Horsepower vs. Tractive Effort

[Anonymous]

I'm trying to figure out relative strengths of steam vs diesel. It's getting harder for me to compare the two. Seems that GE and EMD market their locomotives based on horsepower. Then I look at old steam locomotives and I hear words like "developed x pounds of traction effort at the drawbar at 35 miles per hour."

Sooooo....
What horsepower is, say, the UP Big Boy rated at? Does horsepower have any effect on size of trains? Are trains bigger now than they were?

And, completely off subject, what exactly is the definition of "track capacity"? My gut reaction is that one train occupying track means that the track is "at capacity". Two trains occupying the same space would be a bad thing- the track would be "over capacity". I read about UP exceeding "track capacity"....

Erik

[daveklepper]

Others may come up with more precise definitions, and certainly tractive effort times speed times a constant can give you a horsepower figure, but I don't have the constant in my head for this equation. Basically, tractive effort is an indication of how much friction and inertia the locomotive can overcome and thus the wieght ot train in can pull, which will decrease markedly with each increase in the grade. Horsepower is overall power and related to the top speed with a given load and grade. In your automobile gasoline engine, torque and horsepower are somewhat analogous terms. I think others can do better than this and I look for better entries . Dave Klepper

[tree68]

I've seen horsepower listed for steam locos - recently read of one with 6000 HP, and tractive effort listed for diesels.

Tractive effort is a result of available horsepower and weight on drivers - just as a muscle car can have so much power that it'll spin it's tires easily, so too can a loco. 4500 HP with "no" weight on the driving wheels will have "no" tractive effort. With the advanced anti-slip mechanisms now in use, I would imagine that it's a lot easier to forecast the tractive effort as a function of horsepower than it used to be. That's why you see HP/ton used to compute power requirements.

Someone else can supply the actual HPs of some steamers.

As for traffic capacity - you have to factor in time. Obviously one train on a section of track puts that section at capacity at that moment. The question is how many trains can that section of track handle over a given period of time. A double-track raceway with 70 mph speed limits will have a greater capacity than a single track with loads of slow orders and limited passing sidings.

The yards also count here. It does you no good to have track that will handle 100 trains per day if the yard facilities will only handle 50.

[oltmannd]

The comparison between steam and diesel is difficult because they are inherently very different. The best way to understand the difference is to look at a steam locomotive as being a constant tractive effort machine and a diesel as a constant HP machine. The relationship between TE and HP is: TE(lbs) X speed(MPH)/308 = HP.

A given boiler pressure will exert a given tractive force based on piston diameter and rod and wheel geometry. HP will increase proportionally with speed until the boiler just can't make enought steam (the boiler's HP limit).

Since the diesel has an electrical transmission, the engine can always make full HP. As speed increase, tractive force goes down.

A diesel with a 3000 HP engine will beat a steam loco with a 3000 HP boiler because the diesel can get 3000 HP to the rail all the way down to a very low speed where as the steam loco will have to get up to a fairly high speed before it's making 3000 HP.

However, taking a steam and diesel locomotive with 100,000 lbs of tractive effort, generally, the steam locomotive will have a higher boiler HP than the diesel and the steam loco would beat diesel as it would meet and then exceed the diesel's HP at a moderate speed.

[Anonymous]

http://www.n0kfb.org/rail/railphs.htm
"Horsepower is pull times speed. You can have pull with _no_ HP. Example: A 15,000 ton coal train sitting on a 1% grade and being held there by the engine brakes alone has 300,000 lb pull on that first coupler. But there is no horsepower being produced. The factor of adhesion for a steel wheel on a steel rail is between 20% and 30% of the weight on the wheel. So to prevent that coal train from pulling (sliding) those locos back down the grade the locos need to have at least 300,000 lbs of adhesion. This means they must weigh at least 1,000,000 lbs because 30% of 1 million is 300,000 lbs. The maximum weight per wheel, to prevent crushing the rail, is 35,000 lbs (70,000 lbs per axle). So we need a loco with at least 29.6 wheels, each of which is weighted to 35,000 lbs. 30 wheels is 15 axles or four 4 axle units minimum. It could also be 3 six axle units which would be 18 axles. Or it could be two 6 axle units and one 4 axle unit for a total of 16 axles. The point is that you have to have that one million pounds on the wheels and you are limited to no more than 35,000 lbs per wheel.

