Experimental Investigation of a Novel Dual-mode Power Split System

for Passenger Vehicle

Wei Du, Shengdun Zhao

, Liying Jin, Yangfeng Cao, Jinzhou Gao and Hao Li

School of Mechanical Engineering, Xi’an Jiaotong University, Xi’an 710049, China

904416827@qq.com, wojiaolihao114@stu.xjtu.edu.cn

Keywords: Hybrid electric vehicle, Power split, Planetary gear, Input-split, Compound-split.

Abstract: Hybrid systems are becoming ever more widely used because they can improve the fuel economy of

automobiles. This paper proposes a new hybrid system based on the three row planetary gear train, which can

switch between input-split and compound-split mode according to different road conditions. This paper

introduces its structure and operating mode, and uses the 1.8L displacement Toyota Prius as an example.

According to its design parameters, a prototype is designed and a control strategy based on logic threshold is

established. The operating characteristics of the system were analyzed through a bench test. The experimental

results show that compared with the 1.8L Toyota Prius power train, the novel dual-mode power split hybrid

power train system can reduce fuel consumption by 11.4%.

1 INTRODUCTION

In the next 50 years, the world population will grow

from 6 billion to 7 billion, and car ownership will

increase from 700 million to 2.5 billion (chan, 2001).

If all these cars continue to use engines, the

consequent environmental pollution problems will be

unimaginable (chan, 2002), and these factors force us

to rapidly develop alternative energy-driven

automotive technology. At present, hybrid

technology is one effective way to improve fuel

consumption and reduce emissions.

According to the definition, the hybrid electric

vehicle needs to have two power sources of engine

and electric motor, and can achieve high energy

utilization rate through efficient and precise control

system, which makes it have low fuel consumption,

low emission and excellent advantages such as

dynamic performance (Ehsani, 2007; Lin, 2003;

Sciarretta, 2007 ; Pisu, 2007). According to the

connection form of the internal combustion engine

with the motor and the transmission, the hybrid

electric vehicle can be divided into three types: a

series hybrid vehicle (SHEV), a parallel hybrid

vehicle (PHEV) and a series-parallel hybrid vehicle

(SPHEV) (Meisel, 2006).

In the first generation of the Prius, Toyota

creatively proposed the use of a planetary gear power-

split system for hybrid vehicle (Meisel, 2011). The

system consists mainly of an engine, a planetary gear

train and two motors. Using the planetary gear train

as a power coupling device, the decoupling of the

engine rotating speed and torque can be realized by

two motors and a planetary gear train with two

degrees of freedom, and the power of the engine is

divided into two parts: electric power and mechanical

power. However, the power-split mode of this

configuration is relatively simple, and it is not

possible to switch modes to adapt to different road

conditions (Miller, 2006 ; Liu, 2008).

The dual-mode hybrid system is a hot research

problem in the field of hybrid technology because it

has two power-split modes and can improve fuel

economy in a wider range of transmission ratios. This

paper proposes a novel dual-mode hybrid system. The

Prius was used as an example to design a prototype.

The control strategy based on the logic threshold was

used to test the performance of the system. The

experimental results show that the fuel economy of

the novel dual-mode hybrid system is improved

compared with the Toyota Prius powertrain.

108

Du, W., Zhao, S., Jin, L., Cao, Y., Gao, J. and Li, H.

Experimental Investigation of a Novel Dual-mode Power Split System for Passenger Vehicle.

DOI: 10.5220/0011359000003355

In Proceedings of the 1st International Joint Conference on Energy and Environmental Engineering (CoEEE 2021), pages 108-114

ISBN: 978-989-758-599-9

2 DESCRIPTION OF THE

SYSTEM STRUCTURE AND

OPERATING MODE

The novel dual-mode hybrid system proposed in this

paper has two power-split modes: input-split and

compound-split. The system works in the compound-

split mode in the middle and low gear ratio range, and

works in the input-split mode in the high gear ratio

range. The mode is switched by the control of the

clutches.

The configuration of the novel dual-mode power-

split hybrid system proposed in this paper is shown in

figure 1. The parameters of different parts of the

system are shown in table 1.

