How to Build Dream Electric Car | Power and Torque Calculations of Car

How to calculate power and torque electric car power train system designing

In this blog will discuss how to calculate the Power and Torque for  electric car to design perfect power train system.

Electric power train System consist of three main components

• Electric motor
• Motor controller
• Battery pack

In above three components the most important component is Electric Motor because it gives power of torque to drive power electric car.

Electric motor selection is very critical for power train designing, because electric car  performance, acceleration, speed totally depends electric motor.

Hence to get desired performance of electric car, its very important to select right motor because

• High power motor require high power, large size and more capacity battery pack in terms of Power  thus increase the power consumption, weight and investment.
• On the other hand low power motor will affect the overall performance of a electric car in terms of acceleration speed and power.

So its very clearly understood that selection of motor is very important.
By calculating o\power and torque required in very optimized manner for any electric car is vital for perfect designing of power train system in electric car.

Apart above reasons I have some another reasons to calculate power and torque for  for electric car

-As   earlier disclosed in my previous blog that I am going to build my dream Solar hybrid electric car for this I required the power and torque calculation of my electric car

-To keep my total weight of car minimum with the optimum powered electric motor & battery pack. .

One more Interesting thing in this blog  will be, that I will do both theoretical and practical calculation to get required power and torque and I will demonstrate what is the difference between.

• First in this blog will discuss method power and torque that will be purely theoretical method
• Afterwards will do practical calculation to determine power and torque

 I want to disclose that the car selected to convert in solar hybrid electric car is Tata Nano.

Before deciding the right car I have compared Tata Nano with some other models of Maruti and Hyundai and  found that Tata Nano is a perfect car for EV conversion because for the following criteria:

1. Tata Nano is a light weighted car kerb weight of Tata Nano is about 630 kg. This is the lowest weight among all the petrol car in this category considering passenger weight of 70 kg I am designing the total weight of  car is 700 kg.

2. Feature of Tata Nano is its compact size the none of them it's

length is 3099 mm
Height 1652

Width is 1495 mm

Apart from this compact size Tata Nano height I will get a sufficient space  to fit my conversion kit and battery pack in this car.
Another advantage of its compact size is that I want to make a electric car to keep the
weight west size is very much comfortable for me for EV conversion.

 3. Advantage of found in Tata Nano is designing or modification ability that it's easy and interesting to i modify  Nano and I am very short that after modification cute and adorable very much look as a futuristic EV.

4. Another which I like in Tata Nano it's gear ratio which will be very much helpful for me  to optimize motor power to an extend

If we talk about the nano then

• 1st Gear R     –   3.45
• 2nd  Gear R   –  1.95
• 3rd Gear R    –  1.26
• 4th  Gear R   –  0.83
• Reverse R      –  3.08

5. Last but not the least another important fort Tata Nano section low cost.

The Tata Nano easily available in in the price range of 30,000 Rs to 50,000 Rs. second hand, good condition car.
For me it's very important points because i want keep the cost of my dream electric car range of 1,50,000 Rs to 2,00,000 Rs and i can get Nano in  40,0000 Rs as per current discussion with owner.

Now we calculate the torque required for Nano

Some important parameters for calculations

Weight – 630Kg + 70Kg

Frontage Area - 1652 x 1495 mm – 2.5 m2

Wheel Radius – 12”  - 0.3 mtr

Speed – 60Km/hr

Transmission Efficiency – 85%

Before  Starting Calculations Its very Importanat to understand that 3 Types of resistance/ forces act on the car

1. Rolling Resistance
2. Air Drag
3. Gradient Resistance

Rolling Resistance  Air Drag  Gradient Resistance

P & T Calculation  method

For Power -

Linear Wheel Travel = 2πr = 2 x 3.14 x 0.3 = 1.9 m

Speed V = 60 km/hr  60 x 1000 / 3600 = 16.7 m/s

RPM Required = speed/ Wheel Travel = 16.7x 60 /1.9 = 527

Power P = (m x g x v x Rolling Resistance) + (1/2 x Air density x  coefficient of drag x area x V3) + (m x g x Sin10)  

P = (700 x 9.8 x 16.7 x 0.02) + (0.60 x 0.82 x 2.5 x 16.73) + (700 x 9.8 x 0.17)
   
    = 2291 + 5728 + 1166 = 9185

P max  = 9.185 Kw  Required for Nano

For Torque

 P = 2 π N T / 60

T =  P x 60 / 2 π N

   =  9.185 x 60 / 2 x 3.14 x 527 

T =  166 Nm

(In My next blog i will explain this calculation in very detailed manner to better understanding and more clarity)

And now will calculate  Nano torque in a very practical manner to get best optimized power & torque required for my dream electric car

For which we use one very important data of engine

Nano Engine Tmax  =  51 Nm @ 3900 RPM

1st Gear R – 3.45

Tmax  =  51 x 3.45 = 176 Nm

If we multiply transmission efficiency @ 85% then

Tmax = 176 x 85%

Tmax = 150Nm

Is the actual practical Torque to Run Nano

T = 166 Nm (Theoretical )

T  = 150 Nm (Practical)

Now there is 2 Points to understand before selecting motor

1. Nano Top speed is about 100 Kmph and in my dream EV I dont required this much speed.

2. I am using gear box or transmission ration so have have option to select gear ration according to my travelling requirement

Overall i will chose the motor below 150 Nm torque for maximum optimization of power and hence efficiency.