Final Project: Design of an Electric Vehicle
Electric Vehicle Model
This Model and project pertain to the modelling of a working Electric vehicle in Simulink and with the help of parameterisation in MATLAB.
The main aim of the project is to understand the Working of an Electric vehicle with the help of a DC motor and the various parts the electric vehicle would use for it to run perfectly.
The main software requirement is MATLAB2019b and the main modeling is done Simulink using the simscape and the normal library.
Theory
An electric vehicle (EV) is defined as a vehicle that uses an electric motor or a traction motor for propulsion as opposed to an internal combustion (IC) engine used by conventional vehicles. Electric vehicles use electricity for power instead of fossil fuels as conventional vehicles would and because of this difference, electric vehicles do not produce any exhaust gases or pollution. The power needed for the operation of an EV can come from a variety of sources such as batteries, generators, or a collector system. Batteries fitted into the body of an EV are capable of high energy densities in order for them to meet the power demands and range requirements. Generators are machines that generate electrical energy from an input of mechanical work and are a part of the electric motor, and they operate during braking to produce the phenomenon of 'regenerative braking'. Collector systems are off-vehicle systems that allow for the charging of an EV from homes or public charging stations. The electricity used for charging in these systems is generated from solar panels which can be used by the grid or by homes of EV owners so that they can charge their vehicles more conveniently.
EVs are increasing in demand in recent years due to the need to reduce vehicle emissions which are damaging to the environment. Government incentives and private companies are working on schemes to quicken the adoption of electric vehicles. Since EVs do not produce any emissions, they are more environmentally friendly than conventional vehicles are. The generation of the electricty used to charge the vehicles also comes from solar energy (a renewable energy source) and not fossil fuels which makes them sustainable. EVs can meet the same performance standards of conventional vehicles and even surpass that of theirs and this has been seen with the automotive company Tesla. The Tesla Model 3 is the best selling EV in the world and is capable of a range of approximately 350km on single charge. It also has an acceleration of 0 - 60 mph in 5.6 seconds and a top speed of 130 mph.
EVs offer several advantages over conventional vehicles besides the lack of pollution and lack of need to rely on fossil fuels. One is that, EVs are more efficient meaning that they offer more MPGe (miles per gallon gasoline equivalent), meaning that you will spend lesser on recharging an EV than you would to refuel a conventional vehicle. This because the more efficient the EV is, the more distance you travel with the available energy. In the US, Tesla owners can charge their EVs for free at their private charging stations for life which results in significant savings in fuel. Regenerative braking in EVs adds to their efficiency as the unused energy in braking gets reused purposefully. Another benefit is that EVs have fewer moving/rotating parts and this means lesser maintenance costs than you would have with IC engine vehicles e.g. changing oil, transmission fluid. This, in the long term would give better savings than you would get with conventional vehicles. Lesser moving parts means there is lesser friction between moving parts and hence quieter operation.
In spite of these feats, it will be difficult for a worldwide adoption of EVs for several reasons. One reason is convenience. In order for EVs to dominate the automotive market, their needs to be more convenient ways for consumers to be able to charge their vehicles. There needs to be several charging stations on the highways and roads and this can be supported if everyone's homes were generating solar power. It needs to be as easy to charge an EV as it is to fill an IC engine vehicle with fuel at a petrol station. Another reason is the cost of EVs. EVs are still a relatively new technology and only account for 2-3% of the worldwide market. This is because they cost considerably more than conventional vehicles due to the high cost of batteries and electric components. But, the cost of batteries is getting cheaper over time and manufacturers are developing newer and more efficient batteries. The goal is to produce a 'million-mile' battery which, if made will be a revolution in the automotive sector. Another disadvantage is that the components in EVs operate at high voltages and currents and this can be a hazard if the components explode or don't work as they should.
A compromise between IC engine vehicles and EVs exists as Hybrid Electric Vehicles (HEVs). HEVs combine an IC engine with an electric propulsion system to produce a 'hybrid' between the two different classes of vehicles. The purpose of this is to use the power available from the electric powertrain to improve the fuel economy of the vehicle so that it does not rely solely on the petrol/diesel it uses as fuel. This in turn will also improve the overall performance of the HEV. The IC engine can be used to power the electric motor to charge the battery or use directly in the electric drive motor with a combination of a motor and generator in one. HEVs also implement regenerative braking to recharge their batteries among other features used in EVs. Compared to EVs, HEVs represent more of the automotive market with a 19% market share on all automobiles in the world. This is because HEVs still run on fossil fuels which is still more convenient as a fuel source in most parts of the world. The all-time best selling HEV to date is the Toyota Prius.
In this report, I will be modelling an electric vehicle with a DC motor and battery system using the SIMULINK software. The components I will need for my model exist as blocks in the SIMULINK block library. I will be modelling this EV for a drive cycle and measure the velocity characteristics and the current and SOC characteristics of its battery system.
