An Efficient Energy Regeneration System for Diesel Engines

EVS25

Shenzhen, China, Nov 5-9, 2010

Ying HUANG1, Fuyuan YANG1, Minggao OUYANG1, Lin CHEN1, Guojing GAO1, Yongsheng He2 1 Department of Automotive Engineering, Tsinghua University, 100084, Beijing China, [email protected] 2General Motors Global Research and Development, Warren, Michigan 48309, USA

Abstract

In order to further improve the fuel economy of vehicles, an efficient energy regeneration system for diesel engines is designed and constructed. An additional automatic clutch is added between the engine and the motor in a conventional ISG (Integrated Starter and Generator) system. During regenerative braking, the clutch can be disengaged and the engine braking is avoided. Control strategy is redesigned to determine the braking torque distribution and coordinate all the components. The generated electricity is increased and the fuel economy is improved. An experiment is designed to test the performance of the new energy regeneration system, and the result shows that during a regenerative braking test over 112% more electric energy can be regenerated without correcting for the fuel consumption at idle. Theoretical analysis also shows that the regenerated energy would be increased at least 63.1% for a single regenerative braking event with the new system, if we correct for the fuel consumption at idle.

Keywords: ISG, Regenerative Braking, Automatic Clutch, Control Strategy

urban driving cycle [1]. Therefore, braking energy 1 Introduction regeneration system has a great potential to enhance the energy efficiency and improve the Fuel economy and environmental friendliness are fuel economy. becoming the major topics in vehicle technology Energy regeneration technology is mainly used in nowadays. Research shows, almost the half of hybrid propulsion systems. ISG (Integrated Starter energy is consumed by braking in the EPA75 and Generator) is a popular solution to achieve

hybrid power and energy regeneration due to its In order to further improve the energy simple and low cost configuration. regeneration efficiency, an additional automatic Dr. Tian [2] has carried out some relevant research clutch is added between the engine and the motor on regenerative braking technology in diesel [3]. Therefore, the rotor of ISG system is no engine ISG system. In the conventional ISG longer directly connected to the crankshaft of the system wherein the rotor is directly connected to engine. During regenerative braking, the clutch the engine crankshaft, engine braking cannot be can be disengaged and the engine braking is avoided during braking process. This consumes a avoided. Besides, the ISG motor can be supported significant part of the braking energy and reduces at both sides to increase the rigidity. The vibrating energy regeneration efficiency in the hybrid power impact from the crankshaft to the ISG rotor is system. decreased. The working environment for the ISG To solve the problem, a new energy regeneration motor is improved and its lifetime can be extended. system configuration is constructed in this paper. Figure 2 shows the new ISG hybrid system for Control strategy is investigated. Experiments and diesel engines, with the system setup in the lab (a) theoretical analysis are carried out to verify the and the actual picture (b). An ultra-capacitor is new energy regeneration system and its control used as the energy storage unit and a permanent strategy. magnet brushless motor is used as the ISG motor.

Ultra- Inverter 2 New Energy Regeneration Capacitor

System Configuration

Figure 1 shows a schematic of a conventional ISG Dyno hybrid system. In the conventional ISG system, the ISG motor is integrated at the end of engine Automatic Engine Clutch Motor crankshaft. The rotor is connected to the (a) crankshaft directly. The energy storage unit, either a battery or an ultra-capacitor, is connected to the motor through an inverter. This kind of configuration is compact and simple. But the engine speed and the motor speed remain the same all the time, which makes engine braking unavoidable and consumes energy during regenerative braking.

Energy Storage Inverter Unit (b)

Figure 2: New ISG hybrid system

Transmission The characteristics of relevant components and System dynamometer are specified in Table 1.

Engine Motor

Figure 1: Conventional ISG hybrid system

Table 1: System specifications contribute to the total braking torque demand through brake energy regeneration. But the engine Compon braking torque still exists in the conventional ISG Parameters Value ents system.

Tbk = Tm ech + Treg + Tf eng (2) Engine Type Diesel Engine, 4 where Treg is the regenerative braking torque Cylinders, 2.5 L Max. Power / kW from ISG motor. 87 Rated Speed / rpm The total braking torque demand T b k is 3800 interpreted from the driver’s intention. The engine Max. Speed / rpm braking torque Tf eng is determined by the engine 4250 working condition. ISG system can distribute the ISG Motor Type Permanent Magnet, braking torque between mechanical braking Motor Synchronous system and energy regeneration system, in order Rated Power / kW to maintain the total braking effect and maximize 15.7 Rated Voltage / V the regenerative energy. 250 Base Speed / rpm In the new ISG system, the automatic clutch 1000 can be disengaged during braking. Engine Max. Speed / rpm crankshaft and vehicle transmission system are 4000 separated. Therefore, engine braking is avoided Ultra-ca Equivalent 21 and no longer provides braking torque.

