Final Project: Design of an Electric Vehicle

Objective:

Lets design and model an electric power train for Tata Tigor EV.

Vehicle specifications required for our modelling:

Sl.No	Parameter	Value
1	Kerb weight	1235 Kg
2	Length*breadth*height	4m*1.7m*1.5m
3	Top speed	120kmph
4	Acceleration (0-100kmph)	12.63seconds
5	Tyre dimension	175/65 R14
6	Maximum restart gradeability	29%
7	Range per charge	300km

Assumptions:

Sl.No	Parameter	Value
1	airdensity(ρ) kg/m3	1.225
2	aerodynamic drag coefficient (Cd)	0.28
3	Rolling resistance coefficient (μrr)	0.02
4	Gear ratio	18
5	Gear efficiency	0.9

Vehicle dynamics

Force calculations:

Max total tractive force required, Ftrac = Acceleration force(Fa)+Aerodynamic drag force(Fd)+Rolling resistance force(Fr)+Grade force(Fg)

Acceleration force(Fa)

= Total vehicle mass * acceleration

= (kerb weight + passenger weight + luggage) * acceleration

= (1235+(70*5)+200)kg * 2.2 m/s²

= 3927N

Aerodynamic drag force(Fd)

= 0.5* airdensity(ρ) *aerodynamic drag coefficient (Cd) * vehicle frontal area (A) * velocity²(V² )

= 0.5 * 1.225kg/m³ * 0.28 * 2.6m² * 33.33² m/s

= 495.3N

Rolling resistance force(Fr)

= Rolling resistance coefficient (μrr) * Total vehicle mass(m) * acceleration due to gravity(g)

= 0.02 * 1785kg * 9.8m/s²

= 350N

Grade force(Fg)

= Total vehicle mass(m) * acceleration due to gravity(g) * sin(angle of max gradeability in degrees)

= 1785kg * 9.8m/s² * sin(tan^-1(29/100))

= 4872.23N

Hence The total tractive force, Ftrac = Fa+Fd+Fr+Fg = 3927+495.3+350+4872.23 = 9644.5N

Power calculations( at extreme limits) :

Max Acceleration power = Fa*V = 3927 * 33.33 = 130KW

Max Power to over come air drag = Fd * V = 495.3 * 33.33 = 16.5KW

Max power to over come rolling resistance = Fr * V = 350 * 33.33 = 11.6KW

Max power for hill climbing(at speed 10 m/s ) = Fg * V = 4872.23 * 10 = 48.7KW

Hence The total tractive power, Ptrac = 206.8 KW

Motor selection

Maximum Torque to be developed by the motor to meet the maximum tractive force is

Tmax = (Total Ftrac * wheel radius) / (Gear ratio * Gear efficiency)

Wheel radius, r = (0.65*175mm) + 14 inches = 0.45 meters

Tmax = (9644.5*0.45)/(18*0.9) = 268Nm

Angular speed, ω = Pout/Tout = Ptrac/T = 206800/268 = 772 rad/sec = 772 * 9.55 = 7370 RPM

Therefore we will have to choose a motor which can deliver a peak power of 207kw and peak torque of 268Nm.

Lets select below motor from the IPM MV275 series, manufactured by Dana TM4:

MV275 Internal Permanent Magnet Motor

Battery design

The analysis and calculations for battery pack design is made with the help of excel as below:

ASSUMPTIONS
Energy Consumption [Wh/km]	270
4 - wheeler range [km]	300
Battery DoD	85%
Battery voltage [V]	380
Battery total energy [Wh]	95294.1176
Battery capacity [Ah]	250.773994

We have considered below 7 types of cells from different manufactures to analyse and find the best suited cell for our battery pack.

TYPE OF Li-ion CELLS CONSIDERED
Manufacturer	Panasonic	A123-Systems	Molicel	A123-Systems	Toshiba	Kokam	Kokam
Type	cylindrical	cylindrical	cylindrical	pouch	Pouch	pouch	pouch
Model	NCR18650B	ANR26650m1-B	ICR-18650K	20Ah	20Ah	SLPB78216216H	SLPB7570270
Source	[4]	[5]	[6]	[7]	[8]	[9]	[9]
Length [m]	0.0653	0.065	0.0652	0	0	0	0
Diameter [m]	0.0185	0.026	0.0186	0	0	0	0
Height [m]	0	0	0	0.227	0.103	0.272	0.272
Width [m]	0	0	0	0.16	0.115	0.082	0.082
Thickness [m]	0	0	0	0.00725	0.022	0.0077	0.0077
Mass [kg]	0.0485	0.076	0.05	0.496	0.51	0.317	0.317
Capacity [Ah]	3.2	2.5	2.6	19.5	20	31	15.6
Voltage [V]	3.6	3.3	3.7	3.3	2.3	3.7	3.6
C-rate (cont.)	1	10	1	1	1	2	2
C-rate (peak)	1	24	2	10	1	3	3

