MPG vs HP Compromise

[98G]

That is a misleading or confusing way to phrase or think about the problem. The aerodynamic losses make up the smallest contribution, and so even if you could make them totally go away, it would only have a very small effect. However, playing with aerodynamics is way easier to do, compared to changing the efficiency of an engine, especially for laymen.

You can look up various studies on the topic, and you will find that the vast majority of wasted energy happens at the engine. I don't remember the exact numbers, but I'll throw out something close (within 5-10%?). The engine wastes 70% of the fuel's power (in other words, for every gallon of fuel you burn, you only get 30% of that energy out as the crankshaft rotating). A further 20% total is wasted by the drivetrain (e.g. transmission friction, axles, etc.). And the last 10% is wasted by the aerodynamics and rolling resistance of the truck.

What this is telling you, is that if you get 5MPG, even if you could make aerodynamics totally go away, you could only gain 10%. In reality, adding big airfoils between the cab and cargo box, or rear "cones", bumps the total MPG by like 1-2%. Fleets do it because that 2% over a million miles driven per year adds up to big savings. One FMTV overlander driving 2000 miles per year, at 5MPG, makes a 4 gallon ($20) difference. (PER YEAR!)

That directly conflicts with my understanding of it.

My experience with aero is bicycle related.

At 18mph (IIRC) aerodynamics take over from rolling resistance as the primary impedance. (Bicycles...but should be at least somewhat applicable)

And from firsthand experience on a motorcycle, raising up and catching the wind at 150mph slows you down as briskly as firmly grabbing the brakes...

And one more example - towing a heavy (13k) but dense trailer behind my pickup truck gets substantially better fuel efficiency than towing a relatively light (3200lbs) travel trailer with enormous frontal surface area. I attribute this to aerodynamics.

A quick search on the topic yielded the attached screenshot.

All that being said, I am also of the opinion that tweaking an LMTV based overlanding vehicle for enhanced fuel efficiency is an exercise in futility and not worth the trouble.

Edit to add the complete link of my screenshot -

International Trucks - Fuel Economy Aerodynamics

Fuel Economy Aerodynamics

www.internationaltrucks.com

.

 

[Awesomeness]

That directly conflicts with my understanding of it.

My experience with aero is bicycle related.

At 18mph (IIRC) aerodynamics take over from rolling resistance as the primary impedance. (Bicycles...but should be at least somewhat applicable)

And from firsthand experience on a motorcycle, raising up and catching the wind at 150mph slows you down as briskly as firmly grabbing the brakes...

And one more example - towing a heavy (13k) but dense trailer behind my pickup truck gets substantially better fuel efficiency than towing a relatively light (3200lbs) travel trailer with enormous frontal surface area. I attribute this to aerodynamics.

A quick search on the topic yielded the attached screenshot.

All that being said, I am also of the opinion that tweaking an LMTV based overlanding vehicle for enhanced fuel efficiency is an exercise in futility and not worth the trouble.

It's not in contradiction. It just goes to show how wasteful the engine is (and how counterintuitive this subject can be). All that force you're feeling on your motorcycle, in terms of energy, is tiny compared to how much the engine is wasting. If you need further convincing, using your own braking analogy, imagine how much heat is generated in a brake caliper/drum by stopping, and then compare it to how the engine is able to heat up a few thousand pounds of combined engine/transmission/oil/coolant by just driving for several minutes.

That blog post you found is just completely wrong, or there is more to what they are saying that isn't shown in the picture.

Here, I found one of the actual military scientific studies on the fuel economy of the FMTV. It includes this actual breakdown of the power loss (below), as well as a bunch of stuff they considered/tried (some of it pretty uncommon, like hydrogen injection, but also air dams, electric radiator fans, etc.).


The way you read this is that the engine took in 38.18kWh worth of fuel, but only generated 12.05kWh of crankshaft output, resulting in a 68.4% loss (e.g. "31.6%" efficient). You can see that "Vehicle" loses 6.475 to "aero and rolling resistance energy", which is 16.9% of the original 38.18.

