Complete vehicle model to optimize mild hybrid and thermal strategies and to predict CO 2 emissions: development, validation and analyses

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1 Complete vehicle model to optimize mild hybrid and thermal strategies and to predict CO 2 emissions: development, validation and analyses H. Dupont 1, Dr. H.-J. Nuglisch 1, M. Lankes 2 1: Continental, 1, av. Paul Ourliac, 3136 Toulouse 2: Continental, Siemensstrasse 12, 9355 Regensburg Abstract: This paper describes the development and the validation of two complete vehicle models, achieved within the LMS AMESim platform and aiming to help in the understanding of the contributions of different sub-systems to CO 2 emissions. The models that include the main components of the vehicle (powertrain, coolant and oil circuits, electrical system) take into account the physical interactions and enable both to predict fuel consumption and to develop and validate control strategies for the mild hybrid part. Some measurements have been performed on European driving cycle and allow correlating fuel and electrical consumptions. Keywords: complete vehicle, mild hybrid, CO 2 assessment, super-capacitors 1. Introduction More and more stringent CO 2 emission targets in conjunction with increasingly severe emission regulation (EURO 5 and EURO 6) will be set by the European Community. To assist the OEMs to reach these ambitious goals by maintaining end customer expectations for comfort and fun-to-drive, the supplier industry has to understand and to optimize the control of the different components considering the multi-physical interactions that take place. The complexity of this task cannot anymore be imagined without support. For this reason Continental Automotive has developed some complete vehicle models based on the LMS AMESim software including all physical processes and subsystems (vehicle, engine, transmission, mechanical frictions, coolant and oil circuits and electrical system). This paper describes especially the s activities performed within the framework of a partnership project with Lotus Engineering concerning the development of a concept car for CO 2 reduction measures. The development of the models according to the characteristics of the democar will be presented, followed by the validation with results. 2. Simulation approach The s are based from one side on a physical model at a system level developed within LMS AMESim platform and that includes all physical processes taking into account the interactions between the components and, from the other side, on a Matlab/Simulink model that contains the control functions of various parts. This approach allows both to perform s in order to predict CO 2 emissions, with the identification of the contributors and different analyses as it will be presented at the end of this paper, and to validate and develop functions that are then tested by rapidprototyping on the vehicle in a dspace microautobox. This is illustrated by the figure 1. correlation with measurements Physical model co- rapid-prototyping Figure 1: Simulation chain tool Functions model compilation for dspace µ-autobox This method offers an optimal way to reduce development cost due to vehicle testing and enables a first calibration of the functions. Continental - Complete vehicle model s Page 1/7

2 3. Physical model development In this part the democar and the model will be simultaneously described. A complete model has been created within AMESim. The development has been performed simultaneously to the build-up of the vehicle with the design of the engine and the selection of the components. Thanks to the data coming from Lotus for the engine part, from Continental divisions for the hybrid components and from different suppliers it has been possible to parameterize the necessary components. To complete the model some hypotheses have been made for the missing data, as for instance for engine friction. conditions (for instance crankshaft or piston friction depends on oil temperature). The components taken into account are: pistons, crankshaft, camshaft, oil pump, high pressure fuel pump and tensioner (figure 2). 3.1 Engine The main characteristics of the engine are presented in table 1. Lotus Sabre Number of cylinders 3 in line displacement (cm3) 1498 Type DI / turbocharged Valve mechanism DOHC Bore (mm) 88 Stroke (mm) 82.1 Compression ratio 1.2:1 Maximum output / Engine speed (kw / rpm) 117 / 525 Maximum torque / Engine speed (N.m / rpm) 24 / 18-4 Table 1: Characteristics of the engine The engine submodel chosen in AMESim is a tabulated one; it simulates transient conditions by successive steady-state operating conditions. It only requires tables of indicated engine torque, fuel consumption, heat-wall transfers and exhaust temperature. The full load conditions are provided by the targets defined for the engine. The main difficulty was to estimate the fuel consumption during the engine design process and before having performed any tests. This has been done thanks to previous results obtained with engines of similar technologies and making some hypotheses. The other tables have been built in the same way. 3.2 Vehicle The vehicle chosen for the project is an Opel Astra. For the characterization of the vehicle in AMESim, the weight, tire characteristics, aerodynamic parameters and coast-down curve are needed. 3.3 Mechanical and auxiliaries frictions The frictions are not included in the maps defining the engine but are added separately, with the decomposition of the contributors. This allows adjusting frictions according to the operating Figure 2: Auxiliaries and mechanical frictions 3.4 Coolant and lubrication circuits Based on the architectures of the circuits (figure 3), on the characteristics of the main components and thanks to dedicated s from Lotus Engineering giving flow repartition, the following models have been built in AMESim (figures 4 and 5) Oil cooler T2 Heater matrix 12VDC electric coolant pump (151 l/min) Turbo Radiator Temperature sensors (2x) Head Block T1 Surge tank Block stat Figure 3: Schematic of cooling circuit Head stat Thermostat housing The specificities of the Sabre engine are the following: - integrated exhaust manifold (figure 6) - split cooling (double thermostat housing) - electric coolant pump - oil cooler - piston cooling jets The control of the thermal part (pump, fan and thermostats) is performed in rapid-prototyping through a microautobox. Continental - Complete vehicle model s Page 2/7