pacitor Capacitance / F ''' TTbk mech T reg (3) Resistance / mΩ 54 If the total braking torque demand keeps the Max. Voltage / V 375 same, the engine braking torque in the Dynamo Type HORIBA Dynas3 conventional ISG system should be compensated meter HT350 by the mechanical braking system and the energy Rated Power / kW regeneration system. The regenerative braking 350 Torque Precision / torque is increased. And more energy can be Nm 1 regenerated. As a result, new energy regeneration system provides a possibility to increase the regenerative braking efficiency of the ISG system. 3 Control Strategy 3.2 Control Strategy 3.1 Braking torque distribution In the new energy regenerative system, with more control components, the system complexity is During the braking process in a conventional increased, and the control strategy should be vehicle without energy regeneration, the driver’s redesigned in order to coordinate all these braking torque demand T b k is satisfied by the components and make the best use of its mechanical braking torque Tm ech from the advantages. braking system and the engine braking torque When the system enters a state of regenerative Tf eng . braking, the engine should stop fuel injection Tbk = Tm ech + Tf eng (1) immediately. At the same time, the automatic With an ISG system, ISG motor can also clutch should be disengaged. After the

disengagement is accomplished, the engine braking torque disappears, and the total braking torque demand should be redistributed. The redistribution of braking torque is determined by the state of the automatic clutch, as shown in Figure 3. When the clutch is engaged, the situation stays unchanged as in the conventional ISG system. The regenerative braking torque is still T reg. When the clutch is disengaged, the engine braking torque no longer exists. And this part Figure 4: Engine braking torque characteristic should be compensated as much as possible by the regenerative braking torque, so that the driver keeps the same feeling about the vehicle deceleration. Thus more braking energy can be regenerated. The engine braking torque is determined by the current engine speed and oil temperature through a calibration map, shown in Figure 4. Meanwhile, the regenerative braking torque is limited by the capacity of the ultra-capacitor and the motor speed. The ultra-capacitor cannot be overcharged and the Figure 5: Maximum motor braking torque motor braking torque cannot exceed its maximum value, shown in Figure 5. ECU recalculates the 4 Experiments and Results distribution of the braking torque, and controls the An experiment is carried out to test the mechanical braking system and the ISG motor performance of the new energy regeneration accordingly. system and its potential to increase the ’ Tbk Tmech - regenerative braking efficiency. Engine Speed 2D Tf_eng At first the engine runs at a speed of 3400 rpm. Oil Temperature The required torque output drops to 0 suddenly,

Treg Disengaged ’ + Lim Lim Treg and the engine control system enters a state of

Engaged Clutch State “Overrun”. Regenerative braking starts and two

SOC of Ultra-capacitor strategies are used: Motor Speed Automatic clutch control is enabled. The clutch would be disengaged during the state of Figure 3: Braking torque redistribution algorithm “Overrun”, and engine braking is avoided. Automatic clutch control is disabled. Engine braking is available during the state of “Overrun”. The result is shown in Figure 6. With strategy ①, the automatic clutch is disengaged successfully at a speed around 3000rpm. And the curves of engine speed and ISG motor speed separate from each other. The regenerative braking torque is

increased from around 30N·m to more than quantity Q is: 60N·m. The clutch is engaged again when the ISG 1 motor speed drops to near idle speed. During the Tnn() whole process, the regenerative energy with Qbt fengidleidle_ (4) 119550 i strategy ① isWKJregen1 256 , and that with where b is indicated specific fuel consumption strategy ② when automatic clutch is disabled i isWKJregen2 115 . Without considering the fuel (ISFC, gkWh/( )) consumption at idle, strategy ① can regenerate The regenerated energy W with strategy ①: over 112% more energy. reg1

t1 Tnt' () Wdt reg e reg1 0 9550

t1 [(())]()TT ntnt reg f_ eng e e dt (5) 0 9550

With strategy ②:

Q2 0 (6)

t2 Tnt() Wdt reg e (7) reg2 0 9550 The regenerated energy with strategy ① is

increased by Wreg . Considering the rotational

inertia of the engine is much smaller than that of Figure 6: Energy regeneration process during the vehicle, the deceleration of the vehicle can be "Overrun" (Comparison of two strategies)

