Why are tractor-trailer rig so much more fuel efficient
 

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

if i were to guess i'd say it has to do with the gearing. along with that the motors never really work too hard, maybe 3,000rpm max? that is just a guess i know its not very high though.

consider the weight to frontal area ratio of a transport truck to a 3000 pound car. 25x the weight with maybe 8x the frontal area. Their total air drag-to-weight ratio is much smaller. That same truck gets approx the same milage when it is dead empty, too (may save 1mpg if LUCKY). This reduces the weight/FE ratio greatly.... lots of reasons, really. Most prominant probably being that weight has a much smaller affect on FE than you'd think.

i could see this being true to the fact that once you have the truck moving it wouldn't take as much to keep it moving, just like any vehicle. my mileage shows really low accelerating but then levels once im maintaining speed. maybe the same applies to trucks?

And trains. I would guess that, given a flat straight surface, a 200 car train takes about the same amount of energy to keep moving as a 150 car train. The only difference is in the curves and grades.

Yep...An object in motion tends to stay in motion until outside energy changes the velocity.

That could be friction, a curve, gravity, wind etc....

Once you get 50k+ lbs moving, it wants to keep going in the line set forth. so once it is at speed, it does not take that much energy to do relatively small changes. A higher gear can be used then to improve efficiency.

Jim

As far as grades are concerned, you get an advantage with more weight going down hill to counter the disadvantage going up hill.

Also, if gearing had anything to do with the reason that cars get proportionately poor mileage compared to large trucks then it would simply be a matter of changing the gearing of the cars to increase their mileage. The fact is, cars are already geared as they need to be to most efficiently do their job, as are trucks.

cars can do this, but markets don't let it. the greatest power to weight ratio for 3000 pounds or so expected to do the 80s mph (typical car) is the 3 main boxer four.60mpg without einstein and close to nature has never been a surprise from that engine, even on old trannies and flaws thorughout, as all of them today are old even when they were new. Deisels in such small strokes is still a nuisance. The 8 cylinder diesel with the crazy crank thay have used for 50 years belongs in diesels and returns alot for power to weight as well. I see those coming around slowly, and liking it. The durmax, even ford has a little 4 something liter (rather backwards mentality, but hey, ford and long strokes where they don't belong seems appropriate to see such a tiny diesel)
Believe it or not, a deisel truck could do better with the 8 cyl. Tortured by a six cyl diesel my whole life..it remains the typical mainstream for big american rigs and it is terrible.
This question has been asked many times

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.

Why would an 8 be better over a six? I always thought that an I6 oil burner was the best of all worlds when it comes to packaging and power.

Your analysis aside, I think big rigs get better mpg per pound because they have to. Meaning their purpose is to haul in order to make money, passenger cars are simple transportation for individuals.

Therefore the industry put forth much more effort to make those monsters more fuel efficient in an effort to reduce operation costs. Same would hold true for trains. There is less pressure to make improvements in those fields because it serves the greater good, product and services. Whereas commuter cars are expendable and simply a profit center from which to reap benefits.

This also translates to replacement parts. More passenger cars are repaired at shorter intervals than tractor trailers. My guess is for the same reason.

well the frontal area is bigger but isn't 10 X bigger, also when you have 8 gears (think that is the amount) you don't get high rpm's

gearing varries... up to 14, i think.

Trucks I agree with, but passenger vehicles are seldom geared to efficiently do their job. They're often geared to make them seem peppier or smoother. Only those marketed as fuel efficient are actually geared properly for efficiency.

The inline engines are actually still used because of reliability. The 7 main bearings in an inline 6 supports the abuse better than the 5 in a V-8. Of course an inline 8 would have 9, but the length of the engine becomes an issue and they do not produce that much more torque for the weight added. The higher compression of a diesel does also add a higher amount of force on most all of the rotating parts :)

Jim

They are more aerodynamic.

Some Road Ranger trannys have 18!

Well then why don't we ecomodders raise, or lower, our final drive gear ratios by changing differential gears?

Trucks have more gears because diesels have a much more limited RPM range. And once you are up to hiway speed the only gearing that matters is your final drive gear. More gears has nothing to do with hiway driving for a car unless your terrain is so hilly that you would actually be downshifting on the hiway.

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

IIRC, the ones I used to drive (decades ago) had about 20 speeds: 10 in the gearbox, plus a 2-spd rear end.

This is another reason why trucks get better fuel economy. The large number of gears means the engine can be run closer to its optimum efficiency. In addition, long-haul trucks just get out there on the highway and cruise at a steady speed for hours, which is a lot more efficient than the typical passenger car pattern.

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.

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.

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

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.

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.

Sure you can
 

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

Oh? Have you told Intel about this?

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.

It's also due to not having throttling losses.

