Experience - steps to success

Working on this vehicle has been a great experience. We test drove it quite a few times and each test made us know much more interesting things about it. Initially, we couldn’t reach our estimated power and distance but some changes to the design and distribution of apparatus made it efficient. Some of the major problems we faced are mentioned in brief.

One major change which turned the tide was the introduction of chain drive system replacing the belt driven system. The rubber belt was used to connect the motor shaft and the rear axle with the help of a pulley. As the number of tests increased, the belt began to expand due to the heat produced due to the friction between the motor shaft and the belt. This led to increased power consumption by motor which led to fast discharge of batteries. Also the belt had to be tightened by changing the position of motor which is a tedious job. The chain drive system answered all the queries of the previous system by decreasing friction, no requirement to displace motor, decreasing discharge and no jerk movements or slipping of the belt due to initial start-up.

The selection of a reliable braking system is a very important part of achieving proper vehicle control. We started with disc brakes for both front and rear axles. The front axle brakes are making the design of the steering system complex and it had to be replaced. So, they were changed to drum brakes which are placed inside the tire rim by giving us proper space for the steering. We also replaced the rear brakes with drum brakes but due to lack of professional knowledge on this kind of complex braking, we could not achieve 100 % braking. This had a drastic effect on the morale of the group as it resulted in an accident breaking the entire rear axle into half. Finally, we settled the problem by the use of mechanical disc brakes to rear axle which are mounted on the rear axle beside the pulley. It had a simple operation and selection of proper disc brake helped us by achieving efficient braking.  

Some practical changes include placing the thermocol sheets under solar panels and metal apparatus to damp the sound created during the vehicle run. Using a better insulated wiring in place of regular wires to withstand the heat produced by the high current flow. Distributing the weight of the apparatus innovatively by not clustering all the parts of a specific apparatus at a single place.

Minimum mechanical requirements - Braking

Braking system makes an important mechanical entity to any automobile. An excellent braking system is the most important safety feature of any land vehicle. The main requirement of the vehicle’s braking system is that it must be capable of locking all wheels on a dry surface. Ease of manufacturability, performance and simplicity are a few important criteria that are to be considered for the selection of the braking system. The two main types of braking systems under consideration in this report are Drum and Disc brakes. In case of drum braking there is a high possibility of mud and debris to gather in the space between the shoe and the drum. Same problem is faced in mechanical disc brakes, but not in hydraulic disc brakes. Hydraulic brakes are found to be suitable for all type of terrain. Since, drum brakes are of more cost and they are heavier in weight which greatly increases the weight of solar car we can eliminate it. On the other hand, using hydraulic brakes can be an asset as it is cheap and it is readily available but the drawback was using this system the overall weight of the solar car is increased which makes it harder for the motor (linked to battery to solar panels) to run the car. The discs of brakes are made of paralytic grey cast iron. The material is cheap and has good anti-wear properties. Cast steel discs have also been employed in some cases, which wear even less and provide higher coefficient of friction; yet the big drawback in its case is the less uniform frictional behaviour. Two types of discs have been employed in various makes of disc brakes, i.e. the solid or the ventilated type. Disadvantages of ventilated type discs include usual thickness and heavier than solid discs, In case of severe braking conditions, they are liable to wrap, accumulation of dirt in the vents, which affects cooling, resulting in wheel imbalance, Expensive, Difficult to turn. Turning produces vibrations which reduces the life of the disc. Any of these make no much difference on the solar vehicle mentioned in this report as its overweight cannot go beyond 450kgs to 500kgs. Although in the practical version of the solar vehicle done through this report hydraulic drum brakes are used for the front axle and mechanical disc brakes are used for rear axle for experimentation  it is advisable to opt for hydraulic disc brakes for both the front and back axles as they are economical and reliable. 

Desirable Materials that can be used for construction for solar vehicle

In this report, selection of different materials for the chassis and body works is done considering the physical properties of some selected materials. A right material is of utmost importance when it comes to designing a chassis because if a material of correct requirement is not chosen, the chassis could break on loads leading to fatal conditions of the driver. The following are the important considerations for the selection of proper material for the chassis. The material must have high yield strength, high machinability, easy weld ability, low cost, light weight and high elongation at failure.

