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Old 06-17-2020, 05:32 PM   #111 (permalink)
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indicator

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Originally Posted by nemo View Post
Just some thought, Ideas and questions.

Pot for throttle on fly by wire cars.

Would engine load be a reliable indicator of drag?

Wouldn't you want to testing drag and downforce at the same time if your goal is FE. One can certainly create downforce at the cost of drag.
If all variables could be fixed as constants, and drag was the only variable in the system, then throttle position could represent an aerodynamic change in load.
The rigid SAE protocol for coastdown testing might serve as a template for an accurate accounting of all these associated,potential variables. It's quite demanding, such so, that I'll not attempt it anymore.

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Old 06-17-2020, 05:43 PM   #112 (permalink)
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Quote:
[Hucho] shows a graphic of a VW Transporter with the cavity, along with a drag curve as a function of cavity length.
I can't find the picture of your friend's VW camper, who drove it to Mexico once a year. Could you repost that?

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Originally Posted by California98Civic
Quote:
Originally Posted by MeteorGray
... I use my throttle stop not to test aerodynamic ideas at the moment, but to use on long highway trips as a CRUISE CONTROL. ...
Dig it!
"He got a broomstick on the throttle
To keep his throttle foot a-dancin' 'round
With a cupful of cold black coffee
And a pocketful of West Coast turn-arounds

And they call me Speedball
Speedball Tucker
Terror of the highways
And all them other truckers"
Jim Croce - Speedball Tucker
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Old 06-17-2020, 05:53 PM   #113 (permalink)
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fit for purpose

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Originally Posted by freebeard View Post
You may no be alone.

I'm not sure (obviously) how to put it in words, but it is the mind-set that had Elon Musk produce the Cybertruck. Buckminster Fuller did as well as anyone:



All I was saying was that that Template is not always fit for purpose.
Think of it as a go-no-go for sectional density ( area rule / bump placement). If the body cross-section varies any greater reduction than that of the 'template' you are introducing high-pressure gradients/regimes in the flow direction, which are intolerable to the turbulent boundary layer.
And if your cross-section doesn't contract as much as the 'template,' you are introducing added surface area and surface friction drag.
The 'template' is at the bottom of the drag 'bucket' for streamline bodies of revolution, which are the lowest drag bodies known for 3-D flow. Any shorter and higher drag due to pressure drag. Any longer and greater drag due to aggravated surface friction losses. In this context, the 'template' is the 'optimum' for low drag, as stated by Hucho. His word not mine.
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Old 06-17-2020, 06:03 PM   #114 (permalink)
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theory

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Originally Posted by California98Civic View Post
That seems right to my differently-trained eye. Coming from a different field, I think of the Template as theory, which means it was derived from empirical research in a specific domain. Those findings inform a model, which everyone should know from the start is both useful and limited. It is fit for some purposes and not others. But aknowledging that a theory is limited--as all research is--is not the same as blanket dismissal.

The throttle stop testing method is smart. However, during any given testing effort wind, tire temp, ambient temp, road temp, reliability of the throttle stop material, drivetrain temp, accuracy of your GPS, and alternator charging will all still be variables. Several of those factors are not factors in a 50 mph to 0 mph coast down test. These are different methods, useful for different things and conditions.
Hucho would refer to its constituents parts as ground rules for the fluid mechanics of road vehicle aerodynamics. I'd read his book a dozen times before I felt confident enough to come forward with the 'template.'
I've practiced the Richard Feymann admonition to find fault in my experience of vehicle aerodynamics. The 'template' is an artifact of what remains after struggling with the now obvious simplicity of aerodynamics.
The tyranny of singularity appears to be the real issue surrounding the 'template.' Everyone seems to be convinced that aerodynamics is 'hard.' And streamlining couldn't possibly be as simple as following a simple, off-the-shelf technology.
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Old 06-17-2020, 06:13 PM   #115 (permalink)
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Bob Parson's aero Vanagon

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Originally Posted by freebeard View Post
I can't find the picture of your friend's VW camper, who drove it to Mexico once a year. Could you repost that?



