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Old 07-12-2011, 10:38 PM   #1
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Default Building T/C for Fuel Ecomomy

in response to another post wondering how much hp/fuel it takes to push a TC through he wind.

As most of you know, I tried to build a TC that made 12-14 mpg, I got 11.8 once, but that was with LSD and a tail wind over 400 miles.

Since then, I've a keen interest in the topic, as fuel hovers around 4 bucks, and my avg of 10mpg, a minor 10% gain would push me to 11 and that would be great.

my whole build is blogged on the Escapees forum at this link..

Blizzard's GMC 6500 Conversion Project -RumRunner - Escapees Discussion Forum

What I'd really like to discuss here is Ideas, fuel additives, gearing, aero mods you dream it, lets discuss it.

blizzND
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Old 07-13-2011, 12:27 PM   #2
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Default Hi, Blizzard.

Greetings,

I took a quick look at your project (need to study it more when there is time) and it is very cool. Also, very smart.

We want to do a MDT conversion with an eye on fuel economy. We need to watch the whole project cost as well, because we are very part time in our motorhome use. My wife and I think a type-C design with a bed over the cab and about 19 to 22 feet behind the cab would give us room for what we want and keep the whole thing light enough that diesel mileage would be "reasonable".

I wish I knew more about how to get special designs done without spending a fortune. I don't have my own shop so I need to hire things out. I used to do some welding in the far distant past but don't own any equipment now.

I do know that even a small amount of attention to aerodynamic design and reduction of cross-sectional area (as you have done) leads to significant savings in fuel costs. Significant in the 3 to 12 percent range; not the 40 to 50 percent range.

I look forward to participating in what seems like an important issue for motor/toter home users. I believe that the days of cheap fuel are gone.

Regards.
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Old 07-13-2011, 05:52 PM   #3
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On the subject of mileage and fuel additives-

A number of years ago, a buddy of mine got involved in one of those schemes like amway (where everybody is a "distributor" selling to someone else who sells to someone else... but I digress) selling these fuel additive "pills" for your fuel tank which were supposed to boost mileage and were supposed to work in gas or diesel. At the time I was running around the midwest putting on a bunch of miles with a dually with 8.1 gas motor and a 10,000# trailer so he recruited me to test these "pills" for him. I had 140 gal fuel capacity at the time, so I would run one "tankfull" with straight gas and check mileage, then switch to the pills and check mileage again. Did 3 tests runs on gas, and 3 test runs with the pills, alternating. Consistently got 10% better mileage on the pills (10 mpg increased to 11 mpg), which as Blizz notes, 1 mpg is a big difference when you are only in the 10 mpg range to start. Sounds good, but the kicker was that if you paid "retail" for the pills, the net cost per mile came out exactly the same either way. BUT that was back when gas was around a buck a gallon (do you remember way back then boys and girls?). So with fuel at 4 bucks a gallon now, the math may work out now. If the pills cost the same now, with fuel at 4x the price, it would work out to a net 7.5% gain in per mile cost, which is really what matters, not the actual mileage.

Here's the kicker though- the company got sued by the state of Texas for some sort of scam or fraud, basically the state accused them of selling "snake oil" and ran them out of business. But I'm here to tell you the stuff really worked. I've been racking my brain trying to remember the name of stuff, and it may come back to me yet, but for the moment I'm drawing a blank. The pills were greenish brown, about the size of a horse pill and smelled like mothballs. You used about one per 20 gallons on fillup. Anybody out there remember something like that, or know of something similar today?

Blizz- On the subject of fuel economy, I know you spent (presumably) a decent amount of money on a bunch of minor (taken separately) aerodynamic improvements on RumRunner. I remember sideskirting, a nearly full belly pan, and some sort of wind deflectors on the trailing edge. Were you able to do any sort of before and after on those mods, or were they just part of the build and you only have "after"? In particular those little wind deflectors intrigue me as those can be easily added to virtually any rig. Any thoughts on those you'd like to share in this thread?
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Old 07-13-2011, 08:29 PM   #4
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Default AeroTabs

The Aerotabs on the back of the RumRunner didn't make a noticeable change on the fuel mileage, as I had hoped, but what it really did is cut down considerably (BIG TIME!) on the buffeting when passing other trucks or meeting big trucks, vans etc. It also helps to keep the back ot the RV much cleaner which is always a good thing too!

AeroTab sells them as a kit for around 240 dollars which comes with like 80 tabs. they also work good up front on your fenders or around windows to cut down on wind noise.