It also includes statements like "A study by the Transportation Research Board found that cutting resistance by 10% yielded a 1-2% improvement
in fuel economy [8]. In established vehicle fleets, managing architectural gaps and joints can translate directly to fuel consumption reductions. There are many OEM’s offering products to reduce aerodynamic drag that can be integrated in a short time frame. Most of these are easy to install, require
minimal maintenance and are used everyday. However they are predominantly designed with Class 8 vehicles traveling at highway speeds in mind. Speed is an important part of the drag equation. At 112.6 km/h (70mph), a vehicle has four times the force working against it than at 56.3 km/h (35mph)
[9]. Given that the top speed of the M1078 is 94 km/h (58mph) and its average operating speed is much lower, minimal fuel economy gains are estimated."

 

[Mullaney]

That is a misleading or confusing way to phrase or think about the problem. The aerodynamic losses make up the smallest contribution, and so even if you could make them totally go away, it would only have a very small effect. However, playing with aerodynamics is way easier to do, compared to changing the efficiency of an engine, especially for laymen.

You can look up various studies on the topic, and you will find that the vast majority of wasted energy happens at the engine. I don't remember the exact numbers, but I'll throw out something close (within 5-10%?). The engine wastes 70% of the fuel's power (in other words, for every gallon of fuel you burn, you only get 30% of that energy out as the crankshaft rotating). A further 20% total is wasted by the drivetrain (e.g. transmission friction, axles, etc.). And the last 10% is wasted by the aerodynamics and rolling resistance of the truck.

What this is telling you, is that if you get 5MPG, even if you could make aerodynamics totally go away, you could only gain 10%. In reality, adding big airfoils between the cab and cargo box, or rear "cones", bumps the total MPG by like 1-2%. Fleets do it because that 2% over a million miles driven per year adds up to big savings. One FMTV overlander driving 2000 miles per year, at 5MPG, makes a 4 gallon ($20) difference. (PER YEAR!)

.
So - using just plain old common sense and logic @Awesomeness - I need to call BS on that thought. The trucking industry doesn't generally spend money on air dams and wind directors. If they did, they would have "field tests" done and then a group of ten (maybe) trucks outfitted with these supposedly fuel saving devices. Then every ounce of fuel would be monitored to prove the concept before they spend hundreds of thousands of dollars to outfit every truck in their fleet.

UPS has great proof of that with their OTR fleet. They have air dams under the trailers and roof fairings on top of those trucks. Lots of college boys with sheepskin on their walls come up with these bright ideas and the company pays them big bucks when they come up with ideas that save money.

Unrelated to UPS, here is an article about managing air:

Part 2 – Sustainable Vehicle Practices

Tractor Aerodynamics | Trailer Aerodynamics | Tires | Engines | Alternative FuelsAlternative Powertrains | Cargo Management | Higher Productivity Vehicles Truck manufacturers first began to develop more fuel efficient vehicles in the 1970s as a result of the energy crisis. Since that...

truckingresearch.org

Had to add a picture... AND yeah, I know the square box M107x and M8x trucks are like a box cutting through the air. Just thought the picture might stimulate somebody's brain to come up with something that might help air flow.



EDIT: After your second post - The key seems to be established fleets and highway speeds.
Kinda gives insight in to the UPS idea too.

 

[Awesomeness]

.
So - using just plain old common sense and logic @Awesomeness - I need to call BS on that thought. The trucking industry doesn't generally spend money on air dams and wind directors. If they did, they would have "field tests" done and then a group of ten (maybe) trucks outfitted with these supposedly fuel saving devices. Then every ounce of fuel would be monitored to prove the concept before they spend hundreds of thousands of dollars to outfit every truck in their fleet.

UPS has great proof of that with their OTR fleet. They have air dams under the trailers and roof fairings on top of those trucks. Lots of college boys with sheepskin on their walls come up with these bright ideas and the company pays them big bucks when they come up with ideas that save money.

