JIANG Zhong-lin(蒋忠林), WU Wei(吴维), YUAN Shi-hua(苑士华), HU Ji-bin(胡纪滨)

(National Key Laboratory of Vehicular Transmission, Beijing Institute of Technology, Beijing 100081, China)

Simulation of a vehicle hydraulic propulsion system

JIANG Zhong-lin(蒋忠林), WU Wei(吴维), YUAN Shi-hua(苑士华), HU Ji-bin(胡纪滨)

(National Key Laboratory of Vehicular Transmission, Beijing Institute of Technology, Beijing 100081, China)

The characteristics of a hybrid hydraulic vehicle driven by the hydraulic common rail propulsion system with a hydraulic free-piston engine and a hydraulic transformer were studied. A mathematical model of the propulsion system was established and a control method of the propulsion system was proposed. Extensive simulation results of hybrid hydraulic vehicles with the hydraulic common rail propulsion system were presented. The hydraulic common rail propulsion system achieved the switch power control and the constant power propulsion. The control method based on the propulsion, break and speed limit requirement was verified. Our results showed that the hydraulic common rail propulsion system gained an ideal acceleration process.

hydraulic common pressure rail; free-piston engine； rotate-plate hydraulic transformer

Energy shortage is one of the foremost concerned topics today. Because of the ability to recycle the vehicle kinetic energy, the hybrid hydraulic propulsion system with accumulator has received growing attention in the auto industry. Oil consumption reduction with the hybrid hydraulic propulsion system has been proved feasible by theoretical analysis and experiment[1-2]. Automotive is the second large energy consumption aside from industry. Automotive manufactures are trying to improve efficiency and reduce emission. Compared with hybrid electric vehicles, the hybrid hydraulic propulsion system have a much higher power density and can capture much more kinetic energy[2]. By the use of the hydraulic common pressure rail (HCPR), the decoupling of the engines and the loads is achieved. The HCPR propulsion system can make the engine always work at a higher efficiency[3].The hydraulic common pressure rail propulsion system is studied in this paper. The HCPR propulsion system mainly consists of the switching control hydraulic free-piston engine[4], hydraulic bladder accumulator, and the rotate-plate hydraulic transformer[5]. The mathematical models of the HCPR propulsion system were established. The control method based on the propulsion, the break and the hydraulic transformer speed limitation was proposed. It is aimed to provide a new effective idea for the automotive propulsion system.

1 Configuration of the hybrid hydraulic vehicle

The configuration of the HCPR propulsion system is presented in Fig.1. The HCPR is composed of the high and the low pressure rail. The pressure change is restricted by the hydraulic accumulator. The power source of the HCPR propulsion system is the hydraulic free-piston engine. The rotate-plate hydraulic transformer separates the HCPR from the variation of the load. A fixed displacement hydraulic pump/motor is mounted behind the hydraulic transformer. The traditional mechanical drivetrain is sensitive to the load. Thus, it is difficult to operate the engine at a high efficiency in all conditions. However, there is only simple coupled power between the input and the output for the HCPR propulsion system. The HCPR can avoid the mutual interference between different loads. It ensures the stable operation pressure for the hydraulic free-piston engine. The hydraulic free-piston engine used in the HCPR propulsion system is controlled by a switch method[4,6]. The idling does not exist for the hydraulic free-piston engine.

Fig.1 Configuration of the propulsion system

2 Modelling hybrid hydraulic propulsion system

The hydraulic free-piston engine and the rotate-plate hydraulic transformer of the HCPR propulsion system have been modelled based on the previous study about these components in the National Key Laboratory of Vehicular Transmission of the Beijing Institute of Technology. The models have been validated against the experiment data. Some component mathematical models were simplified to improve the numerical calculation efficiency.

2.1 Hydraulic free-piston engine

The mathematical model of the hydraulic free-piston engine only considers the main features of the switching control process. It includes the approximate constant displacement, stable operation cycle. There is no hydraulic oil output or fuel consumption during the idle. The high efficient operation of the hydraulic free-piston engine is achieved in the high piston frequency[7]. Therefore, only the maximum piston frequency is selected to control the hydraulic free-piston engine. The hydraulic free-piston engine steady operation is described by

(1)

(2)

whereQois the output flow rate of the engine;nfis frequency of the engine;qois the free-piston engine displacement;Qfis the total fuel consumption;vfis the fuel consumption of one cycle.

2.2 Rotate-plate hydraulic transformer

The rotate-plate hydraulic transformer mainly completes the pressure conversion from the HCPR to the load. Its model includes the transformer conversion ratio control model and rotational cylinder model. Its conversion ratio control system is simplified as a second-order system:

(3)

wherekTis the system gain; ωisthesysteminherentfrequency; ζisthesystemdampingratio.Thesystemparametersareacquiredbythetestofthehydraulictransformer.