Some of the materials under consideration include AISI 4130 (DIN 1.7218) chromyl steel (preannealed), AISI 1020 (DIN 1.0402) steel and Al-6063-T1. The problem with AISI 4130 (DIN 1.7218) steel was even though it gave good strength and lighter than mild steel (MS), it is expensive and not easily weld able. Welding AISI 4130 (DIN 1.7218)  steel is not only costly but could not be trusted as it has to be annealed before and after welding yet gives fractures without notice. 

AISI 1020 (DIN 1.0402) steel is cheap, easily available and weld able and with some decent specifications but when analysed for chassis and various components like rear axle, etc., it showed a high deflection of 2- 9mm with very less factor of safety and addition of members to improve strength makes the chassis heavy. Aluminium alloy 6063-T1 gives enough yield strength to withstand all subjected stresses and loads. Though expensive, we cannot compromise on the quality on material for chassis and it is advised to look for a competitive price. Thus, Al-6063 satisfies all other requirements.

Body Works is an important part of the vehicle design. External appearance is an important feature which not only gives grace and lusture to the vehicle but also dominates sale and marketing of it. Each product has a defined purpose. It has to perform specific functions to the satisfaction of customer. The functional requirement brings products and people together. However, when there are a number of products in the market having the same qualities of efficiency , durability and cost, the customer is attracted towards the most appealing and economical product. Three materials such as aluminium, carbon fiber and glass fiber can be considered for aesthetic considerations of the design.

Aluminium shows good properties like light weight, does not rust easily, and has good machinability but is costlier than steel and is very abrasive. Carbon fibre contains some ideal qualities like High stiffness, high tensile strength, low weight, high chemical resistance, high temperature tolerance and low thermal expansion. However, they are relatively expensive when compared to similar fibres, such as glass fibres or plastic fibres. Thus, budget exceeded in its place. Glass fibre is light weight, easily mouldable, easy machining, Fire resistant, Low maintenance, Anti- magnetic, good electrical insulator. However, it is costlier than aluminium but fits into economic range. Selection of glass fibre as the material for moulding the body of the vehicle is an educated choice since glass fibre is cost effective, light weight, has good strength, it fits into the requirement slot for manufacturing the solar vehicle.

Mechanical Prop.	Value (Metric)
Density	2700 kg/m3
Hardness (Brinell)	42
Ultimate Tensile Strength	152 MPa
Tensile Yield Strength	90 MPa
Elongation at Break	20%
Modulus of Elasticity	69 GPa
Poisson’s Ratio	0.33
Melting Point	616-654°C

simple steps to assemble solar vehicle parts.

As all the important aspects for construction of the solar vehicle is covered, there is no requirement of any professional mechanic to supervise you when constructing. The only requirement is a well-equipped workshop which has a good welding, electric cutting machine to cut pipes, plates and solid rods and a drilling machine to drill into metal with different bits. The point wise assembling will make it easier for the assembling to be done within a few hours once all the equipment, materials and simulations are in hand.

 i.     Get the mechanical simulation done in CAD software and evaluate using CAE softwares according to selection process does in the above segments with individual designs and requirements. This simulation is useful to maintain the mechanical properties of the vehicle according to the standards. This also helps in giving a fancy touch to the design of the vehicle.

ii.     Accumulate the mechanical materials according to the quantities mentioned in the simulation like the amount of Al-6063-T1, glass fibre, nuts and bolts, members, spanners, screwdrivers, grease, rack and pinion steering, four suspensions, chain drive system, two disc brakes, two disc brake callipers, hydraulic brake set, four wheels, brake pedals, acceleration pedal, four pedestals with ball bearings, comfortable seats etc.

iii.     Order for all the electrical equipment with above mentioned specifications like BLDC motor, motor controller, solar panels, MPPT solar charge controller, Lithium ion batteries, and required quality of electric wires in abundance. It is also advised to get a soldering set, wire stripper and some high rating legs for the electric wires, two DC voltmeters to measure the voltage at batteries and motor, two DC ammeters to measure the current flowing through batteries and motor, three MCBs.

