Some thoughts while stuck in traffic

By Marcus Wilson 31/05/2012

Monday morning was one of those strange days where for some non-obvious reason there was far more traffic than normal. Maybe there had been an accident somewhere, or there was some event on, but, for whatever reason, it took nearly half an hour to crawl through the Hillcrest roundabouts.

All that start-stop on the car is a massive waste of energy. You get the car going forward, which requires energy, then you stick the breaks on and lose it all. That kinetic (movement) energy turns to heat in your brake pads and does nothing except contribute to global warming. That’s for a conventional car, anyway.  The designers of electric and hybrid vehicles know about this, and that’s why these cars have regenerative braking. The idea is that forward motion of the car is transferred not to heat, but to electrical energy, which can then be used again to get the car moving.

Practically, the concept is quite simple; braking is done in the first instance by the electric motor, which, if driven mechanically, will act as a generator. Ensure that the electrical path back to the battery is in place (otherwise the car will just freewheel) and then motor will act as an electrical brake – the turning motion of the engine will generate electricity and take the kinetic energy from the car.

Easy. So how much energy is involved. Here’s a few quick estimates for a crawling car in a traffic jam. At 20 km/h (or 5.6 m/s), a 1500 kg car will have kinetic energy (calculated by half times mass times velocity squared) of about 25 000 joules. Everytime you brake from this to zero, your brake pads take this away (in a conventional vehicle). If you are doing this say twenty times in a long queue for an intersection, that’s getting on for about  500 000 joules of heat energy created.

What can this energy do? Boiling a litre of water (taking it from say 20 degrees to 100 degrees) would take 4200 J/kg/K  times 1 kg times 80 K = 340 000 joules of energy. Here, 4200 J/kg/K is the specific heat capacity of water – the amount of energy needed to raise a kilogram (litre) of water by 1 degree Celsius or kelvin is 4200 J. In other words you could make several cups of tea for the energy you’ve wasted in the brakes. In terms of the electricity ‘unit’  (One unit, or kWh = 3.6 million J,  your brake pads have consumed about 0.14 units of electricity. That comes to about 4 cents worth at domestic rates.

But one important thing to remember with kinetic energy is that it grows with the square of the speed. So a car doing 100 km/h will have twenty five times the kinetic energy of one doing 20 km/h. Breaking just once from 100 km/h down to zero will also use a similar amount of energy to what’s been calculated above. If you’re one of those people who like to brake at the last moment think about each brake pad boiling water for a cup of tea when you do this.

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