3 Figure 4: Schematic of cooling circuit within AMESim Figure 5: Schematic of oil circuit within AMESim Figure 6: Cylinder Head with Integrated Exhaust Manifold 3.4 Electrical system The vehicle integrates a mild-hybrid system that allows Stop & Start, recuperation and boost assistance. The electrical architecture is composed as follow: - an Integrated Starter Generator (ISG) - a DCDC converter - a super-capacitor module - a classical board net with a 12 V battery The main characteristics are given in table 2. Supercapacitor nominal capacitance 21 F nominal voltage 64.8 V number of cells 24 specific energy 3.6 Wh/kg volume 31.5 l weight 34 kg Integrated Starter Generator Voltage range V Motor type asynchronous Power rpm DCDC converter maximal power 2 14 V (15 A) Table 2: Main characteristics of electrical components The control of the mild hybrid part is operated by rapid-prototyping through a microautobox. The model that contains the strategies is developed in Matlab/Simulink by the Continental division Electric Drive/Drive Train. It is known as the Integrated Powertrain Management (IPM ). With a relatively low power the ISG is not able to really provide boost assistance for the acceleration of the vehicle but helps at low engine speeds to the launch of the car. This is especially useful when the engine is cold to limit pollutant emissions. Two models have been built within AMESim: - a model featuring only a standard 12 V board net with a conventional generator - a second one that reproduces the mild hybrid architecture described above. This model works in co- with Simulink using the IPM. The purposes of these models are different: the first one has been built at the beginning of the project, validating the different parts (vehicle, power-train, cooling circuit, frictions). It has been used for instance for the choice of the gearbox and to predict first fuel consumption of the vehicle if the mild hybrid strategies are not active. The second model is used to predict fuel consumption with the benefit of the mild hybrid strategies. In both cases, the 12 V circuit integrates several resistors for the electrical pump, the fans and the average electrical consumption of the vehicle in both engine states: running or not running (figure 7). Figure 7: Schematic of 12 V - board net Continental - Complete vehicle model s Page 3/7

