2-8-8-2 - Tractive Effort Verses Speed Calculations - Finding The Proper Balance

I just read wikipedia.org's article on tractive effort.

If I understand the physics correctly, tractive effort is for starting like low gears in a transmission. There seems to be a trade off between acceleration and top speed. I guess the grades and loads dictated whether tractive effort, or top speed was most important. I experience the same thing in model trains.

Some go fast but others will pull more.

 

From a previous discussion.

http://www.frisco.org/vb/showthread.php?t=3355&highlight=horsepower

 

Tractive effort is the pull that a locomotive exerts at the coupler, usually expressed in pounds.

Starting tractive effort will refer to the pulling force from a standstill, or tractive effort may be stated at a certain speed. As in locomotive X has Y pounds of tractive effort at Z mph.

Tractive effort is influenced by a variety of factors, including but not limited to weight on the drive wheels, drive wheel diameter, gearing & prime mover horsepower for a diesel, or boiler pressure and cylinder dimensions for a steamer.

Since tractive effort is the amount of pull and MPH is the amount of speed, horsepower can be calculated from the two using the formula HP = TE x MPH / 375.

This does not mean that you can take the maximum speed of a locomotive, calculate it by the maximum tractive effort and get a real horsepower number. Instead the tractive effort used has to be developed at the MPH used to get the actual drawbar horsepower.

Tractive effort, maximum speed and acceleration are all different measurements, but they can be affected by some of the same variables. In general, you are correct. If you try to increase the tractive effort up to the adhesion limit of the drivers by "lowering" the gear ratio on a diesel, the top speed will be reduced.

But the locomotive will accelerate faster with a given load.

James

 

It seems like a 190,000 tractive effort would crawl and a "rocket" would not be able to pull even just an empty tender.

How did Baldwin, Alco, and others get tractive effort and acceptable speed?

Just by increasing the boiler which in turn increased the amount of steam?

It seems like you have to find a good give and take between tractive effort and miles per hour speed.

 

Well, the old hot rodder's adage is "There's no replacement for displacement." TE and speed are not always mutually exclusive, it just takes a large locomotive to do it. UP Big Boys had a TE of 135,375 lbs and a top speed of 80 MPH. A GE AC6000CW has a TE of 166,000 lbs @ 11.6 mph (continuous) with a top speed of 75 MPH.

But you are correct to a point, engineering is the art of finding the best compromise of a variety of conflicting specifications to design something to perform the specified task. If you try to optimize any one design feature, other features tend to be minimized out of necessity. If we are talking about steam loco TE vs top speed, you could certainly pick locomotives from about any era and see some that were designed to pull heavy trains at low speeds and some that were designed to pull lighter trains at high speeds. For instance, Frisco's 2-8-8-2's and PRR's first E-6 Atlantic were built the same year, but for two very different uses.

However, by the end of steam, many advances had been made in the design of steam locos that made it possible to combine a relatively high TE with a reasonable top speed. Even the Frisco 4500's had a TE of 71,200 compared to the 2-8-8-2's TE of 83,300. That is just 15% less TE for the Northern, a loco capable of much higher speeds than the old mallets. Differences in design did include greater steaming capability and high pressure, but there were also advances in balancing, piston valves & ports and front end design (nozzle, petticoat pipe, stack, etc). I really can't point to one thing and say, "That's what made the difference", it is the result of a combination of many changes.

It is amazing that the public perceives steam locomotives to be crude devices, but a last generation steamer was a highly developed, finely tuned machine - the product of decades of R&D. In some circles, the work continues on steam loco development, check out Martyn Bane's site for modern day steam: http://www.martynbane.co.uk/modernsteam/modernsteamlocos.htm

Whew! I realize that was kind of a long explanation, but I hope it helps! - James