MFD Fuel economy measurement. Discussion.

[WelshGas]

OK, the MFD fuel economy measurement, and here I’m talking about the Memory 2 Average MPG, is presumed to be over optimistic compared to a Fuel Tank brim to brim calculation. But is it?

A Fuel Tank brim to brim calculation will not take account of the fuel used to run the Diesel Coolant Heater, in winter, and the Diesel Parking Heater although this might be a small component. However, it also doesn’t take account of the fuel used in the DPF regeneration cycle. All these additional factors will decrease the calculated fuel economy figure ( MPG ).

Now, the MFD calculated MPG is often greater than the Brim to Brim calculation, meaning it seems to be over optimistic. But is it?
I presume the vehicle doesn’t measure the fuel consumption directly but is calculated from the Fuel Rail Pressure and the time each Injector is functioning and data on the Injector performance at certain pressures.
Now, if the MFD Fuel Consumption is based on this assumption and Injector function on the Power Stroke only and NOT Injector function on the Exhaust stroke during DPF Regeneration then it’s MPG calculation might be much more accurate than we believe as it is purely based on Fuel used / Miles covered, disregarding ALL fuel used for other purposes ie: DPF Regeneration/Coolant and Parking Heater.

I don’t know if my reasoning is correct, but does anyone else have any thoughts.

 

[2into1]

I thought its calculation would include DPF regeneration but exclude the Parking Heater (as that doesn't involve 'miles' so it's fair to exclude it from a MPG calc). I hadn't thought about the coolant preheater in Winter. It morally ought to include it (as it is meant to improve efficiency overall) but I bet it doesn't.

No doubt when the van's are put through the EU consumption tests, they are supplied having just completed a dpf regeneration. so as to inflate the numbers even more than normal.

The MFD is about 10% optimistic in my T6 (so 44mpg claimed is really 40). That excludes tanks where the parking heater was used. In my last T5, it was more or less spot-on.

 

[Amarillo]

I don’t know if my reasoning is correct, but does anyone else have any thoughts.

10-15% is a significant error. My MFD consumption shows a marked increase in fuel consumption during a regeneration: I first noticed this on a pristine motorway between Plovdiv and Burgas, first I thought I was going uphill, but while undulating the altimeter showed no great height gain. Then I thought we must have a headwind, but the wind was behind us. So I do believe that regeneration is included in the MFD consumption data. Could it be that the MFD consumption data corrects for the alternator, i.e. ignores use of the van's electrics and charging the leisure battery. Could air conditioning, lights and other electrics account for 10-15% of the fuel use? Certainly it seems likely that use of the parking/coolant heater is excluded from the MFD consumption, otherwise we'd see a big drop in MFD (2) data after a cold night with the parking heater running.

 

[WelshGas]

10-15% is a significant error. My MFD consumption shows a marked increase in fuel consumption during a regeneration: I first noticed this on a pristine motorway between Plovdiv and Burgas, first I thought I was going uphill, but while undulating the altimeter showed no great height gain. Then I thought we must have a headwind, but the wind was behind us. So I do believe that regeneration is included in the MFD consumption data. Could it be that the MFD consumption data corrects for the alternator, i.e. ignores use of the van's electrics and charging the leisure battery. Could air conditioning, lights and other electrics account for 10-15% of the fuel use? Certainly it seems likely that use of the parking/coolant heater is excluded from the MFD consumption, otherwise we'd see a big drop in MFD (2) data after a cold night with the parking heater running.

Is the fuel for Regeneration included?
When an actual DPF Regeneration takes place I would expect the power output of the engine to be affected detrimentally so an increase in power via an increase in fuel consumption, to maintain the power output and hence speed, would occur. Hence a temporary fluctuation in MPG but still excluding fuel pumped into the DPF.

After all, wouldn't you expect a raging inferno in the DPF/Exhaust system to have some effect on power output?

I thought its calculation would include DPF regeneration but exclude the Parking Heater (as that doesn't involve 'miles' so it's fair to exclude it from a MPG calc). I hadn't thought about the coolant preheater in Winter. It morally ought to include it (as it is meant to improve efficiency overall) but I bet it doesn't.

No doubt when the van's are put through the EU consumption tests, they are supplied having just completed a dpf regeneration. so as to inflate the numbers even more than normal.

The MFD is about 10% optimistic in my T6 (so 44mpg claimed is really 40). That excludes tanks where the parking heater was used. In my last T5, it was more or less spot-on.

That is my question though. Does the MFD figure include fuel injected for a DPF Regeneration? It doesn't have to as that fuel is Not converted into power to cover miles but used for another purpose and the amount used for that "Other Purpose " will vary greatly depending on the " Type " of miles. Less for Motorway more for short journeys.