4 3.5 Transmission For different reasons a manual transmission has been chosen for the democar. The model helped in this choice by comparing fuel consumption over the European driving cycle with different existing gearboxes and by evaluating the potential of a customized gearbox. Resulting from this evaluation the choice has been made for a series 6-speed gearbox. 4. Correlations with measurements In this part results coming from measurements and s are compared in order to validate the models. The tests have been performed for the New European Driving Cycle with different configurations. 4.1 Comparison with deactivation of hybrid strategies In order to characterize the benefit of the mild hybrid functionalities, a cycle has been performed ly with the deactivation of all IPM strategies: Stop & Start, recuperation and launch assistance. In this case, the ISG works like a conventional generator; that means providing the required current for the electrical consumers through the DCDC converter. In the same time the voltage of the super-capacitors remains more or less constant at the initial value. These measurements are compared to the results obtained with the first model described previously, with a conventional generator and usual 12 V-board net. This comparison is an approximation since the ISG and the conventional generator do not have the same efficiency to produce the required energy. Figures 8 & 9 show respectively instantaneous and cumulated fuel consumption with both measured and simulated results over the NEDC. Good correlations are obtained for the urban and the extra-urban parts with relative variations versus measurements below 5 %. Fuel consumption (mg/s) Figure 8: Instantaneous fuel consumption with deactivation of mild hybrid strategies Cumul. fuel consumption (g) Figure 9: Cumulated fuel consumption with deactivation of mild hybrid strategies The variations for the different cases are given in table 3. conso (g) variation expe simul % UDC % EUDC % NEDC % Table 3: Variations between and experiments with mild hybrid functionalities inactive The figure 1 compares the current required by the electrical loads. For the, the global resistance that represents all electrical components except coolant pump and fan has been adjusted to have an average value consistent with the measurements. This value is constant whatever the operating conditions. Low voltage current (A) Figure 1: Low voltage current with deactivation of mild hybrid strategies 4.2 Comparison with activation of hybrid strategies In this second part the correlations concern a cycle performed with the activation of all the functionalities of the ISG. The s are performed by coupling AMESim and Simulink. The correlations for fuel consumption are not as good as they were in the previous case. The variations reach 6.6 % at the end of the cycle, with 9 % for the urban part and 4 % for extra-urban one (table 4). Continental - Complete vehicle model s Page 4/7

5 conso (g) variation expe simul % UDC % EUDC % NEDC % Table 4: Variations between and experiments with mild hybrid functionalities active The difference increases all along the cycle, without noticeable deviation but rather a constant slight under-estimation of simulated instantaneous fuel consumption (figures 11 & 12). Nevertheless, the predictions give the right tendencies and are quantitatively precise enough to be used for control function development and pre-calibration. Cumul. fuel consumption (g) Fuel consumption (mg/s) Figure 11: Cumulated fuel consumption with activation of mild hybrid strategies Figure 12: Instantaneous fuel consumption with activation of mild hybrid strategies As an example for the model results using mild hybrid functions, the charging / discharging of the super-capacitors is analyzed. The super-capacitor voltage (figure 13) indicates clearly the different states of the ISG according to the operating conditions. Super-capacitors voltage (V) Figure 13: Super-capacitors voltage The voltage decreases during the two first idle phases due to the electrical loads of the running engine to allow the catalyst heating. It drops strongly when the vehicle accelerates due to the launch assistance strategy and recovers one part of the energy during the decelerations thanks to the recuperation. When the voltage reaches 4 V, to keep a sufficient quantity of energy for further stop & start of the engine or launch assistance, the ISG reloads the super-capacitors. At the end of the cycle, the strong deceleration enables to recover such a quantity of energy that the final state of charge exceeds the initial one. Simulation results show again good correlations but with a slight under-estimation of energy consumption during launch assistance phases. This appears in the comparison of the ISG torques (figure 14), the tendency is correct but the curves are rather different and the amplitudes especially for recuperation diverge. ISG torque (Nm) Figure 14: ISG torque during urban part The main reason for this is the braking torque of the driver that is too strong in the : the model driver adjusts both acceleration and braking ratio with PID controllers according to the deviation of vehicle speed compared to the setpoint. As a result, in the s the vehicle follows very precisely the speed setpoint (curves are difficult to distinguish), this not the case in the reality where the real driver tries to smoothen the vehicle speed within the allowed tolerances (figure 15). Continental - Complete vehicle model s Page 5/7