And that.

why
 

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).

If an auto company manufactures cars (or does anything else) to enrich anyone other than the shareholders IN THAT COMPANY, they will be sued. So where is the cash flow to GM from the government (ok, I'll give you that one), the oil industry, parts manufacturers, labor, insurance companies, pension funds, .... etc.? The institutional shareholders (representing, no doubt, the retired Texas kindergarten teachers) constrain GM, Ford, and any other U.S. automaker (or automaker that trades its stock in the U.S.) from doing anything other than that which is perceived to maximize next quarter's net income.

Agreed. I have a little over 286,000 on the current engine, and when it finally croaks, I'll be upgrading it to a newer (OBDII) engine and transmission, and can find the best ratios then. Still, it'd be nice if all manufacturers made it easy to swap 5th like Saturn did.

Yes but that isn't the whole story. You saw the drag doesn't scale comment? Right?

Read an old article called

"Economies of Scale; comparing the boat, the train, and the semi"

Like it or not larger more expensive LARGER devices always get more attention to efficiency. The train does get more of its fuel converted into power to its wheels independant of of outside drag compared to a car, as does a boat to its prop.

The Most Powerful Diesel Engine in the World

How many cars do you see with verified 50% efficiency motors? The theoretical max is only 56%!

It is very commonplace for trains that have motors in excess of 40% efficiency, (most in "hi end" consumer range vehicles hover in the 30% area when lucky). The train also gets more of its power to its wheels as the drivetrain is more efficient compared to a typical car setup and this is before outside drag. This should show that there are things that can be done and that there is hope for improvement in the automotive sector.

Admit it, we usually build larger devices better with more recovery systems in place. There are large ships with 3 recovery phases, you would never see that possible on a car, train or a semi. Nor would you ever see 50% efficiency on the motor end either.

A few are mentioning recovery in normal channels
5 Ways to Reclaim Engine Heat and Deliver More Efficient Cars « Earth2Tech

Honda only gets about 3.8% back But every little bit counts. If we can get consumer level motors up to 50% efficiency that would easily add 50% or more to mpg values on the vehicle without changes to the drivetrain. Not to mention the side effects of needing a smaller motor for the same power, having less massive suspension, etc.

I saw the comment, but I don't think I really understand what was meant. For practical purposes, cars, trucks, and trains are at pretty much the same scale.

In any case, I didn't say that those two points were the whole story, but they are a big part of it. Take your semi, mount railroad wheels on it, and see how much more it's able to pull. Or consider some of the early "railroads" that were powered by animals, or even humans. The whole point was to reduce rolling friction losses by running on smooth rails.

enrich
 

A good question,deserving a good answer.Kinda complicated,kinda convoluted,I'll give it a go: Both the Texas Municipal Retirement System,and the (Texas) Teacher Retirement System (just for example) are invested in all automakers,worldwide,all oil companies,worldwide,all major auto component manufacturers,oilfield service companies,metal companies,chemical companies,tire and rubber companies, insurance companies,virtually everyone associated with auto manufacture.They are also into Parts.They also (as a state) control traffic lights,collect sales tax on new and used car sales,and gasoline and diesel fuel taxes.They profit in the oil patch,they profit at the gas pump,they profit with the drivers license,they profit with vehicle registration,they profit from titles,they profit from annual inspections.When one of their $30,000 Hummers sells for $100,000 they aren't bashfull about charging their taxpayer neighbor full retail,and they are not bashfull about collecting state sales tax for a $100,000 when the vehicle is worth $30,000.--------------------- When the taxpayer's child goes to public school,the teacher never tells the students how much they screw the parents,and how stupid they are to pay what they pay and they never teach their students anything about efficiency,so the US can cuts its umbilical cord to the Middle East,then when little Johnny graduates from Texas' Public High School,he can join the Marines or the Army and go to Iraq,cause he can't afford college without the GI Bill.And because the teachers own Halliburton and other cost-plus military contractors they can be war profiteers fetching any price they like from the taxpayer parent back home.The federal govt,has it's own fuel taxes,and a profitable burgeoning bureaucracy dealing with all aspects of our automotive culture.Since schoolteacher,city,county,state,and federal shareholders make so much money with "business as usual",I suppose it's lost on them,that they could ask their own corporations for change.Since that hasn't happened since 1973,it leaves one to wonder whats important to public servants.And since we're left to learn of such things at obscure websites on the internet,instead of mass media,also owned by the government and industry(commercial broadcasting),it doesn't bode well for the future,regardless of who's in the White House.The real power lies elsewhere.

Its the men behind the president that have the power. We are in what Muslini described as a corporate government, which is facism. We can vote but no signifant differences between parties at the federal level, they put on a good show but do little beyond party line.

