Research concerning the autonomy of the electric vehicles, simulated and measured, in the case of driving at the high and the very high speed, specific to the WLTC test.

In the current research, the autonomy and the energy consumption are the most important attributes expected from the electric road vehicles. Because the electric car manufacturing industry is in a continuous progress, this offers its customers a wide range of constructive variants through which their operational requirements can be met. It also has high expectations of the electric car manufacturers in the current energy context, to develop new high-performance models that can be considered as a real alternative to the current internal combustion engine solutions. So, the research part continues, and is a priority for all car manufacturers. Moreover, the computer simulation is also included as an intermediate stage before the final projects are validated. In the first part, there are detailed theoretical aspects of the electric vehicle to be tested and the test procedure applicable to it, specific to the WLTC cycle, for the high and very high-speed test area. Also, is described the vehicle model simulated using the AMESIM modelling platform. In the second part, are presented the experimental and simulated results performing a comparative analysis. Following the analysis, it has been possible to draw graphs showing the evolution of the state of charge and the mode of exhaustion of the autonomy, in the case of the two driving zones analyzed, respectively at high and very high speed.


Introduction
Vehicles equipped with an electric power source attract a great deal of interest today, so electric vehicle manufacturers are proposing various design variants that can meet even the highest expectations [1].
Another component that needs to be developed is the charging infrastructure for the traction batteries, which, unfortunately, both in Europe and worldwide, is not able to handle the number of vehicles produced.In this context, the development of electric vehicles, namely the increase in their number, is strictly dependent on the degree of development of the charging infrastructure [2].
Two performance criteria are perhaps among the most important expectations that users have of the electric vehicles, respectively the autonomy and the power consumption.The autonomy represented by the total number of the kilometers travelled after a single traction battery charge is different from one manufacturer to another and obviously lower or higher depending on the destination of the vehicle [3].The autonomy has two main characteristics with which it is closely related, respectively the dependence on the traction battery power and the conditioning generated by the comfort systems that are part of the respective vehicles [5].Also, a characteristic of the traction battery with an effect on the autonomy is the health status which should be maintained at a high level for as long as possible [4].
The simulation of the autonomy of the electric autovehicle being researched using the AMESim platform is based on the legislation applicable to the electric vehicles by the type-approval authorities, respectively European Commission Regulation 2017 / 1151 of 1st of June of 2017 supplementing European Parliament and Council Regulation No 715 / 2007 on type approval of vehicles [8].The speed profile for the WLTC cycle has four zones characterized by a different dynamicity and a different percentage of driving time, respectively a low speed of 600 seconds, a medium speed of 400 seconds, a high speed of 500 seconds and a very high speed of 300 seconds.It can therefore be seen that the vehicle is put through a wide range of the dynamic stresses in order to reproduce as faithfully as possible all the situations it may encounter in the exploitation.The driving time per cycle is 1800 seconds, the distance travelled approximately 24 kilometres, with a maximum speed of 130 km/h, an average speed of approximately 48 km/h and a stationary percentage of 14%.
The AMESim platform is used for the simulation, as it is one of the most favoured modelling and simulation platform by the vehicle manufacturers before the implementation phase.The AMESim platform has a wide range of simulation functions used by the vehicle manufacturers and other industrial fields such as machine building, shipbuilding, aviation, etc., and it can satisfy a very wide range of them in terms of the virtual and the calculation model design as well as the simulation of efficiency through the assisted tests.
The platform can be used to simulate the types of the vehicles, the engines, the mechanical and the automatic gearboxes, the axles, the differentials, the mechanical and the electrical components of the listed systems, the emission treatment systems, the injection systems, the cooling systems, the lubrication systems, etc.Another feature is the integration of the specific test methods defined by the legislative regulations on the type or the system approval, which really bring great advantages in the research phases prior to the development and the implementation of the components or the systems [7].
In addition to the technical benefits conferred by the simulation platforms, the development costs are also significantly reduced, making the simulation a key component with a fundamental implication in the industrial research and in the industrial development [6].

