MFD MPG Lies

[ShooglyStu]

I often coast into junctions, roll down hills and the like to increase mpg. Like others have mentioned on this thread I am infamous among friends and family for being the person who drives slow and does whatever I can to use as little fuel as possible.

If you coast, i.e. roll with the clutch disengaged, then the engine requires fuel to prevent it from stalling.
However, if instead you keep the clutch engaged and lift off the accelerator then the fuel will be cut off completely. This will give you the lowest fuel consumption.

 

[ShooglyStu]

My speedo under reads by 4%, not sure if that effects it

Your speedo should over read for legislative reasons.
Fuel consumption uses your odometer value and that should be fairly accurate, unless you have changed the size of the tyres from original.
Even if you have changed tyres, the MFD will use the same odometer value as you use for calculating mpg (unless you use an independent GPS derived value), so this is not a reason for the MFD to be different.

 

[roadtripper]

Just for reference on a long roadtrip run last weekend I got 43 MPG indicated and 44 MPG calculated - that's on a 150PS 6.1 Caravelle on the factory suspension and 18" wheels and I was using the "since last refueling" figure from the van.

I don't normally calculate accuracy as it's the relative figures I care more about to pick up my driving style slipping or something in the vehicle changing that might need looking at, but this thread reminded me I hadn't done a check on the van. Pretty happy with the result, seems I can broadly rely on it.

 

[roadtripper]

In terms of efficient speed a few things are at play.

Air is a fluid and broadly speaking as speed through the fluid increases the drag from the fluid increase cubically - so a 2 times increase in speed results in an 8 times increase in drag (2 x 2 x 2). There's a lot more nuances in a real world situation but it's a reasonable estimate for the general principle.

Engines tend to have a peak efficiency point or band so the practical result is a combination of where that is in a particular engine, coupled with the gear ratios available, that minimises losses due to drag. Some solutions will come out with impractically slow speeds though!

The 56mph point is often very efficient, not due to physics, but due to regulations. As that is the point that comparable efficiencies are calculated for figures that must be displayed in advertising it's not a surprise that many vehicles are designed to have peak practical efficiency there. As that's a common open road speed all the way up to HGVs it's not actually a bad thing because that's roughly where a peak efficiency should be anyway.

And yes I'm aware that certain companies "optimised" the results at that point in ways that were not in fact real, that's an unfortunate consequence of having so much at stake on the results.

From having the XC70 for over a decade I find the 56mph sweet spot is exactly that, if I drive up to 60mph and let the auto box lock up in fifth the speed can then drop back into that region without the gearbox unlocking at modest torques and the engine revs drop into the 1800rpm region and the fuel consumption becomes dramatically lower. MrsRT calls it the "magic fuel region" as we can drive for 200 miles and have more indicated range at the end than when we started. I realise that's not really true but of all the cars I've owned this one seems to have the absolute ideal match of engine gearbox and drivetrain for efficiency for what it is, a heavy Swedish AWD.

 

[GruB]

If you coast, i.e. roll with the clutch disengaged, then the engine requires fuel to prevent it from stalling.
However, if instead you keep the clutch engaged and lift off the accelerator then the fuel will be cut off completely. This will give you the lowest fuel consumption.

Poor wording. I never disengage the clutch. I should have said rolling.

 

[mmi]

Air is a fluid and broadly speaking as speed through the fluid increases the drag from the fluid increase cubically - so a 2 times increase in speed results in an 8 times increase in drag (2 x 2 x 2). There's a lot more nuances in a real world situation but it's a reasonable estimate for the general principle.

A smartass here sorry - I would say the drag (force) increase is a function of speed "only" squared.

Drag Equation | Glenn Research Center | NASA

Drag Drag depends on the density of the air, the square of the velocity, the air's viscosity and compressibility, the size and shape of the body, and the

www1.grc.nasa.gov

However, the power required to overcome the drag (force) is also a function of speed - thus the power indeed increases cubically vs. speed - from above v² multiplied by v from below = v³

What is Power? | Glenn Research Center | NASA

For scientists, power is the rate at which work is performed by a force acting along a distance. As an example shown on the slide, the Wright 1903

www1.grc.nasa.gov

Then as energy is a function of power.... would expect to see more energy/fuel consumed when more power is applied... to overcome the drag.

 

[GruB]

For the record - this is a steep hill and I would really struggle to keep it at 56mph going up.

On my TCR coming down my quickest recorded speed was 49mph

 

[bobnudd]

After all these years, and I find out she's been lieing all along, how could I have been so foolish! I'm going straight to autotrader in the morning to get a divorce!

 

[Bav]

After all these years, and I find out she's been lieing all along, how could I have been so foolish! I'm going straight to autotrader in the morning to get a divorce!

What a bitch! Did you not get suspicious by her constantly cosying up to the fuel pump?

 

[bobnudd]

I literally caught her with the pump hanging out of her, splashing his stuff all inside her. And down the side. And on my shoes.

 

[Bav]

I literally caught her with the pump hanging out of her, splashing his stuff all inside her. And down the side. And on my shoes.

Utter humiliation

 

[bobnudd]

At first I was almost enjoying it, then they wanted me to pay. That's the humiliating part. Oh well plenty more vans in the sea, etc. Perhaps an ABTe........now there's humiliation.

 

[dubber36]

On my TCR coming down my quickest recorded speed was 49mph

You need some bigger gears. You need to crack 50mph.

 

[mmi]

How much fuel is used for DPF regen and does this get accounted for by the MFD display?

