Monday, February 22, 2010

Car versus Bicycle - relative efficiency

I replaced a headlamp bulb on my car at the weekend. The bulb is a 55W bulb. There are two of them, of course, which makes 110W in total.
The mean power output of a cyclist on the flat stages of the Giro d'Italia is 132W (source). Now, bear in mind these are world-class athletes in a race; your average commuter or club cyclist won't be producing anywhere near that.

In other words, it takes more power just to light a car than it does to power a bicycle.

A typical 1.6 litre petrol engine will produce a maximum power output of around 75KW. Of course, the engine isn't delivering anywhere near its peak output most of the time, but it does give you an idea of how incredibly inefficient cars are for moving people. Even at 1/10th of that power, 7.5KW is still roughly 2 orders of magnitude greater than the 'bicycle engine'.

It's not surprising either, given the physics. An average small-medium car weighs about 1.3 tonnes (1300kg). An average bicycle weighs 13kg. An average reasonably fit person, maybe 75kg. So in a car, 95% of the engine power (and therefore 95% of the fuel) goes on propelling the car rather than the load. That's 5% efficient. Bear in mind the engine is not even 50% efficient, so there is a truly huge waste of energy. Even with a full load of four people with luggage, the efficiency is unlikely to get above 25% (again ignoring engine inefficiencies).

With a bicycle, the ratio is pretty much the opposite. 80% of the power is being used to move the load rather than the vehicle. You could argue that the human body is an inefficient engine, but don't forget a) a completely sedentary driver in a car still needs to eat; b) people need excercise to stay healthy. So the car driver would need to expend the calories that could be used to propel a bike in other ways .

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