Driving on dirt

25 Feb

Yesterday afternoon, a group of us took Trev out to a dirt track and measured the energy required to drive on dirt. Trev with one person in it has a mass of about 400 kg, which is about the target mass of our African Solar Taxi with three people in it.

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As expected, the energy required to drive on dirt, even at low speed, is significantly higher than the energy required to drive on bitumen.

Measuring energy use was straightforward. Trev uses microcontrollers to measure the positions of the accelerator and brake pedals, control the motor speed, monitor the battery, and even flash the indicators. These microcontrollers are connected by a communications system that passes messages around the car. To measure performance we connect a laptop to the car to record the messages, then drive.

We drove at speeds up to about 30 km/h, but had to slow for creek crossings, potholes and turning around at each end of the 700 m track. When we finished driving, we summarised the logged data to create a table of distance, speed, battery voltage, battery current, battery power and battery energy at one-second intervals. The chart below shows a plot of distance (horizontal axis) and energy (vertical axis).

Energy versus distance

The slope of the line is the energy use per kilometre. We changed drivers after about five kilometres. The second driver was heavier and drove slightly more aggressively; the line becomes steeper, indicating greater energy use per kilometre.

When Trev was driven from Darwin to Adelaide in 2007, the energy required to recharge the batteries was about 62 watt-hours per kilometre (Wh/km) for a trip that was mainly cruising at about 90 km/h. In the standard drive cycle tests, which represent a mixture of urban and highway driving, the Mitsubishi i-MiEV and Nissan Leaf electric cars use 135 Wh/km and 173 Wh/km respectively.

Yesterday afternoon, Trev used between 60 and 70 Wh/km from the battery. The energy used to recharge would be higher because of energy losses in the charger and the battery—about  70-80Wh/km.

Driving on dirt roads uses significantly more energy than driving on bitumen, even at low speeds. Larger wheels may help. But for the African Solar Taxi to be viable, we are going to have to make it as light and efficient as possible.

2 Responses to “Driving on dirt”

  1. Enias Marama February 25, 2013 at 8:31 pm #

    good work guys, there are poor rural women down here who can not wait for the day the African Solar Taxi will land here.


  1. Second design workshop | African Solar Taxi - March 7, 2013

    […] discussed the recent weekend session in which we drove Trev on dirt roads and measured its energy use. Trev with one person weighs approximately 400kg, which is about the […]

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