Invisible wrote:
> OK, so here's one for you...
> 
> If I put my car into top gear and set the engine to 3,500 revolutions 
> per minute, my car travels forward at almost exactly 75 miles per hour. 
> So... how far forward does it travel for a single revolution of the 
> engine??
> 
> Similarly, at that speed my car achieves roughly 50 miles per gallon of 
> fuel. So how much fuel does it inject into each cylinder??
> 
> [My car has a four-cylinder, four-stroke internal compustion engine, and 
> is normally-asperated. And runs on petrol, in case that wasn't obvious. 
> Presumably the timing is therefore set so that exactly one cylinder 
> fires for every revolution of the engine.]
> 

The fuel consumption of a vehicle is often quoted (in metric) as litres 
per 100km.  Suppose a medium sized sedan gives 10L/100km to keep the 
maths easy.

So we have fuel consumption as a volume per distance.  That means it can 
be reduced to an area.  Think of it as the cross-section area of a 
cylindrical line of fuel that the vehicle is consuming as it hurtles 
down the road.  Of course when you accelerate the engine guzzles more 
than this average and while cruising at some optimal speed (around 
80km/hour ?) it would sip a bit less so the shape is tapered.  It isn't 
a simple cylinder but rather a lumpy one.

Lets ignore these variations and work out how large this cross-section 
is on average.

Well do the calculation and you get 0.1 mm^2.  That is one tenth of a 
square millimeter!  Depending on the size and resolution of your screen 
and the font size that you use it is about the area of a full stop.

So as you cruise along a motorway your vehicle is slurping a stream of 
fuel about the same size across as a largish grain of sand, burning it 
to produce hot gases and farting them out the rear.

I find the fact that such a seemingly miniscule amount of fuel can be 
used to propel quite massive vehicles along at such speeds quite 
amazing.  The energy density and convenience of petroleum fuels is just 
astonishing.  As someone once said if you were to write down the ideal 
properties of a vehicle fuel you would pretty much define petrol.  High 
energy density but not (terribly) explosive, liquid over a wide range of 
temperatures, cheap (well relatively), easy to handle, not (too) toxic 
or corrosive etc.

Second thought though is that when you add up the consumption of tens of 
thousands of vehicles along a section of major road each day it does 
start to amount to quite a lot.  Those fractions of a square millimeter 
accumulate to something like the cross section of a fire hose or water 
main.  Imagine the threads of each vehicle weaving with others to form a 
massive cable down the center of the road.  Smaller roads might equate 
to a garden hose each.  Sum this across a city and it is a huge web 
spread down every road and tangled across every intersection.

Over the course of a year the sum cross section along a major road would 
be larger than the cross section of a petrol tanker truck.  So it is 
like every highway and major road being lined with tankers parked nose 
to tail.

And all of this is with a liquid that was produced millions of years 
ago.  It is the captured sunlight that fell on forests in the time of 
the dinosaurs.

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