Components
The main components of an EV are:
- batteries
- power converters
- motor controllers
- motor/generator
- transmission system
- wheels/tires
Lithium-Ion Battery
Most electric vehicles use Lithium-Ion Batteries (Li-Ions or LIBs). Lithium-ion batteries have higher energy density, longer life span, and higher power density than most other practical batteries. Complicating factors include safety, durability, thermal breakdown, and cost. Li-ion batteries should be used within the safe temperature and voltage ranges in order to operate safely and efficiently.
Increasing the battery's lifespan decreases effective costs. One technique is to operate a subset of the battery cells at a time and switching these subsets.
In the past, Nickel Metal Hydride batteries were used among EV cars such as those made by General Motors. These battery types are considered outdated due to their tendencies to self-discharge in the heat. Also, the batteries' patent was held by Chevron which created a problem for their widespread development. These detractors coupled with their high cost have led to Lithium-ion (Li-Ion) batteries leading as the predominant battery for EVs.
Lithium-ion batteries' price is constantly decreasing, thus, making electric vehicles more affordable and attractive on the market.
Figure 1: Electric Bus Powered by Lithium-Ion Batteries
Electric Motor
The power of a vehicle's electric motor, as in other vehicles, is measured in kilowatts (kW). 100 kW is roughly equal to 134 HP, but electric motors can deliver their maximum torque over a wide RPM range. This means that the performance of a vehicle with a 100 kW electric motor exceeds that of a vehicle with a 100 kW internal combustion engine, which can only deliver its maximum torque within a limited range of engine speed.
Energy is lost during the process of converting electrical energy to mechanical energy. Approximately 90% of the energy from the battery is converted to mechanical energy, the losses being in the motor and drivetrain.
Usually, direct current (DC) electricity is fed into a DC/AC inverter where it is converted to alternating current (AC) electricity, and this AC electricity is connected to a 3-phase AC motor.
For electric trains, forklift trucks, and some electric cars, DC motors are often used. In some cases, universal motors are used, and then AC or DC may be employed. In recent production vehicles, various motor types have been implemented, for instance: Induction motors within Tesla Motor vehicles and permanent magnet machines in the Nissan Leaf and Chevrolet Bolt.
Configurations and Blocks Used
There are Numerous blocks used in this Model. Some of the blocks are joined together to make a Subsystem as well. These blocks can either be simscape or physical system blocks.
1. Vehicle Body Block
This is the basic Block that we have used in the Model. This block consists of 4 tires of the electric vehicle and a vehicle body.
Figure 2: Model of the vehicle body.
In this model, we have used a simple gear to connect with the Axles. The tire has 4 inputs for them. These are
A - Axles
S - Slip
H - Hub
N - Normal Force
These tires are then connected to the vehicle body.
The vehicle body as we can see in figure 2 has various inputs that can be modeled for the vehicle to have a basic setup. It consists of
1. NR - Normal force Rear
2. NF - Normal Force Front
3. H - Hub Connection
4. V - Speed of the Vehicle Body
5. beta - The grade if the vehicle has
In this model, I have put wind velocity at a constant of 2m/s and the grade to be a constant zero throughout the cycle.
In the vehicle body model, we have connected a simple gear system to transmit the power from the motor to the wheels. This gear has a specific gear ratio that is predefined.
Figure 3:Gear Ratio
Figure 4: Vehicle Body Parameters
2. Motor
For this Model, I have used a simple brushed DC Motor.
After researching on the internet i found some documents that helped me understand the meaning of the terms that are important for a DC Motor.
a. Armature Inductance:
The inductance i.a. of the armature is the reason why the power of a motor can be smaller than the product of RMS voltage and RMS current, and a power factor needs to be applied.
This basically means that the armature inductance governs the amount of power required by the motor to be delivered to the wheels. This power may be very small so as to move a vehicle or very high as to move the vehicle at top speed. Basically, armature is the moving part of the motor that produces the emf which then turns to power the motor.
b. Torque constant:
This is basically the torque a motor is limited to produce in an hour.
Figure 5: DC Motor Parameters.
3. PWM Averaged Model
For any Motor to run we need a controller to control the voltage and current supply from the battery. For this model, I have used a PWM averaged model that takes the input from the duty cycle and feedback from a longitudinal driver.
This averaged model also consists of ports for connections of the battery pack and the motor.
Figure 6: PWM Averaged Block
In this block, we have used Controlled voltage and current source for the motor output. A voltage sensor has also been used here for sensing the battery voltage at any given point in time. One of the connection ports leads to output to the duty cycle through the driver model.
4. Longitudinal Driver
This Model consists of the commands that a driver will give to the vehicle and based on those commands the various scopes are added to monitor the output from the driver. This Block is called a longitudinal Driver.