Unrelated to UPS, here is an article about managing air:

Part 2 – Sustainable Vehicle Practices

Tractor Aerodynamics | Trailer Aerodynamics | Tires | Engines | Alternative FuelsAlternative Powertrains | Cargo Management | Higher Productivity Vehicles Truck manufacturers first began to develop more fuel efficient vehicles in the 1970s as a result of the energy crisis. Since that...

truckingresearch.org

Had to add a picture... AND yeah, I know the square box M107x and M8x trucks are like a box cutting through the air. Just thought the picture might stimulate somebody's brain to come up with something that might help air flow.

View attachment 867779

EDIT: After your second post - The key seems to be established fleets and highway speeds.
Kinda gives insight in to the UPS idea too.

All these points still confuse the ideas of "which systems on the truck are easily modifiable to get fuel mileage gains" and "which systems on the truck could theoretically get large/meaningful fuel mileage gains". You can easily add aerodynamics and get some trivial gains, or you can [theoretically] modify the engine at great difficulty and get significant gains. The military doesn't mess with the aerodynamics because the small MPG gain isn't worth the practical (tactical?) downsides. Companies everywhere have been trying to improve engine efficiency for over a century, and it's really hard to do.

 

[simp5782]

All these points still confuse the ideas of "which systems on the truck are easily modifiable to get fuel mileage gains" and "which systems on the truck could theoretically get large/meaningful fuel mileage gains". You can easily add aerodynamics and get some trivial gains, or you can [theoretically] modify the engine at great difficult and get significant gains. The military doesn't mess with the aerodynamics because the small MPG gain isn't worth the practical (tactical?) downsides. Companies everywhere have been trying to improve engine efficiency for over a century, and it's really hard to do.

You have obviously never seen any of the trucks of the 37th, 66th, or 76th transportation companies.

 

[Awesomeness]

You have obviously never seen any of the trucks of the 37th, 66th, or 76th transportation companies.

They have heavily modified FMTVs?

 

[98G]

All these points still confuse the ideas of "which systems on the truck are easily modifiable to get fuel mileage gains" and "which systems on the truck could theoretically get large/meaningful fuel mileage gains". You can easily add aerodynamics and get some trivial gains, or you can [theoretically] modify the engine at great difficulty and get significant gains. The military doesn't mess with the aerodynamics because the small MPG gain isn't worth the practical (tactical?) downsides. Companies everywhere have been trying to improve engine efficiency for over a century, and it's really hard to do.

Consider the following-

1) A diesel engine at idle uses very little fuel. If the engine resistance were the primary energy consumer, then you wouldn't see the fuel consumption drop off so drastically.

2) Towing a high surface area lightweight trailer uses considerably more fuel than towing a much heavier trailer that has substantially less surface area.

3) Air drag is speed dependant. 60mph has roughly twice the losses as 40mph.

4) "drafting" behind another vehicle can substantially reduce fuel usage. Like by 30% (IIRC)

But we're off into the weeds... an LMTV is a brick and there's just not a whole lot to be done to reduce drag, even though that's where a lot of the fuel is going.

 

[simp5782]

They have heavily modified FMTVs?

The army dabbles in aerodynamics

 

[Awesomeness]

Consider the following-

1) A diesel engine at idle uses very little fuel. If the engine resistance were the primary energy consumer, then you wouldn't see the fuel consumption drop off so drastically.

2) Towing a high surface area lightweight trailer uses considerably more fuel than towing a much heavier trailer that has substantially less surface area.

3) Air drag is speed dependant. 60mph has roughly twice the losses as 40mph.

4) "drafting" behind another vehicle can substantially reduce fuel usage. Like by 30% (IIRC)

But we're off into the weeds... an LMTV is a brick and there's just not a whole lot to be done to reduce drag, even though that's where a lot of the fuel is going.

1. It's not primarily engine "resistance" (e.g. friction). When the fuel is burned, most of the energy is converted to heat in the resulting gases, and then released through the exhaust or absorbed by metal parts on it's way out. Take a look at the study above and it explains a bunch of their testing methods and where they think improvements could be made to MPG (some of which they also test).