6 Vehicle speed (kph) speed setpoint Figure 15: Comparison of vehicle speeds This difference has an influence on the recuperation as well with a stronger torque for a shorter period. Nevertheless the amounts of energy dissipated or recuperated are globally the same even if the profiles are different. The final deceleration is however especially well correlated (figure 16). ISG torque (Nm) Figure 16: ISG torque during extra-urban part The electrical energy consumption of the low voltage circuit depends on the state of the engine. The current that reaches approximately 27 Amps (25 Amps simulated) when the engine is running, drops to a value close to zero as all the nonnecessary consumers are switched off or reduced to a minimal value during idle phases when engine is stopped and during decelerations when injection is cut-off (figure 17). Low voltage current (A) Figure 17: Low voltage current These under-estimations of electrical loads can explain one part of the fuel consumption variation. Further analysis will be performed to identify the reasons of these differences. The comparison with measurements shows globally good enough correlations to allow both uses of the model: on one hand for predicting fuel consumption and CO 2 emissions for various operating conditions and, on the other hand, for helping in the development and validation of strategies for the control of the ISG. Moreover the model enables to identify the respective contributions to the CO 2 emissions and to understand the interactions between the components and the physical phenomena. The benefit of the ISG according to the comparison of both test configurations, it appears that the mild hybrid strategies bring together 9 % reduction of fuel consumption over the cycle, with a main part gained in the urban cycle, especially thanks to the Stop and Start function (table 5). Idle phases count for 3 % of the duration of the urban part with 24 s and for 1 % of the extra-urban part with 4 s. expe simul UDC -15.2% -2.% EUDC -1.9% -6.3% NEDC -8.9% -13.4% Table 5: Fuel consumption benefit of mild hybrid functionalities by NEDC phases The Stop & Start strategy enables fuel consumption savings of 6.2 % compared to the total fuel energy used during the cycle; the results for this strategy give 7.7 % savings (table 6). These rather high values are partly due to the fact that the engine is stopped from the third idle phase of the cycle; that means quite early. For the, the fuel consumption at idle speed is slightly over-estimated, increasing the benefit. fuel energy used kwh energy generated through recuperation kwh proportion of fuel energy % 1.6% 1.4% energy used for engine starting and vehicle launch assistance kwh.84.8 proportion of fuel energy % 1.3% 1.2% energy saved during S&S kwh proportion of fuel energy % 6.2% 7.7% Table 6: Energy analysis over NEDC The quantity of energy produced during decelerations reaches approximately 1.5 % in both cases. A large part of this energy is reused for starting the engine and helping the vehicle launch with the efficiency of the electric motor, one small part remains in the super-capacitors since the final voltage is higher than the initial one (table 7). The values are similar for measurements and s. Continental - Complete vehicle model s Page 6/7

7 expe simul initial voltage V final voltage V energy variation Wh proportion of fuel energy %.13%.18% Table 7: Variation of super-capacitors energy over NEDC 5. Conclusion The models show globally good correlations for the fuel consumption and the electrical characteristics of the vehicle in both configurations, the conventional and the mild hybrid vehicle. The behaviour in various situations shows that some deviations appear, further analyses will be performed to improve the correlations but the contributions of mild hybrid strategies are correctly reproduced, allowing using the model for CO 2 predictions and development of mild hybrid strategies. 6. Acknowledgement The authors acknowledge the contributions of the whole project team, especially the calibration and test centre support. 7. References [1] Dr. Robert Fischer, Dr. Kurt Kirsten: "The turbo Hybrid Holistic approach for a modern gasoline hybrid drive", 27 th International Vienna Motor Symposium 26. [2] D. Coltman, J.W.G. Turner, R. Curtis, D. Blake, B. Holland, R.J. Pearson and A. Arden, H.-J. Nuglisch: "Project Sabre: A Close-Spaced Direct Injection 3-Cylinder Engine with Synergistic Technologies to achieve Low CO 2 Output", SAE- Paper [3] Dr. A. Graf, B. Köppl: "CO 2 reduction with demandoriented Power Control", VDI Berichte Nr. 2, Glossary CO 2 : Carbon dioxide IEM: Integrated Exhaust Manifold IPM : Integrated Powertrain Management ISG: Integrated Starter Generator NEDC: New European Driving Cycle OEM: Original Equipment Manufacturer PID: Proportional Integral Derivative UDC: Urban Driving Cycle UEDC: Extra-Urban Driving Cycle Continental - Complete vehicle model s Page 7/7