Hello,

Comparing with a car using combistion engine: 1200 kg car with 66 kW power. By the simple calculation the car has 66000/1200 = 55 W/kg. The vehicle you are trying to design has 50000/700 = 71 W/kg of mass. So based on this simple calculation - from this point of view there is no problem with the motor power dimensioning.

But the electric vehicle like mahindra reva with 1150kg of weight is using only 30kw of motor..that is my concern..

how can i optimise the motor sizing?

A Beetle hatchback is 103 BHP, which is 76.8 KW. But such a figure includes power lost in the gearbox, alternator, differential, and water pump. So your figure of 50KW looks reasonable for the same 0-60 mph specs, as this set of power losses will not exist in an electric car. The mass of the car is also reduced, because there is no gearbox, alternator, differential, and water pump masses to be carried.

All of the power must come from the batteries and electric motor(s). There is no petrol to provide power! However, this peak power is only required during fast acceleration, and not during coasting. Regenerative braking can increase your distance travelled on one charge. If the electrics are distributed at 50 volts, your connection to the one battery will have to be dimensioned for 1 000 amps! If you have four electric motors (one per wheel), then you could have each separately supplied via wiring dimensioned for 250 Amps. An Earth return will save the cost and weight of return wires. You could even mount four batteries near the four wheels, saving on copper costs and their weight. You will need to use Li batteries to further save mass.

Thanks Ian , but kindly read my comment above. i am concerned about motor sizing..I think 50 kw motor is more than what exactly required for the vehicle with gross weight 700 kg..

If you only drive the back wheels (2-wheel drive, and not a 4-wheel drive as suggested above), you say you will need 25 Kw per wheel for high performance. If you want to make a low performance 2-wheel drive car, you can use only 15 Kw per wheel. Remember that that the user will be somewhat tolerant of the loss of performance over a petrol car because there will be no gear changes to slow the acceleration phase. Have you interviewed your potential users? What do your users want? A 2-Wheel drive? A 4 Wheel drive? A 2-Wheel drive with software upgrade to 4-wheel drive? A rubber-on-tar burner? (How quickly must it accelerate 0-60 Mph?) What are their requirements?  Is it to be a 2-seater or a 4-seater? A 2-seater (1 front and 1 back) can have half the cross-sectional area of a 4-seater. How streamlined is your vehicle? What range do your users demand? How about reducing the range of the car? (This means your batteries will be less, and the mass less). How about solar panel power-augmentation? Is the vehicle for use in sunny India or some soggy climate? How many sun-days per year? How steep are the steepest hills in your target market? Or are the vehicles only to be used on airport tarmacs? How much stop-start driving?

Your lecturer has deliberately given you a typical engineering problem, which is under-specified. Please state all your assumptions and show all your calculations. I assumed that the motors are 100% efficient. What is your assumption?

https://en.wikipedia.org/wiki/Electric_car

http://www.ev-propulsion.com/EV-calculations.html

These link above might be describe one in another way the calculation of the EV battery and motor capacity. I put the paragraph that described more:

If we had a 144 volt pack of 200ah batteries, and a 1000 amp controller, using the above formula we could have  193 HP, (at a 5C draw)  and if we had 288 volt pack of 100ah batteries we could have potentially  386 HP ! (these are calculated without efficiency included, figure about 85% efficient)   Only one problem, that much electrical power put into the motor could easily destroy it rather quickly !  The common "in the field" estimate of KW power a 9" motor can handle (for short periods) is 100 KW.  So using the above formulas, 144 volt system should be limited to about 700 amps, and the 288 volt system to 350 amps. Still, 135 hp is pretty good for a small car.  NOTE: use high power levels at your own risk for motor damage.

You need to take into account peak power vs continuous power as well. For example, a typical might , on average use 20-30% of the peak output of an engine. A semitruck might use 50 or 60 or more. A piece of farm equipment or a stationary pump might use 80 or 90 or more.

This is in fact take into account in the KINDS of engines used for these applications. If you take a 1.6L 4 cyl engine rated at 100hp from a small hatchback and connect it to a pump or a generator that requires 90hp to drive it, it sounds like that will work. And it will for a short while but you will rapidly destroy the engine. You COULD use a big V8 like you find in a performance car, like Fords 420HP "Coyote" V8 used in their Mustang. That way you wouldn't be running it at the high duty cycle. But that would not be coat effective. You'd be better off running a small 100hp or so industrial or agricultural engine that was made to run constantly like that.

Similarly electric motors assume some duty cycle. You need to understand that those relevant specifications.

You have to be very clear with the types of motion when you said 'to propel a vehicle'. There are two types of vehicle motion - i. constant velocity and ii. Accelerating/decelerating.

With constant velocity (acceleration, a =0) on a flat road, a vehicle will just require to overcome the resistive forces like drag, aerodynamics forces, rolling resistance and frictional forces in the bearings as such. So from Newton's Law, F = ma, with a = 0 giving (Tractive Force) - (Resistive Forces) = m(0) or (Tractive Force) = (Resistive Forces)

But, when a vehicle is accelerating it has to overcome the 'inertial forces' such as inertial mass of the vehicle and rotating inertia of the rotating components of the vehicle like the wheels, gears and others. So (Tractive Force) = (Resistive Forces)

+ ma

when car moves at straight road ,their is acceleration ,author wise if you take of power --- the car will stops after while ----- their is no .Acc = 0

Hi

The best way is to compare the similar engine power. 700 Kg is similar to Maruti 800 or Alto whose engine power is roughly 26 kW.

Hope this helps.

Amol

Vibhav Vats were you able to solve your problem, because i need to decide a motor for a 900kg vehicle, and my calculations are telling me i require 99Kw motor. Can i know how you did the calculations as welll