Torque Coordinated Control in Engine Starting Process for a Single-Motor

Special Issue Article

Advances in Mechanical Engineering 2017, Vol. 9(7) 1–10 Ó The Author(s) 2017 Torque coordinated control in engine DOI: 10.1177/1687814017705965 starting process for a single-motor journals.sagepub.com/home/ade hybrid electric vehicle

Minghui Hu, Guochang Jiang, Chunyun Fu and Datong Qin

Abstract To tackle the excessive output torque ripple during engine starting process of parallel hybrid electric vehicles, the engine starting process is divided into three phases in this study: engine cranking phase, speed synchronization phase, and after synchronization phase. The expressions of the vehicle jerk are derived from the vehicle dynamic formula in each phase, and the influences of various parameters on the jerk are analyzed. The coordinated control strategy for the clutch pressure and motor torque and that for the motor torque and engine torque are proposed. The simulation model for the single-motor parallel hybrid electric vehicle is established using MATLAB/Simulink, and the effectiveness of the proposed control algorithms is verified. Besides, a bench test is conducted and the test results show that the selected change rates of clutch pressure, motor torque, and engine torque can effectively coordinate the relation between clutch, motor, and engine. It can be concluded that the proposed control strategy satisfies the requirement of vehicle ride performance in engine starting in-motion process.

Keywords Hybrid electric vehicle, engine starting process, torque coordinated control, simulation, experiment

Date received: 9 January 2017; accepted: 30 March 2017

Academic Editor: Haiping Du

Introduction appropriate torque coordinated control strategy has become one of the key problems in HEV design. In this Hybrid electric vehicles (HEVs) represent an effective article, we put forward a torque coordinated control solution to significantly reduce the consumption of fos- 1 strategy to reduce the jerk. This method employs the sil fuels and carbon emissions which have two or more torques of different parts as the control parameters and mechanical power sources and have different driving coordinates the change rates of clutch, motor, and modes for different driving situations, such as pure elec- engine to ensure smooth engine start. tric driving mode for low vehicle speed. To make full Various solutions have been proposed in the litera- use of the plug-in hybrid electric vehicle (PHEV) power- ture to tackle the above-mentioned problem. The causes train, frequent transitions between different modes are of jerk have been analyzed and discussed through necessary to optimize the vehicle operation.2–4 When switching mode from pure electric driving mode to engine driving mode, the internal combustion engine State Key Laboratory of Mechanical Transmissions, Chongqing University, needs to start in motion. During this process, the total Chongqing, China torque of powertrain may experience a big change if Corresponding author: there is no appropriate control in place, and it is highly Minghui Hu, State Key Laboratory of Mechanical Transmissions, likely to cause a torque fluctuation to the powertrain as Chongqing University, Chongqing 400044, China. well as a jerk to the vehicle. Thus, how to devise an Email: [email protected]

