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Old 11-18-2019, 01:46 AM   #1 (permalink)
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What kind of MPGs would a brick wall get?

https://jalopnik.com/what-kind-of-fu...1839901794/amp

Despite some inaccuracies and bad comparisons I noticed in the article, it explains well that engine efficiency plays just an important part in fuel economy as coefficient of drag does. So some cars that are shaped like bricks (my Jeep) can still get good FE without making too many concessions to aerodynamics. Not something too many ecomodders consider as most cars here have small engines and have a fairly low cd already.

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Old 11-18-2019, 11:17 AM   #2 (permalink)
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@ what speed? Near zero, brick wall aero isn't all that bad. Supersonic, it sucks.
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Old 11-18-2019, 12:47 PM   #3 (permalink)
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Quote:
...engine efficiency plays just an important part in fuel economy as coefficient of drag does. So some cars that are shaped like bricks (my Jeep) can still get good FE without making too many concessions to aerodynamics.
Provided you handicap the more aerodynamic vehicle with a less efficient engine.

Consider a brick with radiused edges.
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Old 11-18-2019, 01:13 PM   #4 (permalink)
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Originally Posted by Piotrsko View Post
@ what speed? Near zero, brick wall aero isn't all that bad. Supersonic, it sucks.
The author of the article calculated that a brick wall with a Prius engine would still get 29 mpg at 50 mph.
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Old 11-18-2019, 02:18 PM   #5 (permalink)
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So I went back and read the article. Instead of a brick,he is using a flat plate as analogous. A brick, on it's long axis would have better fineness ratio, like a truck van.

His starting point was imagining his lifted SUV as having little brick walls all over it. Not a bad article, except for
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Imagine a tiny little bus taking chickens to work in the city. Oh no, the bus is running late and Meryl Cheep is going to be late to the egg factory!
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Old 11-18-2019, 04:00 PM   #6 (permalink)
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A wall is worse than a cube as well right? Basically holding up a sheet of plywood is worse. As pointed out it's all about speed. I can hold up that sheet and walk with it and not notice the aero drag but try an old it up in the back of a pickup on the highway and there will be an injury involved. I'm glad he mentioned the size as well because while I couldn't hold a sheet of plywood up at speed I could hold a playing card up which both have the same Cd.
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Old 11-18-2019, 04:35 PM   #7 (permalink)
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Quote:
Originally Posted by Taylor95 View Post
https://jalopnik.com/what-kind-of-fu...1839901794/amp

Despite some inaccuracies and bad comparisons I noticed in the article, it explains well that engine efficiency plays just an important part in fuel economy as coefficient of drag does. So some cars that are shaped like bricks (my Jeep) can still get good FE without making too many concessions to aerodynamics. Not something too many ecomodders consider as most cars here have small engines and have a fairly low cd already.
They used .22 as an engine efficiency in the caculator while claiming a .4 engine efficiency in the article. 40% must be "peak" efficiency not seen at 50 mph.

I was going to try and see how reducing engine efficiency while leaving the aero good to see what effect that had compared to the aero. You have to take efficiency down to .105 to get 29 mpg. I don't know of any properly functioning ICE that is quite that bad. Take the speed up to 70 and run the same numbers and the aero is now a 40% worse penality than thermal efficiency.
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Old 11-19-2019, 09:13 AM   #8 (permalink)
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The efficient USE of a vehicle is second consideration. MPG is far down the list as to what matters

1). How many family members? Must fit all persons aboard (with luggage).

2). Distances for necessities. Not work or school.

Etcetera

Where one lives in relation to necessary re-supply is prime as to the convenience of personal transportation. Overrides all others, as it’s a desire NOT a necessity to own a car.

A 5-mpg motorhome gets “better” fuel economy than a Prius with a roof rack tent.

Be sure you’ve defined, what is mpg?

It’s work performed against an energy input. One or two people being transported ISN'T work, per se. Its waste. Where’s the income offset or addition? Every single trip.

As to shape, the volume that encloses the most space in the most compact form is the winner.

How much space is mostly a function of how many children in your family.

An unmarried man, like an unmarried woman, is pretty well a drone. Not necessary; limited usefulness before disadvantages outweighs other.

More to the point, again, FE not being central, aero should impose no penalty first (as above), second, it should maximize safe operation (crosswinds, etc).

A drivetrain that can maintain 60-mph in rolling terrain is adequate for a loaded vehicle (as empty operation is proscribed). Thus, it’s more about the transmission & drive ratio than engine size or fuel.

The small Ford Transit van with passenger outfit, is close enough.

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Last edited by slowmover; 11-19-2019 at 09:29 AM..
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Old 11-19-2019, 01:56 PM   #9 (permalink)
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Quote:
As to shape, the volume that encloses the most space in the most compact form is the winner.
The counterpoint is the external shape the volume must squeeze through. Generally 8.5x14x∞.
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Old 11-19-2019, 03:38 PM   #10 (permalink)
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Quote:
Originally Posted by Taylor95 View Post
So some cars that are shaped like bricks (my Jeep) can still get good FE without making too many concessions to aerodynamics.
Within reason. Slapping an efficient engine in an SUV doesn't also add a more efficient drivetrain, lower weight or lower rolling resistance.

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Transmission type Efficiency
Manual neutral engine off.100% @MPG <----- Fun Fact.
Manual 1:1 gear ratio .......98%
CVT belt ............................88%
Automatic .........................86%

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