22 Feb 2013

Efficiency

I’ve recently (re)taken up cycling in a fairly major way, and have been surprised by how much I’ve enjoyed it. One of the things that’s making it more fun this time around, as compared to previous dabblings in years past, is the various ways that you can measure and quantify your progress – not to mention your suffering – and compare it with others, etc.

For example, a recent ride taken with a few friends:

Time: 01:54:50
Avg Speed: 13.5 mi/h
Distance: 25.8 mi
Energy Output: 826 kJ
Average Power: 120 W

Now, 120 W is really not especially great from a competitive cycling perspective; better riders routinely output 500-ish watts. But it struck me as being pretty efficient: for all my effort, the ride actually only required the same amount of power to propel me on my way as would have been required by two household light bulbs.

So that got me thinking: just how efficient is cycling?

My 25.8 mi / 41.5 km roundtrip ride required 826 kJ, if we believe Strava; that’s mechanical energy at the pedals. (I unfortunately don’t have a power meter on my bike, so this is a bit of an estimate on Strava’s part, taking into account my weight, my bike’s weight, my speed, the elevation changes on the route, etc.)

That’s about the same as the energy released by 1.7 grams of combusted gasoline, per Wolfram Alpha. If I ran on gasoline, I’d be able to carry enough in my water bottle to ride across the U.S. more than 3 times (7,813 miles worth).

Of course, cars aren’t perfectly efficient in their use of gasoline, and I’m not a perfectly efficient user of food calories. Strava helpfully estimates the food-calorie expenditure of my ride at 921 Calories, which is 3.85 MJ, leading to a somewhat disappointing figure of only 21.4% overall efficiency. (Disappointing only in the engineering sense; from an exercise perspective I’d really rather it be low.)

Though it’s about on par with a car, interestingly enough. The Feds give anywhere between 14-26% as a typical ‘tank-to-tread’ efficiency figure for a passenger car, with most losses in the engine itself.

So if I were able to drink gasoline and use it at least as efficiently as a car, my water bottle would get me about a thousand miles. (1,094 mi or 1,760 km, using the low-end 14% efficiency figure for a car.) Still pretty good, considering that my own car would only get about 5 miles on the same amount of fuel (24 fl oz at 25 MPG).

Of course, a car isn’t an especially fair comparison – it has a lot of overhead both in terms of mass, rolling resistance (more, lower-pressure tires), and air resistance (higher cross-sectional area). Some sort of small motorbike would be a better comparison, and there I suspect you’d start to see an even playing field.

Maybe that’s my argument for getting a motorcycle…

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