EV design – introduction

Vehicle design is a very complex domain since it involves a lot of attributes, like: interior space, dynamic performance, active and passive safety, connectivity, etc.. The same principles apply to either electric vehicles or conventional vehicles (with internal combustion engines) design.

In a series of articles we are going to focus on the high level design of an electric vehicle drive unit and battery pack. The same calculation methods can be applied whether we design an electric vehicle from scratch or if we do a conversion from a conventional vehicle.

The current article is an introduction, the full content being:

  • EV design – introduction
  • EV design – energy consumption
  • EV design – battery
  • EV design – electric motors
  • EV design – mode management
  • EV design – vehicle systems and components
  • EV design – simulation model
  • EV design – simulation results and evaluation

EV market analysis

The entry point in the electric vehicle design are the performance parameters. In our case we are going to focus on:

  • top speed
  • 0-100 kph acceleration time
  • range

In order to come up with suitable parameters, we can study the current market of electric vehicles and also some statistical data. In the table below you can find a list of some electric vehicles and their main parameters.

VehicleRange
[km]
Battery
Type
Battery
capacity
[kWh]
Battery
Voltage
[V]
Motor Max
Power
[kW]
Motor Max
Torque
[Nm]
Motor
Type
Top
Speed
[kph]
Time
0-100 kph
[s]
BMW
i3
257Li-ion42.2360127250PM
Sync.
1507.3
Mitsubishi
I-miev
160Li-ion1633049196PM
Sync.
13012.1
Kia
Soul EV
249Li-ion3037581.4285PM
Sync.
14511
Nissan
Leaf
273Li-ion40350110320AC
Sync.
1467.9
Renault
Zoe
303Li-ion4140080225AC
Sync.
13711.4
Tesla
Model S 75D
375Li-ion75350305525AC
Sync.
2284.2

*Sync. (Synchronous)

Source: bmw.co.uk, mitsubishi-cars.co.uk, kia.co.uk, media.nissan.eu, renault.co.uk, teslamotors.com

Note: The table data might not be up to date since vehicle manufacturers release new models with different parameters

From the vehicle models summarised in the table we can extract some trends in electric vehicle design:

  • battery chemistry: Lithium-ion (Li-ion)
  • electric motor type: PM or AC/Induction synchronous
  • range: above 200 km

Regarding electric vehicles, government studies are also a good source for daily commute distances. Depending on country, the daily average commuting can vary between 11 miles in UK up to 40 miles in US.

Average distance of commuting journeys in miles, by main mode (1988 - 2015)

Image: Average distance of commuting journeys in miles, by main mode (1988 – 2015)
Credit: UK Department of Transportation

If, for example, we want to design an electric vehicle which can sustain a week’s commuting in the US, without any charge, its range should be around 270 miles.

Electric vehicle conversion

In our case we are going to focus on the the conversion of a sports car with internal combustion engine into an fully electric vehicle. The model of choice will be 16MY Jaguar F-type, with the main parameters defined in the table below:

Engine3-litre V6 DOHC V6, aluminium-alloy
cylinder block and heads
Maximum torque [Nm]450
Engine speed @ maximum torque [rpm]3500
Maximum power [HP]340
Engine speed @ maximum power [rpm]6500
Transmission typeautomatic, ZF8HP, RWD
Gear ratio1st4.71
2nd3.14
3rd2.11
4th1.67
5th1.29
6th1.00
7th0.84
8th0.67
Final drive (i0)3.31
Tire symbol295/30ZR-20
Vehicle mass (kerb) [kg]1741
Aerodynamic drag, Cd [-]0.36
Frontal area, A [m2]2.42
Top speed [kph]260
Acceleration time 0-100 kph [s]5.3

From data available on-line, we can also extract the static engine torque values at full load, function of engine speed:

Engine speed points (full load) [rpm]100020202990350050006500
Engine static torque points (full load) [Nm]306385439450450367

The engine torque and power at full load is plotted in the image below:

Jaguar F-type 3-litre V6 DOHC V6 torque and power at full load

Image: Jaguar F-type 3-litre V6 DOHC V6 torque and power at full load

The high level requirements of the electric vehicle is to have the same (or better) dynamic performance compared with the internal combustion engine version.

Based on the analysis of the current EV market and the performance parameters of the base vehicle (internal combustion engine), in the table below, we are going to summarise the high level requirements of our electric vehicle conversion.

BatteryNominal voltage [V]400
ChemistryLithium-ion
PowertrainElectric motor typePM Synchronous
VehicleRange [km]250
Top speed [kph]260
Time 0-100 kph [s]5.3
Kerb weight [kg]1741

In order to achieve the required performance, the electric vehicle will be all-wheel drive (AWD), with an drive unit on each axle. The drive unit will contain the power electronics (inverter), electric motor, transmission (fixed gear) with decoupling element (clutch / dog clutch) and differential.

EV conversion powertrain layout

Image: EV conversion powertrain layout

Traction force calculation

One key requirement of our battery electric vehicle conversion is to reach 100 kph from standstill in 5.3 seconds. From this requirement, knowing the vehicle weight, we can calculate what is the total traction force and torque. Also, we can check if the wheel (tire) friction can sustain the required traction force.

The input data in our calculation is:

  • vehicle total weight (vehicle kerb weight x mass factor + driver weight): 1908 kg
  • vehicle initial speed: 0 kph
  • vehicle final speed: 100 kph
  • vehicle initial time: 0 s
  • vehicle final time: 5.3 s
  • tire radius: 0.33565 m
  • tire friction coefficient: 1.0
  • gravitational acceleration: 9.81 m/s2

According to Newton’s second law of motion:

\[F = m \cdot a \tag{1}\]

From (1) we can write the equation of the required traction force as:

\[F_{t} = m_{v} \cdot \left ( \frac{v_{f}-v_{i}}{t_{f}-t_{i}} \right ) \tag{2}\]

where:

Ft [N] – total traction force
mv [kg] – total vehicle mass
vf [m/s] – final speed
vi [m/s]– initial speed
tf [s] – final time
ti [s] – initial time

Replacing the input data into equation (2), gives the total traction force required to achieve 0-100 kph in 5.3 s:

\[F_{t} = 1908 \cdot \left ( \frac{27.78-0}{5.3-0} \right ) = 10000.8 \text{ N}\]

The total required traction torque can be calculated as:

\[T_{t} = F_{t} \cdot r_{w} \tag{3}\]

where:

Tt [N] – total traction torque
rw [m] – wheel radius

Replacing the input data into equation (3), gives the total traction torque required to achieve 0-100 kph in 5.3 s:

\[T_{t} =10000.8 \cdot 0.33565 = 3356.77 \text{ Nm} \]

In terms of friction, we can calculate the available friction force as:

\[F_{f} = G_{v} \cdot \mu_{f} = m_{v} \cdot g \cdot \mu_{f} \tag{4}\]

where:

Ff [N] – friction force
Gv [N] – vehicle weight
g [m/s2] – gravitational acceleration
μf [-] – friction coefficient (wheel-road)

Replacing the input data into equation (4), gives the available friction force:

\[F_{f} = 1908 \cdot 9.81 \cdot 1 = 18717.48 \text{ N}\]

Since the available friction force is bigger than the total traction force, assuming that there is no slip between the wheels and road, the total traction force can be applied at the wheels in order to achieve the 0-100 kph acceleration time.

In the next article we are going to focus on the energy consumption of the vehicle over a standard homologation cycle. This will serve as a basis for the high level design of the battery.

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