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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old March 25th, 2008, 12:59   #1
tnp
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Default BSFC (Brake Specific Fuel Consumption)

Being a newbie, I've been reading a lot of threads. There are many mentions to the most efficient range of the RPM curve. A little Googling ran me into some neat terms: BSFC (Brake Specific Fuel Consumption) being the most interesting. I've seen graphs of BSFC vs Engine RPM vs Torque, and BFSC vs BMEP (Break Mean Effective Pressure, or Engine Load) for various engines - but not ours...

Using the minimum fuel for the power produced should equate to the highest fuel economy. While I appreciate the different posts that users state where they believe those points are, it would be great to see some actual data. This would validate a lot of things, such as what really are the best shift points (under various conditions), how quickly to accelerate, among other things. Then, one could use VAG-COM to validate these assumptions and even calculate/predict the best shift points and acceleration modes under particular conditions.

Does anyone have these for a Beetle TDI 1.9L? Or, would anyone know how we can get this?

Thank you!
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Old March 25th, 2008, 14:08   #2
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There is a chart of what you are wanting around here somewhere.....
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Old March 25th, 2008, 14:16   #3
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Found it. DBW had it in his pics.

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Old March 25th, 2008, 14:21   #4
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That confirms what´s been discussed many times, these cars are most fuel efficient at 55mph.

Edit: The BSFC is always at peak torque, isn´t it?
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Old March 25th, 2008, 14:22   #5
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Who drives that slow anymore though :-\
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Old March 25th, 2008, 14:24   #6
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I think the car runs great at 120kph, and that´s where I set the CC
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Old March 25th, 2008, 14:25   #7
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Ya, at 70 MPH and up it feels good.
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Old March 25th, 2008, 14:36   #8
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Quote:
Originally Posted by nokivasara
I think the car runs great at 120kph, and that´s where I set the CC
Indeed, the cars do run just fine/great at ~75 mph and faster. I drove a BMW 530d at about 130mph (210kph) on the Autobahn in Bavaria and it also felt just fine and "ran great" but certainly wasn't as efficent as it could be. I've only ever driven that fast in Germany late at night. The rest of the time I spent on the highways was "with the flow" at about 80mph (70 - 90 average and of course some care are slower).
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Old March 25th, 2008, 14:39   #9
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Quote:
Originally Posted by Bob_Fout
Who drives that slow anymore though :-\
45 to 55 mph for approx. 18 miles every morning. Of course that's at 3:00 AM and on a stretch of rural highway where the speed limit is 45. It's fuel efficient and it makes it easier to dodge all the deer here in PA!
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Old March 25th, 2008, 14:42   #10
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But if you were driving faster you´d be past the deer before it runs up on the road!
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Old March 25th, 2008, 14:45   #11
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Quote:
Originally Posted by nokivasara
But if you were driving faster you´d be past the deer before it runs up on the road!
LOL! A lot of trucks run the same road and you should see what's left of those deer when they collide. Meat explosions!
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Old March 25th, 2008, 15:51   #12
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Quote:
Originally Posted by Bob_Fout
Found it. DBW had it in his pics.

This chart only shows BSFC at full load at the given RPMs. However, nobody drives at full load under normal circumstances.

A more useful map of BSFC over the entire range of operating speeds and loads (BMEP) is below, for an ALH TDI:


It can be seen that the minimum BSFC of 197 g/kWh occurs at around 1750 RPM and just shy of the full-load curve. However, a steady cruise, say in 5th gear at that RPM, would be far, far less for a correspondingly higher (worse) BSFC.
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Old March 25th, 2008, 15:57   #13
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The most economical shift point is the tallest available gear for a given road speed at almost any RPM above 1000 RPM (note the curves of constant BSFC curve back upwards as engine RPMs are reduced closer toward 1000 RPM; this is most noticeable right around 1000 RPM around the 8-10 bar BMEP area.
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Old March 25th, 2008, 16:15   #14
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A more useful illustration is to superimpose curves of constant power onto a BSFC map (given in the blue curves below):



Each curve represents a constant horsepower developed by the engine. If you are driving on a given road with a constant grade, speed, and ambient conditions, regardless of which gear you are in, it requires roughly the same amount of power to overcome aerodynamic drag, rolling resistance and driveline losses. Therefore, regardless of what gear you're in, you're riding anywhere along the same blue curve; exactly where you sit on that blue curve being only dependent on the gear you're in and therefore the RPM at which the engine is turning over.

If I take the example of operating on a certain speed such that the power is 20 HP, The lowest BSFC occurs when running at the gear that corresponds to about 1250 RPM. If I run at either a higher- or lower RPM from this point, my BSFC will increase. This is about the only point in the entire engine map where it would be disadvantageous to operate at an even higher gear if one were available (lower RPM) because of the worsening BSFC. Almost everywhere else, the lowest BSFC is achieved at the lowest possible RPM at a given power.
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Old March 25th, 2008, 16:54   #15
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Are you sure?

Very interesting!
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