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mpg readout

J

johnd

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Location
Tenbury Wells
Car
2.2 i-CTDi Exec
Anyone out there know how the computer senses fuel flow? It needs this to calculate average mpg, along with distance travelled. So I guess there is a sensor fitted somewhere in the fuel delivery system?
 
I would think the fuel is metered via each injector. The injector would only allow a certain amount of fuel through over a given time. So the ecu would know how long the injectors were open for, and then know how much fuel has been used.
 
I think it works of the dash mate and the DBW system, it calculates speed on the speedo and then also fuel levels using the dash display... as you may notice the live mpg meter changes as you press the pedal...
 
I ***umed it was the ECU knowing the ratings of the fuel pump and working it out from that. That way it would explain the rubbish calculations of the MPG.
 
Dan Robinson said:
I ***umed it was the ECU knowing the ratings of the fuel pump and working it out from that. That way it would explain the rubbish calculations of the MPG.
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True - I just worked out the mpg myself after brimming the tank to see what had been used. Computer displays 58mpg but I've worked out 55mpg. Not a vast difference, but enough!
 
Don't knwo if this helps, but I know that it is all relevent to the speed ring sensor, if this isn't workling, and the speedo isn't working you wont get any MPG readings.
 
Speed ring sensor?

***uming that is what i think it is, then that would be used in conjunction with the fuel throughput in order to get MPG.
 
Dan Robinson said:
I ***umed it was the ECU knowing the ratings of the fuel pump and working it out from that. That way it would explain the rubbish calculations of the MPG.
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I thought the fuel pump just increased the fuel pressure to a set parameter, ready to be injected into the cylinder by the injectors. There is normally a fuel return to the tank, so some of fuel pumped ends up back in the tank.
 
You would need a speed reading for the ecu to calculate distance travelled. Not sure on the Accord what picks up this reading, could be on the gearbox or the driveshafts.
 
So basically none of you know and are all guessing :rolleyes:

John, do it the old fashioned way. I don't have the MPG in my pre-facelift but from what I seem to read all the time from member posts, the MPG trip computer has a habit of telling porkies in this car.
 
Gents,

I am certainly no expert!!!! and am only going off personal experience, when I bought my cdti from "dodgy dealer - Auto100.co.uk - BE WARNED!) and drove it off the speedo wasnt working along with the MPG, trip computer etc, and he told me they had to replace the speed ring sensor, which is something to do with the gearbox, which he had out to replace the clutch.

To be honest, was just trying to give an input on my limited knowledge, but as it came out this dealers mouth, could all be a load of tripe anyway!
 
F6HAD said:
So basically none of you know and are all guessing :rolleyes:
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Yeah, but an educated guess. Having a look around it does seem to be down to the opening and closing time of the injectors and the distance travelled. The fuel used can't be measured anywhere else especially with common rail diesel.
Although the ecu calculations used on most makes of car for mpg seems very inaccurate.

I only use the mpg readout as a guide. As Fahad said above, the old fashioned way is the only accurate way.
 
Rob said:
Yeah, but an educated guess. Having a look around it does seem to be down to the opening and closing time of the injectors and the distance travelled. The fuel used can't be measured anywhere else especially with common rail diesel.
Although the ecu calculations used on most makes of car for mpg seems very inaccurate.
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Rob, I think you are the closest! My bet is on some kind of sensor in the common rail area but I really would like to know how it works since fuel flow (as a precise measure of consumption) is notoriously difficult to monitor.
 
The only way I can think of measuring directly other than injectors or fuel pump parameters would be a turbine, or an ultra sonic dooh dah like they have in Gas meters The former is unreliable nad prone to failure. The latter would be too expensive and I doubt its reliability in a moving vehicle.
 
I was thinking about this afternoon during antenatal class (don't tell the wife I wasn't listening). The other parameter needed would be the fuel pressure, because this would effect the amount of fuel passing through the injector.
John, like you say fuel flow is difficult to measure, I think this why the MPG readings are unreliable.
 
wasn't listening
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Listening to what fella? Bunch of sanctimonious earth mothers - I learnt nothing... Don't panic - sit back and feel the power of dreams :lol:
 
as everyone else is guessing, allow me to add some useless trivia...In piston powered aircraft they often use impeller type fuel flow sensor which is usually fitted to fuel line down stream from the fuel pump....if the aircraft has a return line to the tank, another sensor is fitted and the difference between the two is then calculated to give actual fuel consumption figure typically in gallons per hour.
How all this relates to an Accord.....who knows :huh:
 
That would be what I meant by turbine... they must make them pretty durable to work in aircraft!?
 
That would be what I meant by turbine... they must make them pretty durable to work in aircraft!?
 
Dan Robinson said:
That would be what I meant by turbine... they must make them pretty durable to work in aircraft!?
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Haven't seen one fail yet...they do have a bypass system built in just in case the impeller seizes.
 
Just a simple guess would it be just a case of just an equation of the fuel gage and the amount of mile drove :rolleyes:
 
As I said earlier I did a bit of research and it appears to be a measure of the pulsing of the injectors and distance travelled. An injector can only pass a certain amount fuel, at a certain pressure. The ecu is programmed with this figure and also knows how far the car has travelled during this time. The ecu them just does a simple calculation for the mpg.
 
So what measures fuel pressure?

and Pressure Sensor is not a valid answer :lol: Where is it and what type? A Transducer? Simple switch? High/low limit switch?
 
Dan Robinson said:
So what measures fuel pressure?

and Pressure Sensor is not a valid answer :lol: Where is it and what type? A Transducer? Simple switch? High/low limit switch?
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surely it must be a transducer...possibly a diaphragm which is displaced with fuel pressure and this displacement is measured ie. more pressure- more displacement of diaphragm- bigger output signal
 
There should be a fuel pressure sender on the common fuel rail which feeds the injectors. As for what type it is, I haven't a clue. It can't be a simple switch though, otherwise it wouldn't be able to measure the exact pressure, only that the pressure was above a set value.
 
Dan Robinson said:
Listening to what fella? Bunch of sanctimonious earth mothers - I learnt nothing... Don't panic - sit back and feel the power of dreams :lol:
Click to expand...

I think you're probably right Dan. Will wait for the day and let nature take it's course. :)
 
guys have i got the wrong end of the stick but how is a petrol measured
 
Who cares ?:lol:
 
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