Typically - or at the minimum - for each DPF regen it's 0.9 litres on top of the fuel consumed for normal travel.

[T6_measured] Fickle fuel consumption on MFD – encore

Is the data showing approx 4% of fuel normally used for the journey is being consumed when performing a dpf regen?

www.t6forum.com

Instead of displaying real fuel consumption during the DPF regeneration, the MFD compensates the display after the DPF regeneration by showing approx. 15% increased fuel consumption as long as needed to catch up with the extra dose used in the DPF regen. The compensation takes at the minimum about 60 km (an hour). The cutoff then is actually quite steep

Full story here - short version in picture 4.

[T6_measured] Fickle fuel consumption on MFD – encore

In March 2020 finally got the long-awaited ECU (Engine Control Unit) software update. Unfortunately it took a while to get a good chance to do some measurements to try to figure out what has been changed. So finally a few weeks back dusted the VCDS and collected all kind of intriguing data. The...

www.t6forum.com

The MFD was reset at the start and by the end was reading 47mpg again.
So, a smaller difference but still pretty big.

The MFD fuel reading can be adjusted - however, adjusting doesn't change the compensation behaviour above.

 

[ShooglyStu]

Typically - or at the minimum - for each DPF regen it's 0.9 litres on top of the fuel consumed for normal travel.

Instead of displaying real fuel consumption during the DPF regeneration, the MFD compensates the display after the DPF regeneration by showing approx. 15% increased fuel consumption as long as needed to catch up with the extra dose used in the DPF regen. The compensation takes at the minimum about 60 km (an hour). The cutoff then is actually quite steep

Thank you very much for your detailed analysis. In summary, the DPF displayed mpg figure does include the extra fuel used for a DPF regen, but the displayed figure spreads the extra fuel used over 60+km more than the actual distance of the regen (~20km) in order to reduce large fluctuations in the displayed value.

So, that eliminates the DPF regen as a possible cause of the discrepancy between actual and displayed mpg.

In March 2020 finally got the long-awaited ECU (Engine Control Unit) software update.

What was this for? Is there a way for me to find out is my van has it (using Carista)?

 

[mmi]

What was this for?

Please have a read below.

[T6_measured] NOx emissions, AdBlue consumption, DPF regeneration - Before and after ECU update

Introduction An update campaign for T6 EU6 diesel engines took place in 2019-20. Affected engines were on van of model years MY16-MY18, including some of MY19. For M1 class (passenger carriers, VIN WV2...) the update was an official recall. For N1 class (VIN WV1....) the update was not...

www.t6forum.com

Is there a way for me to find out is my van has it (using Carista)?

Yes. The software ID and version can be seen in "Detailed ECU info"-report (under Carista Service menu).

The current/latest software can be found by engine part number here:

E.g.

Welcome to VAG-Flashinfo!

VAG Flashfiles Informations - Version check of VAG ECUs - Update check for Audi, Seat, Skoda and Volkswagen

vag-flashinfo.de

 

[ShooglyStu]

Yes. The software ID and version can be seen in "Detailed ECU info"-report (under Carista Service menu).

Thanks again. My van has the old 4047 version of engine software 04L906056KA

Since your detailed measurements of fuel consumption over DPF cycles were made using the latest version of ECU software, the same conclusions cannot be assumed for the old version of software

 

[Murdoch]

What I find hammers my economy is the van trying to do the dpi regen on short journeys

2 litre manual 150 bhp - consistently around 32 mpg or less unless I’m driving under 65 on motorways

 

[mmi]

Since your detailed measurements of fuel consumption over DPF cycles were made using the latest version of ECU software, the same conclusions cannot be assumed for the old version of software

Thanks for your interest - excellent comment. Fortunately there is no need assume - I had noticed the behaviour already in 2016, but just couldn't understand the phenomena and how to get hold of that for a while. Out of curiosity I did a lot of different data recordings - on different Transporters (engines CAAC, CXFA, CXGB, CXHA, CXEB). Of course, my first assumption was that there is something wrong with the engine because the observed fuel consumption on MFD was so unstable. Well, when faulty injectors were found I thought it would have solved the mystery. See below

[T6_measured] Injector failure

Keyword “loud knocking metallic noise from engine”. VW has published in September a service note which can be found with vehicle's VIN. The sound clips from my engine (150PS, EU6) with comments on 21 Oct 2017 in thread Tick over noise EDIT: audio files from the thread copied here as...

www.t6forum.com

Now being a bit wiser on this I have also checked a good amount of those early recordings - and not surprisingly - the phenomena can be seen also before the update. Actually quite recently I even have had "access" to identical van as the report was about - but still having non-updated software - exactly the same thing going on there, too.

I'm afraid the phenomena actually might be present even on all VAG diesels - in spite of them having different ECU manufacturers (Delphi, Bosch). At the moment I have a recorder onboard mate's old Passat with CFFB engine - and he is meticulously noting down MFD readings . And, no surprise, the same thing but better camouflaged is going on - the compensation is simply stretced over longer period - thus not so obvious at first on MFD - but comparing MFD readings with ECU reported values it sticks out like a sore thumb

 

[ShooglyStu]

I'm afraid the phenomena actually might be present even on all VAG diesels - in spite of them having different ECU manufacturers (Delphi, Bosch).

Thanks for your confirmation.
It is quite common to have more that one supplier for each ECU in a vehicle, even for the same model. However, in this case all suppliers with have to satisfy identical software requirements so that the ECUs behave identically and are interchangeable.