Design of an electrical car

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This article is a laboratory study on the construction of an electric vehicle. The main aim of the experiment was to build and create an electric vehicle. This was done by identifying the myriad fundamental problems in car production, such as balance, energy losses and other forces of nature that obstruct its functionality. In the course of deployment, an electric car concept with a mass of approximately half a kilogram driven by a small electric motor was used. An study of the power transmission and distribution mechanism has been researched and analyzed with respect to efficiency and the level of production of the full amount of power. A wooden track was used to determine the energy losses of the car by determining the speed it gains from the upper part to bottom of the track. In the determination of the coefficient of friction between its tires and the surface, the track is gradually inclined to an angle in which the car ceases to be stationary. It is surprising how much energy the car loses, and this was noted as the main challenge of the design. The forces inhibiting movement were comparatively very high. To improve the model, the fabrication ought to be improved to minimize unnecessary losses. The optimum motor rotation rate ought to be determined to ensure the highest efficiency and power output. Regarding the physical design, computer aided design was recommended to ensure the car is streamlined and balanced. Table of Contents

Summary2

Table of Contents3

Notation5

Introduction6

Background6

Objectives and Goals6

Methods and Discussion7

Motor Torque7

Requirements8

Test 1 Procedure9

Calculations9

Power In10

Power Out10

A Graph of Power against Rotational Speed11

Discussion11

A Graph of Motor Efficiency against Motor Speed12

Discussion12

Test 2 Procedure12

Slope Test13

Procedure14

Calculations14

Grip Test14

Procedure15

Tyre 1 (80 grams)15

Tyre 2 (85 grams)16

Gear Ratio16

Theory16

Question 1: Time taken by each gear ratio to complete the track16

Question 2: Reasons why the car will not finish the lane in the calculated time17

Question 3: Risk assessment analysis17

Discussion18

Possible Source of Errors18

Conclusion18

Future Recommendations19

References21

Table of Notation

ɲ : efficiency

Ʋ : the coefficient of friction

g : gravitational acceleration (Assumed to be 9.81 throughout the report)

Ø : Angle of Inclination

ɯ : Angular Velocity

N : Rotational velocity

Electrical Car Design Lab Report

Introduction

Background

The development of efficient, reliable, fast, and economic electric cars has been an issue of great concern to many stakeholders. Among their advantages, there are environmental friendliness, low noise pollution, and aesthetics among many other factors. However, unlike the conventional gasoline-powered cars, the challenge of sufficient power storage, efficiency and speed have inhibited the development of previous models. It is in that regard it was decided to develop a prototype of an electric sports car using locally available materials.

In the process of developing this prototype, the main challenges encountered in development of real electric cars are expected. Therefore, the solutions developed may be applicable to real cars; there can be car-manufacture industry for the development of reliable and efficient models to help transform the automotive industry. Through the process of developing a feasible model, one had to apply a set of scientific principles in the implementation of car with an ability to move on track. For easy implementation and analysis, the experiment was divided into four stages, which were accomplished separately. They included chassis design, choosing of gears, fitting of wheels, and testing the model. In this regard, the experiment will encompass the testing of crucial parameters such as the grip losses as a result of frictional losses, the motor torque, which is the power output levels, and the slope test, which is the energy losses of the car during movement.

Objectives and Goals

The primary goal of the experiment was the development of an electrical car prototype, a project which was divided into the following objectives:

To determine the efficiency of the model through comparing the power input with power output;

To define the total friction losses by comparing the energy possessed by the car before descending downslope with its energy as it arrives at the lowest poi of the slope.

To determine the coefficient of friction between the tires used and the surface.

Methods and Discussion

The energy conservation principle was applied during the slope test. Theoretically, the potential energy of the car should have been transformed into kinetic energy save for the losses (Brennan 2017). For the grip test, the coefficient of friction which inhibits movement was measured by adjusting the slope. The motor efficiency test applied electrical principles that the input electrical energy should be equal to the output energy save for the losses.

The project was implemented in four distinct stages. The first stage was the chassis design. During this stage, the exact position occupied by the chassis was closest to the middle of the car to enhance its equilibrium percentage. It had diminutions of 85 mm width and a height of 160 mm. The second phase was the choice of gears. Different compensation values were picked and applied to find the wheels of rotation. A gear ratio of 1.46 and a wheel rotation of 7,500 rpm were attained. The third stage was the fitting of the wheels. Four holes were drilled on the chassis, both on the back and on the front, such that they were parallel to facilitate a straight movement for the car. The last stage was the testing of the prototype. The car was placed on the track and its motor switched on. It moved in a linear movement as the motor rotated.