Adhesion as described above and Tractive Effort are closely related and can be though of as the same thing in many cases in that the amount of adhesion limits the maximum TE that can be used. TE is usually quoted at a specific speed. TE is the pull at the drawbar.

HP is the amount of pull (TE) times the speed. So while our coal train is just sitting there on the grade there is no HP required. But try to move it at 1 mph up that hill and HP is required. The required HP is the TE needed (300,000 lbs) times the speed (1 mph or 1.47 ft per second) divided by the definition of a HP (550 lb-ft per second). So the HP required is 801 HP! Yes just 800 hp will move this coal train up the hill. Amazing isn't it? But will one 800 hp unit do it? No! Because that one 800 hp unit must have at least 1 million pounds on its drivers to prevent it from sliding back down the hill. You MUST have the adhesion required. This means each wheel of an 800 hp 6 axle unit would have to have 84,000 lbs on it. Oh my the busted rails that would leave behind! As I said above, the minimum number of axles we need to spread out the required weight is 15 axles. Now it doesn't matter whether we have one 800 hp 4 axle unit and three 4 axle slugs, or whether we have four 200 hp units. It is all the same to the coal train. "
quoted from Al Krug; a discussion on locomotive and train dynamics

[Anonymous]

Thanks! Now it makes sense....

Eruj

[Modelcar]

....Now that is some serious data....! I'm with you until the next to last sentence and there I get lost. Trying to understand how the 800 HP of the "one" unit can get the traction when the required weight [for traction], is spread out say, over 3 more engines.....or something like that. Gee, smoke coming out my ears. [?]

[mvlandsw]

Has anyone been able to hold 15,000 tons on a 1% grade with just the engine brakes?

[Anonymous]

QUOTE: Originally posted by Modelcar

....Now that is some serious data....! I'm with you until the next to last sentence and there I get lost. Trying to understand how the 800 HP of the "one" unit can get the traction when the required weight [for traction], is spread out say, over 3 more engines.....or something like that. Gee, smoke coming out my ears. [?]

these ideas do seem to roll around on eachother. i find it hard to keep it all in place.
i hope that i have properly understood your question and that the following is of use.
(the full article is at the website cited.)

does it help to imagine the three engine consist spoken of here as being one very long single unit? i believe the author's point is that if TE requires more weight than four axles and their traction motors can safely transfer to the track, while still remaining within the load limit of the rail, then the locomotive is made heavier and lengthened some so that two three axle trucks can be used to spread the weight across more but less heavily loaded bearing points. should even more TE be required than six axles can bear to the rail then additional trucks are brought in by way of added locomotives. since only added bearing points are required the added weight can be as slugs whose axles and traction motors will apply the added adhesive force to develop the added TE.

question:
is this principle being used in the design of steam atriculated locomotives?

[oltmannd]

QUOTE: Originally posted by ptt100

It is also not true that steam locomotives are constant force machines. Steam locomotives are only constant force machines until (1) the engineer "hooks up" the Johnson Bar. This reduces the amount of steam entering the cylinders to allow the locomotive to accelerate. The locomotive can't accelerate above a certain speed (usually below 20mph) at full steam, because the cylinders can't remove the spent steam effectively. This causes backpressure which prevents acceleration and causes increased fuel consumption. (2) The other limitation is boiler capacity. The engineer must reduce the steam at some point to maintain effective boiler pressure.

So in reality, steam locomotives are only constant tractive effort machines for a small speed range. After that, they become constant HP machines just like diesel electrics up to a certain speed. After that, they loose HP due to increasing backpressure and mechanical friction.