Figure 1: Configuration of the novel dual-mode power-split

hybrid system.

Table 1. System parameters.

Component Parameter Value

Vehicle

Weight/kg

1467.4

Wheelbase/mm

2700

Wind resistance

coefficient

0.29

Tire radius/m

0.282

Engine

model

5ZR-FXE

Maximum

power/kW

Maximum

torque/Nm

142

Displacement/L

1.8

MG1

MG1 rated

power/kW

MG1 rated

speed/rpm

3000

MG2

MG2 rated

power/kW

MG2 rated

speed/rpm

3000

Battery

capacity/kWh

1.3

The system has two types of power-split modes:

input-split and compound-split. The corresponding

vehicles have reverse gear, low speed running,

medium and high speed running, rapid acceleration,

braking and other working conditions. The control

system selects different operating modes based on the

power demand signal from the driver, the current

vehicle speed, the battery SOC value, and the current

operating state of the engine and the motors.

According to the working state of the engine, MG1,

MG2 and the engagement or disengagement of the

CL1 and CL2, the system has a total of 10 operating

modes, as showed in table 2.

Table 2: The operating modes of the system.

Operating mode Engine MG1 MG2 Engine and system CL1 CL2

Reverse gear stop stop run disconnected disengaged engaged

Single motor drive stop stop run disconnected disengaged engaged

Dual motor drive stop run run disconnected engaged disengaged

Engine cold start follow run stop connected disengaged engaged

Engine hot start follow run run connected disengaged engaged

Charge run run run /stop connected disengaged engaged

Input-split run run Run connected disengaged engaged

Compoun

-split run run run connecte

engage

disengage

Mild brake stop stop run disconnected disengaged engaged

Heavy braking stop run run disconnected engaged disengaged

Experimental Investigation of a Novel Dual-mode Power Split System for Passenger Vehicle

109

3 CONTROL STRATEGY

Control strategies of existing power-split hybrid

vehicle mainly include logic threshold-based control

strategies, genetic algorithm based control strategies,

fuzzy rule based control strategies, and adaptive

dynamic control strategies (Salmasi, 2007; Sciarretta,

2007). In order to make it easier to be compared and

analyze the experimental results, this paper chooses

the control strategy based on logic threshold. The

control strategy based on the logic threshold is

essentially to map all the states that the vehicle may

encounter during driving to different operating modes

of the hybrid system, and at the same time, set the

discriminating conditions and thresholds for the

transition between different modes. For the novel

dual-mode power-split hybrid system proposed in this

paper, there is a total of 10 effective operating modes

as showed in table 2. The control strategy based on

the logic threshold needs to select the corresponding

operating modes according to the set rules and

determine the power distribution of the engine and the

two motors.

The mode switching rules based on the logic

threshold are as follows: The target operating point

and the permissible working range of the engine are

set according to the efficient operating range of the

engine. Set the battery SOC value working range to

ensure its performance and extend its service life.

According to the current gear position information

and torque demand collected by the sensors,

combined with the battery SOC value, select the

operating mode of the system. The heavy braking

energy recovery mode or the mild braking energy

recovery mode is selected according to the vehicle

speed and the battery SOC. The threshold values of

the main parameters are set as follows.

3.1 Battery SOC Value

The SOC (state of charge) value of expresss the

power level of battery, which is usually represented

by a number between 0 and 1. In the process of using

the hybrid vehicle,as to prolong the service time of

battery, it is necessary to shallowly charge it.

Therefore, in the control strategy based on the logic

threshold, the upper limit SOCmax=0.8 and the lower

limit SOCmin=0.4 are respectively set. In any pattern,

if the current SOC of battery is <SOCmin, the engine

is started and enters the charging mode or the hybrid

driving mode. When the vehicle needs to decelerate

braking, according to the real-time SOC of battery,

the proportion of electric brake participation during

braking is determined; when SOC>SOCmax, the full

hydraulic braking mode is adopted.

3.2 Vehicle Speed Threshold V_ref

Both the engine and the MG1 and MG2 have their

maximum speed limits, and each has a speed range

corresponding to the efficient working area.