Therotationalcylindermodelisproposedby

(4)

whereJis the rotational inertia;TPis the driving torque generated by each plunger;cis the viscous drag coefficient;TDis the dynamic friction torque;TSis the static friction torque; θisthecylinderrotationalangle.

2.3Hydraulicaccumulator

TheHCPRconsistsofthehydraulicaccumulatorsandthereliefvalves.ThehydraulicaccumulatorisusedtoabsorbthepressurerippleoftheHCPRandstorethepressureoil[8].Thehydraulicaccumulatormodelispresentedby

(5)

(6)

wherepA0is the pre-charge gas pressure of the accumulator;VA0is the accumulator nominal volume;pAis the oil pressure in the accumulator;VAis the oil volume in the accumulator; γisadiabaticexponentofthegas;ΔEistheenergychangeoftheaccumulator; εA0istheinitialcompressionratiooftheaccumulator,andεAtisthecompressionratiooftheaccumulatorattimet.

2.4Vehicle

Thevehiclemodelisusedtodescribethedynamicscharacteristicsofthevehicle.Thevehiclemodelusedinthispaperonlyconsidersthelongitudinaldynamicscharacteristics.Thelongitudinaldynamicsmodelisdescribedby

FT=Ff+Fi+Fw+Fz,

(7)

whereFTis the traction force at the drive wheels,Ffis the rolling resistance force,Fiis the climbing resistance force,Fwis the air resistance force, andFzis the acceleration force. Namely:

(8)

wheremis vehicle mass;fis rolling resistance coefficient; θisroadinclinationangle; CDisairdragcoefficient; Aisfrontalareaofthevehicle; uisvehiclespeed;andσismassfactorofrotatingparts.

3 Controlling hybrid hydraulic propulsion system

The control method of the HCPR propulsion system is responsible for the propulsion, the brake and the rotate-plate hydraulic transformer speed limitation.

The control system is illustrated in Fig.2. The hydraulic free-piston engine power modulate is achieved by the control of the piston frequency to ensure the HCPR maintains a stable pressure. Due to the advantage of the hydraulic free-piston engine switching control, the idle fuel consumption is completely eliminated. The plate rotation angle of the rotate-plate hydraulic transformer is determined by the vehicle acceleration and velocity requirement. The cylinder speed of the rotate-plate hydraulic transformer is limited by the control of the plate rotation angle. Further, in order to ensure the pressure of the HCPR beyond the minimum working pressure, the output power of the rotate-plate hydraulic transformer is also limited.

Fig.2 Control system of the propulsion system

4 Results and discussion

The prime parameters of the vehicle are listed in Tab.1. The maximum output power of the single piston hydraulic free-piston engine is about 15 kW. The hydraulic transformer is a rotate-plate hydraulic transformer. There are 18 plungers in the rotate-plate hydraulic transformer. The diameter of the plungers is 19 mm. The radius of the pitch circle is 49.3 mm. The inclination angle of the plate is 18°.

In order to meet the vehicle performance, eight sets of hydraulic free-piston engines are configured on the vehicle. The operation characteristics of the HCPR propulsion system under various operation conditions have been analysed in detail.

Tab.1 Vehicle parameters

4.1 Releasing of driving energy and acceleration

The vehicle acceleration characteristics are shown in Fig.3. The graphs describe the vehicle speed, motor inlet/outlet pressure, plate rotation angle of the hydraulic transformer and hydraulic transformer speed respectively. It indicates that the vehicle with the HCPR is able to accelerate from 0 to 100 km/h within 10.9 s. It reveals that the new propulsion system has a similar acceleration performance with the same size mechanical drivetrain vehicle.

Fig.3 Acceleration characteristics

The propulsion system is in the initialization phase during 0-13 s, as shown in the first curve. The pressure of the high-pressure side is established during the initialization phase. The high pressure pump is needed to provide the start-energy for the hydraulic free-piston engine[7]. Then, the pressure of the high-pressure side is established by the hydraulic free-piston engine. After the initialization, the vehicle begins to speed up by adjusting the plate rotation angle of the rotate-plate hydraulic transformer. During 13 s-21 s, the plate rotation angle of the rotate-plate hydraulic transformer is set to be the maximum transform ratio position to get the maximum acceleration. The input pressure of the motor reaches the maximum operation pressure. However, the rotate-plate hydraulic transformer is prone to speeding at this stage. To prevent the rotate-plate hydraulic transformer from speeding, the plate rotation angle is adjusted to control the speed of the hydraulic transformer. Meanwhile the output power of the rotate-plate hydraulic transformer increases with the increasing speed. The pressure of the high-pressure side gradually decreases. When the uninterrupted oil supply still unable to ensure the pressure of the high-pressure side maintains beyond the lower pressure threshold, the output power of hydraulic transformer is limited by controlling the plate rotation angle, as shown in 21 s-40 s. It ensures that the pressure of the high-pressure side of HCPR is always higher than the lower pressure threshold. The hydraulic transformer is operated in a constant power mode. The vehicle accelerates with a constant power.