Creative Commons CC-BY: This article is distributed under the terms of the Creative Commons Attribution 4.0 License (http://www.creativecommons.org/licenses/by/4.0/) which permits any use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/ open-access-at-sage). 2 Advances in Mechanical Engineering various experimental tests.5 Y Tong6,7 put forward a M Yang et al.15 focused on the estimation of the engine dynamic coordinated control method, that is, ‘‘the open clutch torque and proposed estimation algorithm as control of engine torque’’ + ‘‘the estimate of engine well as control method. D Kum16 proposed optimal dynamic torque’’ + ‘‘the compensation of motor tor- control strategy of engine-start process that achieves que.’’ But in practice, the engine dynamic torque is hard the balance between drivability and quick start. to estimate. E Ji8 and Y Wang9 proposed a method to The above works mainly focus on how to estimate limit the engine torque changing slope and use the the transient engine torque and how to control the motor torque as compensation, but the method is only engine or clutch. However, the key to the problem is to effective in after synchronization phase. Y Yang et al.10 find out the coordinated relationship between the controlled the engine torque using a proportional–inte- engine, motor, and clutch. This article analyzes the gral–derivative (PID) controller and controlled the dynamic equations of the engine starting in-motion clutch using a fuzzy control algorithm; however, this process and obtains the relationship between the clutch method also needs the real-time engine torque informa- pressure change rate and motor torque change rate tion. Unfortunately, this approach cannot be com- before synchronization as well as the relationship monly used due to patent protection. J Zhang et al.11 between engine torque change rate and motor torque proposed a new algorithm which realizes smooth mode change rate after synchronization. Then, the control switching from pure electric driving mode to engine feasible regions are pointed out, and the control strat- operating mode. This algorithm obtains engine pseudo- egy based on the control feasible regions is proposed. target speed according to speed and throttle opening, Furthermore, in the control strategy, (1) the engine but the engine is treated as a quick torque response ignition shock has been avoided by igniting the engine machine, and the control algorithm has only been veri- before speed synchronization and (2) the clutch torque fied by simulation. J Sun et al.12 used model predictive abrupt change has been properly managed in the syn- control (MPC) methods to coordinate the clutch, chronization phase. Both the simulation results and test motor, and engine. In his study, the clutch is taken as results verify that the coordinated control strategy is an optimized variable and the clutch friction loss is effective in reducing the jerk and improving the ride taken into consideration. The simulation results show performance. that the proposed control strategy has less vehicle jerk A single-motor parallel HEV is considered in this and less clutch friction loss. L Wang13 improved the study, as shown in Figure 1. In this vehicle, a clutch is driving comfort by means of the adaptive sliding mode installed between the engine and the motor. The vehicle control technique. In this work, the error caused by the is in the pure electric driving mode (EV mode) when system parameters’ uncertainty and error between the the clutch is disengaged. This HEV has several driving actual engine torque and engine target torque are esti- modes, including the EV mode, engine-only driving mated, and these errors are employed as the control mode, and hybrid mode (the motor works as a traction variable of the sliding mode controller. J Oh et al.14 and motor or a generator). During start-up or at low

Figure 1. Structure of the single-motor parallel HEV. Hu et al. 3 vehicle speed, only the motor drives the vehicle (EV mode) while the engine is shut down to increase the fuel economy and to reduce emissions. When the vehicle speed reaches a certain value (or other situations), the motor needs to start the engine. As the clutch engages and the engine speed increases after ignition, the engine-only driving mode or both the engine and motor (hybrid mode) drive the vehicle. The transition from the pure electric driving mode to the engine mode or hybrid mode is called ‘‘engine starting in-motion.’’ The rest of the article is organized as follows. Section ‘‘Characteristics of the key components’’ explains the characteristics of the key components. Section ‘‘Dynamic equations for engine starting in- motion’’ divides the engine starting process into three phases, and dynamic analysis is performed for each phase. Section ‘‘Vehicle jerk analysis’’ presents the control relationship of the engine, motor, and clutch, resulting from the vehicle dynamic equations. In section ‘‘Control strategy and simulation verification,’’ the control strategy and HEV model developed by MATLAB/ Simulink are introduced. Section ‘‘Simulation and experimentation’’ demonstrates the simulation and bench test results, followed by some discussions about the results. The last section concludes the article.

Characteristics of the key components To quickly start the engine and minimize the vehicle Figure 2. Engine characteristic curves: (a) engine static output jerk in the engine starting process, the detailed knowl- map and (b) engine reverse drag torque. edge of the characteristics of every key component is necessary. is set to 288 V. Different parameters (e.g. motor output torque, motor speed, motor efficiency, power supply voltage, power supply current, and inverter efficiency) Internal combustion engine characteristics are tested at motor speeds of 0 and 100–4000 r/min (at In order to precisely control the powertrain load torque a 100 r/min interval). variation during engine starting in-motion, and to According to test results, the maximum power, max- smoothly start the engine, a test bench of a 1.6 L four- imum torque, and rated speed of the motor are, respec- cylinder electronic injection engine is built. The test tively, 25 kW, 120 N m, and 2000 r/min. The motor bench evaluates the steady-state engine output torque external characteristics is shown in Figure 3 at the throttle valves of 1%, 2%–40% (at a 2% inter- 8 val), 45%–95% (at a 5% interval), and 99%, at < Tmmax = 120 Nm, 0\nm\2000 r=min the corresponding engine speeds of 1000 and 1250– : 238, 750 Tmmax = , 2500 r=min\nm\6000 r=min 6000 r/min (at a 250 r/min interval) respectively, as nm shown in Figure 2(a). This bench tests the reverse drag ð1Þ torque at different speeds when the throttle valve is 0%, in order to obtain the loss torque characteristics of where Tmmax denotes the maximum motor torque and the engine, as shown in Figure 2(b). nm represents the motor speed.

