How do railroads determine how much power they need for a given trip. I’m curious about coal trains on Crawford Hill and at Palmer Lake.
You’re dealing with two things here. Tractive effort and speed. Tractive effort (TE) is the pulling power in pounds that a locomotive has. To provide an example, a standard DC traction diesel has between 18% and 25% of its actual weight available for traction, dependind on the condition of the rail. So if you have good dry rail with a 200,000 pound locomotive, you have 50,000 pounds TE available to start your train.
HPT (Horse Power per Ton) is a measure of how much power it will take to move the train depending upon the type of service is intended. A pig will take 4 to 5 HPT while your coal train may operate at .8 to 1.3 HPT to get the train over the road according to its service contract.
Kenneo - why is the pig a higher ratio? Weight, weight distribution or head end power? Any of those?
Mook
Acceleration. Pigs have to compete with trucks. so they’re always overpowered. They need to get up to speed quickly.
On some routes pig trains travel faster than coal drags.
Next time you see a pig train notice the large spaces between trailers. This causes extra aerodynamic drag.
The intermodal train has a higher HP/ton ratio because it has to maintain a higher average speed than the coal train. Mineral freight customarily has a low HP/ton ratio because it doesn’t have to move very quickly (usually restricted to 40 MPH) while intermodal has a high HP/ton ratio because it’s often considered priority freight.
So any correlation between the SD70’s and the Dash 9’s on coal trains and other power (some 9’s, but also SD40’s) on the freight. I hardly ever see an SD70 on a freight and not even that many Dash 9’s, altho a few more. Usually muliples of other power. Or does it just work out that way?
I read some time ago that speed needs horsepower. A person can move a loaded freight car, just not very fast. A 3 HP lawnmower engine could probably move a sizeable train, but again, not very fast. As mentioned, all a coal drag needs is enough power to get it over the road in an acceptable manner and time.
Gearing notwithstanding, consider that passenger trains also have a high HP/ton ratio, both for acceleration and top speed.
Thanks Everyone. Now when I see 3 SD70MAC’s on the front and 2 SD70MAC’s remote at Palmer Lake, perhaps I can calculate how much the train load of coal weighs.
Taxilab
Just count the cars and multiply by 131 tons. That’s more accurate.
You can’t judge the weight of a train from the power. The same train that takes 5 SD70’s at Palmer Lake, might only take 2 units someplace else. Same train, same weight, different power.
Dave H.
Our power desk people used to look it up in a book. It’s got something to do with the maximum grade on a particular route.
All,
Two points of clarification. One intermodal trains have relatively high wind resistance, and wind resistance increases as the square of speed which is part of the reason they need the high HPPT. The other is that tractive effort falls off as a function of speed. TE at 60 MPH is half of what it would be at 30 for the same unit, which is more important reason that pigs have high HPPT.
Second is that HPPT depends on ruling grade to be encountered and whether 4 or 6 axle and whether AC or DC power.
Mac
It is my understanding that a 300 HP 18 wheeler loaded with 40 tons is about 7.5 hp/ton. Obviously we use greater power. However we dont use T/E we depend on torque to keep moving on grades.
There are alot of Intermodal Trains that slam thru my town at some 60+ mph with 4-6 or more locomotives at the front. If the coal trains having pushers are allowed to get up and run they too can do 60+ loaded, but they take alot longer to get moving than a stopped Intermodal. Either way the entire town shakes. =)
Some of the fastest trains I have seen was the DD40X (8 axle and BIG) with the 7 car Executive train I think 80 mph is not too far fetched because at those speeds tend to lift the debris and carry it along the track.
OOPS Hold on, let me grab my calculator and my logarythem tables… O.K. I’m ready
Randy Stahl
Here is an easy way to think about it without getting into the physics (yes I have taken physics).
Horsepower or just power) dictates the top speed of any thing against whatever force is resisting it. Be it a ruling grade or wind resistance. If you need to move fast against these conditions, it takes more power. If not, you can slow down and still make it, but it takes longer.
Intermodals and passenger trains trains need to maintain a high speed average- so they need higher horsepower per ton. As a side benefit they also get to accellerate faster too.
If you want a numeric demonstration of the inherent efficiency of a steel wheel on steel rail, try calculating the horspower/ton values in your auto or in a semi and compare that to the value for coal trains. Big difference.