The integrated control of the engine and the rotate-plate hydraulic transformer can make the full use of the power storage function of the hydraulic accumulator and the constant power output characteristic of the rotate-plate hydraulic transformer. The idea acceleration process is achieved by the HCPR propulsion system. The HCPR propulsion system makes the motor reach the maximum power in the shortest time. It takes full advantage of the maximum power of the HCPR. The acceleration processes realized by the HCPR propulsion system satisfies the vehicle dynamic requirement much more closely.

4.2 Kinetic energy recovery and brake

The HPCR propulsion system has some braking characteristics different from the conventional vehicle propulsion system. The HCPR propulsion system decouples the load from the hydraulic free-piston engine. However, the HCPR propulsion system can’t make use of the engine brake as there is only simple coupled power exists between the input and the output. Then, only traditional mechanical brake and kinetic energy recovery brake can be used for the HCPR propulsion system. The pressure transform process of the hydraulic transformer is bidirectional. Besides exporting energy to the hydraulic motor, the hydraulic transformer can also be used to recover kinetic energy to the accumulator.

Fig.4 Kinetic energy recovery and brake

Operation characteristics of the propulsion system between 0-250 s of MVEG cycle are given in Fig.4. The graphs describe the vehicle speed, pressure of the high-pressure side of the HCPR, motor inlet/outlet pressure and plate rotation angle of the hydraulic transformer respectively. It declares that the pressure of the high-pressure side of the HCPR declines with time when the vehicle speed get zero. There is a leakage in the hydraulic free-piston engine and other hydraulic devices. When the vehicle breaks, the pressure of the high pressure rail rises. The vehicle kinetic energy is converted into the hydraulic energy.

During braking, the inlet and outlet of the fixed displacement hydraulic motor is interchanged, as shown in Fig.4. The hydraulic free-piston engine is turned off and the hydraulic motor works as a pump to recuperate the kinetic energy.

4.3 Fuel consumption and efficiency

Fig.5 Integrated condition characteristics curve

The operation characteristics under the urban dynamometer driving schedule (UDDS) cycle of the vehicle propulsion system with HCPR is given in the fist graph of Fig.5. The graphs describe the vehicle speed, accumulator pressure, engine frequency and plate rotation angle of the hydraulic transformer respectively. At place 1 as marked in Fig.5, the vehicle speed declines with time. Part of the vehicle kinetic energy is converted into hydraulic energy. The pressure of the high-pressure side rises. It is helpful for fuel consumption reducing. The simulation results indicate that the fuel consumption can be reduced about 34.6% by kinetic energy recovery brake relative to the mechanical brake under the UDDS cycle.

The rotate-plate hydraulic transformer used in the HCPR propulsion system realizes the constant power adjustment. Meanwhile, idle does not exist for the engine with the switching power control. There are only on operation and off operation. The hydraulic free-piston engine discharges high pressure oil only in the on operation. Thus, it avoids the idle fuel consumption. The pressure change of the HCPR and the frequency change of the engine are shown in Fig.5. The figure indicates that the engine begins to work when the pressure of the high-pressure side bellows the lower threshold. The engine stops running when the pressure of the high-pressure side reaches the upper threshold. The engine work time is only 20% of the entire driving cycle, which means that switching control method of the hydraulic free-piston engine has a great significance for the vehicle fuel economy. Simulation result also claims that fuel consumption of the vehicle propulsion system with HCPR is 3.28 L/100 km. As a result, the fuel economy improvement of a vehicle propulsion system with HCPR is about 60.6% in comparison with the mechanical drivetrain vehicle. The vehicle with HCPR propulsion system has obvious advantages on fuel economy over the traditional vehicle.

5 Conclusions

A novel hydraulic propulsion system was proposed. Extensive simulation results of the hybrid hydraulic vehicle with the HCPR propulsion system were presented.

The results indicate that the switching power control and constant power accelerate can be achieved by the HCPR propulsion system.

The results of the study provide a new effective idea for the automotive propulsion system. Future work should be carried on the components parameter optimization and the HCPR propulsion system prototype test.

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(Edited by Cai Jianying)

2013- 03- 11

Supported by the National Ministry Fundamental Research Foundation of China (A2220060053)

TH 137 Document code: A Article ID: 1004- 0579(2014)02- 0178- 06

E-mail: wuweijing@bit.edu.cn