2. This is mixing two ideas, surface area (drag, wind resistance) and mass. In simplified terms, the surface area only matters when you're trying to keep the trailer moving at a high constant speed, and the mass only matters when you're trying to accelerate it from a stop. Hypothetically, the same trailer takes has the same fuel mileage to tow on the highway whether it's empty or a million pounds full, because the wind resistance didn't change. (Yes, in real life there are some changes due to rolling resistance of the tires, bearing friction, and the way a driver may hit the brakes going down a hill then hit the gas on the way up to keep a constant speed.) In stop and go driving, where you don't get going that fast but are constantly accelerating and decelerating the trailer, a heavy trailer is going to suck a ton more fuel.

3. Actually it's worse than that, because it's exponential. To go from 35 to 70MPH is a 4x change. That's why the high speed axle gears aren't necessarily a guaranteed (or even "likely") fuel mileage improvement, like everyone seems to think they are. The top speed of an FMTV is 58MPH with the stock 3.90:1 gears, and 74MPH with the 3.07:1 "high speed" gears. If you were taking it easy, driving 50MPH with 3.90:1 gears, and than 65MPH with 3.07:1 gears, that's actually a 1.7x increase in wind resistance, because of the speed change. The problem gets more complicated when we then bring in the engine HP/torque curves, and transmission gears. If these speeds are at different RPMs then the engine is behaving at different levels of efficiency. If the transmission is in different gears at these two speeds (e.g. 6th @ 50MPH w/ 3.90:1, and 7th @ 65MPH w/ 3.07:1), then it can put the engine at extremely different RPMs, and thus extremely different fuel economies.

4. Perfect drafting would be the elimination of all aerodynamic drag, because the lead vehicle was taking all the abuse, plus a tiny amount of bonus created by a slight negative pressure zone created behind the lead vehicle. You have to keep the rear vehicle inside the bubble of turbulent air created by the lead vehicle (and how far this laminar flow separation extends is dictated by the aerodynamic shape). Since wind resistance is exponentially higher at higher speeds, you can get bigger results the faster you go. Think back about that 16.9% number from the actual FMTV study, which included both aerodynamic and rolling resistance - that's the best you could expect do get, under perfect conditions, so realistically you wouldn't expect an FMTV to get more than maybe a 5-10% change if it was drafting really well in normal driving conditions (e.g. maybe 15' separation, on a straightaway, etc.). Here you can see a picture of how close you'd have to stay to get full effect.

 

[jtcawrse]

So what I'm gathering from this feedback is to keep the higher horsepower and try to drive at 1440 RPM for the best mileage.

The MPG is in my right foot, not necessarily in HP, aerodynamics, etc...

 

[Awesomeness]

So what I'm gathering from this feedback is to keep the higher horsepower and try to drive at 1440 RPM for the best mileage.

The MPG is in my right foot, not necessarily in HP, aerodynamics, etc...

That's an oversimplification, but yes. The other key point is that while you can make the MPG way worse pretty easily, you're not going to improve it more than a couple percent with any particular mod (e.g. air dams, high PSI in the tires, fuel additives, etc.). So if the truck starts out at 5MPG average, you can get to 5.2MPG, maybe 6MPG with a LOT of work/money, but not 10MPG, according to the science.

 

[Mullaney]

.
NASA's Dryden Research Center has dabbled in truck aerodynamics.
See Attached PDF

 

[simp5782]

That's an oversimplification, but yes. The other key point is that while you can make the MPG way worse pretty easily, you're not going to improve it more than a couple percent with any particular mod (e.g. air dams, high PSI in the tires, fuel additives, etc.). So if the truck starts out at 5MPG average, you can get to 5.2MPG, maybe 6MPG with a LOT of work/money, but not 10MPG, according to the science.

It is possible with a nice tailwind EB on I80 from Laramie across Nebraska.

 

[98G]

You'd be shocked what you can do for mpg if you go all out.

My 1ton pickup truck normally gets 17mpg on a good day, empty, cruising at hwy speeds. (6.7 cummins and 6spd manual transmission)

There's a few places in the US where there's a substantial distance between fuel opportunities. Due to poor planning on my part, I found myself low on fuel in the middle of nowhere TX. I ran completely out and dumped 1.5 gallons into the tank from a partial jug.

I made it 42 miles from the point I dumped the jug into the tank to the point that I reached a fuel station. Gentle acceleration, going into neutral and shutting the engine off while going downhill, not going faster than 35mph unless gravity-induced downhill, keeping my foot off the brakes, etc.

