Why are tractor-trailer rig so much more fuel efficient

[meanjoe75fan]

I've heard it mentioned indirectly, but the reason tractor-trailers are less draggy per weight is known in aerodynamics as the "square-cube rule."

Imagine you take a Metro and double all its dimensions. The vehicle's volume would have gone up 8X (2^3 for 3-D), but the CdA only goes up 4X (2^2 for 2-D). Thus, it would incur 4X the drag of the stock Metro while carrying 8X the load--half the drag per unit of cargo.

This comes into play a lot in aviation, because weight is 3-D, while lift (proportional to wing area) is 2-D. Because of this, it would be possible to build an airplane that's too damn big to leave the ground! (Allowing for current propulsion systems.)

It would be interesting to see if similar classes of private motor vehicles, (i.e. all sedans) with similar HP/weight ratios, become more efficient per pound of cargo capacity as they get bigger...

[MadisonMPG]

*did not read whole thread

Diesel is already more fuel efficient. Trucks have a higher load/Cda ratio. Stay at 70mph for nearly all of their trip. Made for hauling, not for city driving or spirited driving. So all of the gear ratios (18 or so of them) are made for highway. They rarely change speeds and they do all of the Hypermiling things because they have to.

[bestclimb]

Originally Posted by trebuchet03

Doh! That should have been 140MW - which is
Oh, don't forget the 1450hp APU (you do want a pressurized cabin and hydraulic controls, right?) - which is roughly 1MW...

Good stuff in the rest of the post, The APU is mostly on on the ground, to provide air to start the engines, during takeoff, and approach. In cruse it is off (at least on a 727 and 737, a 747 I don't imagine it would be much different). The cabin pressure comes from bleed air from (I think) one or both of the two inboard engines.

[dwtaylorpdx]

Over the rod trucks use low rolling resistance high pressure skinny tires, proportionally a prius should probably have 2" wide tires... Like my Neighbors 80 year old 40 - 50 MPG Model A. His has a 3 spd tranny and a two speed rear end and if you look the thing over carefully it looks like it has a form of water injection from the radiator. For grins we ran it through DEQ, it had a little more CO than my BMW but everything else was fine.

Dave

[dwtaylorpdx]

I read once that a 747 Engine (one) produces the same KW as a standard Nuclear reactor in the US...

Sounds almost right

Dave

[Frank Lee]

Tire width is more or less proportional to the weight of the vehicle and the design tire psi.

The old-time cars with skinny tires didn't weigh much.

That, and over time understanding of tire dynamics has improved.

Semi tires aren't 20" wide because they don't spec 30 psi.

Prius would not have 2" wide tires unless they spec'd em with way higher psi than they figure the consumer will accept.

[RH77]

Originally Posted by slowmover

Have a look at Jay Leno's 1961 FLXIBLE with a 6V-92 Detroit.

The Detroit Series 92 (especially in 6V92TA form) is a favorite of mine in the history of diesel transport. The local Metro bus line replaced old Series-71 equipped GMs with Gilligs in the early 80's. The engine was marketed as a "fuel miser" and was built on the reliability of the previous-gen 71-series. It lacked power, but got the job done. Electronic engine and transmission control improved FE and emissions until the engine was discontinued and replaced by the Series 60, 4-stroke.

Environmentally, the 2-stroke, turbo-aftercooled plant was an emissions nightmare, but served the purpose for the era and offered superior FE compared with older equipment (well into the 90's). Plus there's just something unique about the sound of a 6V-92. Day after day, I heard the passing buses pull a hill from a standing start, which became a familiar routine of "neighborhood sounds" for years.

Anyways, the more gears the better -- 6-speed automatics are becoming more common in cars (borrowing on the torque curve argument)...

RH77

[dwtaylorpdx]

Originally Posted by Frank Lee

Tire width is more or less proportional to the weight of the vehicle and the design tire psi.

The old-time cars with skinny tires didn't weigh much.

That, and over time understanding of tire dynamics has improved.

Semi tires aren't 20" wide because they don't spec 30 psi.

Prius would not have 2" wide tires unless they spec'd em with way higher psi than they figure the consumer will accept.

That's kind of what I was alluding to, Design compromise for comfort...

Of course I do drive a jeep wrangler with 4 corner leaf springs, so Its obvious I am numb or like pain...

Dave