Design and Strategy of Series-Parallel Hybrid System Based on BSFC

Huien Gao, Liang Chu, Jianhua Guo, and Dianbo Zhang

(1.State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130022, China; 2.Jilin Provincial Institute of Standards, Changchun 130022, China)

Abstract: In this paper, a drive control strategy is developed based on the characteristics of series-parallel plug-in hybrid system. Energy management strategies in various modes are established with the basis on the minimum brake specific fuel consumption (BSFC) curve of engine. The control strategy, which is based on rules and system efficiency, is adopted to determine the entry/exit mechanisms of various modes according to battery state of charge (SOC), required power and required speed. The vehicle test results verify that the proposed control strategy can improve vehicle economy efficiently and makes a good effect on engine control.

Key words: series-parallel; plug-in hybrid electric vehicle (PHEV); based on rules; based on system efficiency

Plug-in hybrid electric vehicle (PHEV), as a new generation of hybrid electric vehicle, is becoming increasingly popular in the global automotive market[1]. PHEV is derived from traditional hybrid electric vehicle. Because it has a larger battery capacity and can be charged directly from the state grid, it can significantly reduce the vehicle fuel consumption and emissions[2]and therefore has become a feasible energy saving and environmental protection solution. PHEV system can be categorized into three types, i.e., series type, parallel type and series-parallel combined type[3], in which the series-parallel combined type can be further divided into power-split type and series-parallel type.

At present, many scholars at home and abroad have conducted a lot of research on the series-parallel and power-split PHEV. Carla and others[3]studied the energy consumption, emissions and cost of PHEV; Torres et al. conducted multi-objective optimizations for the power component size of PHEV based on the vehicle fuel consumption and acceleration[4]; Zhou Nenghui et al. proposed a multi-stage multi-objective control strategy for PHEV based on its characteristic[5], different battery status and demands of the vehicle power battery to improve the fuel efficiency. Qiu Lihong et al. established a control strategy based on logical threshold values for different drive modes[6]of 4WD PHEV under different working conditions, and it was simulated with cruise. Chen Shanglou et al.[7]presented an energy management strategy which has been applied to a series-parallel hybrid electric vehicle. The equivalent fuel consumption rate is adopted as a uniform standard of fuel economy. Zhang Song et al.[8]proposed an energy management strategy of DCT-based series-parallel PHEV.

Series-parallel PHEV has the advantages of both series PHEV and parallel PHEV. However, there are relatively few researches on this type of PHEV, especially no such mass-produced models in China.

Based on the above research, this paper takes series-parallel PHEV as the research object, and investigates its drive control strategy, hoping to provide some technical references for OEM to develop series-parallel PHEV.

1 Power System Configuration and Working Mode

1.1 Power system configuration

The power system configuration of the series-parallel PHEV is shown in Fig.1[9].

The target model is defined as Grade B and the relevant parameters of the vehicle are shown in Tab.1.

Tab.1 Relevant parameters of the vehicle

The main components of the target model include Atkinson cycle engine, generator, drive motor, lithium-ion battery, clutch, torque coupler, final drive, etc. The detailed parameters of the main components are shown in Tab.2.

1.2 Working mode

According to the characteristics of series-parallel PHEV power system, the working mode of the vehicle under driving conditions can be divided into EV mode, series mode, parallel mode.

Tab.2 Detailed parameters of the main components

① EV mode. In EV mode, the engine and generator are in “off” position. The driving force of wheel is provided by the drive motor and all of the vehicle energy comes from the battery. The energy flow in EV mode is shown in Fig.2.

② Series mode. In series mode, the engine, generator and drive are in a working state. The driving force of the vehicle is provided by the drive motor. The engine drives the generator to generate electricity to provide the energy for the vehicle. The power battery works in the state of discharge or charge as required so as to adjust the working point of the engine. The energy flow in series mode is shown in Fig.3.

Fig.2 Energy flow in EV mode

③ Parallel mode. In parallel mode, the engine is in working state and the generator is in “off” position. The clutch is engaged. The drive motor determines its working state according to the drive requirement of the vehicle. The battery’s working state also depends on the drive requirement of the vehicle. The driving force of the vehicle is jointly provided by the engine and drive

motor. The vehicle energy comes from the engine and battery. The energy flow in parallel mode is shown in Fig.4.