CELL LEVEL CALCULATIONS
Manufacturer	Panasonic	A123-Systems	Molicel	A123-Systems	Toshiba	Kokam
Type	cylindrical	cylindrical	cylindrical	pouch	pouch	pouch
Model	NCR18650B	ANR26650m1-B	ICR-18650K	20Ah	20Ah	SLPB7570270
Energy [Wh]	11.52	8.25	9.62	64.35	46	114.7
Volume [l]	0.017553	0.03451	0.017716	0.26332	0.26059	0.171741
Energy density	237.53	108.55	192.4	129.74	90.2	177.16
gravimetric [Wh/kg]
Energy density	656.31	239.06	543.01	244.38	176.52	327
volumetric [Wh/l]

BATTERY LEVEL CALCULATIONS
Manufacturer	Panasonic	A123-Systems	Molicel	A123-Systems	Toshiba	Kokam
# of cells in string [-]	106	116	103	116	166	103
String energy [Wh]	1221.12	957	990.86	7464.6	7636	11814.1
# of strings [-]	79	100	97	13	13	9
BP energy [kWh]	96.46848	95.7	96.11342	97.0398	99.268	106.3269
BP capacity [Ah]	252.8	250	252.2	253.5	260	279
# total cells [-]	8374	11600	9991	1508	2158	927
BP mass [kg]*	406.139	881.6	499.55	747.968	1100.58	293.859
BP volume [l]*	146.988822	400.316	177.000556	397.08656	562.35322	159.203907
BP peak current [A]	252.8	6000	504.4	2535	260	837
BP peak power [kW]	96.064	2280	191.672	963.3	98.8	318.06
BP continuous current [A]	252.8	2500	252.2	253.5	260	558
BP continuous power [kW]	96.064	950	95.836	96.33	98.8	212.04

From the above analysis we can conclude that building the pack with Molicel would be the most suited selection for our application.

Hence the battery pack configuration will be: 103S97P

Pack capacity: 95KW or 250Ah

Simulink model

A complete EV vehicle was modelled in simulink using the above calculated parameter configurations as shown below:

Vehicle subsystem:

This block implements all the vehicle dynamics related design parameters such as mass, rolling resistance, air drag, grade angle which decides the total tractive force required by the vehicle.

Motor and controller subsystem:

For sake of simplicity here we have chosen a simple DC motor model with above calculated ratings..

Longitudinal driver subsystem:

This block will act as a PID controller working in closed loop which compares the feedback actual vehicle speed with the reference speed input and calculate the error and provide the acceleration or decceleration commands accordingly to the motor controller.

Drive cycle source:

It is a predefined set of data of speed vs time which can be used to analyse the vehicle performance. Here we have used FTP-75 drive cycle which has a total range of 17.7km and total time of 2474 seconds.

Top speed: 25m/s

Average speed: 7.1m/s

Battery subsystem:

For simplicity here we have chosen a li-ion battery available in Simulink and provided our above calculated ratings. This battery system model is designed to find out the SOC and battery voltage and current characteristics during the drivecycle. Hence we are taking the motor armature current provided by the H-bridge(Power supplied internally) and feeding that current to the battery via a controlled current source. Therefore the battery will get charged or discharged accordingly and we will get the SOC as desired.

Simulation Results & Analysis

• The developed simulink model was simulated for the time period of the FTP-75 drive cycle 2724 seconds to analyse if the vehicle speed is following the reference speed correctly.
• 

From the result we can conclude that the model is precisely tracking the reference speed..

• The developed simulink model was simulated for a range of 300 km to analyse the battery SOC and mileage of the vehicle.

Considering our drive cycle FTP-75, one drive cycle is for 17.77km

17.77*17 = 302.09 km

Therefore, we have to provide 17 cycles of FTP-15 drive cycles as input

Total execution time = time for one cycle * 17 = 2747seconds * 17 = 46699 seconds

From the result we can see, SOC starting from 100% has dropped to 15% towards the end of the simulation. Also the the battery voltage and current fluctuation is within the desired limits. Hence, we can conclude that the chosen battery capacity 95KWh and motor ratings are adequate to meet the desired traction power and to provide the assumed range of 300Km/charge.

• We have successfully designed and configured a complete EV system and modelled the parameters and implemented in Simulink.