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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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