Therefore, different speed thresholds need to be set so

that the vehicle always runs in the most suitable mode

while driving. Set the vehicle speed threshold

v_ref1=30km/h, v_ref2=60km/h. The engine is

inefficient at idle speed. Therefore, when SOC value

of the battery is greater than its lower limit value, the

engine only run when the vehicle speed is greater than

the threshold value v_ref1. When the vehicle speed is

lower than v_ref1, it operates in pure electric mode.

The input-split mode is suitable for low speed.

Therefore, when the vehicle speed is in the range of

(v_ref1, v_ref2), the input-split pattern is

preferentially chosen; if the vehicle speed is greater

than v_ref2, switch to compound-split mode.

3.3 Demand Torque T_req Threshold

When designing and optimizing a hybrid system, not

only its fuel economy but also the dynamic

performance of the vehicle must be met. The real-

time torque required during vehicle travel is derived

from the opening of the throttle and brake pedals and

represents the instantaneous power demand. The

control strategy of this paper linearly converts the

signal of the accelerator pedal opening and the

demand torque, and assigns it to the engine and two

motors with the goal of fuel economy. While the

engine, MG1 and MG2 have their upper limit of

output torque and the optimal output torque range, so

we should set the threshold for the demand torque as

the evaluation standard for mode switching.

First,it is judged that it is currently in the drive

mode, the parking mode or the braking mode

according to 𝑇_𝑟𝑒𝑞 > 0, 𝑇_𝑟𝑒𝑞 = 0,or T_req<0. In

the drive pattern: if the battery SOC>SOCmin, the

vehicle speed v<30km/h, the vehicle runs in pure

electric drive mode. When 𝑇_𝑟𝑒𝑞 < 30𝑁𝑚, a single

motor drive, when T_req>30Nm, double motor drive.

When the instantaneous torque demand T_req>90

Nm, the engine starts to work and provides torque.

When T_req=0 and the duration is greater than 3s, the

vehicle enters the parking or coasting mode. At this

time, if the battery SOC value is greater than the

lower limit threshold, the engine stops. In the braking

mode, the mechanical brake, single motor auxiliary

electric brake or dual motor auxiliary electric brake

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mode is selected rely on the vehicle speed and battery

SOC value.

3.4 Validation Test

Figure 2 is the model of the novel dual-mode power-

split hybrid system, including MG1, three-row

planetary gear train, engine input shaft and MG2

motor input shaft. In order to simplify the structure,

the MG1 rotor is designed as a hollow structure,

directly connected to the ring gear of the second

planetary gear train. Considering the experimental

verification of the hybrid system, the two clutches

showed in figure 1 need to be equipped with hydraulic

or electronic control units separately, which is

difficult and costly. At the same time, the structure of

the three sets of planetary rows is too complicated.

Therefore, in the structural design and subsequent

experimental research, the structure consisting of two

sets of planetary rows with dual output ports is used,

and the reduction ratio of the third row of planetary

transmissions is realized by the sprocket. During

system testing, the current operating mode can be

selected as input-split or compound-split through

different output port.

Figure 2: The 3D model of the novel dual-mode power-split

hybrid system.

Figure 3: The model of the hybrid system test bench.

In order to test the speed and torque of each

component of the hybrid system under different

working modes and to test the fuel economy of the

system, the experimental platform of the hybrid

system is built according to figure 3. The physical

diagram is shown in figure 4. It includes a main motor

for simulating the engine, an experimental device, a

torque sensor for testing the output speed and torque

signal, an adjustable inertia disk for simulating the

vehicle's driving inertia, and a magnetic powder

brake. The hybrid system includes a flux switching

permanent motor(FSPM) as MG1,a permanent

magnet synchronous motor(PMSM) as MG2, and a

power split device composed of two rows of planetary

gear trains that are connected to the subsequent test

unit through a sprocket of the first or the second

output port. The control system mainly consists of

three servo motor drivers, two upper computers and

one motion control card. The upper computer

connected to the control card sends the speed and

torque control commands to the servo motor driver by

means of the motion control card, and reads the

current and speed signals collected by the driver. The

industrial computer is used to control the magnetic

powder brake, and read the speed and torque signals

measured by the torque sensor.