iv.     Mark the Al-6063-Ti meatal and cut it to make a chassis according to simulation and also give additional members for seating, motor mounting and steering mounting.

v.     Mark two circular rods of equal length for both front and rear axles, fix the disc brake for the front axle with nut and bolt, fix two pedestals from both sides of the axle and fasten it tight without horizontal movement. Weld two solid circular plates perpendicular to the axle at both ends of the axle which fit exactly to the rims of the wheels. Mark holes on the plates, drill them and fasten the wheels to the axle. Weld two small metal parts in the forward direction with holes. Repeat the same with no small metal parts with an additional chain drive gear in exact middle of the axle.

vi.     Attach both the front and rear axles to the chassis with the pedestals with nut and bolt. Also connect the rack and pinion steering set to the front axle by bolting the two ends to the small metal parts with holes and fix the base of the steering to the main body. (If the steering is not tightly fixed, the vehicle cannot change direction). Attach the steering to the extra member provided in point number IV.

vii.     Make a cabinet in the rear to hold the electrical equipment like motor controller, solar charge controller and batteries. Mount the motor in such a way that the motor chain drive gear should be exactly in line with rear axle gear and fix the motor.

viii.     Chain the motor and the rear axle tightly and lubricate it with grease. Build the members and form the body of the vehicle as per the individual designs and mount the solar panels on top of the vehicle. It is advised to use some thermocols under the panels to reduce heat and noise while travelling.

ix.     Complete the wiring of all the electrical equipment with safety measures where ever necessary. Do not short any wires. Fix the brake callipers to both front and rear axle with the support of members and attach them with pedals at foot. Also connect the accelerator to motor controller and connect it with pedal near the driver’s feet.

x.     Place the seats and fix them. First switch on the MCBs at batteries and motor respectively and go for a test run. If the motor is running satisfactorily, fix the rest of glass fibre according to individual designs. Now the new “on-the-run charging solar vehicle” is ready and good to go.

Mileage and Role of Batteries in solar vehicle

Batteries form the main source of power from the solar panels to run the BLDC motor. In the simulation a battery which can be recharged with the help of solar power is designed. So, a battery must satisfy the property of charging and discharging which is considered in the simulation. The Lithium ion batteries with the specification mentioned later in the paragraph are considered for the vehicle. When battery is in charging mode electrical energy is converted into chemical energy and while in discharging mode chemical energy is converted into electrical energy. The selection of batteries in this report is done considering the need to supply sufficient power to the motor, cost and weight of the batteries. There are two types of batteries which can be chosen to run the vehicle. They are lead acid batteries and lithium ion (cobalt) batteries. In this report, the lithium ion batteries are considered due to the long discharging time, less weight and low maintenance. The main disadvantage of lead acid batteries are heavier (weight) than the lithium ion batteries and they require regular maintenance. In this report four lithium ion batteries of 12V and 33Ah are considered which are connected in series to achieve a total of 48V and 33Ah. The calculations on charging time and discharging time are the most important in perfect analysing of the working of the solar vehicle. The calculations are based on the specifications of motor, load torque, solar panels and batteries.

Capacity of the batteries = 33Ah

Current from the solar panels (average) = 8.3A

Charging time of the batteries = capacity in Ah / Charge rate in A                                 (9)

Therefore, Charging time = 33/8.3 = 3.974 Hours.

The time mentioned above is the suitable considering only the ideal conditions. In practical the lithium ion batteries has an efficiency of 90%. Considering the practical conditions:

Charging time of the batteries = capacity in Ah / (Efficiency * Charge rate in A)        (10)

Therefore, Charging time = 33 / (0.9*8.3) = 4.417 Hours.

Discharging time = (Capacity in Ah * Battery voltage) / Applied load in watts            (11)

Considering the motor uses an average continuous current of 15.62 amps during the running of the vehicle, the applied load on the vehicle becomes 749.76 watts.

Therefore, Discharging time = (33 * 48) / 749.76= 2.11 hours.