Experimental
The AMESim platform was used for modelling and simulation.The test was carried out on a mid-size vehicle equipped with an electric motor and gearbox.All data of the simulated and modelled vehicle are the same as the data of a real vehicle, which has been tested on a dynamometer to validate the simulation results.
The model used aims to determine, on the basis of mathematical algorithms, the mode of discharge of the state of charge of the traction battery and the electrical energy consumption according to the established relationships, which are implemented according to the operating conditions imposed by the WLTC, but in this study only the aspects linked to the electric vehicle's range are highlighted, energy consumption being the subject of another study.
The vehicle tested is a medium-class vehicle whose general architecture is shown in the diagram below and whose technical specifications are as follows: -  Figure 2 shows the model of the electric vehicle made for the simulation, the visually identifiable elements being: -an element that simulates the simulated vehicle,  The simulation model's main input quantities are the components listed above, but it also uses other sub-components to obtain information on inertia, torque, engine speed and temperatures, which form the basis of the internal calculation algorithms [9]. Figure 3 shows the vehicle component modeled in the simulation, the role of the ports present on the vehicle is as follows:  The ports 1 and 3, through which are received the torque signals resulting from the braking of the front and for the rear axle;  The ports 2 and 4, through these are received the informations about the drive torque and return the angular velocity information, specific to each axle;  The port 5, is strictly mechanical, through which the external forces applicable to the vehicle are entered and through which information on the speed and the acceleration is returned;  The ports 6 and 7, through which the informations are received about the wind speed, as well as respectively the ramp and the rolling slope.Information on whether the test conditions correspond to the required speed profile was recorded at a frequency of 10 Hz [3].This type of information validates the quality of the test and makes it possible to deviate from the upper or lower limit speed profile by up to 2 km.
In Figure 4, the speed profile was maintained throughout the test period, indicating that the vehicle was capable of meeting the dynamic demands to which it was subjected.In order to correctly analyse how the reduction in the state of charge of the traction battery, which implies a reduction in range, was achieved, several journeys were made and average values were calculated for each of the four specific zones of the WLTC cycle.
Based on the information provided by the electronic control units controlling the traction battery, it was possible to analyse the energy balance, i.e. the ratio between the energy supplied by the traction battery to the traction system and the energy recovered by the traction battery after regenerative braking.
The traction battery in the study vehicle can only withstand a slow and fast AC charge.The charging mode used on the traction battery prior to testing was slow to ensure a correct and full charge up to 100% [2].

Results and discussions
Based on the simulation it was possible to draw up graphs regarding the evolution of the state of charge of the traction battery for each high and very high-speed zone specific to the simulated running on the WLTC cycle.Therefore, it was analysed the simulated behaviour of the state of charge in the traction battery in relation to the load and the speed of the electric motor, obviously generated by the travel speed and the cycle dynamics.The simulation was run until the traction battery state of charge was fully depleted.Based on this information, it was also possible to highlight the speed zones where the electric vehicle performs best in terms of economy, characterised by a low electricity consumption.
In figure 5 it is represented the variation of the regenerative load over time of the vehicle submitted to the test at the high and at the very high speed specific to the simulated running on the WLTC cycle, for one test cycle.It is observed how the SOC state of charge decreases very much over the time, despite the regenerative braking with a very high inertia at the high and at the very high speed, after the studies revealing that the temperature also plays a very important role in the evolution of the regenerative state of the charge of the electric vehicles.Figure 6 shows the variation of the regenerative load over the time of the vehicle tested at the high and at the very high speeds specific to the driving simulated on the WLTC cycle for 15 test cycles.It is observed how the SOC state of charge decreases very much over the time, despite the regenerative braking with very high inertia at the high and at the very high speeds, and the studies show that the temperature also plays a very important role in the evolution of the regenerative state of charge of the electric vehicles.Also, it was preferred to have several test cycles, considering that these tests were carried out under different temperature and under different climatic conditions, and in certain speed ranges, at the high and at the very high speeds of the vehicle being tested.This makes that the accuracy of the results of these tests are much closer to the truth, considering that there are many factors influencing these measurements.
In Figure 7, in such situations, if we speak of an average speed of 55.7 km/h and a maximum speed of about 96.9 km/h, the energy recovered is usually practically insignificant, with the state of charge and thus the range decreasing gently and gradually.The very high-speed running zone, graphically represented in the figure above, has a special character in conditions to maintain the state of charge of the traction battery.At these speeds, the average speed is 93 km/h and the top speed 132.2 km/h.As a general rule, in this zone, the vast majority of production electric vehicles reach their maximum design speed, characterized by a maximum load where energy recovery is non-existent.
At this driving speed, the charge level of the traction battery drops rapidly and range is immediately affected.This is the worst case scenario for the energy stored in the traction battery, due to the very high consumption generated by the electric traction motor.
The tests applied to the studied electric vehicle were repeated until the total autonomy of the traction battery was exhausted, respectively until the speed limit could no longer be respected by the vehicle.

