Research on hydraulic braking energy recovery system of heavy vehicles

The carbon emission of a heavy-duty diesel vehicle is almost equal to the sum of 100 small cars. In the global environment where energy conservation and emission reduction are advocated, the pollution caused by heavy vehicles is non-negligible. A hydraulic brake energy recovery and regeneration system can store the braking energy, convert the energy into kinetic energy and output it to the vehicle transmission system when starting or accelerating, thereby reducing energy consumption and achieving the role of energy conservation and emission reduction. Firstly, this paper mainly summarizes several vehicle energy recovery technology schemes and determines the hydraulic braking energy recovery scheme after a comprehensive comparison. Secondly, the overall structure of the hydraulic brake energy recovery and regeneration system is designed, and the working mode is analyzed. Finally, according to the working characteristics of the main components in the system, the mathematical model is established, the key parameters are simulated and studied, and the appropriate selection and matching scheme is provided.


Introduction
Energy conservation and emission reduction are hot topics today.Both the vibration of the flexible suspension system and the braking acceleration can produce energy that is convenient to recycle.Though the energy recycled from the suspension system is safer, it requires a supporting electric motor and battery.Further, compared with the energy recycled from cars braking, the energy from the suspension system is much less.Heavy-duty vehicles can recycle more energy because of their heavier weights and longer vibration of the suspension system, but they are only in a low degree of electrification.As a result, the energy regenerative suspension system is usually under research and theory, and they have not got the chance for extension yet.
Being different from the energy regenerative suspension system, the braking energy recovery system has a higher popularizing rate and a wider application.The braking systems today could be divided into three types: drum brake, disc brake, and ceramic brake.Their operating principle is the use of the force of friction.However, friction means not only the abrasion of the parts but also the waste of energy.For example, a car that weighs 1.5-ton brakes from the speed of 60 mph would consume about 0.16 kilowatthours of energy.In addition, trucks can usually weigh from 14 to 100 tons, which equals tens of times those of small cars.Their braking could produce considerable recovery, which means amazing benefits.On the other side, trucks in China usually have a displacement of 8-12l, as some trucks overseas have a striking displacement of 16l.In addition, trucks made much more pollution than cars because of the terrible efficiency of diesel engines and the absence of diesel particulate filters.So, here comes the opportunity for brake energy recovery systems to realize their values.Thus, the brake energy recovery system is critical in solving related environmental problems.
As a youngster in the development of the vehicles industry, electric vehicle attracts much attention in the territory of energy saving, and quantities of intelligent technology are used, one of them being the brake energy recovery system.One-pedal driving, designed and applied firstly by Tesla, allows drivers to control their cars only by the accelerator.It has been used widely by many auto manufacturers on their electric vehicles now.However, as a new type to control a car, the different mode from traditional cars could lead to the risk of more accidents.One-pedal driving is facing queries from buyers.General motors gave another way out: a switch behind the steering wheel, which could push by hand, controls the brake energy recovery system, which means that it would not impact the brake pedal.This new function had its first show on the Cadillac lyriq.Above all, the brake energy recovery system is rather mature on electric vehicles now.
Benefiting from the connection of electric motor, battery and automotive transmission system, it is convenient to load a brake energy recovery system on an electric car.However, it is infeasible for an internal combustion engine vehicle to the difference between those two kinds.Today, mainstream internal combustion engine vehicles usually use a hydraulic braking energy recovery system, which is connected to the output shaft of the gearbox, which can make the recycling of energy a reality by the transition between kinetic energy and internal energy of the compressed liquid.But they are only applied to heavy-duty vehicles like trucks and buses.There are also some other ways out in the planning and design stage.
This text aims to elaborate on the structure, principle, methods of control and efficiency of recycle of the brake energy recovery system on heavy-duty vehicles: (1) The comprehensive comparison among different kinds of brake energy recovery systems and the analysis of the advantages of hydraulic braking energy recovery systems.
(2) Summaries about the principle of the system and design of the whole system in proper ways.
(3) Design the control strategy of the system and introduce the working mode under different conditions.
(4) Elaboration about the selections of elements and the efficiency of recycling in the simulation environment from the calculation.

