The design and development of an Integrated Propulsion System – Phase 2: the functional electric behaviour strategy

The paper presents the integrated propulsion system implementation steps on a L6E road vehicle. The implementation followed several technical levels which are properly described within the paper. The functional behaviour in terms of electric propulsion was defined and developed. All the dedicated components were investigated, set and customized, to equip a L6E vehicle. Currently the electric machine, the battery pack, the user interface, the speedometer, the pedals, the power electronics etc. are implemented on a real existing L6E road vehicle, after they were defined and virtually tested.


Introduction
The current needs of automotive industry in electrification are great.Combining the requirements of world governments with the economic and environmental concerns, the electric propulsion systems designed for road vehicles are one of the possible key factors to push the electrification forward.
When talking about reducing harmful emissions, most automotive engineers think to the hybrid or electric powertrains, since the internal combustion engine road vehicles represent a significant source of carbon dioxide emissions at the tailpipe.In addition, even though the hybrid vehicles have less local carbon dioxide emissions while operating, and the electric vehicles produce zero tailpipe emissions, the electricity they use is produced in a polluting energy plant.
The governments around the world are implementing increasingly stringent standards and regulations on road transport emissions.They are also providing financial support to customers for the purchase of electric vehicles.Most countries have a strict deadline for implementing the restriction on sales of road vehicles equipped only with internal combustion engines.The car manufacturers are facing the biggest challenge since the turn of the twentieth century, to electrify their entire manufacturing fleet.The customers are more inclined to use electric vehicles.Expanding charging infrastructures convincingly offers a convenient alternative to fuel stations.The advancements in battery technology are growing dynamically, both for the chemistry used and for battery packages modularity.The energy efficiency of the electric propulsion systems is better than of the internal combustion engine.In addition, by using electric propulsion systems the dependence on fossil fuels is reduced and the running costs are lower because the electricity is often cheaper.
With the benefit of the current framework, the development of different designs for the propulsion systems is very appropriate.When these propulsion systems are `integrated`, even modular, their development potential increases, and the users are the ones who impose the limits.The road electric vehicles integrated propulsion systems are those that have arrangements of their systems such as the 2. The approach: from the concept idea to requirements and testing The integrated propulsion system was defined by the specific goals and requirements: the performance objectives, the energy efficiency, the costs.All these three targets include different steps.Starting with the components` selection, the electric machine, the battery package, the power electronics, the controller were defined.Continuing with the integration and packaging, the physical layout was designed together with the integration plan of the components to the vehicle chassis.The control system needs advanced algorithms to manage the power flow between the components and optimize their performance, while the thermal management and the regenerative braking control are mandatory.The virtual testing represents the first testing level, while using a customized test bench for validation the results help to ensure that the integrated propulsion system meets the standards and the regulations.

The electric machine
The electric machine (figure 1) for this solution is a brushless direct current motor (BLDC) in-wheelhub motor, manufactured by QS Motors [7], having the nominal power of 2 kW, and the peak power of 4 kW, using customizable voltage of 48 / 60 / 72 Volts for a rotary velocity of 665 / 833 / 1000 min-1, the maximum torque of 160 Nm, the declared efficiency between 85 to 92 %, maximum current of 30 Amps, peak current of 80 Amps (reaching also 100 Amps for a short period of time), the angle between the Hall transducers of 120 deg.It weighs 15 kilograms.The electric machine has one axle, being dedicated to being mounted inside wheels or nearby the wheels.Following the already developed wheel architecture, the electric machine will be coupled to the wheel.

The energy storage system
The energy storage system is customized for a second vehicle designed especially for the in-wheel electric machine solution, operating at 48 Volts.It consists of four different LiFePO4 in-series batteries, of 12.8 Volts, 50 Ah and 640 Wh each.The main parameters of the batteries are presented in table 1.Each battery has integrated battery management system, including a state of charge indicator.Its manufacturer declares that its life cycle is expected to be more than 2000 cycle of charge-discharge at maximum of 80 % deep of discharge.The temperature of the battery is very important to its state of charge and its end of life.Therefore, the manufacturer clearly presents that the appropriate charging temperature should be between 0 and 45 degC (figure 2).