Fig.3 Energy flow in series mode

Fig.4 Energy flow in parallel mode

The working status of various power components in the three working modes are shown in Tab.3.

Tab.3 Working status of various power components

2 Research on Drive Control Strategy

2.1 Energy management strategy

2.1.1Energy management strategy in EV mode

The energy management strategy in EV mode is relatively simple. The vehicle energy comes from the power battery. There is no problem on distribution of multiple energy sources. The energy management strategy is shown in Fig.5.

In this model, the relationship between the components of power assembly is shown as whereTreqis the required torque;Tmotis the motor torque;Pbatis the battery power;Pmotis the motor power;ηeleis the efficiency of electrical system;ωmotis the motor speed;vis the vehicle speed;ieleis the transmission ratio in EV mode;ris the wheel radius.

(1)

Fig.5 Energy management strategy in EV mode

2.1.2Energy management strategy in series mode

The energy in series mode is mainly from the engine. At the same time, according to the power battery’s state of charge (SOC), the engine’s working point can be adjusted through the charge/discharge of power battery, which involves the distribution of two energy sources. Therefore, it is necessary to study the control of the engine’s working point and the power battery’s charge/discharge power in the series mode.

① Minimum BSFC curve of engine

In order to enable the engine to work in the high efficiency zone on its universal characteristic curve, it is required to find the minimum brake specific fuel consumption (BSFC) curve, that is, when working along this curve, the engine works at the highest efficiency at the same output power. To find the minimum BSFC curve, firstly draw an equipower line on the universal characteristic curves which are obtained by engine performance tests according to the standard GB/T 18297; then, find the tangent point of the equal fuel consumption curve tangent to the equipower line and this point is the most efficient working point of the engine at this power. Draw the tangent points of all equipower lines in turn and connect them to get the minimum BSFC curve of the engine[10]. The minimum BSFC curve of the engine is shown in Fig.6.

Fig.6 Minimum BSFC curve of the engine

② Control of the engine’s working point

After the required vehicle power is determined, the intersection point between the equipower line of the required vehicle power and the minimum BSFC curve of the engine will be the preset working point of the engine, as the red dot shown in Fig.7.

Fig.7 Preset working point of the engine

In order to make the engine run more efficiently, the working point of the engine should be close to the high efficiency zone on the minimum BSFC curve as much as possible through the compensation function of the power battery[11]. That is, when the required vehicle power is greater than the power of engine working in the high efficiency zone on the minimum BSFC curve, the power battery should be discharged at a certain power to adjust the working point of the engine to move to the high efficiency zone on the minimum BSFC curve[12]; when the required vehicle power is less than the power of engine working in the high efficiency zone on the minimum BSFC curve, the working point of the engine should also be adjusted to move to the high efficiency zone on the minimum BSFC curve and the excess energy will be stored in the power battery. It should be pointed out that when the working point of the engine needs to be adjusted through battery charge/discharge, it is required to consider the SOC of power battery to avoid over-charge or over-discharge of power battery, so as not to affect the battery life. Adjustment of the working point of engine in series mode is shown in Fig.8.

Fig.8 Adjustment of the working point of engine

③ Charge/discharge power control of power battery

As the working point of engine needs to be

adjusted through charge/discharge of power battery, it is required to investigate the charge/discharge power of the power battery. The SOP of power battery is often used to show the continuous charging peak power of the power battery in the current state. This parameter is mainly related to the capacity, internal resistance, SOC and temperature of the power battery, etc[10]. In this paper, only the effect of SOC on SOP is considered and their relationship is determined by experiments. The power battery is tested in accordance with the requirements of relevant national standard[13]and some test results are shown in Fig.9 and Fig.10.

Fig.9 Test curves under 25% SOC

Fig.10 Test curves under 30% SOC

The corresponding relationship between SOC and SOP can be obtained by sorting out the power test data under different SOC test conditions, as shown in Tab.4.

