Box van rear end mod

[aerohead]

The EPA granted a conditional SmartWay designation to Rocketail based upon 1/8th-scale wind tunnel testing submitted by Rocketail, and on-road fuel economy testing by Messilla Valley Transportation in New Mexico.
The EPA did not conduct nor solicit any 3rd-party, independent evaluations to corroborate results and conclusions.
Messilla claims an improvement from, 9.20971 mpg, to 9.5238 mpg, 3.58-gallons saved per 1,000 miles, a 3.36% improvement.
According to the NHTSA, a 10% drag reduction will net a 4.8% improvement in mpg for a Class-8 tractor trailer.
A 3.36% mpg improvement implies a 7% drag reduction via forward lift and wake modification according to an 'all-star team of industry professionals, engineers, and testing partners.'

[aerohead]

I've looked at the wind tunnel testing report by ARC, posted at Rocketail.com.
Some observations.
1) while actual Reynolds number for tractor trailers exceeds 5-million, ARC is limited to 1.1-million.
2) 'wing' performance is very Reynolds number dependent.
3) 'wing' performance is predicated upon free-flight conditions.
3-B) from a simple, turbulent boundary layer thickness calculation, the boundary layer at the rear of the 2016, Wabash, 53-foot trailer is 9.24-inches in depth, up to the 99% inviscid flow at infinity velocity.
4) ARC's baseline Cd for the tractor trailer drifts around a bit at zero-yaw.
5) At supercritical Reynolds number the drag coefficient would be constant up to transonic flow.
6) the testing was conducted with a top-mounted, nearly-full-span ' TOP A ( No Flex) wing mounted to the trailer van.
7) this 'TOP A ( No Flex) wing is never mentioned by Rocketail.
8) although the 'TOP A and side wings clearly add frontal area to that of the trailer van, there is no accounting for it in the drag summary tables.
9) ARC suggests a 2.20% improvement in mpg at 65-mph. A high of 2.7% at 75- mph.
10) Diesel fuel has a thermal expansion coefficient of 0.02093.
11) the average winter-to-summer temperature variability in the USA is 45-degrees F.
12) At a delta-45-degrees, a 100-gallon saddle tank could experience a natural volumetric variation to 'FULL', of 2.093-gallons at the pump, depending upon ambient temperature, above-ground fuel storage/ below-ground storage, fuel turnover rates, and how recently fuel was received from the refinery.
13) If 'volume' is the metric used to gauge mpg, mass equivalency would be required to ascertain actual energy consumed.
14) 22-separate on-road tests of Rocketail open up a virtual Pandora's Box of uncertainties with respect to all quanta.
15) If Messilla Valley truckers aren't disciplined enough to even operate their 'Trailer Tails' one would wonder as to their discipline during the Rocketail investigation.

[aerohead]

Working with ARC's test data, published at Rocketail.com's website, revealed that the added projected frontal area of the 3-wings never made it into the calculations. It appears that ARC simply used the gross frontal area of the Wabash, 53-foot dry van for the 'rig', with no consideration of mirrors, under-airdam clearance, actual wheel exposure, mudflaps, tire gaps, etc., for all the test data, with and without the wings.

[racprops]

People, I am very much into improving MPG, sadly at best is seems a LOT of work gives little improvement in air drag, and to really get results one needs a wind tunnel big enough to test your car/truck as models do not always react as does real cars in the real world does.

The number are at best 30% improvement, so on a car that gets 32MPG you can only get 9.6 more MPG or 41.6….and ONLY if you get everything 100% correct.

I think you really want to work on engine, fuel systems, and drive line…

Here is a real mind blower, the 2017 Chevy Express 2500 VAN (full size) with a 2.8 Turbo Diesel running a 8 peed auto is RATED at 30MPG at highway.

A stock BOX getting 30MPG….

Rich

[JSH]

Originally Posted by racprops

People, I am very much into improving MPG, sadly at best is seems a LOT of work gives little improvement in air drag, and to really get results one needs a wind tunnel big enough to test your car/truck as models do not always react as does real cars in the real world does.

The number are at best 30% improvement, so on a car that gets 32MPG you can only get 9.6 more MPG or 41.6….and ONLY if you get everything 100% correct.

I think you really want to work on engine, fuel systems, and drive line…

Here is a real mind blower, the 2017 Chevy Express 2500 VAN (full size) with a 2.8 Turbo Diesel running a 8 peed auto is RATED at 30MPG at highway.

A stock BOX getting 30MPG….

Rich

I'm curious where you found a rating on the 2.8L diesel. All the window stickers I've looked at don't give a EPA MPG rating.

[racprops]

Well for starters https://ecomodder.com/forum/showthre...van-36421.html

I cannot seem to find the site that also seemed like a factory sales site with that rating.

I know I saw it along with:

The 2.8L Duramax is an inline 4 cylinder, turbocharged diesel featuring a high pressure common rail injection system and dual overhead camshafts. It produces a peak 181 horsepower and 369 lb-ft of torque in both Colorado/Canyon and Express/Savana applications. The GMC Canyon/Chevrolet Colorado utilize a 6 speed automatic transmission (GM 6L50) while the Chevrolet Express/GMC Savana transmit power through an 8 speed automatic transmission (GM 8L90).