The battery package charger
The battery package charger is very important to the presented electric powertrain design solution.Not only the choice of charging technology, but also the selection of the correct charging method is a feature that must be considered during the charging procedure.The most popular charging strategies to recharge Li-Ion batteries are the constant current / constant voltage (CC / CV) and pulse current charging methods.However, these methods do not consider the internal processes of the battery that influence the charging capacity and its aging.As a result, some promising charging strategies that are based on more complete Li-Ion battery models are under research.But many solutions are using too many components that effect the sustainability of the charger.Therefore, to minimize the number of the components and to reduce the size and the costs of the battery charger, the inverter and the electric machine were used.After studying several strategies, together with the requirement to keep the electricity consumption low, a charger manufactured by Yewy was chosen (figure 3 The controllers' behavior is strict related to the driver system design and development.The electric machine driver system represents the set of electronic components that converts the energy from the batteries into a sequence of electrical power pulses applied to the electric machine to control its behavior.The electric machine driver system development needs parameters like the electric machine power, the peak power, the regenerative braking power. For input, the electric machine driver system uses: (i) the signals received from the encoder that is located inside the electric machine, respectively from the Hall transducers that indicate the position of the magnets inside the motor, (ii) the throttle pedal position, that is used for switching on and off the internal transistors to run the electric machine proportionally to its position.
The electric machine driver system consists of the transistors and the integrated circuit.The transistors must be selected to support the electric machine power, as the lower their internal resistance is, the lower the dissipated current will be.Six different groups of transistors have been used (SW1…SW6).They can continuously switch a voltage of 60 Volts and a current of 70 Amps.For achieving a control load of 300 Amps, the optimal solution consists of connecting several transistors in parallel (figure 6).The obtained values are presented in table 2.  The integrated circuit receives and interprets the information from the hall transducers located in the electric motor.The DC electric machine with permanent magnets is using three phases A, B and C. The motor starts when the driver provides voltage on two of the three phases, accordingly with the information received from the encoder.
The group of transistors are switching in turn in a certain sequence, being divided into UP and DOWN.The positive voltage is provided on one phase when the UP transistors are open, while the negative voltage is provided when the DOWN transistors are open.When applying a positive voltage to terminal B and a negative voltage on terminal C, the electric motor receive control.The Hall transducers are sending the information for continuously run of the motor, while the driver system is applying a certain voltage to another terminal.The control circuit for the electric machine with permanent magnets is presented in table 3.For each of the phases the terminals will receive positive voltage, negative voltage, or will be unpowered.This voltage sequence will allow the magnetic field produced by the coil to be attracted by the next magnet and to be repel by the previous one, resulting the torque to start the motor.This sequence is given by an integrated circuit, which converts the information received by the Hall transducers into the power sequence for the electric machine.The electric machine rotary velocity is controlled by each terminal voltage and the electricity supply sequence.The pulse width modulation is responsible for control it, the limits being between 0 and 100 %.For example, for a 48 Volts supply and a pulse length of 50%, the resulting voltage is 24 Volts.The pulse width is controlled by the acceleration pedal.
The output driver is responsible to send information to the electronic control unit about the battery voltage, the electric machine rotary velocity, the current energy consumption, the error messages.

The user interface -the speedometer
The user interface consists of the speedometer that is providing information about the battery voltage, the trip distance, the trip time, the vehicle speed, the absorbed current.Its programming is easy to handle, all its features being intuitive to set.Its operation voltage is between 48 to 96 Volts.

Implementing and testing the powertrain components on the prototype L6E vehicle
The above presented powertrain components were implemented and integrated to a prototype L6E vehicle.The implementation was followed by the in-real-life testing activities of the subassemblies and of the new vehicle.The results allow to evaluate the performances the achieved integrated propulsion system has.Using two electric machines, one for each wheel of the driven axle, the expected results are expected to be better than to an already existing similar solution.
Firstly, the e-wheel with integrated suspension system was tested using a customized dedicated test bench.Then, the e-wheel with integrated suspension system was tested on the prototype L6E vehicle, especially developed for testing this solution.The prototype L6E vehicle consists of its own chassis, a steering system, a braking system, two different axles, where the rear axle is driven by the two BLDC electric hub motors, coupled directly to the e-wheel, while the front axle is steered.The battery package (figure 8

Conclusions
The paper presented the technical level that were achieved for designing and developing an electric powertrain, together with choosing each component and implementing them to the prototype L6E vehicles.The current solution of the integrated propulsion system for electric vehicles, where the suspension system is in the wheel, clearly improves the road behavior in terms of maneuverability and dynamic performances using two electric machines, each driving a wheel.The electric machines are coupled to the wheels, as one of the parts of the suspended mass, belonging to the rear rigid axle.This design considerably reduces the energy consumption because of the simplified kinematics, the lighter rear axle where no conventional suspension is needed and the torque distribution between the driven wheels.
The functional behavior strategy for this vehicle was defined.Different configurations for the driver system were investigated.The number of transistors and their arrangement were used to calculate the power losses.All the components that interact and are connected to the power electronics were considered as contributors to the power losses.The L6E prototype is currently under testing and its optimization is mandatory.

Figure 6 .
Figure 6.Power losses evolution for the transistor and for the driver.
.a) is placed above the rear axle, directly mounted to the vehicle chassis, being protected by a customized housing that also hosts the power electronics (figure 8.b).The L6e vehicle has only one left seat but it can be equipped with the second seat, placed on the vehicle right side.The interface with the driver is assured by the speedometer.a b Figure 8.The battery package and the power electronics implementation.Before starting the on-road testing, the battery package was charged (figure while the speedometer was set to run for the first time.In addition, the tire pressures were set.Charging the battery package (a), setting up the speedometer (b), setting the tires pressure (c) and beginning the on-road testing (d).

Table 1 .
The electric, mechanical and functional parameters of the batteries.

Table 2 .
The power losses evolution while connecting transistors in parallel.

Table 3 .
The control circuit for the electric machine with permanent magnets.