Why are tractor-trailer rig so much more fuel efficient

[trebuchet03]

I second rmay's comment on scale.

Engines (perhaps even in general) do not scale down very nicely - nice meaning that you keep internal losses proportional to output (such in the case of reciprocating engines) or bypass ratios (Such as Pratt/Whitney's GTF with a bypass of 11:1 - but when scaled down, that figure dropped to 10:1 --- FTW: higher bypass means less fuel consumption).

Just remember - every 747 you see fly by is a 140KW power plant in the sky... The US commercial airline fleet has a combined output around 1 terrawatt (US power production is ~6TW)... And I'm gonna stop before I digress too much more

[bgd73]

alot of informative conversation here...

ever looked at u-joint of a rig, then a 3/4 ton pickup? they are playing with mere inches of difference for up to 150 THOUSAND pounds difference (log trucks can haul with a 550hp engine 150,000 pounds). The claims they cannot do better for vehicles and power to weight is hogwash. And furthermore, a 550 hp with 3.7 rears and a 15 speed is faster than my 13 second sube. A driver knows what to skip for gears and when, when empty. The only thing losing mileage is engineers, I hope they run out sooner than later..pun intended.

To add more to a cure: a 3 main boxer is as close as man has ever gotten to maximizing thoughts of power to weight for car sized vehicles and ICE.. No need to prove it with crazy you-tube vids of unbelievable subaru beatings of the 3 main classic, a bit of reading reaveals a type 1 vw still has the world record of 202 mph above ALL four cylinders. The more I learn the crazier I get...

A karl benz crank in a 4 liter 90 degree v8 would drop the jaws of entire gooked nations...

RIDICULOUS TO FALL INTO THE HELMS OF BABBLE.

Dear God Have Mercy...

[dremd]

Originally Posted by trebuchet03

Just remember - every 747 you see fly by is a 140KW power plant in the sky... The US commercial airline fleet has a combined output around 1 terrawatt (US power production is ~6TW)...

Got a source on that?
My Calculations indicate that 140KW = 187 hp

[instarx]

Originally Posted by bgd73

. And furthermore, a 550 hp with 3.7 rears and a 15 speed is faster than my 13 second sube.

Maybe bob-tailing, but then what would be the point?

[SmartTrucker]

Originally Posted by SuperTrooper

Some Road Ranger trannys have 18!

Yep we have some 18 speeds in our fleet.

On another note we just hired a guy that is hauling 60,000# payloads and he's apparently getting 7.7mpg this is with a flatdeck and a non aero truck in an area with plenty of hills.

If only I could get him to teach our other drivers how to drive and have them listen! The best of whom average maybe 5.8mpg.

[instarx]

Originally Posted by SmartTrucker

Yep we have some 18 speeds in our fleet.

On another note we just hired a guy that is hauling 60,000# payloads and he's apparently getting 7.7mpg this is with a flatdeck and a non aero truck in an area with plenty of hills.

If only I could get him to teach our other drivers how to drive and have them listen! The best of whom average maybe 5.8mpg.

Hmmm, they work for you, right? Offer them a one-time performance bonus if they improve their mileage 10% after riding with the guy. They'll learn it for the bonus, but they'll keep on doing it afterwards because they'll feel how much better it is.

[Duffman]

1) Diesel Engine is more efficient and there is more energy in the fuel
2) Big rig engines operate in an efficient narrow band
3) Low power to weight ratio
4) Better Driving habits and bulk of the time spent cruising
5) Aero Drag is smaller on a per pound basis (being both heavy and long are advantages here)
6) Semis are optimized to reduce cost of operation above all other criteria.

[trebuchet03]

Originally Posted by dremd

Got a source on that?
My Calculations indicate that 140KW = 187 hp

Doh! That should have been 140MW - which is an estimation...

Maths

747-400 Powered by 4 Rolls-Royce RB211-524H2-T with a Thrust of 264KN multiplied by 4 engines is 1056KN of thrust (264KN comes from 747-400 website, see link below). Here's an interesting tidbit according to Rolls-Royce "Every 34 seconds an RB211-524 powered 747 takes off or lands" (jet engines for the Boeing 747-400 - Rolls-Royce).

Power = F*v

Typical Cruising speed of a 747 (Boeing: Commercial Airplanes -- 747 -- 747-400 Technical Characteristics) : Mach .85 = ~290m/s @35K feet

So, worst case scenario, a 747 requiring 100% thrust @ cruising speed...
920*290=~306MW

That's not realistic - turbines are throttled back while Cruising - to what % is debatable, so here's a spread...
% | Power (As function of %thrust)
90 = 275MW
80 = 244MW
70 = 214MW
60 = 183MW
50 = 153MW
40 = 122MW
30 = 91MW
20 = 61MW
10 = 31MW

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

140MW puts us right around 45% throttle, which doesn't seem unreasonable and is also found on this page ( Orders of magnitude (power - Wikipedia, the free encyclopedia) ). Remember, I used the least powerful engine option for these quick calcs

Many of these values have been rounded, several times - this is a back of an envelop calculation as I don't feel like spending much time on this. I'm also aware of the assumptions necessary to make these estimations - you don't need to be an ass and tell me what I already know unless you know the range of drag forces of a 747 while cruising.

alot of informative conversation here...

ever looked at u-joint of a rig, then a 3/4 ton pickup? they are playing with mere inches of difference for up to 150 THOUSAND pounds difference (log trucks can haul with a 550hp engine 150,000 pounds). The claims they cannot do better for vehicles and power to weight is hogwash. And furthermore, a 550 hp with 3.7 rears and a 15 speed is faster than my 13 second sube. A driver knows what to skip for gears and when, when empty. The only thing losing mileage is engineers, I hope they run out sooner than later..pun intended.

To add more to a cure: a 3 main boxer is as close as man has ever gotten to maximizing thoughts of power to weight for car sized vehicles and ICE.. No need to prove it with crazy you-tube vids of unbelievable subaru beatings of the 3 main classic, a bit of reading reaveals a type 1 vw still has the world record of 202 mph above ALL four cylinders. The more I learn the crazier I get...

A karl benz crank in a 4 liter 90 degree v8 would drop the jaws of entire gooked nations...

RIDICULOUS TO FALL INTO THE HELMS OF BABBLE.

Dear God Have Mercy...

Pie?

I really couldn't follow much of that, I felt like I was reading the text in spam eMail o.0

[SmartTrucker]

Originally Posted by Duffman

1)
6) Semis are optimized to reduce cost of operation above all other criteria.

It's too bad drivers aren't.

Also we offer significant per mile bonuses for our drivers already. We'll see how many start making the goal.

[Clev]

Originally Posted by SmartTrucker

It's too bad drivers aren't.

Also we offer significant per mile bonuses for our drivers already. We'll see how many start making the goal.

Very forward-looking. Don't most trucking companies pay per mile, making the incentive for drivers to drive faster?

Is it unacceptable to just govern the truck to 58 mph? Might make passing more difficult, but I see too many of them going 70-75. You'd save money on gas and on tickets, and your insurance company might even give you a break.