Also I would argue though that the teachers like most just want to make money and get ahead and have no real power to change how those companies make their money (those INDIVIDUALS that do have interests in every segment have influence),

Also our edumacational system is also not decided by teachers and was purposely tailored starting in the 30's to train children to become good employees and not question authority figures even if they are wrong (good training for corp life). That was a stated agenda for the schools from government. (watch a circa 1950's school related filmstrip to see that in action)

Oddly enough that credo was probably much better than the dumbed down system that is in place now, churning out kids who don't understand math or the english language. That and there are more mentally challenged children than ever before in our history clogging up the system due to fertility drugs, old farts having kids, mercury laced injections and our wonderfull environmental damages catching up to us.

It takes a while for people to see that our actions do cause damage to our children and our future.

Unless we can organize like the folks in france and britain and have a country wide mass havoc walk on government every time central government gives us the shaft there won't be much change in any of the regards you state. (its hard to even get people to realize they are getting the shaft, my uncle is convinced that farmers should get subsidies and he is a republican farmer, voting for people who have stated they want to remove all subsidies but he thinks they are going to assist getting him more subsidies, don't tell him what the rep said though)

The folks that have power keep it because nobody really pays attention to who they are or what they are doing. Furthermore good citizenship has been lost, folks feel no obligation to get involved in local government, sadly fed level need radical changes to become accessable to normal people again.

As a side note Fed gov is still pushing RFID chips for drivers licenses at states, good thing local Wisconsin government has fought that bogus BS every time it comes up. RFID is in your passport though, big brother needs to make sure you go where you state you are going.

You left out the part about the black helicopters. :rolleyes:

I have my truck on my profile, although I haven't updated in a while. 95% of trucks on the road are 10 speed in line 6cyl, (9-15.0 litres) with turbos around 25-35lbs boost. Mine redlines at 2400, but revving it past 1500 is pointless and wasteful. They use higher fuel pressure and compression, so they are actually squirting much less into the cylinder than a passenger car, combined with the lower rpms, the engine is essentially snoozing down the highway. If you could get a diesel for your civic (maybe a I-3 pushing 250-300 lbs tq.) you may be able to get 80-100mpg. Granted, that little engine would weigh 750 lbs and probably sink the nose of the car into the pavement :-)

[QUOTE=rmay635703;59835Unless we can organize like the folks in france and britain and have a country wide mass havoc walk on government...[/QUOTE]

Ah, yes. France & Britain are such wonderful places. Why, a person doesn't have to work there, just sit back and collect dole money.

Read an interesting thing the other day, about the difference between the American and British attitudes toward wealth. An American sees a rich man driving by in an expensive car, and says "some day I'll have one of those too", while the Briton says "some day we'll take the the fancy cars away from those rich buggers".

difference
 

When so much is invested,and so much stands to be gained,it is probably a terrifying spectre for some to consider actual change.And since at least one US president had the cajones to actually go on public record,stating that"The business of America is business!",it does overshadow everything behind US behavior.Even our wars are based on business,and wrapped in the flag and democracy and notions of mom's apple pie and baseball, to make them more pallatable to the "mindless automatons,wandering aimlessly in the dark,muttering incantations."(Congressional Record,1972:declassified U.S.Navy document).------------------------------- And so to protect the investor class,its been stated by capitalists,that if change should come,they need about 12-years to move their investments into the emerging "market" and out of the "declining" markets.That would explain the lack of real change,or the slowness to change.And in all fairness,if we expect "them" to mass-produce consumer goods for us,to the tune of hundreds of thousands of units a year,it is only respectful to try to sympathize with their position.-------------------------- As to the teachers,they're not under attack.They have lives and bills to pay like everyone else.They are probably all underpaid for the hours they give,and have no say over curriculum or textbooks,or recently the "scripts" they're given to regurgitate in the classroom.The ones I'm intimate with have no knowledge of what is done in their stead by fund managers who manipulate the portfolios of their pension funds.What planet these people come from I know not.It is my opinion that very sinister things come from this entire bedplay.------------------------------ You may be aquainted with John Gotta,and his experiences and insight into public education.Quite remarkable!----------------------------- When we're so busy,leveraging credit to live beyond our means,and then cower in fear of losing those material possessions,had at the cost of debt,something seems to happen to people,something devisive,and they appear willing to strike out at anything,and any neighbor they can be led to believe, threatens their insecure lifestyle.They do it to themselves.The whole world laughs at them,but they're to busy with their folly to pay attention.----------------------------------- Personal autonomy is foremost the highest sin.A reasonable,modest,high mpg automobile might allow consumers(we're no longer citizens) to help break free of the bonds of debt and begin the slow climb up Mazlo's hiearchy towards self-actualization.Wouldn't want to do it!