# The Naked Scientists Forum

### Author Topic: How does my car calculate its fuel consumption?  (Read 11411 times)

#### chris

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##### How does my car calculate its fuel consumption?
« on: 08/04/2009 21:15:13 »
I recently purchased a new(er) car. It has an average and an "instant" fuel consumption display of the dashboard.

But how does it work? How does it calculate how much fuel is being burned? The "instant" consumption registers moment to moment rates of fuel use to the nearest mpg. How?

Chris

#### techmind

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##### How does my car calculate its fuel consumption?
« Reply #1 on: 08/04/2009 23:07:32 »
Well it's got an electronic tachometer (mileometer) which records how far you've travelled, and I imagine the engine-management computer knows proecisely how much fuel it's injecting into the cylinders (it's computer controlled after all)... so it's fairly simple really!

The real question is how the fuel is delivered/metered to the cylinders - to which I don't didn't know the answer.

http://en.wikipedia.org/wiki/Fuel_injection

Wikipedia says that the amount of fuel injected is controlled by a solenoid valve which varies it's 'on' pulse-width according to the amount of fuel required... but you'd still need to know the flow rate. Hmmm...

Anyone work in automotive?

#### Bored chemist

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##### How does my car calculate its fuel consumption?
« Reply #2 on: 09/04/2009 16:07:58 »
My guess is that the engine management system adds just enough fuel to the engine to use up the oxygen in the air (or some defined fraction of it) so, with the engine capacity and the RMP (both known) you can calculate how much air goes through the engine and, if you know how much of the oxygen you want to use (my guess is nearly all of it) then you can tell the rate at which you need to add the fuel.

#### Raghavendra

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##### How does my car calculate its fuel consumption?
« Reply #3 on: 10/04/2009 09:12:37 »
pressure

#### Bored chemist

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##### How does my car calculate its fuel consumption?
« Reply #4 on: 10/04/2009 17:38:14 »
pressure

#### nicephotog

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##### How does my car calculate its fuel consumption?
« Reply #5 on: 11/04/2009 11:18:56 »
...(or some defined fraction of it)...(my guess is nearly all of it)

14:1 OR 15:1 is a conventional vehicle (carburetor).

15 parts atmosphere to 1 part fuel

NOTE: 1. Stratified Charge Mode
http://autospeed.com/cms/A_107830/article.html?popularArticle

As stated before
E = engine revolutions RPM
D = displacement

Note: over 720ο revolution of the engine all cylinders fire once for a four strole.
------- 4 stroke p/minute ------ "divide by 2 because of RPM AND 720ο"
((E/4)(molecules yeilded at e.g. 20ο Celsius from D size))/2

« Last Edit: 11/04/2009 15:52:34 by nicephotog »

#### chris

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##### How does my car calculate its fuel consumption?
« Reply #6 on: 11/04/2009 15:12:28 »

Chris

#### Ethos

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##### How does my car calculate its fuel consumption?
« Reply #7 on: 11/04/2009 15:32:13 »
I recently purchased a new(er) car. It has an average and an "instant" fuel consumption display of the dashboard.

But how does it work? How does it calculate how much fuel is being burned?

In the case of your vehicle, as others have already said, your computer performs this function. However, in older vehicles, computers are not used but there is still a way to monitor fuel consumption. Intake Manifold vacuum is a good indicator for monitoring the efficiency of the internal combustion engine. There are still companies selling vacuum gauges that can be adapted to the carburetor vacuum port for exactly this purpose........................Ethos
« Last Edit: 12/04/2009 02:03:29 by Ethos »

#### lyner

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##### How does my car calculate its fuel consumption?
« Reply #8 on: 12/04/2009 10:10:35 »
pressure
Is this word association football?

#### chris

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##### How does my car calculate its fuel consumption?
« Reply #9 on: 13/04/2009 22:47:26 »
Well he might inadvertently (or otherwise) have a point because the engine is a diesel and hence the injectors are operated by pressurising the fuel to lift the injectors from a seat allowing fuel to be sprayed in.

I presume that the engine management computer can alter the pulse-width of the line pressure supplied to the injector to tailor fuel supply to demand and operating requirements (i.e. how heavy the right foot is).

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##### How does my car calculate its fuel consumption?
« Reply #9 on: 13/04/2009 22:47:26 »