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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 April 3rd, 2008, 11:51   #31
EddyKilowatt
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Quote:
Originally Posted by GoFaster
It's better to perhaps spend a wee bit more on fuel, but get acceptable drivability and vibration properties, and not smash up turbochargers, clutches, and engine mounts.
QFT!

I keep two maps in mind as I drive... one is the BSFC map posted above by TDIMeister... the other is this cool one that I orginally saw in a post by Drivebiwire, which gives a rough idea of Wear Rate vs Load % and RPM:



My understanding was this map is based on actually running an engine under the stated conditions and measuring wear products in the engine oil...

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Old April 5th, 2008, 12:03   #32
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Sorry for being dense, but is red bad and green good, or vice versa?
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Old April 5th, 2008, 20:47   #33
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The legend shows blue as the worst wear - Not sure if I am reading it correct but I guess its showing load vs rpm and the wear at that range. Light load and low rpm is good and heavy load and low rpm is bad. 3D like that its a bit hard to understand. My load is always low and I run 1700 to 2500 rpm. Passengers and luggage is usually under 370lb, my UOA's reflect this.
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Old April 6th, 2008, 03:21   #34
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The reason I was confused is that the graph shows heavy load + low RPM to be good (at least below 1,300 rpm) which doesn't make sense to me.
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Old April 6th, 2008, 12:07   #35
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That confirms what´s been discussed many times, these cars are most fuel efficient at 55mph.
You mean if speeds under 55 MPH didn't exist? You will get better FE at 54, even better at 53, better yet at 52, etc.

Sorry folks, there isn't a magic RPM/load where a higher speed (in a given gear) will yield better fuel economy. And by fuel econom, I mean miles per gallon. I don't mean BSFC or thermal efficiency.

You might produce more HP per gallon at a higher RPM/load, but you will be burning more fuel per mile traveled than you would at a lower speed.

Think about it this way: A tractor trailer towing a 40,000# trailer getting 5 MPG is a more efficient use of fuel than a Ford Explorer getting 12 MPG, but the Explorer is still getting better miles per gallon than the tractor trailer. Unless the point is to haul a 40,000# trailer, the "thermal efficiency" argument doesn't make the tractor trailer use less fuel to get from one place to another.
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Old April 6th, 2008, 12:08   #36
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tnp, what you want is a ScanGauge. This shows your real-time fuel efficiency, as well as other useful information.
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Old April 6th, 2008, 13:00   #37
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Quote:
Originally Posted by EddyKilowatt
... the other is this cool one that I orginally saw in a post by Drivebiwire, which gives a rough idea of Wear Rate vs Load % and RPM:


I really wished Drivbiwire would include more information or attribution as to the source of this chart, or at least some context like a caption, as I try to do when I post charts. For all I know, this could have been created in MS Paint. Unlabeled axes (which one can deduce what they are from the magnitudes and the semi-ambiguous chart title) is not helpful. It would also seem odd to me that wear would be less at, for example, 100% load @ 4450 RPM than 83% load at the same RPM.
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Old April 6th, 2008, 13:08   #38
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Now, if they built a car that was electric, driven off of a battery that was charged by a TDI engine/generator combo, then it would seem to be most efficient to have the engine start up when the voltage dropped below a certain point, run at a high load at peak thermal efficiency until the battery was charged back up, then shut the engine down. This would yield the most energy for the fuel burned, which would (since the battery is storing that energy for later use) yield high miles per gallon as well.
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Old April 6th, 2008, 13:46   #39
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McBrew, that's the operating principle of "range extender" hybrids. However, this is not a panacea. Even if the engine could be only operated almost always at- or near the best-point BSFC, there are losses as the work from the engine (assumed to be at an efficiency of ~43% for a TDI at the best point BSFC) goes through the generator (for simplicity's sake 90%), power electronics (95%), battery charging (80%), battery discharging (80%), power electronics (95%), electric motor (90%). The "round-trip" efficiency is around 47% based on the individual efficiencies I estimated, and this is in the the ballpark, which means that the 43% efficient engine is actually only about 20% efficient once the energy from the engine comes back to the drivetrain via the electrics.

