Energy Management Control Strategy of Series Hydraulic Hybrid Vehicle

Abstract

Aiming at a series hydraulic hybrid vehicle, the mathematical model and Matlab/Simulink simulation model of the vehicle are established, and the energy management strategy based on rules is put forward. The simulation results show that the control strategy not only realizes four working modes of hydraulic hybrid vehicle, but also improves its fuel economy by 11.9%. It is found that the volume of accumulator and the working pressure range of accumulator are two important parameters that affect the fuel economy of hydraulic hybrid vehicle. Under the condition of a certain accumulator volume, increasing the working pressure range of the accumulator can not only increase the energy storage of the accumulator, but also reduce the idling times of the engine and further improve the fuel economy of the vehicle.

1 Introduction

Energy problems and environmental pollution have become the focus of attention in today’s world, and developing a low-carbon economy with low energy consumption and low emissions is becoming the common choice of all countries in the world. With the rapid increase of automobile ownership in China, it is of great practical significance to solve the problem of energy saving and environmental protection of vehicles.

Series hydraulic hybrid power is one of the main energy-saving and emission-reducing technologies in the automotive industry in the world today, and energy management strategy is the core technology to realize energy-saving and emission-reducing of series hydraulic hybrid power vehicles. At present, the control strategies for energy management of hybrid electric vehicles include rule-based energy management control strategy, real-time optimization-based energy management control strategy, global optimization-based energy management control strategy, and intelligent control strategies such as fuzzy logic, neural network or genetic algorithm, among which the rule-based energy management control strategy is the simplest algorithm [1,2,3,4,5,6,7,8,9].

In this paper, the mathematical model of a series hydraulic hybrid vehicle and the whole vehicle simulation model of Matlab/Simulink are established, and the energy management strategy based on rules is put forward, which gives full play to the advantages of charging and discharging energy of the accumulator in the control rules and verifies its practicability.

2 Composition and Working Principle of Series Hydraulic Hybrid Vehicle

The series hydraulic hybrid vehicle consists of an engine, a closed volume speed regulating circuit with an accumulator and the transmission system of a traditional vehicle, as shown in Fig. 1. The power output by the engine is transmitted to the variable pump through the clutch, and the variable pump converts mechanical energy into hydraulic energy. The hydraulic oil in the hydraulic pipeline drives the variable pump/motor to convert hydraulic energy into mechanical energy, which is transmitted to the gearbox and the drive axle, and finally the wheels are driven. The accumulator arranged between the variable pump and the variable pump/motor is an energy storage device, which can drive the vehicle independently and recover the braking energy when parking.

Fig. 1

Schematic diagram of power system of series hydraulic hybrid vehicle

3 Vehicle Dynamics Model

3.1 Engine Torque Output Model

The output torque of the engine output shaft is

$${T}_{{\text{out}}}={T}_{{\text{max}}}\cdot {\alpha }_{{\text{throttle}}\_\mathrm{ position}}$$

(1)

where T_out is the output torque of the engine output shaft (${\text{N}}\bullet {\text{m}}$), and T_max is the maximum torque of the engine, ${\alpha }_{{\text{throttle}}\_\mathrm{ position}}$ is the throttle opening of the engine.

The torque balance equation of the engine output shaft is

$${T}_{{\text{out}}}={T}_{{\text{load}}}+{J}_{{\text{engine}}}\alpha $$

(2)

where ${T}_{{\text{Load}}}$ is the load torque, ${J}_{{\text{engine}}}$ is the inertia of the engine integrated accessories, and $\alpha$ is the angular acceleration of the engine output shaft.

3.2 Pump/Motor Dynamic Model

The torque balance equation of the pump/motor output shaft is

$${T}_{{\text{p}}/{\text{m}}}=\Delta PDx+{J}_{{\text{p}}/{\text{m}}}\alpha +{T}_{\eta }$$

(3)

where, ${T}_{{\text{p}}/{\text{m}}}$ is the input torque of the pump/motor, $\Delta P$ is the pressure difference between the inlet and outlet of the pump/motor, $D$ is the maximum displacement of the pump/motor, x is the variable coefficient, $J{\text{p}}/{\text{m}}$ is the moment of inertia of the output shaft of the pump/motor, $\alpha$ is the angular acceleration of the output shaft of the pump/motor and ${T}_{\eta }$ is the moment of efficiency loss.

