Efficiency map of the drive train of a car

[bradyj7]

Hi there,

I'm reading a report about the efficiency of the drive train of an electric car. The author recorded the speed and acceleration of the car over a period of time and created the graph below to illustrate the efficiency.

Could anybody tell me what the relationship is between the speed/acceleration plot a car in general? Why would one use this plot to illustrate the efficiency and can one determine an average efficiency from the raw data?

Thanks for your help

https://dl.dropbox.com/u/54057365/All/efficiency.JPG

 

[mfb]

Could anybody tell me what the relationship is between the speed/acceleration plot a car in general?

It is a relation between speed, acceleration and efficiency. For each point in the diagram, you can calculate the required power (based on the air drag and acceleration), and measure the power the car really needs. That ratio gives an efficiency.

To get an average efficiency, you need some data how the car is used.

 

[bradyj7]

Hello mfb,

Thanks for your reply. Are you saying that the author used 3 variables when plotting this graph? speed, acceleration and efficiency? I thought that it was just a colour point map of the velocity and acceleration?

Would you know the formula to calculate the required power?

Many thanks

 

[mfb]

Are you saying that the author used 3 variables when plotting this graph?

Right.

As an example: If you want to know the efficiency at 40km/s with an acceleration of 1m/s, you look at the corresponding position in the graph: The area is red/dark red, indicating an efficiency of about 90%.

Would you know the formula to calculate the required power?

Depends on the car itself: its mass, size and geometric shape.

 

[bradyj7]

I understand it now. Thanks for the help.