then, it is connected with the data acquisition system to measure the pressure value of the steering pump and the steering hydraulic cylinder under different working conditions.The way of turning in place is that the loader turns from the left limit to the straight-line driving position and then to the right limit, and then from the right limit to the left limit.The dynamic characteristics of steering pump and steering cylinder It can be found in Table 1 that the optimization results can reduce the maximum pressure of the steering cylinder by 13.5% and the maximum stroke difference between the left and right cylinders by 16% under the steady state condition, and the optimization effect is obvious.

Experimental Study of Steering Hydraulic System
The steering hydraulic system of the loader is a coaxial flow amplification hydraulic steering system, which can improve the working efficiency of the wheel loader and greatly reduce the energy consumption.The pressure signal is transmitted to the pressure sensor during the steering process, and the signal is converted by the data acquisition system.The pressure sensor needs to be installed, respectively, in the steering pump outlet position, the working pump outlet position, the left of the hydraulic cylinder large cavity oil circuit, and the right of the hydraulic cylinder large cavity oil circuit.Then, it is connected with the data acquisition system to measure the pressure value of the steering pump and the steering hydraulic cylinder under different working conditions.The way of turning in place is that the loader turns from the left limit to the straight-line driving position and then to the right limit, and then from the right limit to the left limit.The dynamic characteristics of steering pump and steering cylinder pressure of steering hydraulic system are obtained by measuring the steering working state in real time under typical working conditions, as shown in Figure 5.
pressure of steering hydraulic system are obtained by measuring the steering wor state in real time under typical working conditions, as shown in Figure 5.As can be seen from Figure 6, the pressure of the steering system rises rapidly w the steering begins at 0.5 s.This is because the steering accelerates from zero to a cer speed, making the outlet pressure of the steering pump of the hydraulic system reach opening pressure of the steering safety valve.As the steering progresses, the pres quickly drops to a lower point.When steering at 2.6 s, the pressure of the stee hydraulic system gradually increases.The outlet pressure of the steering pump rea the opening pressure of the safety valve.When the steering reaches the maxim steering angle, the pressure of the large chamber of the steering cylinder continues to to the opening pressure of the overload oil refill valve.Since the loader is a large ine load, there is a large impact after the beginning of steering, and the impact of the left right steering limits the block.At the beginning of the steering, the impact pres reaches the overflow pressure of the steering safety valve, and the pressure d smoothly with the steering.There is a large pressure shock both at the beginning o steering and at the end of the steering.The difference between the outlet pressure of the steering pump and the pressure of the left-turning cylinder has a large sudden change when the steering rea As can be seen from Figure 6, the pressure of the steering system rises rapidly when the steering begins at 0.5 s.This is because the steering accelerates from zero to a certain speed, making the outlet pressure of the steering pump of the hydraulic system reach the opening pressure of the steering safety valve.As the steering progresses, the pressure quickly drops to a lower point.When steering at 2.6 s, the pressure of the steering hydraulic system gradually increases.The outlet pressure of the steering pump reaches the opening pressure of the safety valve.When the steering reaches the maximum steering angle, the pressure of the large chamber of the steering cylinder continues to rise to the opening pressure of the overload oil refill valve.Since the loader is a large inertial load, there is a large impact after the beginning of steering, and the impact of the left and right steering limits the block.At the beginning of the steering, the impact pressure reaches the overflow pressure of the steering safety valve, and the pressure drops smoothly with the steering.There is a large pressure shock both at the beginning of the steering and at the end of the steering.As can be seen from Figure 6, the pressure of the steering system rises rapidly when the steering begins at 0.5 s.This is because the steering accelerates from zero to a certain speed, making the outlet pressure of the steering pump of the hydraulic system reach the opening pressure of the steering safety valve.As the steering progresses, the pressure quickly drops to a lower point.When steering at 2.6 s, the pressure of the steering hydraulic system gradually increases.The outlet pressure of the steering pump reaches the opening pressure of the safety valve.When the steering reaches the maximum steering angle, the pressure of the large chamber of the steering cylinder continues to rise to the opening pressure of the overload oil refill valve.Since the loader is a large inertial load, there is a large impact after the beginning of steering, and the impact of the left and right steering limits the block.At the beginning of the steering, the impact pressure reaches the overflow pressure of the steering safety valve, and the pressure drops smoothly with the steering.There is a large pressure shock both at the beginning of the steering and at the end of the steering.The difference between the outlet pressure of the steering pump and the inlet pressure of the left-turning cylinder has a large sudden change when the steering reaches The difference between the outlet pressure of the steering pump and the inlet pressure of the left-turning cylinder has a large sudden change when the steering reaches the maximum steering angle of the left and right.In the hydraulic oil that enters the steering cylinder through the priority valve to achieve steering, a large pressure loss is generated after passing through the steering device, and the loss pressure is about 5 MPa.The reason may be that the outlet flow of the steering pump is greater than the flow required by the steering hydraulic system, and too much flow spills through the priority valve into the hydraulic system circuit of the working device.In addition, the return pressure of the left and right steering cylinders reaches 2 MPa, and a considerable part of the energy is also lost.When the engine is idle and the steering wheel is fast, the outlet flow of the steering pump is completely used for steering, and the steering hydraulic oil produces about 2 MPa pressure difference after passing through the steering gear.When the loader makes right steering to straight driving without constant speed steering, it is found that adjusting the steering wheel causes the steering hydraulic cylinder to fluctuate, resulting in greater pressure loss.The bad working road environment of the loader will inevitably lead to energy loss, but the continuous rotation of the steering wheel can be avoided as far as possible.