Maybe my warped theory but I can see the rational for excluding DPF regeneration as well as Coolant Heater and Parking Heater fuel usage as none of them contribute to " Miles Covered or Engine Runtime ".

 

[Loz]

I am sure it includes all fuel passing through the injectors as calculated by the ecu and will include dpf regen extra fuel.
The overall mpg will not be totally accurate as it is based on wheel rotation and as we know the speedo under reads, which is a statutory requirement, but this could be corrected if you slightly oversize your tyres to the point where the speedo matches true (gps) speed.

 

[SailSki]

Winding back up the thread a touch - just posting a correction - when using acontinental car (or taking a UK car in Europe) I am used to seeing the consumption in litres per 100km, so I was horrified to see 15-18 recently. Just looked again and realised the it is actually showing km per litre, so a figure of 15-18 isn’t bad at all....(43-50mpg)

 

[WelshGas]

I am sure it includes all fuel passing through the injectors as calculated by the ecu and will include dpf regen extra fuel.
The overall mpg will not be totally accurate as it is based on wheel rotation and as we know the speedo under reads, which is a statutory requirement, but this could be corrected if you slightly oversize your tyres to the point where the speedo matches true (gps) speed.

I agree the wheel diameter is a factor so the MFD calculation could be more correct with new tyres and less with worn but the software used was sophisticated enough to know when on a rolling road and to adjust matters to give a better emissions score so would know when fuel was bing used for power and not for DPF regeneration.
On my 180 if doing a long trip the MFD readout is within 1 or 2 MPG compared to a brim to brim measurement, so significantly less than the 10-15% that @Amarillo quotes.

 

[Amarillo]

I agree the wheel diameter is a factor so the MFD calculation could be more correct with new tyres and less with worn but the software used was sophisticated enough to know when on a rolling road and to adjust matters to give a better emissions score so would know when fuel was bing used for power and not for DPF regeneration.
On my 180 if doing a long trip the MFD readout is within 1 or 2 MPG compared to a brim to brim measurement, so significantly less than the 10-15% that @Amarillo quotes.

A quick Google search suggests that the MFD calculates fuel use by measuring time of flow through the injectors.
https://www.rac.co.uk/forum/showthr...-MFD/page2&s=34ea368ec758685bb08f1bd134eca2bf

In one conversation there is a suggestion that on some VWs the MFD calculation can be adjusted through the VCDS!
https://www.my-gti.com/155/volkswagen-fuel-consumption-error-correction

Can anyone confirm if this can be done for the T6?

 

[Loz]

A quick Google search suggests that the MFD calculates fuel use by measuring flow through the injectors.

In one conversation there is a suggestion that on some VWs the MFD calculation can be adjusted through the VCDS!
https://www.my-gti.com/155/volkswagen-fuel-consumption-error-correction

Can anyone confirm if this can be done for the T6?

You can but only in one direction (larger diameter tyres) so it doesn't help us with standard tyres.

 

[2into1]

Thinking about this more....
Often the first hint I get that a regeneration is taking place is through the mfd which will be showing a much lower mpg than expected - you tend to get a feel for your expected mpg. So its certainly the ‘lost power’ part but I suspect it’s also capturing the fuel sent to the dpf.

Don’t forget that the T5s and the T6s have very different approaches to dpf regenerations. T5s miss out on the hovercraft impressions us lucky T6 owners get when you finish your journey part way through a regen!

 

[Bellcrew]

Thinking about this more....
Often the first hint I get that a regeneration is taking place is through the mfd which will be showing a much lower mpg than expected - you tend to get a feel for your expected mpg. So its certainly the ‘lost power’ part but I suspect it’s also capturing the fuel sent to the dpf.

Don’t forget that the T5s and the T6s have very different approaches to dpf regenerations. T5s miss out on the hovercraft impressions us lucky T6 owners get when you finish your journey part way through a regen!

It is a little embarrassing when you stop at friends house and your cool camper sounds like it is about to take off.

 

[briwy]

Our MFD does appear to be fairly accurate when I have bothered to check it. However, the van uses whatever it takes so I really don't bother about consumption too much unless the MFD started to show a significant difference to normal for some reason which could indicate a problem somewhere. I know I could drive slower, always go where it's flat, take stuff out of the van etc but quite frankly what is the point. I didn't buy a three ton non aerodynamic brick expecting good fuel consumption.
When people say "brim" what exactly are they talking about. Stop filling when the pump clicks off or literally filling up to the top of the filler tube? It is not a good idea to fill it right to to very top.

 

[Loz]

When people say "brim" what exactly are they talking about. Stop filling when the pump clicks off or literally filling up to the top of the filler tube? It is not a good idea to fill it right to the very top.

Referring to Brim-2-Brim mpg calculations. Litres of last fill over distance on trip reset each fill. Only really useful in the summer when neither of the heaters is likely to consume fuel.