Direct injection, 2000 bar high pressure common rail (29,000 psi), Piezoelectric fuel injectors

Turbocharged, water cooled variable geometry turbocharger (VGT), intercooled

DOHC (dual overhead camshaft), 16 valve (4 valves per cylinder)
 
Ultra-low sulfur diesel fuel (ULSD), up to B20 biodiesel compatible

181 hp @ 3,400 rpm

369 lb-ft @ 2,000 rpm Max RPMs 5,000 rpm

2.8L Duramax Horsepower & Torque Curve


3.73 duramx van setup. These are the RPM's I've been seeing: 55 mph 1723 RPM 60 mph 1885 RPM 65 mph 2055 RPM With 3:42's 60 mph would equate to 1700 RPM 3:23's 60 mph to 1600 RPM

We are typically running about 7500 lbs GVW in Chicago traffic and also 100 mile runs on the highway. The little motor launches the van quite well, since it has way more bottom end than typical gas v-8's (325 ft lbs @ 1500 rpm, 370 @2000). In the city, it drives about the same as the 5.4 E 250 it replaced. On the highway, no trouble merging and accelerating while loaded. It’s no race car, but it’s fine.

I’ve never driven one of the big diesels, but to attempt to compare this 2.8 to engines over twice its size doesn’t seem to add up. Those big diesel vehicles have about 3 times the torque to weight ratio and about twice the HP to weight ratio of almost all production vehicles in the past 50 years. Yes this van will seem slow compared to those, but it has plenty of power for the typical user. At 180hp, this is more power than most small block V-8’s made until rather recently, and it has way more torque where its counts.

Fuel Economy is exceptional when heavily loaded. We run about 20 to 21 mpg city, and holding a steady 66 mph (at 65 it will downshift to 7th) we just got 33.3 mpg in humid 90 weather with A/C on over 60 miles, while running behind a semi, but not too close. This is probably about the best anyone could get running without any mods, as I had the speed control set at slightly over 65 to keep the tranny in 8th and then added a little extra as needed to maintain driving distance behind the semi. At this speed, the engine is engine running 1600 rpm (about 100 hp). On heavier grades, such as coming out of a river valley, it would downshift to 7th to maintain speed (2100 rpm/ 150 hp), but otherwise would stay in 8th. This is on relatively flat interstate of the Midwest. On open road without trailing a semi, we get a little over 30. At about 55 to 60 mph in heavy highway traffic entering a big city about 31.5 to 32 mpg...the other traffic helps pull and push you along due to drafting etc.

I've been hoping for a small diesel in a full size van for over 15 years. It provides the torque needed to get heavy loads moving, but sips fuel like a compact car.....a perfect combo for tradespeople. The big diesels never made any sense for regular tradesmen, since they don't almost never pull 15000 lb trailers, which I suspect is probably about 98% of the truck and van buyers. For that matter, must truck/van/ suv's are not carrying more than a couple of people, so it's about time a drive train was offered that fits this need quite well.

My only gripes have been the crappy factory shocks (this thing feels like our 2007 Jeep GC with 180,000 miles on it with the original struts) and the fact that the tranny holds low gears way too long. In city traffic this occurs about 35 MPH and it will hold the engine at about 2200 rpm until you accelerate to 37 mph, and on the highway....the trans will not upshift to high gear until 67 mph. On the highway it appears to be about a 10% hit in fuel economy at 65. In the city it’s a little harder to tell, but running 2200 rpm when you only need 1500 rpm or less can't be too good for economy. Again this is running about 7500 lb. GVW.

With the 20,000 miles per year we put on vans, the payback for the diesel upgrade is only about 18 months. If we keep the vehicle for 12 years (my daily driver is a 93), we should save about $30,000.00 on fuel costs over the 6.0 l v-8's that we have in other vans. The mileage is about double of the v-8s, maybe even more than double on the highway.

I do plan to complete some aero mods on the van that should bump the fuel economy up some more, since GM has done very little in this department and there's is a lot of room for improvement. I expect low to mid 30's mileage could be regularly attained with a loaded van, and maybe even better if running lighter.

With the couple exceptions noted above… shocks, trans holding gears too long and poor aero… this is a really good combination. While mileage is still way behind full size service vans sold in Europe (they get about 40 to 45 mpg with 1.4 L turbo diesels), for a heavy full framed vehicle this is exceptional. We will be buying more of these to replace the 6.0l gas equipped vans we have in service.

If put in full size pickups, this drivetrain is going to kill the Ford turbo full size trucks. They are only rated at 24 highway and rumored to get worse mileage than the v-8’s when worked hard.

[freebeard]

You have three opportunities, aero, drivetrain and hypermiling. All three may be optimized already.

You can find slowmover's posts, he's one of those members that got themselves banned, but his attitude was have the whole itinerary planned before you turn the key and have the wheels and steering tuned so there is no side-to-side wandering on the way.

[JSH]

Originally Posted by racprops

Well for starters https://ecomodder.com/forum/showthre...van-36421.html

I cannot seem to find the site that also seemed like a factory sales site with that rating.

I know I saw it along with....

Thanks. I was hoping you found a backdoor to the official test data. Fueleconomy.gov only has mpg ratings for the 6.0L gas engine.

[aerohead]

If the GVWR is around 6000-6500-lbs, the automaker is not required to list mpg on the window sticker.
We do know from the past, that GM does have internal data which was shared by Popular Science in the early 2000s.

[JSH]

Originally Posted by aerohead

If the GVWR is around 6000-6500-lbs, the automaker is not required to list mpg on the window sticker.
We do know from the past, that GM does have internal data which was shared by Popular Science in the early 2000s.

Correct, that why I was hoping someone had found a spreadsheet with all the CAFE certifications. I've found the EPA emissions file tucked away but nothing on CAFE yet.

GM stopped publishing mpg ratings for the 4.3L V6 engine in 2014, the 4.8L V8 engine in 2015, the 6.0L in 2017, and has never officially published numbers on the 2.8L diesel.