Conclusions
As a result of the simulated drive, it was found that, when the vehicles are used at the high and at the very high speeds, characterized by the high and by the very high dynamic loads, the maintenance of the state of charge is difficult to achieve and the effect on the electric autonomy is very high.
With direct reference to the high to the very high-speed range of the simulated WLTC cycle, this can be assimilated to the use of the vehicles in the extra-urban and the highways.Under such conditions, basically without any regenerative braking, the recovered energy contribution of the traction battery is non-existent, so the impact on the electric autonomy is very high.
The simulated evolution of the state of charge of the traction battery with the effect in the electric autonomy, when running on the WLTC cycle, indicates that the electric vehicles with an electric power source have a very good utility in the urban mode and are satisfactory for the extra-urban or for the highway mode.The evolution of the technologies used to produce the traction batteries and their increased power can supplement the use of the vehicles in terms of their autonomy in the extra-urban or in the highway mode.
It was also found from the studies that the temperature, the climate conditions and the number of charging and the discharging cycles of the traction battery are very important for the depletion of the state of charge of the traction batteries of the electric vehicles in the case of the high and of the very high-speed driving.During high-speed driving, the state of charge of the traction batteries decreases gradually and the contribution of the recovered electrical energy is negligible.Such a situation leads to a rapid reduction in range due to the increased power consumption of the electric traction motor.
The most unfavorable situation for the depletion of the traction battery's state of charge is when vehicles are driven at very high speeds.Under these conditions, with a high load and high electric motor speed, consumption is very high, electrical energy recovery is practically non-existent and this type of driving leads to an immediate depletion of the traction battery's state of charge.
It should be noted that the main research objectives in the field of traction batteries should focus on increasing performance in terms of load levels at high and very high speeds.Of course, this performance is not only related to the traction battery, but further research is also needed in the area of optimising the design of electric motors, transmission or passenger comfort system, which play an extremely important role in such situations.
The development of the market for vehicles equipped with electric propulsion systems is driving the need to increase the electrical power of traction batteries, which must fulfil new functions such as dynamics and economy to meet the most demanding needs of modern society.
1303 (2024) 012004 IOP Publishing doi:10.1088/1757-899X/1303/1/012004 2 Consequently, the speed profile to be met by the electric vehicle during simulation is that set by the World Harmonised Light Duty Cycle (WLTC),described in the figure 1.

Figure 2 .
Figure 2. AMESim model for an electric vehicle ESFA-2023 IOP Conf.Series: Materials Science and Engineering 1303 (2024) 012004 IOP Publishing doi:10.1088/1757-899X/1303/1/0120044 -a component called "Driver" that simulates the driver's behaviour and controls the vehicle in the WLTC simulation, -a component that simulates the electric motor mounted on the simulated vehicle -an element simulating the electric vehicle electronic control unit .-a component simulating the transmission -a component simulating the traction battery.

Figure 4 .
Figure 4. WLTC speed profile followed by the tested vehicle.

Figure 5 .
Figure 5. Variation of the regenerative state of charge for the tested car at the high and very high speeds for one test cycle in WLTC In fact, as a result of the studies, it has been shown that a very important role regarding the depletion of the state of charge of the traction batteries fitted in the electric vehicles, in the case of the high and

Figure 6 .
Figure 6.Variation of the regenerative state of charge for the tested vehicle at the high and at the very high speeds for 15 test cycles in WLTC

Figure 7 .Figure 8 .
Figure 7. Traction battery charge level, at the high-speed zone