Motor characteristics Clutch characteristics To obtain the characteristics of the motor and its con- To reduce the fluctuation of total output torque, the troller, a hybrid power system for motor test is designed clutch needs to engage and disengage gradually when and a test bench for the motor and its controller is the engine is switched on and off. The knowledge of developed. In the test, the motor is running on the lim- the clutch transfer torque under different pressure is iting condition and the voltage of the DC power supply required, and the clutch must be controlled smoothly 4 Advances in Mechanical Engineering

engine cranking phase, speed synchronization phase, and after synchronization phase. To simplify the control plant, the following hypotheses are made:

1. Except for the three key components, all other components of the driveline system are rigid bodies and are rigidly connected with each other. 2. There is no response lag for all components.

With these hypotheses, the dynamic equations for each phase can be derived.

Engine cranking phase Figure 3. Motor external characteristics. The engine cranking phase is from the beginning of clutch engagement to the beginning of speed synchronization. In this phase, the clutch pressure rises gradually and the clutch enters into the slipping state in which the friction torque is proportional to the clutch pressure. When the clutch friction torque exceeds the engine reverse drag torque at speed 0, the engine crank starts rotating and then the speed difference between the engine and motor reduces gradually. The dynamic equations for this phase are 8 P > I i2i2 <> r g 0 du r + Im r + mr dt =(Tm Tc)igi0hT Tf > :> Ieae = Tc Tef Tc = fp0SZRc ð3Þ

Figure 4. The torque transmission characteristics of clutch. where r is the wheel radius, u is the vehicle speed; i0 and ig are the final drive ratio and transmission ratio; and quickly. The clutch torque depends on the pressure Tm is the motor torque; Tf is the converted total resist- and the speed difference between the clutch disks, as ing torque; Ir, Ie, and Im are the moment of inertia of expressed by equation (2) the wheel, engine, and motor, respectively; m is the vehicle mass; Tef represents the engine reverse drag tor- Tc = f (n, p0) ð2Þ que before ignition; hT is the transmission efficiency; f is the static friction coefficient of the clutch; P is the where T is the clutch torque, n is the speed difference 0 c oil pressure; Z is the friction face quantity; S is the con- between the clutch disks, and p is the clutch oil 0 tact area of clutch disks; R is the equivalent friction pressure. c radius; and a is the engine angular acceleration. In order to precisely control the clutch torque, we e Since the real-time engine torque is hard to obtain look into the clutch characteristics by means of experi- and different engines have different characteristics, we mentation and achieve the clutch torque at different oil ignited the engine in this phase to ensure that the igni- pressure and speed difference between clutch disks. The tion impact is not transferred to the wheels. experimental results are shown in Figure 4. Because the clutch torque mainly depends on the pres- As seen from Figure 4, the clutch torque is mainly sure, the engine’s ignition does not change the clutch tor- dependent on oil pressure, and the effect of the speed que, and the dynamic equations of the powertrain also difference between clutch disks is insignificant. remain the same. Note that two benefits can result from igniting engine in the clutch slipping condition: Dynamic equations for engine starting in-motion 1. The engine speed increases more quickly, namely the engine accelerating time is minimized. Based on vehicle dynamics, the process of starting 2. The engine ignition shock is not delivered to the engine in-motion can be divided into three phases: wheels.17 Hu et al. 5