Tab.4 Relationship between SOC and SOP

In this paper, SOC threshold value of the power battery is set as 0.3 when it enters CS stage from CD stage. It should be noted that the engine is allowed to charge the power battery by generator only in CS stage in order to reduce the fuel consumption of the engine[14].

In series mode, the charge/discharge power of power battery can be determined by the relationship between the required vehicle power and the engine power at the optimal working point. In CD stage, operating capacity of battery is stronger. However, in order to slow down the battery discharge rate, the maximum operating power of the battery is limited to 25 kW. Determination of the SOP of power battery is shown in Fig.11.

Fig.11 Determination of battery status (in CD stage)

In CS stage, the operating capacity of battery is relatively weak and its maximum operating power is limited to 20 kW. Meanwhile, to maintain the balance of battery SOC, the power battery needs to be charged. The maximum charge

power of power battery is preset to be 7 kW to reduce the fuel consumption of the engine. Determination of the SOP of power battery is shown in Fig.12.

In summary, the energy management strategy in series mode is shown in Fig.13.

Fig.12 Determination of battery status (in CS stage)

Fig.13 Energy management strategy in series mode

In series mode, the relationship among the components of power assembly is shown as

(2)

wherePengis the engine power;ηemis the efficiency from engine to motor.

2.1.3Energy management strategy in parallel mode

In parallel mode, the engine is coupled with the wheel by two pairs of transmission gears, that is, there is a proportional relationship between the engine speed and vehicle speed. It is difficult for the engine to always work in the narrow high efficiency zone. The output torque of engine is adjusted by controlling the drive motor working in the state of motor driven/electricity generation, so as to control the engine to work along the BSFC curve. That is, when the required vehicle torque is below the BSFC curve, the output torque of engine should be increased to make the engine work close to the BSFC curve. The excess engine torque will be absorbed by the drive motor in power generation mode and the electric energy generated by the drive motor will be stored in the power battery; when the required vehicle torque is above the BSFC curve, the output torque of engine should be reduced and the reduced part will be compensated from the drive motor[15]. Adjustment of the working point of engine is shown in Fig.14.

Fig.14 Adjustment of the working point of engine

Determination of battery status in parallel mode is similar to that in series mode and will not be described. The energy management strategy in

parallel mode is shown in Fig.15 and the relationship among the components of power assembly is shown by

(3)

whereηmzis the transmission efficiency from the motor output to the torque coupler;ηezis the transmission efficiency from the engine output to the torque coupler;ibmis the total transmission ratio from engine output to wheel.

Fig.15 Energy management strategy in parallel mode

2.2 Mode selection strategy

The vehicle can work in a variety of drive modes. In order to achieve high-efficiency operation of vehicle, this paper adopts the strategy based on rules and system efficiency to select the drive mode for the vehicle.

In order to minimize the use of the electric energy stored in the power battery and save fuel consumption, the vehicle will work in EV mode preferentially; when EV mode cannot meet the drive requirement of the vehicle, the vehicle will select the series mode or parallel mode based on the system efficiency[16].

2.2.1Entry/exit mechanism of EV mode

The entry/exit mechanism of EV mode is affected by the required vehicle power, battery SOC and vehicle speed. The SOC threshold value of the power battery is set as 0.3 when it enters CS stage from CD stage and the lower limit of discharge is 0.24. At this time, the vehicle will not be able to enter the EV mode. The power characteristic test of power battery shows that when the power battery is in CS stage, the maximum continuous discharge power of the power battery is 40 kW; When the power battery is in CD stage, the maximum continuous discharge power of the power battery is 60 kW. The drive capability in EV mode is limited by the discharge power of the power battery. The maximum vehicle speed in EV mode is also limited to limit the discharge rate of the power battery and improve the service life of the power battery. At the same time, since the economic vehicle speed range is 70-140 km/h when the vehicle is directly driven by the engine, the use of the electric energy of the power battery is preferred to be minimized. In this paper, the maximum vehicle speed in EV mode is set to be 100 km/h. To sum up, the entry/exit mechanism of EV mode is shown in Fig.16.

