Author Topic: Another way of looking at fuel consumption  (Read 518 times)

Offline Allen Smithee

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Another way of looking at fuel consumption
« on: July 30, 2019, 02:55:33 PM »
The other day I was looking at some car fuel consumption numbers which were expressed in both the British Miles per Gallon and the European Litres per 100km. As I looked at the latter my mind went off at a tangent (as it is wont to do) and observed that "l/100km" could also be expressed in "cubic metres per metre" (a volume divided by a distance), and that dimensionally this could be further reduced to just square metres (a volume divided by a length gives an area). For ease of visualisation this could also be expressed as the diameter of a circle

In dimensional analysis the minimised dimensional form of a parameter does actually represent something physical, so I pondered what this diameter might actually represent and the penny dropped - it is the diameter of an imaginary tube which contains the fuel burned by the car.

So for example 56mp(UK)g is 5x10^-7 cubic metres per metre (or square metres). That equates to the area of a circle of 2.5mm diameter.

So the fuel burned by a car doing a constant 56mpg for 100metres would fill a 2.5mm diameter and 100m long.

I found that way of looking at it quite interesting.

If anyone is interested the actual equation boils down to a simple (to 3sig fig) expression. For a fuel consumption of x miles per gallon, the diameter D of the "fuel cylinder" is given by:

D = √(360/x)


Quidquid latine dictum sit altum sonatur

Offline Stuart

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Re: Another way of looking at fuel consumption
« Reply #1 on: July 30, 2019, 06:44:57 PM »

Donít know about fuel consumption but my brain cell now as constipation  :pinkelephant:

My aim is for a accurate part with a good finish

Online Roger B

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Re: Another way of looking at fuel consumption
« Reply #2 on: July 30, 2019, 08:03:43 PM »
So a single carb petrol car would need a main jet around 2.5mm diameter as the available pressure drop is quite small. Diesels could have smaller nozzles as there is one per cylinder and the pressure and hence velocity is higher   :stir:  :toilet_claw:  :wine1:  :wine1:  :wine1:
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