This implies that when the vehicle runs at an average speed of 50km/h the distance travelled by the vehicle turns out to be 50 * 2.11 = 105.5 Km.

Assuming that the conditions are ideal for efficient charging of the batteries through the solar panels. By the time the batteries get totally discharged within 2.11 hours, 47.76% of the battery gets charged back according to equation (12). Due to which the battery can run for additional fifty kilometres.  

Percentage of charging = (time for charging / total time for full charge)*100               (12)

Therefore, Percentage of charging = (2.11/4.417)*100 = 47.76%

For one full charge, if the vehicle runs at a constant speed of 50kmph, the vehicle runs a distance of 105.5kms. Similarly, at 47.76% of full charge and at the same constant speed, the vehicle runs an additional distance of (0.4776*105.5=50.38) 50.38km.

In the same way according to equation (12) while the vehicle runs an additional distance of 50.38km, about 22.81% of the battery recharges which can run for 24.06kms more. Similarly, during the run of 24.06km, about 10.89% of the battery recharges and can run for extra 11.48km. The next stages can be neglected as the batteries get completely drained of charge.

Therefore the total distance covered by the vehicle at a constant speed of 50 km/h in ideal conditions for efficient charging of batteries is (105.5+50.38+24.06+11.48) 191.42km.

Day	Battery (Ah)	Current               (I)	Charging time (hr.)	Efficiency (%)	Actual time(hr)	Current drawn(A)	Load (watt)	Disc. time (hr)	Speed (km/h)	Distance (km)*
1	33	8.34	3.956	90	4.396	15.62	749.76	2.112	50	105.6
2	33	8.24	4.004	90	4.449	17.78	853.44	1.856	50	92.8
3	33	8.1	4.074	90	4.526	16.24	779.52	2.032	50	101.6
4	33	8.15	4.049	90	4.498	15.40	739.20	2.142	50	107.1
5	33	8.32	3.966	90	4.407	15.33	735.84	2.152	50	107.6

* Only for one full charge.
Tab.3.5.1. Speed, distance covered and Time taken for charging and discharging

Dynamic calculations of electric vehicle with BLDC motor

We require a  BLDC motor which can sustain a load torque of 25.4291 N-m (according to equation 3). A BLDC motor with ratings 48V, 29A, 32.92 N-ms considered. The mathematical calculations for the BLDC motor are as follows:

P_electrical = P_mechanical + P_{copper losses}                                         (1)

Where,

P_electrical is input electrical power in watts

P_mechanical is output mechanical power in watts

P_{copper losses} is copper losses i.e. I²R losses in watts

P_electrical = V*I                                                         (2)

Where,

V is supply voltage in volts (48V)

I is current in amps (29A)

Therefore, P_electrical = 48*29 = 1392 W

Load torque need to be calculated to know the amount of torque required to move the vehicle. It is also essential in selecting a perfect motor for the desired qualities.

T_load = F*r*µ                                                                              (3)

Where,

T_load is load torque in N-m

F is the force required to spin the wheel in Newton =251.40N (from force equation)                         

R is the radius of the wheel in meters = 0.2023m

µ is the coefficient of friction = 0.5

Therefore, T_load = 251.40*0.2023*0.5 = 25.4291 N-m

Considering the BLDC motor with torque greater than or equal to the load torque (T_load) with an output speed of 300 rpm and output torque of about 32.62 N-m.

P_mechanical = T_m*ω                                                                   (4)

Where,

T_m is motor torque in N-m i.e. 32.62 N-m

ω is angular velocity in rad/sec i.e. ω_rpm*(2π/60)

Therefore, P_mechanical = (2πNT_m)/60 = (2*3.14*300*32.62)/60 = 1024.268 W

P_{copper losses} = I²R                                                                        (5)

Therefore, P_{copper losses} = 29²*0.139 = 116.899 W

                                                                                                      (6)

Therefore, Efficiency =  = 81.98%

Horse Power (1hp=750W)	1hp
Operating voltage	48 v
Operating current	15.62A
Starting /max current	29A
Maximum torque	32.62 N-m
Maximum output speed	300 rpm