After ignition, the engine torque becomes positive N dTmc MNu Frr j = T + Mu ð5Þ and the engine will accelerate itself. However, the clutch I dt I 2 mc I 2 disk speed on the engine side is still lower than that of with the other disks, and the clutch torque is still determined by the clutch pressure. So only the second equation in Tmc = Tm Tc (3) is changed to Ieae = Tc + Te, where the value of ae P is increased. I i2i2 I = r + I g 0 + mr By this means, the ignition impact can be success- r m r fully prevented from being transferred to the wheels. N = igi0hT C Ar Speed synchronization phase M = D 10:575 In this phase, the speed difference between the clutch C A driving disk (connected to engine) and the clutch slave F = mgf cos a + mg sin a + D u2 r r 21:15 disk (connected to motor) is small enough, but the engine is still accelerating. Thus, the dynamic equation where Fr denotes the converted total resisting force, CD before synchronization does not change. When the represents the air resistance coefficient, and A stands speed difference becomes zero, the clutch switches its for the vehicle frontal area. state from slipping to engagement and the speed syn- As seen from equation (5), the jerk is mainly depen- chronization is complete. The dynamic equation is now dent on three parameters: vehicle speed, torque differ- given by equation (4) ence between the motor and clutch, and change rate of P ! the torque difference. Ignoring the slope resistance and 2 2 I i i du substituting the parameters N, I, M, r, and Fr (depend- r +(I + I ) g 0 + mr =(T + T )i i h T r e m r dt m e g 0 T f ing on vehicle structure) in equation (5) lead to the following expression of vehicle jerk ð4Þ

2 dTmc 7 Before synchronization, the engine is accelerating to j = 1:699 3 10 7:8 3 10 uTmc a certain rotational speed and the dynamic equation is dt ð6Þ + 2:1 3 109u3 + 5:96 3 106u Ieae = Tc + Te. When speed synchronization is complete, the clutch torque is equal to the engine torque The influence of each parameter on jerk is shown in (T = T ), and the engine angular acceleration becomes c e Figure 5. zero (or very small). So the clutch torque will have a Figure 5 indicates that the torque difference change great change before and after synchronization, and this rate has great influence on vehicle jerk, while the torque change can be expressed by DT = I a (DT is the c e e C difference and vehicle speed do not present obvious clutch torque difference before and after synchroniza- influence. Therefore, a proper control of the torque dif- tion). Since this change occurs abruptly, an appropriate ference change rate is crucial. The expression of the control strategy is required. vehicle jerk can be simplified as follows

N dT dT After synchronization phase j = mc or j = 1:699 3 102 mc ð7Þ I dt dt In this phase, the clutch is completely engaged and the vehicle is driven by two power sources. Since the engine The vehicle jerk during mode switching should meet 18 output torque may be unstable, the motor needs to the powertrain design requirement of jjj \10 m=s3. adjust its torque to compensate for the engine torque Thus, using the parameters of the vehicle considered in variation. So the dynamic equation for this phase is still this study, the relationship between the clutch pressure equation (4). change rate and the motor torque change rate should After compensation, the motor’s operating condition satisfy the following inequality is dependent on the HEV driving mode. dTm dp0 0:0001 \588 ð8Þ dt dt Vehicle jerk analysis This relationship is plotted in Figure 6. Vehicle jerk analysis before speed synchronization The shaded region in Figure 6 represents the control Before speed synchronization, the dynamic equation feasible region. As long as the change rates are con- does not change, so the vehicle jerk can be obtained by trolled in this region, the vehicle jerk can satisfy the taking derivative of equation (3) design requirement. Theoretically, the jerks on the 6 Advances in Mechanical Engineering

Figure 6. The relationship between the clutch pressure change rate and the motor torque change rate.