Figure 4: Hybrid system test bench.

In order to verify the performance of the novel

dual-mode power-split hybrid system, the fuel

economy experiment was carried out on the system.

The fuel economy experiment was carried out under

the NEDC cycle condition (Wang, 2013).The target

and actual vehicle speed in the experimental process

are shown in figure 5. It can be seen from the figure

that the presence of the motor effectively improves

the acceleration and deceleration performance of the

hybrid system, so the actual vehicle speed and the

target speed basically coincide.

Figure 5: Comparison of target speed and actual speed.

Experimental Investigation of a Novel Dual-mode Power Split System for Passenger Vehicle

111

(a)

(b)

(c)

Figure 6: The speed, torque and power of the engine:(a)

variation of engine speed with time;(b) variation of engine

torque with time;(c) variation of engine power with time.

The engine speed, torque and power during the

experiment is shown in figure 6. As we can see from

figure 6(a), since there are two motors in the hybrid

system, the vehicle is mainly driven by the motor at

low speed. Under normal urban conditions, the engine

does not work most of the time, and only starts

charging the battery when the battery SOC value is

low.

In order to analyze the performance of the two

motors, the curves of the speed and torque of the two

motors are obtained during the experiment. As is

shown in figure 7, in low speed case (less than

30km/h), MG1 does not work and torque of it is 0

Nm. In this case, MG2 is used as the drive motor. In

the case of medium speed, when the battery SOC is

greater than 0.4, MG1 and MG2 drive the vehicle

together. When the battery SOC is less than 0.4, the

engine is started. At this time, the engine speed and

torque are adjusted by MG1 and MG2, so that the

engine always works in high efficiency.

(a)

(b)

(c)

(d)

(e)

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(f)

Figure 7: The speed, torque and power of two motors:

(a)variation of MG1 speed with time; (b) variation of MG1

torque with time; (c) variation of MG2 speed with time; (d)

variation of MG2 torque with time; (e) variation of MG1

power with time; (f) variation of MG1 power with time.

Figure 8: The SOC value of battery.

Limited by the experimental conditions, the

battery is replaced by a simulation model. According

to the measured power of the engine, MG1, and MG2,

theoretical SOC of the battery can be calculated, and

the variation curve is shown in figure 8. In pure

electric mode, the energy of the motor is provided by

the battery, the SOC value decreases, and when the

vehicle speed increases, the system is in the engine

drive mode. If the SOC value is lower at this time,

part of the energy of the engine is used to drive the

car, and the other part is used to charge the battery to

increase the SOC of the battery. When the system is

in a braking state, braking energy is recovered by

MG1 and MG2 and stored in the battery.

In order to evaluate the fuel economy of the

proposed novel dual-mode power-split hybrid

system, the equivalent fuel consumption of the Prius

hybrid system and the novel dual-mode power-split

hybrid system is compared under the same driving

cycle. As showed in table 3, the novel dual-mode

power-split hybrid system can save fuel consumption

by 11.4% compared to the Prius hybrid system. It can

be seen that the novel dual-mode power-split hybrid

system can effectively improve fuel economy.

Table 3. Comparison of fuel consumption rate.

Fuel

consumption(L/100km)

Energy

saving

rate

Prius 4.3

Novel dual-

mode power-

split hybrid

system

3.81 11.4%

5 CONCLUSION

This paper proposes a novel dual-mode power-split

hybrid system based on a three-row planetary gear

train, which is mainly composed of an engine, a three-

row planetary gear train and two motors. After

analyzing the configuration and operating mode of

the system, taking the Toyota Prius as an example, a

prototype was made and a bench test was carried out.

The experimental results show that the fuel economy

of the novel dual-mode power-split hybrid system is

11.4% higher than the Toyota Prius. It can be seen

that the novel dual-mode power-split hybrid system is

a good hybrid solution.

ACKNOWLEDGMENTS

This work was jointly supported by the Fundamental

Research Funds for the National Key Research and

Development Program of China (Grant

No.2017YFD0700200) and the National Natural

Science Foundation of China for key Program (Grant

No. 51335009).

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