It can be seen from the test curve in Figure 7 that the working device of the loader does not work under full load condition, and then, the pressure of the steering pump only depends on the steering hydraulic system.During the test, when the loader is turning left, the operator tries to turn at a constant speed.At the beginning of the steering, the loading machine produces a large impact force, and the steering pump outlet pressure rises to about 16 MPa.In the process of steering, the pressure decreases with time to reach a stable state and finally stabilizes at about 9 MPa.The difference between the outlet pressure of the steering pump and the inlet pressure of the left-turning cylinder is about 2 MPa during the steering process.There is a large abrupt change when the steering reaches the maximum left-right steering angle.The hydraulic oil at the outlet of the steering pump gives priority to the steering of the steering hydraulic system, and the hydraulic oil at the outlet of the steering pump is completely used for steering at idle speed.The impact pressure reaches the overflow pressure of the steering safety valve at the beginning of the steering, and the pressure drops smoothly after the steering.Finally, the steering pump reaches the safe pressure of 16 MPa for the overflow of the system, and the locking pressure of the steering cylinder reaches 18 MPa when the left and right steering impact.The pressure oil at the outlet of the steering pump has a large pressure loss of about 5 MPa after passing through the steering gear.
The driver's control of the accelerator pedal and brake pedal will affect the working state of the fuel cell and motor system, the hydraulic system, and the power performance of the vehicle.In addition, the basic performance of the vehicle and road conditions will also affect the power of the construction vehicle.Under the control of the energy management strategy, it can be seen from Figure 8a that the fuel cell always operates near the maximum efficiency point during the process of providing energy to the drive motor.This can ensure that the power demand of the vehicle can be met in the shortest time when the vehicle is accelerating rapidly and driving at a steady speed and improve the power performance and energy utilization rate of the vehicle.The fuel cell hydraulic hybrid power system effectively combines the high energy density characteristics of the fuel cell with the high power density characteristics of the hydraulic system, so that the vehicle can easily meet the requirements of the vehicle's power performance when facing the complex and changeable road conditions and a series of operations such as starting acceleration and braking are required frequently.As can be seen from Figure 8b, hydrogen consumption also increases linearly as the operating time increases, but the rate of increase remains almost constant.The energy management strategy based on driving condition recognition enables hydrogen fuel cell engineering vehicles to obtain more efficient power distribution, effectively reducing the vehicle equivalent hydrogen consumption, and improving the vehicle economy.
steering safety valve at the beginning of the steering, and the pressure drops smoothly after the steering.Finally, the steering pump reaches the safe pressure of 16 MPa for the overflow of the system, and the locking pressure of the steering cylinder reaches 18 MPa when the left and right steering impact.The pressure oil at the outlet of the steering pump has a large pressure loss of about 5 MPa after passing through the steering gear.The driver's control of the accelerator pedal and brake pedal will affect the working state of the fuel cell and motor system, the hydraulic system, and the power performance of the vehicle.In addition, the basic performance of the vehicle and road conditions will also affect the power of the construction vehicle.Under the control of the energy management strategy, it can be seen from Figure 8a that the fuel cell always operates near the maximum efficiency point during the process of providing energy to the drive motor.This can ensure that the power demand of the vehicle can be met in the shortest time when the vehicle is accelerating rapidly and driving at a steady speed and improve the power performance and energy utilization rate of the vehicle.The fuel cell hydraulic hybrid power system effectively combines the high energy density characteristics of the fuel cell with the high power density characteristics of the hydraulic system, so that the vehicle can easily meet the requirements of the vehicle's power performance when facing the complex and changeable road conditions and a series of operations such as starting acceleration and braking are required frequently.As can be seen from Figure 8b, hydrogen consumption also increases linearly as the operating time increases, but the rate of increase remains almost constant.The energy management strategy based on driving condition recognition enables hydrogen fuel cell engineering vehicles to obtain more efficient power distribution, effectively reducing the vehicle equivalent hydrogen consumption, and improving the vehicle economy.As an engineering vehicle with heavy energy consumption, articulated loader has a large energy loss in steering hydraulic system.Reducing energy consumption and improving steering efficiency have become one of the research focuses of wheel loaders.It can be seen from Table 2 that the efficiency of the steering hydraulic system is relatively low, only 17%, in the first working condition of in situ steering.The main reason is that the output hydraulic oil of the steering pump is much larger than the hydraulic oil needed for steering when the engine is in a high-speed state, resulting in excess hydraulic oil flowing back to the tank through the priority valve port at high pressure.Not only the high-pressure overflow power consumption accounts for 60.4% of the total power, but also the flow to the steering cylinder through the priority valve port produces a large pressure difference of about 5 MPa after passing through the steering gear, resulting in the loss of work on the steering gear up to 39 kJ.The main reason for the low steering hydraulic efficiency is that the outlet flow of the steering pump under high power is greater than the required flow of the steering pump, resulting in too much hydraulic oil flowing through the priority valve port at high pressure.After the variable orifice is distributed on the steering gear, the pressure difference is up to 5 MPa in the hydraulic oil entering the steering hydraulic system.The efficiency of the steering hydraulic system is mainly affected by the engine speed and the steering wheel speed and is less affected by the loader's full load or no load.In the hydraulic oil entering the steering, a large pressure difference is generated after passing through the variable orifice distributed on the steering gear.The efficiency of the steering hydraulic system is mainly affected by the engine speed and the steering wheel speed and is less affected by the loader's full load or no load.In the process of optimizing the design of the steering system, the unsteady flow of the hydraulic system and the pressure of the hydraulic system are reduced by reducing the stroke difference of the cylinder or the moment arm difference.As an engineering vehicle with heavy energy consumption, articulated loader has a large energy loss in steering hydraulic system.Reducing energy consumption and improving steering efficiency have become one of the research focuses of wheel loaders.

