Why are tractor-trailer rig so much more fuel efficient

[rmay635703]

There are 2 major things going on here

1. Man can't build small things very well (the bigger it is the better and more efficient it can be made because of the extra room to fudge with) Best example is comparing a big rig to a train, the train is also much more fuel efficient. Very large boats strangely enough are fuel efficient but many times have energy recovery systems onboard. (you usually wouldn't have room in the average car to adequately impliment these things)

2. The motor is sized just big enough for the load in a big rig. If a car would accelerate like a big rig and have a motor HP scaled similar to a big rig (while keeping the motor well built and highly efficient) you would indeed get the mileage you should considering of coarse frictional losses which do not scale. Just look at old issues of mother earth news where folks power their car off a lawnmower engine, go slow and get a lot better mileage. Sizing the motor appropriate to the size of the vehicle reduces pumping losses and puts the motor in the area it belongs. Assuming again the motor can still be made well.

[instarx]

Some misconceptions from previous comments:

1. lots of gears on trucks are not there to give good mileage - they're there so the engine can move the truck at all.

2. Low RPMs of diesels do not translate into FE. Even at 1,000 rpm fuel can be pouring into the cylinders to produce power and torque.

3. all things being equal, diesels engines get better mileage per gallon because diesel fuel has 10-15% more energy per gallon than gas - not because diesels have high compression.

Trucks are more efficient than cars per pound moved because of simple physics. It simply takes much less energy to keep even a large load moving at a constant velocity than it does to accelerate it (Newton's First Law, and F=ma). If engines on trucks were made large enough to accelerate 80,000 lbs as fast as a 3,500 lb car they would get about 0.1 mpg (I'm guessing).

Conversely, if car engines were sized proportionally to truck engines cars might get 200 mpg, but no one would drive them because you would have to shift gears 10-15 times between stop lights to get the car moving. If your car engine were sized like a truck engine, a 3500 lbs car would have an engine of 20 cubic inches (0.3 liters)! That's less than half the size of a riding lawnmower engine.

So, trucks use less fuel per pound moved primarily because:

1) it doesn't take 20 times more force to keep 70,000 lbs at 65 mph than it takes to keep 3,500 lbs at 65, and
2) your car is hugely overpowered compared to a truck.

[RH77]

My theory...

Big rigs engines are designed for longevity and economy -- and consequently cost more to produce/purchase. To buy the equivalent in a passenger vehicle costs at least $3000 more (diesels over gassers: specifically pickups, followed by passenger cars).

Some of these trucks stay running 24/7 until the next oil change, with an expectation to get at least 750K/1-million miles out of the powerplant. RPMs are often limited from 1,800 to 2,100 -- to stay within the powerband and for longevity.

RH77

[Who]

80,000 pounds... 400 hp tops...

For a 3000 pound Prius to have that hp/wt ratio it would have 15 hp

I love the inline 6... overlapping powerstrokes without any excess.

[Frank Lee]

Why do yall keep comparing fully loaded semis with empty cars?

Load a car up to capacity and you've majorly increased the "efficiency" of the car right there.

[ALS]

Originally Posted by Frank Lee

Why do yall keep comparing fully loaded semis with empty cars?

Load a car up to capacity and you've majorly increased the "efficiency" of the car right there.

Ditto.
Take a 1.9 TDI Jetta and fill it with a family of four and luggage. At 60 mph it would still pull at least 50 mpg. Driving 1000 miles to their destination they would use 20 gallons of fuel. Actual fuel mileage per person is 200 mpg.
1000 miles divided five gallons of fuel per person.

[metroschultz]

Originally Posted by Clev

If I had RWD, I would, but I don't know enough about FWD transmissions to comfortably replace the gear.

Don't cut yourself short. There are many Honda guys here who could guide you through the process.
Or you can look for a tranny with different gearing and just swap the entire thing in a day or less.

http://ecomodder.com/forum/showthrea...html#post54097

This is the route I took. I still haven't filled my tank yet, but if gage accuracy is to be believed I may get 60+ mpg this tank.
Last tank was ~ 50 mpg.

Look around and see what others have done.
Try it for yourself.
Schultz

[jamesqf]

Originally Posted by rmay635703

1. Man can't build small things very well...

Oh? Have you told Intel about this?

Best example is comparing a big rig to a train, the train is also much more fuel efficient.

Why's the train energy-efficient, though? Think about it: you have two main sources of frictional loss, air friction and rolling friction. Air friction depends mainly on cross sectional area, and the area of a train is only about 3-4 times that of a semi, no matter how long it is. For rolling friction, instead of a semi's rubber tires flexing on the rough road, you've got solid steel wheels on smooth rails.

[PA32R]

Originally Posted by Gregte

Another advantage big trucks have over our American cars is that the diesel engine is more efficient in producing power than the gasoline engine. this is due to the higher compression ratio that a diesel can and does run at.

If larger engines that didn't have to work so hard were advantageous FE wise then our large engine cars would get better mileage than our small engine cars, but alas the opposite is true. An engine that is working harder is more efficient. For instance, my GMC with its 4 cylinder engine has to work harder than the same pickup with the 6 cyl engine but my smaller, harder working engine, gets better mileage.

It's also due to not having throttling losses.

3. all things being equal, diesels engines get better mileage per gallon because diesel fuel has 10-15% more energy per gallon than gas - not because diesels have high compression.

And that.

[aerohead]

Originally Posted by dph815

80,000 pound tractor-trailers rigs get 5-7 miles per gallon. Compare the weigh-fuel mileage ratio and a 5,000 pound auto should get about 80 miles per gallon.

What are the reasons for this and shouldn't autos be able to be just as efficient?

Just thinking,

Danny Harris
Arkansas

Big rigs are commercial vehicles.They are to MAKE money.All truck owners are interested in MPG as it directly impacts profit/loss,and so there is competition within the market to deliver a product with commercial advantage.There still remains some "gentleman's agreement" witholding of technology to protect competition and bolster the used truck market.Bankers hate innovation!---------------------------------- Automobiles are designed to go out of style and enrich governments,oil industry,and parts,labor,insurance,pension funds,labor unions,collision repair,chambers of commerce,etc.,and keep the US at war.---------------------- Since "economy" cars constitute only about 2% of new car sales,nobody really gives a flying --ck about them.Sure,we could of had 80-mpg cars in 1992,but why would we? It would cut into the retirement benefits of retired Texas kindergarten teachers.As long as soldiers are cheaper than foreign oil,we continue to kill for it.To hell with fuel economy.(Big flag wave).