Fig.16 Entry/exit mechanism of EV mode

2.2.2Entry/exit mechanism of series/parallel mode

The common ground of series mode and parallel mode is that the final energy source driving the vehicle mainly comes from the engine. The difference is that the former converts the energy generated by the engine into electric energy and supplies it to the drive motor to move the vehicle, while the latter directly outputs the energy generated by the engine to the vehicle[17]. The series model is designed to decouple the working point of engine from the working conditions so as to control the engine to work efficiently; the parallel mode is designed to avoid the long-term secondary energy conversion during medium and high speed cruising of the vehicle in series mode, which reduces the system efficiency; if the engine is used to directly drive the vehicle and the engine works in the medium load zone with a higher efficiency, the system efficiency will be greatly improved. Therefore, the series and parallel modes are suitable for different driving conditions and the vehicle will choose the drive mode based on the system efficiency under different working conditions.

The system efficiency in series mode is calculated according to

ηc=ηeng_cηgηmηptr

(4)

whereηcis the system efficiency in series mode;ηeng_cis the engine thermal efficiency in series mode;ηggenerating efficiency of generator;ηmis the efficiency of drive motor;ηptris the efficiency of drive system.

The system efficiency in parallel mode is calculated according to

ηb=ηeng_bηptr

(5)

whereηbis the system efficiency in parallel mode;ηeng_bis the engine thermal efficiency in parallel mode.

Selection of series/parallel mode is shown in Fig.17.

Fig.17 Selection of series/parallel mode

3 Vehicle Test Verification

To verify the effectiveness of the drive control strategy proposed in this paper, the energy consumption test of vehicle is carried out according to the requirements of national standards. The calculation of energy consumption in the standards is shown as

(6)

(7)

whereCis the weighted average of fuel consumption;C1is the fuel consumption under condition A (unit: L/100 km);C2is the fuel consumption under condition B (unit: L/100 km);DOVCis the OVC endurance mileage (unit: km), rounded;Davis the 25 (unit: km);Eis the weighted average of power consumption;E1is the power consumption under condition A (unit: Wh/km);E2is the power consumption under condition B (unit: Wh/km);

Calculation ofE1is shown as

(8)

wheree1is the electric energy of vehicle obtained from the power grid after the test is finished under condition A (unit: Wh);Dtest1is the actual running distance under condition A, equal to the calculatedDOVC.

Five NEDC cycles are tested under condition A ande1=4 000 Wh is obtained from the power grid. The test data is shown in Tab.5.

Tab.5 Test data under condition A

Calculation is shown as follows:

E1=4 000/55=72.727(Wh/km)

Three NEDC cycles are tested under condition B ande2=-500 Wh is obtained from the power grid. The test data is shown in Tab.6.

Tab.6 Test data under condition B

Calculation is shown as follows:

Therefore, the weighted value of fuel consumption is shown as

The weighted value of power consumption is shown as

The energy consumption test results show that the comprehensive fuel consumption per hundred kilometers of the vehicle under NEDC condition reaches 3.80 L and the power consumption per hundred kilometers is 4.526 5 kWh, which meets the economic index design requirements of the vehicle.

In order to investigate the control effect on the engine, the engine working points under the five NEDC conditions in the energy consumption test are compared with the engine BSFC, as shown in Fig.18.

Fig.18 Working points of engine

From Fig.18, it is obvious that under NEDC cycle condition, the working point of engine basically varies along the BSFC curve of engine, which indicates that the engine basically works in its high efficiency zone, achieving a good control effect on the engine.

4 Conclusions

To make the engine work in the high efficiency zone, the energy management control strategy is established for various drive modes of series-parallel PHEV, and then the mode can be selected by using the control strategy and system efficiency based rules to make the vehicle work in a mode with higher system efficiency under different working conditions.

In the series mode， for making the engine run more efficiently, the control strategy makes the working points of the engine close to the high efficiency zone on the minimum BSFC curve as much as possible through the compensation function of the power battery. In the parallel mode, the control strategy adjusts the engine operating point near the BSFC curve by controlling the motor output torque

In order to achieve high-efficiency operation of vehicle, the mode selection was proposed based on rules and system efficiency. The vehicle test results show that the control strategy proposed in this paper has a good control effect on the engine and can significantly improve the fuel efficiency of the vehicle.