Conclusions
In this paper, the optimal design of the articulated steering system is carried out by using particle swarm optimization, taking the maximum stroke difference and minimum pressure of the left and right steering cylinders as the optimization objectives, and the position of the hinged point of the cylinders as the design variable.The optimization results show that the maximum cylinder pressure is reduced by 13.5% and the maximum stroke difference between left and right steering cylinders is reduced by 16% under steady state.The dynamic characteristics of the steering hydraulic system under different engine speeds and loads were obtained by in situ steering tests of wheel loaders.It is found that the speed and load have important effects on the pressure response of the system, and the causes and influencing factors of pressure and flow fluctuation are determined.The main reason that the efficiency of the steering hydraulic system is only 17% is that the engine is in a high-speed state, and the output hydraulic oil of the steering pump is much larger than the hydraulic oil needed for steering, resulting in excess hydraulic oil flowing back to the tank through the high-pressure overflow port of the priority valve, and the high-pressure overflow power consumption accounts for 60% of the total work.The flow to the steering cylinder through the priority valve port produces a large pressure difference of about 5 MPa after passing through the steering gear, and the work lost on the steering gear reaches 36 kJ.The test results provide data support for the improvement in steering hydraulic system.The energy management strategy based on driving condition recognition can make hydrogen fuel cell loaders obtain better power distribution under complex test conditions and improve vehicle fuel economy.
As a complex problem involving many factors, the articulated steering of wheel loaders is suggested as follows: The steering system and transmission system are considered together, and it is suggested to carry out system research to make the research closer to the actual working state.It is suggested to analyze the dynamic characteristics and energy consumption of the steering hydraulic system of variable steering pump in order to reduce the energy consumption and improve the steering efficiency.It is necessary to continuously improve the driving condition recognition algorithm and energy management strategy to further improve the performance of hydrogen fuel cell vehicles under complex working conditions.

Figure 4 .
Figure 4. Particle fitness curve in the optimization process.

FitnessFigure 4 .
Figure 4. Particle fitness curve in the optimization process.

Figure 6 .
Figure 6.Pressure of hydraulic cylinder in steering process.

Figure 6 .
Figure 6.Pressure of hydraulic cylinder in steering process.

Figure 6 .
Figure 6.Pressure of hydraulic cylinder in steering process.

Figure 7 .
Figure 7. Test results of dynamic characteristics of hydraulic system.

Figure 7 .
Figure 7. Test results of dynamic characteristics of hydraulic system.

Figure 8 .
Figure 8. Dynamic characteristic experiment of power system.

Figure 8 .
Figure 8. Dynamic characteristic experiment of power system.

Table 1 .
Optimization results of steering mechanism.

Table 1 .
Optimization results of steering mechanism.

Table 2 .
Hydraulic system energy consumption list under steering conditions.