Motor Torque

An electric motor, which was powered by a battery, was used to power the electrical car model. In the power analysis it had to measure the power, torque, and force produced by the motor. A dynamometer was adopted for this purpose, as it aids in measuring the above parameters and in the calculation of varying efficiency with speed variations. The dynamometer works as a generator, since it is connected to the motor, which is surrounded by a wire coil; the resultant magnetic flux as result triggers an electric current, which travels through the coil. However, the electrical load subjected to the dynamometer can be varied by using a variable resistor to appropriate levels. As the electrical load an increase, a higher value of torque is attained by the dynamometer; hence, the motor draws more current. The amount of torque is directly proportional to the current (Kirkpatrick and Francis 2010). The motor and the generator were directly connected with a shaft; thus, mechanical energy was transformed directly to the generator. The dynamometer is fixed to an ordinary gimbal such that it is restricted from other movements but rotation. The setting is connected to a load cell, whose function is to measure the output force by determining the amount of force necessary for stopping the dynamometer completely. This, therefore, should be equal to the torque generated by the motor at that instance. The rotational speed of the dynamometer, on the other hand, depicts the amount of power output; hence, the efficiency at different speeds can easily be determined by comparing the power input with the power output (Mulcahy 1999).

Requirements

one off motor

two off wide wheels

two off narrow wheels

two off axles

one off chassis base at maximum size

four off bearing blocks

one off toothed drive belt

one off track pick up

one off SIM-card

Small tool kit

A list of selected tooth drives

Test 1 Procedure

Set the motor voltage to 3.7 V

Set the motor current to five amps

Note the load and rotational speed measurements

Repeat for motor currents: 7, 9, 11, 13 and 15 amps

Fill out tables one and two with your results

Plot a graph of power vs. rotational speed

Plot a graph of motor efficiency vs. rotational speed. When is the motor most efficient?

Calculations

To calculate the power delivered to the motor,

Power input = current × voltage

The output force through the load cell is measured in grams force, which can be converted to Newton.

Force = × 9.81

The torque produced is calculated through converting the grams force by considering the length of the arm as shown below.

T = F (N) l

In this case, the length of the torque arm was l = 0.022Metres.

The amount of power delivered by the motor is, therefore, a product of the angular velocity of the shaft and the amount of torque.

POUT = ɯ T

The rotation speed is converted to angular velocity, and the efficiency of the motor can be calculated using the following formula:

Efficiency (ɲ) = 100 ×

Speed of rotation, the power output and the efficiency were recorded and tabulated on the tables below for analysis.

Power In

Current A

Pin (Watts)

Load (gf)

Load (N)

Torque (Nm)

3.55

13.135

15.6

0.153

3.37 × 10-3

7.27

26.96

98.4

0.965

21.23× 10-3

9.29

34.37

123.7

1.21

26.62× 10-3

11.48

42.48

161.3

1.58

34.76× 10-3

13.26

49.06

213.9

2.10

46.2× 10-3

15.48

57.28

251.5

2.47

54.34× 10-3

Power Out

Speed of rotation (N)

Angular speed (ɯ)

Pout (Watts)

Efficiency (%)

7200

753.98

2.54

19.33

5800

607.37

12.89

47.90

5100

534.07

14.32

41.66

4160

435.6

15.14

35.64

3415

357.6

16.16

32.94

2630

280.65

15.25

26.62

A Graph of Power against Rotational Speed

Discussion

From the above graph, the maximum power is produced when the motor is rotating at a speed of 3,415 rpm. This attribute of the motor must be determined, since this is the most efficient operational speed, when maximum power needs to be derived.

A Graph of Motor Efficiency against Motor Speed

Discussion

As seen on the graph above, the motor efficiency is lowest at its highest speed. However, upon decreasing the rotational speed to 5,800 rpm, the motor achieves its highest efficiency levels. Therefore, to maximize efficiency, the car should operate at this speed, or a speed, which is closer to this value. However, as the rotational speed increases beyond that, the efficiency diminishes.

Test 2 Procedure

Set the motor voltage to 6 V

Set the motor current to 14 amps

Note the load and rotational speed measurements

Fill out tables three and four with your results. We will use these results later to calculate whether wheel spin will occur in your car.