Figure 7: Longitudinal Driver
This block takes various parameters as a reference to the vehicle. Like Velfdbk and velref.
VelRef takes the input from the drive cycle that we use for the simulation. For this simulation of the electric vehicle, we have used a UDDS Cycle.
Figure 8: UDDS Cycle
5. Battery Pack
For this Electric Vehicle Model, I have used a Lithium-Ion discharge Charge Battery. This Battery has many sensors such as temperature Sensor SOC condition Etc.
Figure 9: Battery Pack Created
For this Battery Pack we have also some parameters set that make this battery pack suitable for our Electric Vehicle.
Figure 10: Battery Parameters
RESULTS
1. For the results we have attached much outputs capturing scopes in the model. For these outputs, we need to make sure that we have the proper connections.
We have used a Normal Body subsystem that has two outputs connecting to the motor and to a feedback scope.
Figure 11: Body to motor Connection
In the above figure, we can see that the body is connected to the motor via the rotor output. This R output is connected to the simple gear we have used in the body subsystem. The cage is connected to a mechanical reference so as to show the stationary nature of the motor. The speed connection of the motor is then connected to scope and to an integrator and a divider block that gives us the distance output that the vehicle will run for the given drive cycle.
NOTE: For the UDDS Drive cycle the Vehicle is running a distance of 12 Kms.
Figure 12: Distance the vehicle has traveled
2. When we move onto the Battery, there are a lot of areas where we can analyze the battery for better results.
Some of the areas I have covered in the results for this analysis.
I have used a temperature measurement block that takes the input of ambient temperatures of the battery and applied a scope to the output to check the thermal management of the battery.
For an average battery under constant operation, it should heat up to a maximum of 45 degrees for charging and 60 degrees for discharging. For this battery pack, the discharging temperature reached up to a maximum of 52 degrees.
Pertaining to the fact that this result is more than sufficient for the analysis we need to consider that the result is for a particular drive cycle and a nominal speed.
We can check the varying discharge temperature using other drive cycles or using a signal builder where we can subject the battery to high speeds.
Figure 13: Battery Temperature Rise For the Drive Cycle.
3. For the State of charge, We can observe some dips and some constant regions. These Dips are mainly observed on acceleration feedback from the longitudinal driver to the PWM averaged block and to the Battery.
The constant regions in the SOC graph are the areas where the battery is in regenerative braking or in the deceleration feedback mode. The ups in the graphs are due to the charging of the battery.
Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object by converting its kinetic energy into a form that can be either used immediately or stored until needed. In this mechanism, the electric traction motor uses the vehicle's momentum to recover energy that would otherwise be lost to the brake discs as heat. This contrasts with conventional braking systems, where the excess kinetic energy is converted to unwanted and wasted heat due to friction in the brakes, where the energy is recovered by using electric motors as generators but is immediately dissipated as heat in resistors. In addition to improving the overall efficiency of the vehicle, regeneration can significantly extend the life of the braking system as the mechanical parts will not wear out very quickly.
Figure 14: Battery SOC Reading
From the battery block, we can also observe the current profile for the UDDS Cycle.
Figure 15: Cycle Current Profile
4. When we talk about the acceleration and deceleration of the graph cycle we know that the vehicle has accelerated or decelerated at many time stamps. Basically the driver has pressed the accelerator and the decelerator many times during the drive cycle. Maybe he was in a traffic situation where he needed to break and move forward on simultaneous occasions.
Figure 16: Acceleration and Deceleration data from the drive cycle
We can observe that the data is scattered due to the multiple uses of the throttle and brake.
Figure 17: A closer view of the acceleration and deceleration data
5. For the main output we have put a scope with two inputs. One input is the vehicle feedback and the other is the speed of the vehicle. For our simulation of the vehicle to run perfectly, we need a match on the drive cycle for the position and the speed.
The maximum speed the drive cycle can attain is 93 Kmph.
Figure 18: Output Graph
From the graph, we can see that the Drive cycle (Upper window) is almost identical to the speed output we get from the simulation.
We can also observe some noise in the speed graph.
Figure 19: Noise in speed graph in comparison to the cycle graph
This noise is observed due to the wind speed we have mentioned in the vehicle body subsystem. For a normal drive cycle, the wind velocity is neglected but we have taken into account a 2 m/s wind velocity for our simulation. Due to the hindrance caused by the wind in the motion, our vehicle tries to keep the graph at a constant with compliance to the drive cycle. This is the reason why we observe such noise in the graph for our vehicle speed.
The model has been analyzed in the simulation and the results have been mentioned.
Figure 20: Electric vehicle Model
CONCLUTIONS
The model has been made and the simulation has also been run. We can conclude that the model is dependent on many factors like drive cycle, Battery modeling, Vehicle parameters, etc.
These factors can be altered in the future to make an even more efficient electric vehicle that would run on more complex cycles with better battery management systems and an even more powerful motor.