MPG v L/100km

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MPG

Miles per gallon (MPG) is a misleading quantity.

Miles per gallon is often referred to as a measurement of fuel consumption but it isn’t. Miles per gallon is a measure of the car’s fuel efficiency: how many miles the car can extract from one tank of fuel. A more efficient car can extract more miles from a tank.

Which saves you more fuel?

  • Switching from a car that gets 20MPG to one that gets 40MPG.
  • Switching from a car that gets 40MPG to one that gets 60MPG.

For a hypothetical 400 mile trip:

  • Switching from 20MPG to 40MPG saves 10 gallons of fuel.
  • Switching from 40MPG to 60MPG saves 3.3 gallons of fuel.

As fuel efficiency increases the amount of fuel saved decreases. A 20MPG increase in the low MPG values has far more of an effect than a 20MPG increase in the high MPG values. The scale is non-linear.

Put another way: the average MPG rating for a car is about 27MPG. For our hypothetical 400 mile trip:

  • Doubling fuel efficiency to 54MPG saves a total of 7.4 gallons of fuel.
  • Tripling fuel efficiency to 81MPG saves a total of 9.9 gallons of fuel.

This dependence on MPG is (yet another) hangover from our archaic insistence on non-standard imperial units.

l/100km

To measure fuel consumption, we need to measure not miles per gallon, but gallons per mile. A (volume) per (distance) measurement already exists and is already commonly used in Europe: litres per 100 kilometres.

Which saves you more fuel?

  • Switching from a car that gets 6l/100km to one that gets 4l/100km.
  • Switching from a car that gets 4l/100km to one that gets 2l/100km.

For a hypothetical 400km trip:

  • Switching from 6l/100km to 4l/100km saves 8 litres of fuel.
  • Switching from 4l/100km to 2l/100km saves 8 litres of fuel.

In this case the scale is linear, a 2l/100km saving always results in the same fuel saving.

TL;DR

We should use the l/100km rather than the MPG scale for measuring fuel consumption.

Mad Science on ‘Fringe’

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Walter Bishop from the TV show Fringe really is a mad scientist.

walter-bishop-fringe-bad-science

Why Walter is a Mad Scientist:

  1. He’s heating a test tube with his Bunsen burner set on a yellow safety flame.
  2. He’s holding the test tube with a pair of tongs, not a test tube holder.
  3. He’s not wearing safety goggles.

The yellow flame comes from incomplete combustion of a hydrocarbon fuel (in this case natural gas), resulting in the production of soot; if this soot is hot enough it will glow, producing the characteristic colour. Heating a test tube with a yellow flame will leave the test tube covered in a layer of this soot that will insulate the contents from the heat. It could also lead to contamination.

When using tongs it’s very easy to accidentally apply too much force and crack the test tube; test tube holders have the advantage that they “fail safe” – loosening your grip doesn’t result in the test tube sliding out, you have to push to release the test tube, not to hold it.

My issue with the safety goggles is, I hope, obvious.

U2 and particle tracks

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U2’s new tour makes use of a very impressive shape-changing video screen designed by Chuck Hoberman. What piqued my interest was the images in this video of the screen in action: I’m pretty sure that they’re using photographs of particle tracks from a bubble chamber.

U2-particle-tracks

If you click on the image to enlarge it I’m pretty sure that you can see a neutral lambda baryon decaying into a negative pion and a proton on both the left and right of the screen.