Principle and structure of the hydraulic brake energy recovery and regeneration system
2.1.Working principle and structure design of the recycling system 2.1.1.Principle of hydraulic type energy recovery and utilization.Hydraulic braking energy recovery and utilization systems mainly comprise the transmission system, control system, hydraulic energy storage device and hydraulic pump/motor.The control system is connected to the vehicle acceleration and braking system.The main function is to detect the operating state parameters of the vehicle and transmit instructions to other working parts to control the whole system.The main function of the transmission system is to transfer hydraulic energy to the original transmission system to power the vehicle; the hydraulic reservoir and hydraulic pump are mainly used to store the recovered energy and convert it into mechanical energy [1].
When the driver presses the brake pedal, The controller receives the message and sends the brake signal, and the hydraulic system starts working as an energy storage device.The kinetic energy of the vehicle passes sequentially through the driving wheel, Drive axle main reducer, final drive shaft, gearbox, Drive the pump to transfer to the hydraulic energy storage device; When the energy of the hydraulic energy storage reaches the upper limit, The internal pressure sensor transmits the signal to the controller, Energy storage system shutdown; When the driver presses on the accelerator pedal, The controller sends an acceleration signal, Valve opening of the hydraulic energy storage device, Highpressure oil gushing into the hydraulic pump, The torque generated is re-input to the drive axle through the supporting gear system, Together with the original car power system to provide acceleration for the car, To achieve energy reuse [2].

Principle of air-pressure energy recovery and utilization.
The pneumatic energy recovery and utilization system mainly comprises the control system, the pneumatic energy storage device and the gear system.The control system is also connected to the braking and acceleration system to detect the driving state parameters and control other systems; the pneumatic reservoir mainly stores and output energy; the gear system functions to recover the vehicle braking energy and store the vehicle in the pneumatic energy storage device.
When the driver steps down the brake pedal, the controller sends the brake signal, valve 1 opens, and the screw connects to the gear connected with the transmission system.When the screw rotates, the screw pushes, the bottom of the airbag tube is compressed inward, and when the brake pedal is lifted, valve 1 is closed, the screw is separated from the gear; when the driver steps down the accelerator pedal, the controller output the acceleration signal, the valve 2 is opened, and the high-pressure gas is input into the turbocharger through the pipeline to push the blade rotation to increase the intake pressure and realize the utilization of air pressure energy [3].

Principle of electric-type energy recovery and utilization.
The electric energy recovery system of electric vehicles and gas-electric hybrid vehicles mainly comprises electric motors, sensors, automatic controllers and batteries.Since the electric vehicle itself already includes batteries and motors, there is no need to add additional related equipment.Sensors and controllers are used to monitor the acceleration and deceleration of cars, batteries are used to store electricity, and the motor can output positive and negative torque as a motor and generator [3].
When the driver presses the brake pedal, the sensor detects the signal and sends it to the control system.The control system transmits instructions, the motor works in reverse as the generator, and stores the electric energy in the battery, which can be directly used when the vehicle accelerates.In addition, in-vehicle driving, the uneven road surface could cause the longitudinal compression and reciprocation movement of the vehicle suspension system, which can be connected with the suspension to repeatedly cut the magnetic sensor line or use the planetary gear to change its movement mode into a fixed axis rotation to generate power for the motor [4].
Because the internal combustion engine car is not equipped with any electric drive system, the braking energy recovery system needs to be redesigned.It is difficult and costly, but it can still use the light battery to recover, and the energy recovery method is the same as above.When the driver presses the accelerator pedal, the sensor and the controller pass the command to the battery and the electric turbine.Electricity drives the turbine blade to rotate actively, and the electric turbine starts to work [5,6].

Scheme comparison of vehicle energy recovery and regeneration system
This paper mainly puts the vehicle energy recovery scheme into two parts, braking recovery kinetic energy and suspension recovery potential energy.The former is divided into hydraulic braking recovery system, pneumatic braking recovery system, and electric braking recovery system , but for the latter, electric braking recovery system is the only available scheme.Among them, the electric braking energy recovery system has been widely used in a variety of electric vehicles, while the hydraulic braking energy recovery system is only used in a few kinds of special vehicles, and the pneumatic braking energy recovery system and feed suspension which due to the limit of technology have not yet been perfect.Table 1 compares the the differences about the life length, temperature effect, safety, environmental protection, technical maturity, comprehensive advantages and disadvantages of several energy recovery schemes.Table 1.Different kinds of energy recovery programs and their characteristics, advantages and disadvantages [1].The potential energy change is less, the technology is not mature, the pure internal combustion engine car needs the supporting design of turbocharger

Overall structure of the control strategy of the recycling system
According to the different structures, the recycling system can be divided into series, parallel and hybrid regenerative braking systems.The engine and accumulator power the series structure, but the engine and hydraulic pump are connected, not directly to the gearbox; the parallel structure is powered by the engine and accumulator, and the mixed structure functions as series and parallel systems and combines to exert their respective advantages.
Because the secondary element variables are adjustable, the vehicle power can be adjusted smoothly, which can allow the engine to be efficient and economic work, but the vehicle changes greatly with relatively low system efficiency; the parallel structure is easy to install, easy to realize and high recovery efficiency; hybrid combines the advantages of the former two, but the control strategy is complex, large vehicle changes, high cost and less application.
For vehicles with a single power source, series and parallel structures can be selected; the hybrid structure can be selected for vehicles combining two or more power sources.For the heavy vehicles studied in this paper, the parallel structure is selected, and the control strategy of the recycling system of the parallel structure is introduced below.
The structure of the hydraulic brake energy recovery and regeneration system is shown in the following figure, mainly composed of hydraulic, transmission and control.The hydraulic system functions to recover, store and release the brake energy; the accumulator and secondary components (pump/motor); the transmission system functions to realize the power transfer between the pump/motor and the original transmission system; the control system functions to detect the operating parameters of the vehicle and realize the operation and control of the recovery and regeneration system (Figure 1).

