Low mass, low energy

Using tonnes of machinery to move one or two people around a city seems ridiculous. But how important is low mass?

Many years ago, before we started designing Trev, I was driving a Solectria electric car to work when my path became blocked by a large “SUV” that had broken down in the middle of an intersection. Pushing the SUV to the side of the road made me realise just how much energy is required to move these massive machines. My electric car was better because it was using energy generated from clean, renewable sources. But it still needed a lot of it.

I sometimes demonstrate the importance of low mass by tying a small child to Trev and another to a conventional car, then asking them to race across the yard. Trev moves, the other car doesn’t. It usually takes five kids towing a conventional car to keep up with one towing Trev. A large SUV took ten kids.

In 2008 I had two high-school students investigate the relationship between vehicle mass and CO2 emissions. They used emissions data from the 2007 Australian Green Vehicle Guide and looked up vehicle masses from manufacturer’s web sites. The results are shown in the following graph.

CO2 emissions vs vehicle mass

The red dots show CO2 emissions for petrol cars, the blue dots for diesel, and the green dots for hybrids. The trends are obvious—halving the mass halves the emissions. The reduction is not entirely due to the reduction in mass; lower mass cars are generally smaller, with smaller engines and less aerodynamic drag. But mass is the dominant factor.

The graph also shows that for a given car mass (and size), there is wide variation in emissions. Improving vehicle technologies is one way to reduce emissions, but choosing an appropriate vehicle with low emissions can be a lot more effective.

Trev has a mass of just over 300 kg, and so it takes a lot less energy to push it along the road. In 2007 we drove Trev from Darwin to Adelaide, cruising at 80-90 km/h. We used 187 kWh of electricity, worth $33, to drive 3000 km. Petrol costs for a conventional car would be ten times this amount.

Electric cars are coming. Most will be based on conventional cars, and so will be heavy and require a lot of energy. A 1300 kg electric car with a range of 120 km might have 200 kg of batteries. Trev has the same range with only 45 kg of batteries. (We are going to increase this to 80 kg for Zero Race, because the charging points are up to 250 km apart.) Reducing the mass of a car is a very effective way of reducing the energy required to move the car, and the amount of materials required to build the car.

Trev. Not only does it use clean energy, it also uses a lot less energy.

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