# Hybrid Electric Transit Bus Powertrain Design

## Main Question or Discussion Point

Hello,

I am in the beginning of sizing all the components in the powertrain and really need your help;

I am trying to size the motor for a hybrid eletric bus

All articles I have read so far mention one lengthy equation that to know the rated power of the motor, one mush use;

PEM = [M(Vf2 + Vb2)/2ta] + ... + ...

Where Vf = the final vehicle speed

Vb = Vehicle base speed

ta = time to accelerate from 0 m/s to the desired speed

here are my questions;

1. In sizing the electric motor for traction application (to be used with a bus), am I in the correct direction? If not, what is the approach?

2. By saying the rated power of motor, does it actually meanthe maximum power that the motor could provide?

3. About the Vf, final velocity of vehicle >>> I am not sure what to use, could it be only sigle value which is the maximum speed of a bus which is 60 kph? Do I need other intermediate values, like 0-10 kph, 0-20 kph, 0-30 kph

However, the bus will have to operate most frequently in 0-40 kph region, shall I use Vf = 40 kph instead?

4. About the Vb, I understand the definition of motor base speed, but in the equation I need the vehicle speed, don't I? So, where do I find this value? What does this value depend upon?

Thank you very much

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anorlunda
Staff Emeritus
Yes rated does mean maximum. Once you calculate the required power, you find a motor that can provide at least that much, possibly more.

There are three considerations:
1. Power to overcome drag and friction at maximum bus speed. This can not be calculated, you need to determine it by experiment.
2. Power to accelerate. Those equations you cite are for that. If top speed is 50, how many seconds to accelerate from 0 to 50?
3. Power to climb hills. You need to determine the steepest hill and the highest speed you will require from the bus. Then use poer=force*velocity where force is the weight of the bus and load, velocity is the vertical speed of the bus, not the horizontal.