"He got a broomstick on the throttle
To keep his throttle foot a-dancin' 'round
With a cupful of cold black coffee
And a pocketful of West Coast turn-arounds

And they call me Speedball
Speedball Tucker
Terror of the highways
And all them other truckers"
Jim Croce - Speedball Tucker
That was one of the 770 photos I lost to PhotoBucket. And I'm not going to pay extortion money to 4- bloodsucking venture capitalists to unlock them. They've already had the pleasure of sodomy.
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Old 06-17-2020, 06:57 PM   #116 (permalink)
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Bummer. I thought I'd squirreled away a copy somewhere, but I guess not. If you remember the length (width and height are given), I've got a .3DS model of a bus. I could try to reproduce it.
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Old 06-18-2020, 12:27 PM   #117 (permalink)
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More difficulties with my throttle stop yesterday when i tested it. I am a little obsessed to reliably and safely get the same throttle position reading whenever I press the throttle to the stop. A little variation is fine, like 17.5 instead of 17.6, but 16.1 seems unacceptable. Like usual, I have perfectionist demands for imperfect tools, materials, and skills... so it takes me a while to feel satisfied. I be able to report specifically on how I reduced throttle variations when/if I get to do throttle stop testing. I want data on throttle position live while I test. Without that reporting, I would wonder about reliability of reported results a little..
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Old 06-19-2020, 11:37 AM   #118 (permalink)
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As noted before, I also have observed variations in the TPS (throttle position sensor) screen of my Scan Gauge when at WOT using a throttle-stop device. These variations seem to depend on on road grade and wind speed and direction, ie, the engine load.

For instance, at a consistent WOT pedal position, I found that my Scangauge reported TPS values from 26% at 70mph to 28% at 75mph. This was during an uninterrupted 10 minute WOT run over a stretch of approximately level and approximately north-south oriented road with a wind that the weather bureau reported was 10mph from the northwest. I was going approximately south at the time, so it was essentially a tailwind. I attribute the range in speed to imperceptible road grade and wind direction-and-speed variations. The only surprising thing to me was the 2% TPS variation. This is obviously something other than a mechanical throttle position being reported by the gauge on its TPS screen.

As part of the same test, I turned the car around and drove in the opposite direction, ie approximately north, which then put the wind as a headwind. On that run, which was only a minute or two so did not see as many road or wind variations, I recorded a speed of 65mph at a TPS of 27%.

This kind of puzzled me, since I didn't see why the throttle position value would change with these variations in load when I know that the throttle is being held in exactly the same position, mechanically speaking.

So, I must conclude that the throttle position sensor screen is not reporting the actual throttle position per se, i.e. mechanically speaking. I think in my drive-by-wire Mazda3, the TPS value is actually a calculated derivative, probably resulting from a combination of such computer-read sensors as the throttle sensor on the throttle pedal, the mass air flow sensor that measures the volume of air going into the engine which is used to determine the load for the proper engine settings, and whatever else the engineers felt useful. Just a WAG.

As noted before, I quit looking at the TPS value on the gauge because it is not useful in trying to measure the precise mechanical throttle position, which does not change with load. I just take it on faith that when the throttle is being held at WOT by the throttle stop, the actual throttle position does not change with load. The speed does change with load, of course, but not the mechanical position of my throttle pedal. What the computer is doing with that mechanical pedal position obviously is up to the engineers and what their design dictates.

Incidentally, I also looked at the LOD (engine load?) screen during my tests, but it gave values that ranged widely that seemed even more useless to me.

The world of computers has passed me by without even a courteous honk.
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Old 06-19-2020, 11:38 AM   #119 (permalink)
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length

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Bummer. I thought I'd squirreled away a copy somewhere, but I guess not. If you remember the length (width and height are given), I've got a .3DS model of a bus. I could try to reproduce it.
I want to believe that it projected 48-inches to the rear. DOT allowed 60-inches, but Bob compromised. None of the Germans living on the Yucatan Peninsula ever believed his mileage claims. They didn't approve of the Subaru engine either. A knock to their Teutonic pride I suppose.
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Old 06-19-2020, 11:53 AM   #120 (permalink)
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throttle

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More difficulties with my throttle stop yesterday when i tested it. I am a little obsessed to reliably and safely get the same throttle position reading whenever I press the throttle to the stop. A little variation is fine, like 17.5 instead of 17.6, but 16.1 seems unacceptable. Like usual, I have perfectionist demands for imperfect tools, materials, and skills... so it takes me a while to feel satisfied. I be able to report specifically on how I reduced throttle variations when/if I get to do throttle stop testing. I want data on throttle position live while I test. Without that reporting, I would wonder about reliability of reported results a little..
If engine power ( load) is a function of air/ fuel mass charge, would not a voltage signal from the mass airflow sensor also be valuable for monitoring load? As well as the number of fuel injector pulses per unit time, at a fixed delivery pressure? That would get everything into units of mass.
Mass is a better metric than volume, as that can vary, but mass is mass regardless of thermal volumetric expansion effects.
Brake Specific Fuel Consumption is normalized to standard SAE atmospheric (density) and fuel temperature ( density).
I'm unsure whether or not the throttle position would reflect this nuance. Any change in road grade, air density, wind, curves, road surface, etc. would effect engine load. It would be hard to parse out the proportion attribution for each category.

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