I’m sure everyone has encountered those ads or tv commercials about whoever it is that has trains that can move some astronomical amount of cargo 500 miles on a gallon of diesel.
In contrast, diesel trucks get some abominable mileage - like 10 to 20 mpg.
I know that some level of difference is to be expected because the train engine is roughly the size of a cargo container, and there is some difference because the train doesn’t have to do as much speed adjustment.
But really? What gives? There has to be some other reason that transport trucks have such godawful gas mileage in comparison.
I don’t know enough about this subject to search for it - all I’m getting are advertisments for various truck companies touting their amazing 30 mpg heavy duty personal trucks. :rolleyes:
Yes, you have to compare on the basis of ton-mile per gallon, because a diesel locomotive is typically pulling thousands or tens of thousands of tons of freight. But rail freight is more efficient because travel on steel rails means much less loss due to friction, and because a dedicated track means less energy wasted in speeding up and slowing down.
Even more efficient than rail is travel by barge on rivers and canals, or by ship on the ocean, again because less energy is lost through friction.
There are a bunch of reasons that trains are so efficient but a major reason is that they move at roughly the same speed over long distances while having a huge amount of inertia on metal wheels traveling over metal rails. Trains don’t have good starting or stopping capability. They just want to continue doing exactly what they are doing at that time no matter how fast or slow. Once the train spends miles to get up to speed, it takes fairly little engine power to keep it moving at that speed. The engine just has to compensate for energy loss through aerodynamic drag (it still presents only one nose to the air no matter how long it is) and drag from the efficient steel wheels on steel rails.
Trucks don’t work the same way. They speed up and slow down a lot more so the energy fed into the system keeps getting added and taken away by traffic, red lights, and road conditions. Trains wouldn’t do very well either if they changed speeds all the time. Tractor trailers are still fairly efficient however compared to passenger vehicles. The trailers can often carry about 40,0000 pounds of goods or the equivalent of about 10 other vehicles yet they get much better gas mileage carrying the equivalent of those cars instead of the cars driving on their own. That is a function of combined mass being more efficient as well.
They aren’t quite the same measures. I believe the trains are talking about ton-miles per gallon, rather than straight miles per gallon. So if the truck has five tons of cargo on it, the comparable number is five times its milage.
Also (in addition to what other people have said), there’s probably some efficiency from economies of scale. Having a big engine haul a lot is probably more efficient than having a number of small engines do it.
Also, aren’t most diesel locomotives diesel-electric instead of straight diesel? I.e. the diesel fuel runs a motor that runs a generator that runs an electric motor that turns the wheels, and this means the diesel motor can be made run more efficiently because it only has to operate at one speed, and there is less transmission-related loss getting that power to the wheels.
I thought there was a thread on the CSX claim but my search skills are bad.
Anyway, Giles is right. Economy of scale is an amazing thing. CSX can make the claim and its easy to support with basic arithmetic. Just knowing the amount of tonnage moved, how much fuel was purchased and the number of miles covered will give you the number.
Also remember that a train’s drive system is a type of hybrid system. The diesel engines turn generators that produce electricity that power the electric motors that move the train. It is a much more efficient system than one engine powering a tractor with one, two, or occasionally three trailers.
It takes a lot of power to get a train moving but once it is moving inertia is a real friend especially when it is riding on smooth steel rails. Cars are stacked behind each other so there isn’t much air resistance. That’s why trucks tend to draft other trucks. It’s fuel saver.
Another factor is how flat railroads are. IIRC the grade on a railroad doesn’t exceed 4% while a grade on an interstate highway can max out at 8%.
Yeah, if you want to compare apples to apples, let’s say a fully loaded semi truck carries 40 tons of freight and gets about 4-5 miles per gallon, which means it can move a ton of freight 160-200 miles on a gallon of fuel. (Numbers are approximate here.)
Actually, before you edited, you did have something I missed. I meant to write 40,000 lbs of cargo, not 40 tons of cargo. Forgot to divide by 2000. So, with a 20 ton cargo figure, my numbers really should have been 80-100 ton-miles per gallon for the semi truck. (Which is the range you got before you edited your post.) So I’m not sure where the 170 comes from, unless there are trucks hauling around 40 tons of cargo.
The reality is that trains get terrible “mpg”, I’m not sure what but one site said around .5 mpg. However, they can carry immense weight so they get great “ton mpg.”
It can’t really move a single ton of freight 436 miles on a gallon of fuel, but it can move, say, a thousand tons 436 miles on a thousand gallons. So this fictional train gets 436 ton miles per gallon, but about .4 miles per gallon of fuel burned.
As far as transporting a lot of stuff, it is extremely efficient but by itself, a train gets horrible fuel mileage.
What’s funny is that when you look at the freight ton miles per gallon, everything starts to look like horrid mileage, especially normal cars.
I find it funny that UPS is at the low end of that comparison chart, and even more funny that aircraft are even worse. Makes me wonder how much freight is going by air that really isn’t that urgent, and how much cheaper it would be to send it by ground or water.
Hint: For those three-letter searches (and many board searches in general–the SDMB search engine leaves a bit to be desired), it’s best to be off to Google. It was the first hit for the following search:
Not a hijack, just an interesting factoid about efficient methods of transportation.
I have a book on marine transportation that reports the interesting data that along about the turn of the last century, (1900 or thereabouts), a steamship using a triple expansion steam engine could move one ton of cargo one mile on the amount of energy in a sheet of octavo notepaper. I think octavo is about 9 X 5-1/2 inches in size.
It should be noted that the diesel-electric drive train of a locomotive is by itself not particularly efficient. The diesel engine drives a generator which powers traction motors mounted on the trucks (wheel assemblies). A standard car or truck transmission with a friction clutch is much more efficient if operated properly as it provides a direct mechanical link between the engines output shaft and the wheels. A friction clutch is not practical however, for very heavy vehicles and high horsepower. This is why very large equipment…earthmovers, locomotives etc generally use a hydraulic or electric drive.
Interestingly, there were a few small switching locomotives built with a direct drive and friction clutch. This one, for example, which I rebuilt a few years ago for a local railroad museum. It had a big, hand-operated clutch and 4-speed gearbox, exactly like a large tractor that ran on rails. Of course, this kind of machine is only intended for low-speed short-distance haulage…I can only imagine the difficulty of trying to change gears under momentum with a string of railcars in tow.
The fuel efficiency of rail haulage is primarily from the near-frictionless movement of steel wheels on steel rails. The permanent guideway helps, as does the fact that railroads are generally limited to grades of 3% or less while highways have hills of 7% or sometimes more.
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