Figure 5. The influence of each parameter on jerk: (a) the influence of vehicle speed and torque difference rate on jerk and (b) the influence of torque difference and its change rate on jerk. Figure 7. The relationship between the motor torque change rate and the engine torque change rate. upper and lower boundaries are 6 10 m/s3 and the jerk on the optimal control line is 0. Equation (10) indicates that the vehicle jerk mainly depends on the change rate of the total torque (motor Vehicle jerk analysis after speed synchronization torque plus engine torque). The above jerk expression Similarly, taking derivative of equation (4) leads to the can be further simplified to following jerk expression N dT dT j = me or j = 1:355 3 102 me ð11Þ N dTme MNu Frr I1 dt dt j = 2 Tme + Mu 2 ð9Þ I1 dt I1 I1 In order to meet the design requirement of 3 where j\10 m=s , the relationship between the motor torque change rate and the engine torque change rate should P 2 2 satisfy Ir igi0 I1 = +(Ie + Im) + mr r r dTe dTm + \738 ð12Þ Tme = Te + Tm dt dt

Substituting the basic parameters in equation (9) This relationship is shown in Figure 7. yields the following vehicle jerk expression, without The shaded region in Figure 7 represents the control considering the slope resistance feasible region. As long as the change rates are controlled in this region, the vehicle jerk can meet the 2 dTme 7 j = 1:559 3 10 6:80 3 10 uTme design requirement. Theoretically, the jerks on the dt ð10Þ upper and lower boundaries are 6 10 m/s3 and the jerk 9 3 6 + 1:83 3 10 u + 5:20 3 10 u on the optimal control line is 0. Hu et al. 7

Figure 8. Speed synchronization phase control strategy.

Control strategy and simulation verification Control strategy In the engine cranking phase, the clutch pressure is increased and the clutch and motor are coordinated, so as to ensure that the clutch pressure change rate and the motor torque change rate are in the control feasible region. In the synchronization phase, the change of clutch torque occurs abruptly, and this sudden change, if improperly managed, can become very large and result in a remarkable jerk. To avoid this problem, we pro- pose that when the speed difference is less than a certain amount Dn0, the engine torque is tuned to zero and then the clutch torque is adjusted to follow equation (13) by controlling the clutch pressure

Tc = Te(ne)+k(nm ne) ð13Þ with Figure 9. The control flow of engine starting in-motion for HEV. k =(Tc Dn Te Dn)=Dn ð14Þ

8,9 where Tc_Dn and Te_Dn are the clutch torque and engine the throttle changing slope and then the engine and torque at the moment when the speed difference is Dn. motor are coordinated to ensure that the engine torque Figure 8 illustrates the torque change routes with change rate and the motor torque change rate are in the and without pressure control. control feasible region. Theoretically, when all components are controlled accurately, the effect of DTc can be ignored. In the after synchronization phase, the engine torque Control flow change rate cannot be accurately controlled. Thus, the Based on the characteristics of the single-motor parallel engine torque changing slope is restricted by limiting HEV and the control feasible regions we have obtained, 8 Advances in Mechanical Engineering

Figure 10. Comparison of the simulation results: (a) simulation results of the conventional strategy and (b) simulation results of the proposed strategy. a set of control flow is established, as shown in which depends on the clutch pressure becomes static fric- Figure 9. tion suddenly, and the clutch torque is reduced as the part of torque for accelerating the engine and suddenly disappears. Note that the motor cannot coordinate timely Simulation and experimentation and as a result gives rise to an inevitable shock. Simulation Figure 10(b) demonstrates the simulation results resulting from the proposed strategy. The engine start- In the simulation studies, different vehicle speeds and ing process starts at 1 s. The clutch pressure is increased continuously variable transmission (CVT) ratios are gradually and then the engine starts rotating and the employed. Figure 10(b) shows the simulation results of engine is ignited when its speed exceeds 850 r/min, after engine starting in-motion when the cruise speed is which the engine speed increases more quickly. When 20 km/h and CVT ratio is 2.4. the speed difference between the engine and motor is In Figure 10, the parameters Tc, Tm, Te, and Tout less than Dn0 (200 r/min in Figure 10(b)), the power- represent the clutch torque, the motor torque, the train enters the speed synchronization phase (at 1.4 s). engine torque, and the final output torque after motor, The engine torque is tuned to zero, and the clutch pres- respectively. sure is decreased and then held again. Note that the Figure 10(a) shows the simulation results resulting clutch pressure cannot be too small to stop the syn- from the conventional control strategy. As can be seen, chronization from happening. Before speed synchroni- the vehicle jerk is about 22 m/s3 and the engine, motor, zation starts, the clutch pressure change rate and the and clutch pressure need to be properly controlled. At motor torque change rate are coordinated according to the synchronization moment, the dynamic friction the control feasible region. Hu et al. 9

Figure 11. The HEV test bench for engine starting in-motion.