3.3 Accumulator Model

According to Boyle-Mariotte’s law, the relationship between the pressure and volume of the gas of the airbag accumulator can be expressed as follows

$${p}_{0}{V}_{0}^{n}={p}_{1}{V}_{1}^{n}={p}_{2}{V}_{2}^{n}={\text{C}}$$

(4)

where p₀ is the inflation pressure, p₁ is the lowest working pressure of the system, p₂ is the rated working pressure of the system, V₀, V₁ and V₂ are the volume of the gas under the corresponding pressure, n is the thermal coefficient, and during adiabatic process $n = 1.4$, C is the gas constant.

The amount of liquid that the accumulator can store is called effective working volume V, and its value is

$$\Delta V={V}_{1}-{V}_{2}$$

(5)

Substituting the values of V₁ and V₂ obtained from Eq. (4) into Eq. (5), it can be obtained that the accumulator volume V₀ is in adiabatic state, that is, under the condition of fast charge and fast discharge.

$${V}_{0}=\Delta V/\left[{\left({p}_{0}/{p}_{1}\right)}^{1/n}-{\left({p}_{0}/{p}_{2}\right)}^{1/n}\right]$$

(6)

The range of inflation pressure is 0.25p₂ ≤ p₀ ≤ 0.9p₁.

The simulation model of accumulator ignores the influence of temperature and high pressure (when p₂ ≥ 20 MPa) on volume V₀.

3.4 Vehicle Driving Model

The total driving resistance of the vehicle is

$$\sum F={F}_{{\text{f}}}+{F}_{{\text{w}}}+{F}_{{\text{i}}}+{F}_{{\text{j}}}$$

(7)

Among

$${F}_{{\text{f}}}=m{\text{g}}f$$

(8)

$${F}_{{\text{w}}}=\frac{1}{2}{C}_{{\text{D}}}A\uprho {{u}_{\text{r}}}^{2}$$

(9)

$${F}_{{\text{i}}}=m\mathrm{gsin\alpha }$$

(10)

$${F}_{{\text{j}}}=\updelta m\left({\text{d}}u/{\text{d}}t\right)$$

(11)

where, ${F}_{{\text{f}}}$ is the rolling resistance, ${F}_{{\text{w}}}$ is the air resistance, ${F}_{{\text{i}}}$ is the ramp resistance and ${F}_{{\text{j}}}$ is the acceleration resistance; m is the mass of the car; g is acceleration of gravity; f is the rolling friction coefficient. ${C}_{{\text{D}}}$ is the air resistance coefficient, A is the windward area, $\uprho$ is the air density, and u_r is the relative speed between the vehicle and the air, and the actual driving speed of the vehicle when there is no wind. $\mathrm{\alpha }$ is $i=h/s=\mathrm{tan\alpha }$, where i represents the road slope, which is equal to the ratio of the slope height h to the bottom length s. δ is the conversion coefficient of the rotating mass of the automobile, δ > 1, and du/dt is the driving acceleration.

The dynamic equation of vehicle driving is

$$\frac{M}{{\text{r}}}=m{\text{g}}f+\frac{1}{2}{{\text{C}}}_{{\text{D}}}\mathrm{A\rho }{v}^{2}+m\mathrm{gsin\alpha }+\updelta m\frac{{\text{d}}u}{{\text{d}}t}$$

(12)

where M is the driving torque that the pump/motor needs to provide for the vehicle, and r is the wheel radius.

4 Energy Management Control Strategy and Working Mode

4.1 Control Strategies

Strategy 1.

Make the engine work in the best efficiency area and emission area.

Strategy 2.

Under the condition of a certain accumulator volume, the working pressure range of the accumulator is increased, that is, the energy storage capacity of the accumulator is increased, so as to give full play to the energy storage and discharge efficiency of the accumulator in the hydraulic hybrid electric vehicle, thus reducing the idling times of the engine and obtaining better fuel economy of the whole vehicle.

4.2 Energy Management Control Rules and Working Mode of Vehicles

The ultimate goal of energy management strategy is to realize the energy saving of vehicles. Therefore, during the whole journey of vehicles, the optimal energy saving effect of vehicles can be achieved by controlling the engine to work cyclically in the two working conditions of optimal fuel consumption and idle speed, and at the same time, making it cooperate with the charging and discharging of accumulators. During the whole journey, the vehicle works in the following four modes:

• 1. Vehicle starting mode

When the vehicle speed is 0 and the working pressure of the accumulator is greater than its minimum working pressure, the accumulator discharges energy and drives the vehicle to start.

