jud - there is only one consumption readout on the Clio, which is not 'instantaneous' but cumulative, restarting whenever the tripmeter is reset. The longer I wait before doing this, the less the figure changes - i.e.it is calculating over an increasing period. But unless it has been going for 50 miles or more, climbing a steepish hill is enough to produce a noticeable change. It seems to update every second or two.
As it cannot know in absolute terms how many litres are in the tank, it can only calculate by feedback from the fuel injectors, unless there is a flowmeter somewhere? I suspect that this device (whatever it is) is wrongly calibrated.
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The quantity of petrol injected into you engine is controlled by the management unit which depending on the demand, i.e.throttle, air flow meter and lambda sensor the product of which controls the duration of the pulse that opens the fuel injector.. This pulse width and the rate it fires ( RPM ) provides the measure of quality and the electronic speedo taken from the final drive shaft give you distance travelled thus base line for the computed fuel consumption. To stop the numbers jumping all over the place the results are dampened significantly. The amount of fuel in the tank is not used for the calculation of MPG
If you had cruise control ( thus removing the human right foot element ) and travelled a 100 miles on a relatively clear motorway the computed fuel consumption and the actual can be very close.
You are now down to the tyres. If new then they have a rolling distance some 1 to 1.5% bigger than there worn value. Add to this you may have fitted alternative alloys and different profile tyres and you may have another correction. Some vehicles computer/EMU have a programmable element for the build of the vehicle and the Alloy/Profile of the rubber.
Lets be honest it is only an indication and the full tank to full tank is the real situation. Having said that I have heard of people noticing a change in the computer MPG that caused them to call into the garage to discover there O2 ( Lambda ) had failed and the car was overfuelling. Why he didn’t see the EMU/Lambda light on as well I do not know. So it can be another diagnostic tool. Regards Peter
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Thanks Peter et al. Although we have not owned the car from new, there is nothing to suggest that it is running incorrectly or that the wheels are not as programmed. Perhaps its first MoT in June will show an odd lambda value. But I presume that although the electronics do an accurate job of maintaining mixture and timing, it does not follow that they know in absolute terms the volume of each dose to the injectors.
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I do not know what flow meter technology Renault use for the Clio, but since the ECU is responsible for delivering the fuel in correct quantities, it must therefore be aware of the instantaneous (and hence by calculation, historical cumulative) figures. This must be the data source for display calculation unless they install a duplicate set of sensors.
In the Old mechanical K jet systems the figure was calculated from the measured depression (vacuum sensor). Since the injection quantity is determined by the Mass flow sensor (which will have a defined relationship with inlet flow and hence depression) the consumption can be computed. My understanding of this system is that it will only give correct readings if the 'mixture adjustment' is set up correctly. ie the consumption figure is not an absolute measure. That apart my Golf |Mk2 with K jet gives a reading within 2% of brim to brim measures.
Can anyone comment on or correct this view?
pmh (was peter)
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When i started my post, the Peter D post was not there!
Apologies for duplication etc etc.
pmh (was peter)
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Agree with Peter D and pmh.
I had the dispay unit (infamous \'missing\' pixels problem) replaced in the Vectra SRi V6 Estate I drove just over three years ago, and the new unit resulted in inaccurate fuel consumption. (40 MPG at a steady 70 MPH, I wish!) On going back to the dealer, they explained that the unit was programmable, and used fuel flow rate for the injectors as the basis of calculation. Five minute\'s worth of TEC2 plug-in, and the result was a unit that gave exactly the same readings as the one on which the readout failed, and which was pretty close to spot on with the full tank to full tank checks that I made.
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