It has been my experience doing hybrid drive cycle simulations in my day job, that the biggest gains from hybridization actually come from shutting the engine off during traffic light stops and charging the batteries only regeneratively when braking. You don't need a high-degree of hybridization to achieve this. BMW's Efficient Dynamics incorporates start-stop and a mild-form of regerative braking without complicated hybrid hardware, and it's extremely effective. Another large benefit of hybridization is to be able to run on electric-only mode for low-speed, low-load conditions, when the combustion engine is very inefficient. Using the engine to charge the battery should only be used very sparingly and in very specific operating conditions, where the total "roundtrip" efficiency with this hybrid "load shifting" is greater than or equal to that of the baseline non-load-shifted condition.
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Old April 7th, 2008, 07:12   #40
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Meister... good points about the losses. I was intrigued by a system I read about that used regenerative braking... but by using hydraulic pumps/motors at the wheels to compress gas (over oil) in high pressure cylinders while braking, and then use that system in reverse to recover the energy while accelerating. This would be, of course, in conjunction with an ICE or electric drive. The efficiency, according to the article, was much greater than electric regenerative braking. I'm sure you've heard of this, but I'm mentioning for the benefit of others reading this thread.
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Old April 13th, 2008, 04:33   #41
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Quote:
Originally Posted by TDIMeister



Each curve represents a constant horsepower developed by the engine.
Why are your blue lines curves? If power is inversely related to RPMs why aren't they straight lines?
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Old April 13th, 2008, 12:47   #42
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I see it more like this:



Maybe I'm overlooking something but I've cross checked this against a torque and power curve map and it appears to be correct (for a 105PS TDI PD).

Was the BSFC map for a 105PS TDI PD (I don't know what motor is in that Beetle).

Moving on from this does anyone have any idea how much horsepower it takes to keep the car rolling at various speeds?

Last edited by bokeh; April 13th, 2008 at 13:13.
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Old April 13th, 2008, 13:39   #43
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You have the red curve of best efficiency correct. However, plot horsepower as a function of BMEP and RPM according to the following equation (you can do this in Excel by adapting the equation below):

POWER = RPM * BMEP {bar} * DISPLACEMENT {cc} * 8.333333e-7

This is give you power in kW. To convert to horsepower, multiply by 1.341022. You will certainly get curves shaped like hyperbolas, not straight lines.
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Old April 13th, 2008, 14:23   #44
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Quote:
Originally Posted by TDIMeister
However, plot horsepower as a function of BMEP and RPM according to the following equation

POWER = RPM * BMEP {bar} * DISPLACEMENT {cc} * 8.333333e-7
Ok. We can forget the stuff in red because that is just constant. Now we are just left with:

POWER = RPM * BMEP * CONSTANT
POWER = RPM * TORQUE * CONSTANT
BMEP = TORQUE * CONSTANT

The scale down the left is BMEP which according to the above is synomous with torque. Seeing as the
x-axis, RPM and y-axis, torque are both linear scales the only possible way to plot from point "A" to point "B" while maintaining a constant power level is a completely straight line (according to the above formulas). How could it be any different?

Also:

BHP = (TORQUE ft/lb * RPM) / 5252

If you check any point on my straight lines using that formular you will find they are correct.

Last edited by bokeh; April 13th, 2008 at 14:41.
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Old April 13th, 2008, 14:41   #45
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You're missing the point.

Pick a given horsepower number at a given middle-of-the-range torque and speed. To keep that same power level with (say) double the torque, the speed is half. To keep that same power level with (say) half the torque, the speed has to double.

That is not a straight line, it is a hyperbola.

A straight line graph, if extrapolated, will cross the zero axis at some point. This implies, if your presumption were true, that it would be possible to generate that power level with zero torque by using some high RPM, or with zero RPM by using some high torque. Obviously this cannot be true.

A constant-horsepower line with axes of torque in one direction and rpm in the other direction must be a hyperbola that can never cross the zero axes no matter how arbitrarily high the other variable is chosen.
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