Why are tractor-trailer rig so much more fuel efficient

[solarguy]

Originally Posted by jamesqf

I think you mean cross-sectional area? It's something people tend to forget when they talk aerodynamics. Cd is a function of shape, so a low Cd is better, but drag is Cd * cross-section. So even though the semi may have a high Cd, the cross-section relative to total weight is much smaller, so drag per pound is less.

No actually. Frontal cross sectional area has already been mentioned several times. Frontal cross sectional area is also proportionally smaller on a semi, but it is not the [I]only[I] factor. A semi with a trailer a mile long will have a different Cd than a "normal" semi even though both have the same frontal cross sectional area. The people who have to deal with Reynolds numbers and long pipes could tell you all about that subtle difference. Also, the high end competitive boat people pay a lot of attention to "wetted area" since their "aerodynamic" losses go up much faster with a lot of exposed surface area compared to moving objects in air.

Admittedly, it is not a major factor.

Finest regards,

troy

[Ptero]

I drove 120,000 lb, 105 foot long, 9 axle trucks with 36 wheels for many years.
You turn your engine power into momentum and maintain it. The torque to weight ratio is tiny - BUT IT'S ALWAYS ON. Think about it.

[never stock]

Another point no one has mentioned is that a diesel engine is not throttled like a gas engine. Engine power/rpm is controlled by fuel flow not by controlling air flow. A throttled engine has lots of pumping loss because it is always trying to pull air thru a variable throttle opening.
Tim

[noeryan]

Ptero, I like your input. Thank you. Aerodynamic losses on a big rig are great but the amount of momentum in it keeps it going. Drag is a force, F. Which can be calculated and reasoned to a particular number at a given velocity. Force is also the change in momentum, F = mass*(change in velocity).

Meaning a heavier object will maintain velocity longer than a lighter object at speed with the same amount of force resisting motion. HOWEVER, the amount of energy lost due to drag is the same amount of energy needed to be replaced by the engine to maintain speed over time. So even though they weigh more, the engine is still working hard to replace the lost energy.

I would think most of the efficiency in driving a big rig comes from the drivers who play the terrain and have TONS of experience being on the road everyday.

IMO, the reason diesels get great gas mileage for their size is because THAT IS THEIR BUSINESS!

[Ptero]

On another thread, I have discussed the constant throttle technique that I use for my Smart Car when hypermiling. Big trucks pretty much use constant throttle technique as well, using gearing to keep the engine within the torque band. It doesn't matter what fuel your engine is using. And forget horsepower. All hypermilers need to know their engine's torque curve, then find the rpm within that curve that provides the best fuel efficiency.

[slowmover]

Are you all forgetting the primary force behind big truck design? Money. The least amount of that to produce the highest returns. Subsidized fuel means it is a lesser consideration. The highest costs to run a truck are: 1) fuel; 2) driver pay; 3) tires.

Government can be suborned to continue fuel subsidies on several fronts. Employees can be exploited endlessly. Tires can be capped and re-capped (so what the failure rate).

The TARE weight of the trucks I drove was 27,000 to 27,500-lbs (tractor at 18,000-lbs and trailer at around 9,000-lbs; flatbeds, mainly). The gross weight limit on federal highways is 80,000-lbs. That means that the load was (for the companies I worked for) at or near 50,000-lbs.

Can your 2700-lb eco car tow an equivalent (and last 500,000 miles?)

The pay is in how much load each truck can move (for many, but not all, types of trucking). The pay-per-mile by the customer usually depended on the value of the load; gravel or brick pays little; fancy alloy steel pays a lot.

Most OTR trucks you see average 5.5 to 7.2 (or so) mpg. Average truck has a 9 or 10 speed transmission (as most drivers aren't skilled enough to properly use a 13, 15 or 18-speed trans). Most truck engines are from around 1600 to 2,000 LBS/FT torque, and 350 to 600 HP.

Truck drivers use what is known as "progressive shifting" to work their way thru the gears; shifting at increasing rpms as the speed increases (and boost comes on).

A skilled driver is worth 30% lower fuel cost (pure profit) than one unskilled (or uncaring).

The most aerodynamic tractor is a flat front cabover. An aero tractor (KW 2000) is okay, and a conventional (Pete 379) is worst. Driver comfort, and safety generally are not so compromised -- inversely -- with a wheelbase that has the driver set back from the front axle and BEHIND the engine.

Driving a big truck is a job. Hypermiling is a joke by comparison. Truck drivers are involved in a daily grind far more dangerous than what police and firefighters are subject to. And they have no union or pension and are pretty much guaranteed to live 10-15 years shorter than average.

The only relative equivalent is not personal transportation, but a bus.

Have a look at Jay Leno's 1961 FLXIBLE with a 6V-92 Detroit. The videos are poor, but the bus originally carried 30 or so passengers at 12-14 mpg highway. THAT'S efficiency. The original configuration would probably have been an 8 or 9 speed coupled to a low HP 2-stroke DETROIT.

Some of them old bus drivers were true gear jammers. I remember in the early 70's going from northern New Mexico to Dallas with a bowlegged old cowboy in the seat -- his little boy sitting behind him -- and passing slow traffic on the shoulder when he couldn't get around them in the hammer lane. That motor never saw south of 2,500 rpm as I recall. I moved back in the seats a ways just to listen to the way he did it, hanging in the aisle to watch his decision process. I'll never forget that beat-up sonnuva . . . .

[some_other_dave]

Originally Posted by slowmover

The most aerodynamic tractor is a flat front cabover. An aero tractor (KW 2000) is okay, and a conventional (Pete 379) is worst

That surprises me! It would seem at first thought that a less flat fronted rig would be more aero. I wonder if the difference has to do with the air having to turn to go over the hood and turn again to go over the roofline? Hmm... Could be a case where cars are small enough for such things to not be (much of) an issue, while the tractor is big enough for it to be.

Interesting!

-soD

[slowmover]

A COE (cab-over-engine) has less total surface area. A COE has packaged all components as efficiently as possible.

The best was an early/mid-90's Pete cabover that had a "front" with the slightest V-shape to it. Subtle.

1991 Peterbilt 372
1991 PETERBILT 372 Cabover Truck w/ Sleeper For Sale At TruckPaper.com

[TEiN]

Originally Posted by slowmover

A COE (cab-over-engine) has less total surface area. A COE has packaged all components as efficiently as possible.

The best was an early/mid-90's Pete cabover that had a "front" with the slightest V-shape to it. Subtle.

I agree on the cabover being way more aero than a conventional. I get the worst drafts from a cabover pulling triples, the best from a conventional logging truck. I measure the "quality" of the draft with the scan gauge, watching instant mpg over a long flat spot. I've gotten absolutely spectacular drafts way back behind a logging truck. Woo, it's almost like free fuel.

[roflwaffle]

Originally Posted by Ptero

All hypermilers need to know their engine's torque curve, then find the rpm within that curve that provides the best fuel efficiency.

That would work if most cars weren't so overpowered compared to trucks, but as it is, the vast majority of cars don't have gearing sufficient to insure the engine is always at a relatively high load and in a favorable portion of the BSFC map, which is why P&G works. A properly geared/sized engine won't need P&G to maximize efficiency, it's already there.