The engine speed and motor speed achieve synchronization at 1.55 s, at which moment the dynamic friction that depends on the clutch pressure becomes static friction. The clutch pressure rises steeply after speed synchronization until it exceeds 2 MPa, after which the powertrain enters the after synchronization phase. In the after synchronization phase, the throttle change rate is limited. The motor torque is used to compensate for the engine torque, and the engine torque change rate and the motor torque change rate are coordinated according to the control feasible region. Figure 12. Test results for engine starting in-motion. In the whole simulation process, the maximum vehicle jerk is 6.2 m/s3, which satisfies the requirement and is significantly reduced compared to that resulting from engine torque fluctuation is compensated for by the the conventional strategy. motor. The second sub-figure in Figure 12 shows that the engine ignition shock is not transmitted to the vehicle Experimental verification body, and the motor works correctly to ensure stable torque output. From the third and fourth sub-figures Based on the simulation studies, a test bench (shown in we see that the motor speed and vehicle speed present Figure 11) is built to further validate the control strat- some fluctuations during the engine starting process. egy. The experimental results are plotted in Figure 12. This is because the accurate values of clutch torque and As shown in the test results, the ‘‘start engine’’ com- engine torque cannot be obtained, which makes the mand is sent at 15.1 s. The clutch pressure immediately motor compensation torque deviate from the optimal rises up, but it is unstable at the beginning due to the value. effect of the resistance resulting from the clutch disk In the whole process of engine starting in-motion, we contact. After 0.25 s, the engine starts being rotated by can see that the maximum vehicle jerk is about 8.9 m/s3, the motor, and the engine is ignited when its speed which well satisfies the design requirement. The pro- exceeds 850 r/min at 15.45 s. As soon as the speed differ- posed control strategy can smoothly start the engine in- ence between the engine and motor becomes less than a motion, thereby improving the ride performance and certain value (namely 200 r/min), the process enters the driving comfort. speed synchronization phase. Then, the engine torque is tuned to zero, and the clutch pressure is adjusted according to the proposed method. Speed synchroniza- Conclusion tion is achieved at 15.6 s, after which the clutch pressure is quickly increased to ensure complete clutch engage- The core problem of engine starting in-motion process ment. Since then, the engine starts driving the vehicle is the torque coordinated control which is a critical task and its torque change rate is limited. Meanwhile, the for HEVs drivability. To tackle this problem, a control 10 Advances in Mechanical Engineering strategy for every phase of the engine starting in-motion 6. Tong Y. Real-time simulation and research on control process based on jerk analysis is proposed in this article. algorithm of parallel hybrid electric vehicle. J Mech Eng The effectiveness of this control strategy is validated by 2003; 39: 158–160. simulation studies and experiments. Both simulation 7. Tong Y. Study on the coordinated control issue in parallel and experimental results show that the vehicle jerk dur- hybrid electric system. PhD Thesis, Tsinghua University, ing the engine starting in-motion process is reduced sig- Beijing, China, 2004. 8. Ji E. Study on the coordinated control for mode-switch of nificantly by means of the coordinate control strategy parallel hybrid electric vehicle. Changchun, China: Jilin proposed in this article. This strategy proves effective in University, 2006. improving the vehicle ride performance and driving 9. Wang Y. Simulation research on the mode switching qual- comfort. ity of hybrid powertrain with two clutch. Changchun, China: Jilin University, 2009. Declaration of conflicting interests 10. Yang Y, Yang W and Qin D. Coordinated torque control strategy for driving-mode-switch of strong hybrid electric The author(s) declared no potential conflicts of interest with vehicle. J Chongqing Univ 2011; 34: 74–81. respect to the research, authorship, and/or publication of this 11. Zhang J, Lu X, Wang L, et al. A study on the drivability article. of hybrid electric vehicle. SAE paper 2008-01-1572, 2008. 12. Sun J, Xing G, Liu X, et al. 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