I've seen a study where they got >30mpg out of a 1ton with a 5.9 cummins. This was a small sample size of about 10 miles and accurately measured from a small calibrated tank. They went all-out on aero, and drove to maximize efficiency.

I've got a buddy that regularly gets >9mpg out of a 5ton with an NH250. I usually see more like 5.5mpg out of these trucks. But he takes great care to optimize his driving for maximum efficiency. @BigRig379 .

 

[coachgeo]

..... The aerodynamic losses make up the smallest contribution, a....

that's interesting.. long ago had read complete opposite. But you might have hit the nail on the head in that past information Id been review may well have been alluding more to what a single person can actually do to their vehicle..... example it is near impossible to reduce the mechanical losses.. but possible to reduce drag with effort.

 

[BigRig379]

You'd be shocked what you can do for mpg if you go all out.

My 1ton pickup truck normally gets 17mpg on a good day, empty, cruising at hwy speeds. (6.7 cummins and 6spd manual transmission)

There's a few places in the US where there's a substantial distance between fuel opportunities. Due to poor planning on my part, I found myself low on fuel in the middle of nowhere TX. I ran completely out and dumped 1.5 gallons into the tank from a partial jug.

I made it 42 miles from the point I dumped the jug into the tank to the point that I reached a fuel station. Gentle acceleration, going into neutral and shutting the engine off while going downhill, not going faster than 35mph unless gravity-induced downhill, keeping my foot off the brakes, etc.

I've seen a study where they got >30mpg out of a 1ton with a 5.9 cummins. This was a small sample size of about 10 miles and accurately measured from a small calibrated tank. They went all-out on aero, and drove to maximize efficiency.

I've got a buddy that regularly gets >9mpg out of a 5ton with an NH250. I usually see more like 5.5mpg out of these trucks. But he takes great care to optimize his driving for maximum efficiency. @BigRig379 .

For the most part I drive 45, going up hills I just let the transmission do the work. The first time I drove mine on a trip I was driving 60-65 and got about 4mpg. After that I stopped driving on the floor and went off rpm’s and tried to stay around 1600-1800. If I’m on the interstate I drive maybe 50. The only mod I’ve done is put 395’s. I may do 16.00 of I bump the power up and put a different transmission in it. But I have too many projects to finish before I even get there.


Sent from my iPhone using Tapatalk

 

[BERZERKER888]

When I get the same question, and they have an attitude or tone in their voice like I am responsible for climate change, I say "Pound for pound I get better fuel mileage than your (insert vehicle name)".
That usually ends the conversation right there because they have to stop and think a little.
I use my truck for work so I usually have a load on it.

I get the same grief.. my response "This is Lea.. he/she identifies as a Prius"... shuts them Greenies up everytime

 

[BERZERKER888]

um.. which oil is best to reduce aerodynamic drag ?...

 

[Ronmar]

um.. which oil is best to reduce aerodynamic drag ?...

Whale...

 

[Xengineguy]

Just my 2 cents,
I believe my m1078 is far from being geared correctly. Last night I tested gps speed and checked engine rpm.
2645 rpm at 61 mph, level grade. Cat recommends for the best balance of performance and fuel mileage
2000 @ 60 with a max cruuse speed of 65. Also a recommended minimum cruise speed of 55 @ 1800.
Makes me wonder if the military designed the vehicle with a top speed as a primary consideration.
The torque and horsepower drops very fast as the rpms are pushed above 2400. My 1078 is 274hp
with 3:90 gears….
I owned a Hawkins class A with a 3116 and it had plenty of power, it blocked more wind and was much heavier
than a 1078. At cruise 65mph it was only turning 1650 rpm. I pulled a 27’ enclosed trailer most of the time.
The fuel mileage was great, 9 to just over 10 mpg. I drove with the flow of traffic mostly 70 mph.
One of the local construction companies has a tri axle dump with a 3126 and regularly weigh in at
70000 lbs when moving there excavator from job to job. He did say it a turd with that kind of load
but it will pull it to road speed.