Have fun - Marty
Hey !! My calculator does’nt even have sqare root, Oh wait ti’s company issue only calculates KILOWATTS. Nice of them to issue me a solar powered calculator… working 3rd shift!
Randy
Of course on the flip side if you work for a shortline, Horsepower to Ton sometimes is thrown out the window, as it’s common to have 1500 HP with 3900 trailing tons on 1% grades, 2000 HP with 7, 000 trailing tons on slighty hilly ground( with which the 8th notch is all you use and don’t even keep track speed). It kills me to see 2 AC4400CWs powering an 8,000 ton train and the crew whining their underpowered[:(!].
Horse power is defined as Work as a unit of force in a measure of time.
Torque is defined as the ability to resist inertia and drag by the engine.
I think that mighty 8000 ton train Rustyrex was talking about had a crew that is used to “Pigs” I would be more worried about ability to control that weight.
My favorite engine for T/E has always been the Y6b class. These can keep it moving no matter how heavy it is or how contorted the railroad is.
an excellent site for an explaination:
I am beginning to think that the troubles with the railroads started with the demise of the steam locomotive. They had a limited number of powered axles and no MU, but any train they could start they could pull as fast as you want provided you could generate enough steam.
The DC-motored Diesels could start any train but they could burn up traction motors lugging. The AC Diesels have double the HP, but they also have double the tractive effort owing to those fancy wheel slip control systems, so it seems the railroads are using the AC’s to move their traffic even slower. The AC-motored Diesels can attack hills at a crawl without worry for their traction motors, never mind that the he-man attempt to climb a hill at 2 MPH is tying up traffic over the entire network.
This idea of pulling 3800 tons up a 1% grade with 1500 HP – my formulas tell me you could do that at 7 MPH at around 70,000 lbs tractive effort – what has 1500 HP and that much tractive effort – an SD-7? Isn’t 7 MPH below the minimum continuous speed for that thing? Does someone make a 1500 HP AC?
Yeah, yeah, fuel is expensive, and locomotives are expensive, and railroads have to reduce costs. Do you see their trucking competition going with, say a 100 HP normally-aspirated Diesel and inching their way up Interstate mountain passes? Heck no! The trucks have 14 litre turbo Diesels and are the fastest things on the road these days – an ordinary non-turbo car can’t keep up with them, especially at altitude.
Yes trucking is expensive, but the fuel mileage of an 18-wheeler has increased from 2-3 MPG to over 6 MPG, partly because of improvements in truck engines, largely because of the streamlined fairings you see on trucks. Given that aero drag is the dominant factor at 60 MPH and that I don’t see streamlined locos or fairings on the flatcars, I am beginning to wonder if piggyback trains use more fuel per ton than all of those trucks running on the highway.
I’m sorry I don’t remember the article except that I read it in Model Railroader. In this article, it was explained how locomotives are assigned to trains and all of the considerations mentioned - weight, grade, speed were listed; but it went into detail about the ratio of tractive effort / weight to horsepower of the locomotive.
Locomotives with a higher ratio between TE / total weight and horsepower - in other words a high HP and a low TE/weight are considered “slippery”, while the opposite is considered “drag”. this system was for both steam and diesel locomotives. I don’t remember the numbers, but they were all ratios between 1 and 2, for example 1.64. In general steam engines were ratios much higher than diesels, in other words, steam engines are slippery, their horsepower can exceed their TE. Horsepower increases with speed and so does torque up to a point where the two balance and then begine to decrease, this is the point of maximum efficiency in an engine. Typically, this point is designed to be the cruising speed or rated speed of an engine and in locomotives an amount of power in balance with the TE. This speed then is at a higher point in steam locomotives, and a lower point in diesel locomotives.
Paul’s example of the 100 Hp truck gowing ten miles an hour up the mountain is a good example.
When a truck is built, it is “spec’d out” or built to the specifications of the buyer based on his needs. Following equations for “grade ability” to specify adequate gearing in the transmission and rear-end, trucks typically have a grade ability well over 100%, and with a larger differential of 15:1 or more and a hole or granny gear could be as high as 200%. A 200% grade is not practical trying to visualize, it is just a number that an engineer can then plug into other equations for power requirements. So, trucks in fact have a TE far in excess of the truck’s weight or power, Where with trains, this is a balancing act. The real advantage of steel wheels and rail i