I'd buy them again, even if they didn't help my mpg.
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Old 07-14-2011, 12:52 PM   #5
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Default power needed for air resistance

Hello again. I have been trying to look at power requirements and the related fuel efficiency as it relates to choices of motorhome construction. Here is some interesting stuff:

Assuming a level road and correctly inflated tires, the power needed to move a vehicle (I used 115 square feet as the frontal area and 16000 pounds as the weight) increases as the cube of velocity times the drag coefficient. For a big truck the coefficient of drag is a factor we can modify with design and typically ranges between 0.77 to 1.2. Somewhat non-real world, wishful thinking could get this down to closer to 0.6. For comparison, a normal car would have drag coefficients somewhere around 0.35 to 0.45 and sports cars 0.25 to 0.35.

Anyway, here are the calculations for HP needed to move a rig at various speeds (first number) for coefficient of drag 0.77 then 1.2

45MPH: Cd 0.77=>73HP Cd 1.2=>103HP
50MPH: Cd 0.77=>95HP Cd 1.2=>136HP
55MPH: Cd 0.77=>122HP Cd 1.2=>176HP
60MPH: Cd 0.77=>153HP Cd 1.2=>223HP
65MPH: Cd 0.77=>190HP Cd 1.2=>279HP
70MPH: Cd 0.77=>233HP Cd 1.2=>344HP

Note that this does not factor in hill climbing or head winds.

So, at highway speeds we can influence our power needs (and the related fuel use) by 30 to 100HP depending on how and where we drive. I did not run the numbers on different vehicle weights but changes in weight relates more to starting, stopping, and hill climbing. From what I looked at, it is not very difficult, by design modifications, to lower the drag coefficient (aerodynamics) from 1.1 to 0.85 which are real world values that apply to rigs belonging to members of this forum.

If you prefer to think in terms of energy, like on your electric bill, 100HP translates into 75kW.

Comments?
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Old 07-14-2011, 07:36 PM   #6
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Several diesel truck sites have discussions on propane injection, . . .I know of a couple of farmers that are using this on their tractors. They all note better performance & better (combined) fuel economy. Just off hand I don't remember which sites I was browsing however, you might Google "propane injection" & see what comes up.
In-so-far-as drag coefficient, . . .anything you can do to make the front facing part more pointy would create better airflow, . . .the problem is, . . .how do you make a proverbial concrete block going down the road, aerodynamic without making it look like a POS????
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Old 07-15-2011, 02:09 AM   #7
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Propane injection for diesels has been around since the 50's, and maybe older. Very common back in the day when every truck was so underpowered that they had to labor up each and every hill. Basically fed some vapor propane into the airstream to improve combustion and add power on demand. It sort of fell by the wayside in the 70's and 80's as power levels came up to adequate on over the road trucks. You do still occasionally see it on a semi, but rarely. However, all of a sudden with the big craze with diesel pickup power (programmers, turbos, etc.) propane injection has come back. More modern kits, but the same basic idea of injecting propane into the air coming into the engine to produce more power, either at the push of a button, or controlled by the engine programmer. Still more of a power adder than a mileage tool, but if you can run a smaller engine and just use the boost when you need it for a grade, overall mileage could certainly improve. There are kits commercially available now.

Here is an article from Banks on propane injection:
Banks Power | Propane & the Diesel

just google "propane injection" and any number of sellers pop up.
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Old 07-18-2011, 05:11 PM   #8
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Here is close to my idea of the perfect smaller T/C it is a E550,







I couldn't find a E550 when I built my truck, Ford only built them a few years 2001-2003???
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Old 07-18-2011, 05:19 PM   #9
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I think it would be great with....
** 22.5's lightweight rims
** properly geared
** a tuned Power Stroke 6.7 and an 6 speed auto w/ OD
I would love to see one E550 with an aerodynamic style box like Road-trek or Pleasure Way etc.




**Mod some big storage units simalar to the first img down nearly to the ground and smooth out the whole bottom of those boxes with sheeting.
** nice aero bumper from a conversion van.


The whole thing weighing in at 13-14K it would be a cool one of a kind jelly burnin road trip rig
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Old 07-18-2011, 09:22 PM   #10
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Don't I see these Ford 450s and 550s as ambulances? They look newer than 2001 to 2003, but if they belong to a fire department they are well taken care of and are probably that old. Very nice looking rig. Thanks for sharing.
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