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If we have the cc of the engine, and its running speed, how can we calculate the mass of gasoline in milligrams sucked in each intake stroke and the mass of gas consumed in a given distance, based on its mass and volume?

user3407319
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    You can't, fuel consumption is based on engine load, not just on revs. – HandyHowie 6 mins ago – HandyHowie Nov 26 '15 at 18:20
  • Yah, ask @HandyHowie how we know that is true ;-) – Pᴀᴜʟsᴛᴇʀ2 Nov 26 '15 at 18:40
  • You'd have to know a lot more information, such as the intake manifold vacuum. The mass calculation is pretty simply, if you can calculate (or measure) the air mass -- it's takes about 14.7 times more air-mass than gasoline-mass for proper combustion. But knowing the air-mass requires knowing what the actual airflow is, which can be measured with a mass airflow sensor (duh) or by computing the airflow curves for steady-state manifold vacuum readings. Or do what one of the answers says and measure actually consumption between fillups. – Julie in Austin Nov 27 '15 at 00:47
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    @JulieinAustin the trouble with the 14.7:1 assumption is that it won't always be true. – Zaid Nov 27 '15 at 03:08
  • Hmm, how far off will it be? Is this how board computers calculate fuel consumption? – I have no idea what I'm doing Nov 27 '15 at 07:26
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    @IhavenoideawhatI'mdoing ... No, they use a MAF, MAP, and/or O2's as well to calculate, using the amount of air going into the engine. Remember the long discussion on air intake based on load we all had? – Pᴀᴜʟsᴛᴇʀ2 Nov 27 '15 at 11:37
  • @IhavenoideawhatI'mdoing On top of what Paulster2 said, it is the ECM that is deciding how much fuel to put in, so it knows the current fuel consumption, because it decided it. – HandyHowie Nov 27 '15 at 12:57
  • @Paulster2 That's what I was asking, if board computers calculate fuel consumption by multiplying measured air mass with the required stoichiometric ratio. And the discussion is still going on, we didn't really come to a conclusion :) – I have no idea what I'm doing Nov 27 '15 at 14:17
  • 14.7 to 1 is the "ideal" ratio because that's how the chemistry works. If there's less air a gasoline ("petrol") engine runs rich, if there's more air it runs lean. ECMs work by measuring the fuel and air and adjusting the mix based on how the combustion worked out based on O2 left over in the exhaust. They should report an error for over-rich or over-lean combustion. In an ideal world with an open-source ECM, you could just ask the car and it would just tell you :) – Julie in Austin Nov 29 '15 at 14:44

2 Answers2

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Just a thought of a way to determine what you are looking for...

Your injectors are rated for a certain flow at a certain PSI. Most cars have a Fuel Pressure Regulator to keep this pressure constant relative to the pressure in the manifold. You should be able to find this flow rating for your vehicle's injectors.

Find a way to 'watch' how long the injector is open. This is going to happen is the millisecond range, I'm thinking an arduino using the injector power wire as an input, constantly polling and recording/outputting changes. Not sure if the arduino would be fast/accurate enough. The car is 12v and the arduino needs 5v, so you would need to drop the voltage somehow and use a diode so your reading doesn't effect injector performance.

Once you have a flow rate and how long it is open, you can calculate how much fuel is injected.

rpmerf
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  • "You should be able to find this flow rating for your vehicle's injectors" - easier said than done! :) Nothing wrong with the approach though, +1 – Zaid Dec 19 '15 at 18:23
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How about filling the tank, driving it however far and re-filling the tank. The greater the distance travelled, the more accurate the amount of consumption per unit of distance.

Rotten Ronnie
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