However, it should be noticed that during the taken as the same, i.e.: tt12 : braking process, the engine maintains idle speed with strategy ①, while the fuel injection is shut off WWWreg reg1 reg2 because of “Overrun” state with strategy ②. So t1 the increase of regenerative energy is Tntntfenge_ (())() e dt (8) accompanied by increased idle fuel consumption. 0 9550 This fuel consumption should also be taken into consideration when the regenerative braking Treg is the regenerative braking torque in the efficiency is evaluated. conventional ISG system. In the experiment

5 Theoretical Analysis described in section 4, TNmreg 30 . Tfeng_ is

the engine braking torque. Its characteristic curve With strategy ①, the engine runs at idle speed is already shown in Figure 4. during the braking process, the consumed fuel

Therefore it can be concluded: this case:

TT W W (9) WE reg f_reg2reg eng reg 1 63.1% (13) Wreg2 Wreg 100% (10) Therefore, even when the fuel consumption at idle W reg2 is considered, the regenerated energy is increased Strategy ① can regenerate energy more than at least 63.1% with the new energy regeneration 100%. system.

However, with strategy ①, more fuel Q is 1 6 Conclusion consumed. If this amount of fuel is used to charge the In conclusion, a new energy regeneration system ultra-capacitor, the corresponding electric energy with an automatic clutch between the engine and can be described as: ISG motor is investigated to avoid engine braking consuming energy during braking process. The EQH11ueISG control strategy is designed to coordinate all the relevant components to achieve an efficient Tnnt() fengidleidleeISG_1 (11) braking torque distribution. Experiment and 9550i theoretical analysis both show that the regenerated energy would be increased significantly with the where is the lower heating value of diesel Hu new energy regeneration system, even with the fuel consumption at idle. This fuel consumption at fuel, and are the effective efficiency and e ISG idle could be avoided by shutting down the engine charging efficiency of ISG when the engine output with an appropriate algorithm in future research. power is used to charge the ultra-capacitor through References ISG, i is the indicated efficiency at idle.

[1] Y.GAO, L.CHEN, M.EHSANI, Investigation of the WEreg 1 E 1 1 effectiveness of regenerative braking for EV and HEV, WW reg2 reg2 SAE International SP-1466, 1999-01-2910. [2] S.Tian, Optimal Control of ISG Hybrid System with Tnntf_1 eng() idle idle e ISG / i 1 Diesel Engines in Transient State, Tsinghua University, Ttnreg 2()/2 2max n 2min 2008 2n 1 idle e ISG [3] G.Gao, Configuration Design and Control of Hybrid ()nn2max 2min i Power System in Hybrid Engine, Tsinghua University,

2nidle 2009 1 m ISG ()nn2max 2min (12) With nrpmidle 900 , nrpm2max 3400 ,

nrpm2min 900 , m 98% , ISG 90% in

Mr. Guojing Gao Authors State Key Laboratory of Automotive Safety and Energy, Tsinghua

Mr. Ying Huang University, 100084, Beijing, China State Key Laboratory of Automotive Tel: +86-(0)10-62781830 Safety and Energy, Tsinghua Email: [email protected] University, 100084, Beijing, China Mr. Guojing Gao is a Master’s Tel: +86-(0)10-62781830 candidate in Tsinghua University Email:[email protected] working on Hybrid Powertrain Control. Mr. Ying Huang is a Master’s candidate in Tsinghua University working on Diesel Engine Management System. Dr. Lin Chen State Key Laboratory of Automotive Safety and Energy, Tsinghua University, 100084, Beijing, China Prof. Fuyuan Yang Tel: +86-(0)10-62781830 State Key Laboratory of Automotive Email: chenlin00 @mails.thu.edu.cn Safety and Energy, Tsinghua Dr. Lin Chen is recently graduated University, 100084, Beijing, China from Tsinghua Univ. He will be Tel: +86-(0)10-62795828 working at section of Powertrain Email: [email protected] Advanced Engineering in SAIC R&D Major: Electronic Engine Control center, focusing on the research of Technology engine performance development.

Prof. Minggao Ouyang Dr. Yongsheng He State Key Laboratory of Automotive Propulsion Systems Research Lab, GM Safety and Energy, Tsinghua Global Research & Development, MC: University, 100084, Beijing, China 480-106-252 Tel: +86-(0)10- 62785966 30500 Mound Rd, Warren, MI 48090, Email: [email protected] USA Prof. Minggao Ouyang is the director Tel: +1 (586) 291-6852, of State Key Laboratory of Fax:+1 (586) 986-0176 Automotive Safety and Energy. His Email: [email protected] research interests include Electronic Dr Yongsheng is a Staff Researcher Engine Control Systems, Hybrid at GM R&D working on Hybrid Power Systems, and New Energy Powertrain, Automotive Engines, and Power System. Emission Control.

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