Analysis of the control mode of the recycling system
According to the actual working process of the hydraulic hybrid electric vehicle, the working process of the vehicle energy regeneration system can be divided into the following processes: Energy recovery method: when the vehicle brakes or decelerates, the controller receives the braking signal, the secondary element presents the working condition of the pump, the hydraulic oil enters the high-pressure accumulator from the oil tank, and the energy is stored in the high-pressure accumulator in the form of hydraulic energy.
Energy release mode: When the vehicle starts or accelerates, the controller receives the drive signal, the secondary element presents the working condition of the motor, and the hydraulic oil drives the secondary element to transfer the energy to the vehicle.
Energy maintenance mode: When the vehicle is driving normally, the car only needs to overcome the wind resistance and ground rolling friction.Thus, the driving force of the car is small when driven by the engine alone [7].

Simulation study of hydraulic braking energy recovery and regeneration system
Hydraulic braking energy recovery system mainly comprises an accumulator, hydraulic pump/motor and other components.System working process: when the vehicle brakes, the accumulator starts to save energy; when the vehicle encounters the starting and climbing hill, the hydraulic motor provides power.
According to different types of heavy-duty vehicles, the main parameters to be determined are: total mass m, main deceleration ratio i0, wheel radius r, and the initial braking speed v0 is determined by general driving conditions.The following introduces the selection matching of the accumulator and hydraulic pump/motor.

Accumulator model
After the braking energy recovery, the energy is saved in the accumulator and generally choose the leather bag type accumulator.The selection of the accumulator relies on the life, working pressure and recoverable energy of the accumulator.

4.1.1.
Working pressure of the accumulator.The accumulator can be regarded as an independent thermodynamic system, obtained by Boer's gas law: ) And p and V are the pressure and the corresponding volume of the accumulator gas, respectively; p0 and V0 are the gas pressure in the accumulator airbag and the corresponding gas volume in the initial state; p1 and V1 are the lowest pressure of the accumulator and the corresponding gas volume respectively; p2 and V2 are the highest operating pressure of the accumulator and the corresponding gas volume.n is the gas index.Since the accumulator can be considered according to the adiabatic process during the liquid filling and discharge process, n=1.4 is taken here.The highest working pressure, p2 is generally used as a nominal pressure, assumed to be the known parameter.
According to the experience: ) Substitute formula (4) into formula (5) to obtain: 4.1.2.Accumulator has recoverable energy.Hydraulic energy stored by the accumulator   : According to formula (1) and ( 6): stay  0 and  2 under certain circumstances, it can be seen that: The maximum stored energy can be obtained by substituting formula (8) into formula (7):

Hydraulic pump / motor model
The key parameter of the hydraulic pump is displacement.Because the speed of the system is not stable, it is calculated from the supply torque of the required pump/motor.By the torque balance equation: is the outlet pressure of the hydraulic pump/motor;   is the inlet pressure of the hydraulic pump/motor;   is the rotational inertia of the hydraulic pump/motor;   is the speed of the hydraulic pump/motor;   is the damping coefficient of hydraulic pump/motor.

The flow equation is as below:
=     −   (  −   ) −     (11)   is the output flow of the variable displacement pump/motor;   is the internal leakage coefficient of variable displacement pump/motor;   is the external leakage coefficient of variable displacement pump motor [8][9][10].

Conclusion
Brake energy recycling is an application direction with both economic and energy-saving significance.In the design of the braking energy recovery system of heavy vehicles, the cost, safety, efficiency and other indicators of hydraulic energy storage have quite obvious advantages, such as low equipment price, extended service life, installation and transformation, etc., which is a relatively mature energysaving application technology.This paper compares the advantages and disadvantages of various braking modes, gives the appropriate implementation scheme of the energy regeneration and recovery system, analyzes the control strategy and control mode, and makes the corresponding response under different vehicle conditions; establishes the mathematical model of the main components of the energy regeneration system, and gives appropriate selection and coordination according to the proposed main parameters.
In this paper, there are also some shortcomings in the braking energy recovery system.In practical work, the energy recovery methods and efficiency in different driving conditions need to be further discussed.In addition, the system should consider the impact of more components.

Figure 1 .
Figure 1.The structure of the hydraulic brake energy recovery and regeneration system.