Route's Energy Use

 

There are 3 forms of forces that act upon your car when driving. One is the drag force, another is the force required to accelerate the car, and the other is used to drive up hills. The drag force is what was measured when calculating the Cd of the car. The same equation we used to calculate the Cd of the car can now tell us the drag force at any speed. The force to accelerate the car is force = mass X Acceleration, and the force to go up hills is mass X gravitational constant. Your batteries in your car store energy. Force X distance is equal to work, which has the same units of energy. The logging GPS saves all the information about your drive needed to calculate all of the energy required to complete the route. Every second, it is recording your speed, distance and altitude. This means, we can calculate the energy required by the laws of physics to drive the route. This is good information for estimating your particular needs, since hills and stops and starts can’t be taken into account when using typical range estimating techniques. My particular 5 mile work commute requires the same energy use as if I drove it at 56 mph without stopping. When I was first considering my electric car project, I was hoping that it would use the same energy as driving at 40 mph. This is an interesting piece of information because the Midget is a light car that doesn’t have a good Cd. I would have thought that the stopping and starting wouldn’t have used that much energy compared to driving at higher speeds.

 

This information is also good for estimating the increase in range that you could get by modifying your car. The Cd and the mass of the car are required for these calculations. You can simply run the estimation with 100 lbs less weight and see how much less energy use you’ll need to complete the route. If your considering changes in your car to reduce the Cd, you can see how much more range you can expect to see.

I’m including my Excel spread sheet as a download so you can try can try this out if you can obtain GPS data. I’ve tried to explain how to use the spreadsheet on the first sheet. It is not fully automatic, and care must be taken when pasting in your route data. The units are in metric because they work out so well, especially when the data is taken on 1 second intervals. Click here to go to the download page.

The graph below is the output of the route energy use spreadsheet showing my commute to work.

 

Energy use calculator output

Additional information