Note: The setting should not stay like this for very long as the motor may burn out.

At this stage, the motor parameters were set at six Volts and 14 Ampere, and the data below recorded.

Current

Pin

Load (gf)

Load (N)

Torque

Rot. Speed

Angular speed

Pout

Efficiency

14

84

Slope Test

A significant amount of energy is lost through friction. This part, therefore, aims at the determination of frictional losses of the model, as it descents down the slope. In this part, the potential energy is compared with the kinetic energy, as the car descents freely downslope. By using a timer to determine the time taken by the car to reach at the bottom of the slope, its speed can be determined and hence its kinetic energy. The frictional forces can be reduced in future to minimize the losses.

Energy lost = Total Potential Energy – Total Kinetic energy

ELOSS = mgh – ½ mv2

Where m= mass of the car

H= the height of the beginning of the track

V = the velocity of the car

G= the gravity acceleration

To obtain the velocity of the car at the lower end of the slope, this formula is used.

s = (u + v) t

u = the in initial speed of the car

s = the displacement or distance travelled by the car

t = the duration taken by the car to get to the bottom of the slope.

Procedure

Remove the electrical pick up from the car

Measure the dimensions of the slope

Measure the mass of the car

Get the average time the car takes to roll down to the end of the slope and calculate its energy losses

Record the results on a table

Calculations

In this experiment, the height of the slope h= 0.76 meters

The displacement distance; s = 2.13 meters.

Given the mass of the car was 0.549 KGs, its potential energy at the beginning of the track was

PE = mgh

PE = 4.09

The kinetic energy at the end of the slope was only a small percentage of the initial potential energy, i.e. only 6.7%, only a very small amount of energy is transformed to kinetic energy hence the losses are significant.

Given the time taken downslope, t was 1.79, the following parameters were calculated. ,

Design

M (kg)

T (s)

V (m/s)

PE (j)

KE (j)

E loss (J)

1

0.549

1.79

1.19

4.09

0.275

3.815

Grip Test

The grip test is used to determine the constant coefficient of friction between the wheel the wooden track. In the implementation of this test, the car wheels are fixed in manner, in which they cannot rotate, and placed on an inclined slope, where they are held by a loose string. The inclination angle is gradually increased to a pint, when the wheels begin to slide, and the angle of slope is measured to determine the tire grip. The grip of the tire is determined through this critical angle. The force due to friction, F, is calculated by:

F = ƲR

Ʋ = the coefficient of friction

R = the reaction force

A larger coefficient of friction implies more grip to the track surface.

Calculating the forces to both sides,

R = mg Cos Ø

M = mg Sin Ø

Hence

Ʋ = = Tan Ø

Procedure

The wheel being tested are placed on the middle of the slope

The wheels are attached to the top of the slope using a piece of loose string

The angle of inclination is increased to a point in which the wheels begin to slip and use the angle to calculate the coefficient of friction.

Record the coefficients of friction for the various tires.

Tyre 1 (80 grams)

Test Number

Ø (degrees)

Ø (rad)

Ʋ

1

32.0

0.56

0.63

2

32.5

0.57

0.64

3

33.5

0.58

0.66

Tyre 2 (85 grams)

Test Number

Ø (degrees)

Ø (rad)

Ʋ

1

35.1

0.61

0.70

2

33.2

0.58

0.66

3

33.0

0.58

0.66

Gear Ratio

Theory

The available gears were the 10, 12, 15 and the 22 teeth types. The motor speed was 11000rpm.

Gear Ratio =

S1 × T1 = S2 × T2

The circumference is obtained through =

D = 5.4 cm

Question 1: Time taken by each gear ratio to complete the track

T1

T2

S1

S2

Time taken to complete

10

12

11000

9166.7

0.695

10

15

11000

7333.33

0.868

10

22

11000

5000

1.274

12

15

11000

8800

0.723

12

22

11000

6000

1.061

15

22

11000

7500

0.85

Question 2: Reasons why the car will not finish the lane in the calculated time

Several factors will cause disparities between the theoretical time and the practical time such as:

Losses due to friction in the gears and on the track;

Energy losses through sound and heat as power is transmitted from one gear to the other;

Drag losses and losses associated with starting at a stationary state;

The weight of the car and its internal components is also likely to cause variations in time.

