Ok, this is really not that serious, but just wanted to find out if anyone could give me a theory to confirm what I've observed.....or just tell me I'm nuts.
The background:
I have a little VW Golf Bluemotion 1.6l turbo diesel that I use for commuting the 40 something kms to and from work. It's now 3 months and 6000km old, and starting into it's 5th tankful. It's supposedly one of these environmentally friendly cars that will save the planet, and the official fuel consumption is rated at 3.8l per 100km (though having a V8 Disco kinda balances it out!)
The odd phenomenon is that for the first quarter of a tank, I can get ridiculously low fuel consumption - between 3.3 and 3.5l/100km (per the in-car trip computer and measured the old fashioned way and I get nice little reports from the lease company that back it up too!). But for the remainder of the tank, the consumption increases so that it shows a average of 3.9 to 4.0l/100km.
I know you're thinking: "why would you care about less than 1l/100km. I don't really, but I am just baffled as to why fuel consumption would increase by over 25% just according to how much fuel is in the tank??
Everything else is as constant as it can be. The fuel, same brand (Caltex account card sees to that), same filling station, the route (I pretty much only use it to and from work, the Disco comes out to play at weekends).
I would have thought that as the weight in the car reduced (not that a quarter of a 55l tank of diesel would weigh much) then fuel consumption would improve?
Or is it that the less fuel in the tank the less dense it is, and the more is needed to burn?
So theories please!
Swaggie
The Bluemotion engine and its operating system is VERY advanced and frugal, as you know. A great design. It is possible that the density of the fuel, IE the fuel is heated somewhat during driving, changes to th point that it has a different characteristic during injection, and knowing how precise the germans can be, this could be enough to actually impact on the consumption to compensate for some miniscule output variations?
??
Got to think like one of those german scientists...
JC
Yup, it's very clever. I opened the bonnet and, no word of a lie, there's a slot that's marked "insert dilithium crystals here"!Originally Posted by justinc
You must use ze loud pedal less!Got to think like one of those german scientists...
JC
RoverLord
temperature, wind conditions and vehicle load can make that much difference...
I used to be paranoid about it when I noticed it on an older vehicle and it took me 2 months to work out that my initial fuel economy on the top 1/4 of the tank was better than the next quater because of the driving pattern.
ID fill up at the same place at the same time in the week and the difference between the week thats that I had to half fill VS the quater fill weeks was not only that I was doing longer (or more frequent runs but that the longer (or more frequent runs were driven during the day and mostly into the seasonal winds compared to the shorter runs that were driven over mostly the same roads but at different times.
Dave
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YarnMaster
You will find it's more to do with the fact that the fuel sender is constant in it's resistance from full to empty whereas the shape of the tank and resulting volume is not.
As the fuel level drops, the shape of the tank changes which results in a changing rate of fall giving the impression of a higher fuel consumption.
The only real way to gauge (no pun intended) consumption figures is to measure over a full tank and base it on the actual fill volume.
In the D2 I usually get around 220 - 240kms for each of the first two 1/4 tank marks, 180 for the third and less than that again from there to the light.
Look under a D2 or RRC and check the tank shape.
We have exactly the same thing on our Freightliners fitted with round tanks but not on those fitted with Hilton square tanks.
The only other acurate way on the fly is to meter fuel going to the rail and then what's going back thru the tank return line and taking into account temperature of both flows. Problem is to measure such a low differential volume, meters would cost a fortune and outweigh the benefit of what it's there for.
Most manufacturers use a formula derived from known injector flows based on nozzle size, pulse width, fuel gravity, tank level, temperature and injection pressure for in car fuel consumption.
Using Capitals, the difference between helping your Uncle Jack off a horse or helping your uncle jack off a horse...
Dave, the load varies by the weight of plus or minus an extra apple in my bag, or if I haven't I haven't had a shave in the morning.
It's always just me, and my laptop bag. With some lunch in the bag on the way to work, the lunch in my belly on the way back
I fill up at the same place, either on the to work, or on the way back. A tankful lasts about 3 weeks, or ~1300km for 51-52l.
I've not tried to look for weather conditions and winds, but for temperature, it's been anywhere from ~3 degrees first thing in the morning back in July to 30 degrees last Friday afternoon, with consistent results in fuel consumption behavior. I don't use the aircon either.
However, reading a technical blog on the car, it seems that the alternator can be switched off, (recharging can be achieved from the brakes or during deceleration), something as simple as an extra electrical load can cause the alternator to be re-engaged.
So perhaps the fuel pump has to work harder due to lower fuel levels (to achieve the injection pressure of 1600bar!), meaning there's an extra electrical load, and the alternator is re-engaged to cope, putting an extra load on the engine, and increasing fuel consumption.......<deep breath>
Makes the Disco V8 look positively stone age.
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