• 2. Accumulator working alone mode

When the working pressure of the accumulator is greater than its maximum working pressure, the engine works at idle speed and the output torque is zero; The accumulator discharges energy and drives the vehicle independently.

• 3. The engine is driven and the accumulator is charged

When the working pressure of the accumulator is less than its minimum working pressure, the engine is controlled to work at the optimal fuel consumption point. At this time, the engine charges the accumulator while driving the vehicle.

• 4. Braking energy recovery

When the vehicle decelerates or brakes, the pump/motor works in the pump condition, the accumulator is charged, the engine is in the idle condition or stopped, and the output torque is zero.

5 Simulation of Series Hybrid Vehicle System

5.1 Simulation Model

The simulation model of hybrid vehicle is established in Matlab/Simulink, which includes engine model, clutch model, variable pump model, variable pump/motor model, accumulator model, safety valve model, reducer model, vehicle running model, energy management control model, engine control model, variable pump control model, variable pump/motor control model and driver model. The simulation model is shown in Fig. 2.

Fig. 2

Vehicle system simulation model

5.2 Simulation Conditions

The main parameters of series hydraulic hybrid vehicle are shown in the Table 1.

Table 1 Main parameters of series hydraulic hybrid vehicle

5.3 Analysis of Simulation Results

In this paper, CYC_HWFET is selected as the simulation road condition to test the fuel economy of the rule-based energy management strategy. The simulation results are shown in Figs. 3, 4, 5, 6 and 7.

Fig. 3

HWFET high-speed cycle condition

Fig. 4

Working pressure change of accumulator

Fig. 5

Engine speed

Fig. 6

Actual output torque of pump/motor

Fig. 7

Fuel consumption

Figures 3, 4, 5, 6 and 7 respectively show the changes of vehicle speed, pressure of accumulator, engine speed, actual output torque of pump/motor and fuel usage when the vehicle runs according to CYC_HWFET.

As can be seen from Fig. 4, the vehicle is started by discharging energy from the accumulator. After discharging the accumulator, it can be seen from Fig. 5 that at this time, the engine starts to work at the optimal fuel consumption point, charging the accumulator on the one hand and driving the vehicle on the other. When the accumulator is charged, the engine works at idle speed, and the accumulator is discharged to drive the vehicle alone. This process is circulated throughout the whole journey of the vehicle, until the end of the journey, the pump/motor works in the pump working condition and starts to recover the braking energy, and the accumulator completes the energy recovery synchronously.

Compared with Figs. 4 and 5, it can be seen from Fig. 7 that when the accumulator works independently, the engine is at idle speed and consumes only a small amount of oil. When the engine works at the best fuel consumption point, it consumes a lot of oil because it charges the accumulator and drives the vehicle at the same time.

Referring to Fig. 3, it can be seen from Fig. 6 that whenever the vehicle is in the deceleration state, the pump/motor will work in the pump condition and recover the excess energy to the accumulator.

5.4 Fuel Economy Analysis

As can be seen from Fig. 7, the hydraulic hybrid vehicle runs according to CYC_HWFET road conditions, and the total fuel consumption is 723.5 g. However, the distance travelled by the vehicle according to CYC_HWFET road condition is 16.51 km [4], and the fuel consumption of the vehicle is 1.00 L according to the gasoline density of 0.725 kg/L. From this, it can be further obtained that the average fuel consumption per 100 kms of hydraulic hybrid vehicles is 6.06 L. Compared with the average fuel consumption of small cars of 6.88 Ls per 100 kms [10], the fuel economy of hydraulic hybrid vehicles has improved by 11.9%.

It can be predicted that the fuel economy will be further improved due to the frequent start and stop of vehicles if the urban circular road simulation is adopted.

6 Conclusion

• 1. Based on the rule-based control strategy, four working modes of hybrid vehicle are realized, including vehicle starting, accumulator working alone, engine driving and accumulator charging, and braking energy recovery. Each of these working modes is permeated with the design concept of energy saving. Under the working condition of CYC_HWFET expressway, its fuel economy can be improved by 11.9%.

• 2. The volume of accumulator and the range of working pressure are two important parameters that affect the fuel economy of hydraulic hybrid vehicle. Under the condition of a certain accumulator volume, increasing the working pressure range of the accumulator can not only increase the energy storage of the accumulator, so as to give full play to the energy storage and discharge efficiency of the accumulator in the hydraulic hybrid electric vehicle, but also reduce the idling times of the engine and further improve the fuel economy of the vehicle.