Question 3: Risk assessment analysis

Type of Risk

Risk level

How to reduce the risk

Risk of getting dust and other moving material in eye, especially during fabrication

High

It is recommended to put on safety goggles

A risk of sustaining cuts when using the saw

Medium

Precautionary measures when using the saw

Sticking of clothes into running machines

Medium

Appropriate gear ( safety apron) ought to be worn in the laboratory

Burning with heat saw

Medium

Care ought to be taken when using the same, and safety gloves can minimize the risk

Burning with vacuum foam

Low

Be cautionary when using the same

Cutting yourself with a knife

Medium

Use the cutting rule

Pinch risk from moving gears

Low

Avoid touching moving gears

Discussion

Initially, the car did not move at all, despite the motor being completely functional. After close scrutiny into the design of the car, it was found that the front wheels were properly fixed in a parallel manner, and this hindered the car from moving in a straight direction. This was resolved by the creation of new holes. The second challenge facing the car was the friction coefficient. To improve the friction coefficient to prevent slips, the weight of the car had to be increased. However, the vertical force increased, hence inhibiting movement. Through modifications and continuous improvements, the car moved across the track completing the distance in varying durations as shown below. The mean time taken to complete the track was calculated as 5.88 seconds.

Trial Number

Time Values (Duration)

1

Failure

2

5.36

3

6.99

4

5.72

5

5.48

Possible Source of Errors

Human errors: Errors may have been incurred during the reading and the recording of data

Conclusion

Despite experiencing various design related challenges, the primary goal and objectives of the experiment were achieved. It was managed to design an electric car, which was able to move across the set track. In addition, the various objectives in the beginning of the experiment were reached, and the various parameters, which influence the workability and efficiency of electric cars, were determined. By establishing the scientific principles behind the parameters, and how various design aspects influence these forces, one was at a position to improve specific attributes for efficiency and enhanced performance. Some of the design aspects entail the arrangement of the wheels in a parallel manner. However, it was not managed to physically determine the most appropriate position to mount the chassis. This inhibited movement, as the car was not effectively balanced. In conclusion, the project idea is to improve the group teamwork and use engineering skills and principles in order to build an electrical car. This project has been done in four stages, and these are designing the chassis, choosing the gears, fitting the wheels, and testing the car. Furthermore, the main objective has been achieved after solving the manufactured issues such as the friction force and fixing the wheels by using the scientific principles. Finally, in the future, the car will be modified to complete the track in a shorter time by replacing the gear ratio with a faster one. The frictional losses for this model were approximately 93% of the initial potential energy of the model before it took off. This is highly significant, but can be attributed to fabrication defects and ineffective mechanisms to minimize frictional losses.

Future Recommendations

Although the car managed to move across the track but at a lower speed, future design modifications are expected to give the car a higher speed and even a higher torque. By improving the friction coefficient through the use of wheels made of a material with a higher friction coefficient, the car will be able to minimize any chances of slip. Another approach, which can be adopted to enhance grip, is to determine the optimum mass of the car. Besides, the larger the mass, the less significant the frictional losses; hence, there is need to use graphical means to determine the most appropriate mass, which does not overload the motor, but minimizes the impacts of frictional losses and the coefficient of friction on the model (Singh and Taheri 2015). Regarding the rate of revolution, power and torque, the experiment can be repeated with smaller speed variations, as this will aid in the determination of the rate of revolution, which transforms to the highest engine power and torque. Also, to ensure appropriate balancing of the various components of the car, the chassis and the engine had to assume a balanced position to for equal distribution of weight on all tires for a straight movement. Accomplishing this manually was a huge challenge, and therefore, it is recommended that balance enhancement software and applications such as CAD be adopted in future to ensure the prototype is balanced and has minimal vibration, as they are known to hinder efficiency.

References

Brennan, J. (2017). Conservation of Energy. [Online] Available at: http://jamesbrennan.org/physics/notes/Energy/conservation_of_energy.htm [Accessed 11 May 2017].

Kirkpatrick, L., and Francis, G. (2010). Physics. 1st ed. Belmont, CA: Brooks/Cole Cengage Learning.

Mulcahy, D. (1999). Materials handling handbook. 1st ed. New York: McGraw-Hill.

Singh, K. B., and Taheri, S. (2015). Estimation of tire–road friction coefficient and its application in chassis control systems. Systems Science & Control Engineering, 3(1), pp.39-61.

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