I agree completely. I was over simplifying in order to answer on a level consistent with the question. I was paraphasing what Wheelihan wrote in a Trains Q&A several years ago. I thought it was a very useful way of thinking about it.

Yes, there are diesels that behave more like steam - the P42 being a good example with a min cont speed over 30 mph. And there are some steam engines that behave like diesels, the N&W Y6b being a good example where the boiler HP curve governs down to a low speed.

[Modelcar]

cbt141....Ok, now you have helped the issue for me...I was under the impression from the former post the power was limited to less than ALL the axles....but now in your explanation power is distributed to every axle and I understand the comcept. I thank you...

[Anonymous]

"but now in your explanation power is distributed to every axle and I understand the comcept"
you're welcome. i'm relieved that it makes sense... as i said these ideas roll around on eachother when i read them.

[Jackflash]

Yes a 15000 ton coal train can be held on a 1% grade with just the
independent brakes, what I want is where did 70,000 axel loading
(35,000 per wheel) come from, right now, there is a pu***o go to
315,000 lb cars, thats over 39,000 lb per wheel, almost 79,000 per
axel. jackflash

[BaltACD]

An electric motor develops it maximum torque and power when it is motionless, as its rotational speed increases the torque and power have declining curves. The 'short time' and minimum continuous speed' ratings that apply to straight electric and diesel-electric locomotives reflect this power curve. If a train is operated at a speed below it's minimum continuous speed with wide open throttle generating maximum horsepower from the prime mover and maximum current from the generator the current developed from the generator will 'overload' the traction motors and cause them to heat up as the traction motors work at increasingly higher power requirements, the slower the train moves. The 'short time' ratings are observed by watching the load meter on the locomotive....If the locomotive can handle 1000 AMPS load at minimum continuous of 10 MPH, then a 9 MPH the load may be 1100 AMPS and the time the locomotive may be operated at that load level would be 50 minutes, at 8 MPH the load may be 1250 AMPS and at that load the locomotive may be operated for 30 minutes....lelectrical loads increase the slower the train moves and the times permitted at those increased load levels decreases. Exceeding the 'short time' ractings will cause traction motor failure from overheating.

A steam engine, being a recriprocating engine, develops is maximum torque and power at a RPM somewhere above its stall (or motionless) speed. The power and torque curves rise to an maximum and then decline at RPM's above the the point where the maximum torque and power figures were achieved. Think of the power curves that apply to the gasoline engines that power our automobiles.

[Anonymous]

BTW: I have heard that a Big Boy steam locomotive was rated at 6000 horsepower - the same as the current SD90. I don't know how the two compare in terms of tractive effort.

Electric locomotives can confuse things. The GG-1 could generate up to 8,000 HP "running simple" (less than 10 MPH) but 4,800 "continuous" at higher speeds.

The SD50 is 3600 HP and (I hear) 100,000 pounds of tractive effort. SD60 generated 3800 HP and 133,000 tractive effort. How is it that tractive effort increases at a faster rate than horsepower?

To what extent do different truck designs (such as radial or high adhesion) affect tractive effort?

[tree68]

QUOTE: Originally posted by ptt100

Agree with with the statement about steam locomotives having a power curve similar to a gasoline engine. Thats why a Y6b is a great stump puller but would suck with intermodal speed trains.

Re: Steam Locos - Remember, too, that driver size varied widely. An engine intended for slow speed service had smaller drivers - 50" or so. A high speed racehorse was likely to have drivers as large as 80". Thus if you put 50" drivers on a Northern, you'd have a classy stump puller. If you (could) put 80" drivers on a Y6b (never mind the engineering considerations - just go with me on this), that baby would fly. A single revolution of a 50" driver will cover about 13 feet. A single revolution of an 80" driver would go over 21 feet. Of course, tractive effort, etc, vary inversely with driver size if all other factors are equal.

Don't quote me on driver sizes for specific locos. This post for consideration of driver size as a factor in speed and tractive effort only....[:D]