Sunday, June 18, 2006

Power of 1860's steam engines

Our automobile has a 300 horsepower engine. How does that compare with an 1860's steam powered locomotive?


Blogger CPRR Discussion Group said...


Well, it is sort of a trick question. A 1925 GE/ALCo/I-R diesel electric had a 300 hp engine, but I'll wager it would pull considerably more than a modern auto with a 300 hp engine, and for that matter considerably more than an 1860's loco. Among the factors are weight, efficency of power conversion, method of power transmission (including gearing), efficency of power transmission, adhesion, rolling resistance of the thing being pulled – that's just for starters. And of course (except very early in the age of steam locomotives), power was generally not expressed terms of in horsepower for steam locos. Certainly there are conversion equations, but in the end it is very much like comparing apples and, say, ears of corn. An auto and a loco have so many differences in form and use that it is difficult to compare them.

That said, I'm guessing that the typical 1860's Central Pacific steam loco could drag that modern 300 hp car backwards in a pulling contest – but that is just a guess. And similarly, the 1925 GE/ALCo/I-R 300 hp diesel electric could drag the 1860s steam loco backwards.


6/26/2006 6:35 AM  
Anonymous Anonymous said...

Efficiency of power conversion should not be a factor, as a power specification is for the output of an engine, and does not take into consideration the input fuel use.

6/26/2006 6:43 AM  
Blogger CPRR Discussion Group said...

If not horsepower, what physical units were generally used to express locomotive power in the 1860's? Kilowatts? Do any builder's records, etc., survive that quote power specifications for any of the CPRR steam engines, or similar?

6/26/2006 6:47 AM  
Blogger CPRR Discussion Group said...


In the 20th century steam locomotive power is generally expressed as tractive effort or tractive force (two terms for the same thing). This is a calculated value based on boiler pressure, cylinder stroke size and driver diameter, although there are some other permutations.

See the following web sites:



Tractive effort From Wikipedia

Tractive Force and Hauling Capacity

There are also other pages that you can Google.

In the 19th century the knowledge was more empirical, and the reference was often in terms of cylinder size (with general knowledge assumed as to what that meant in terms of pulling power). I've also less commonly seen locos referred to in terms of weight.


6/26/2006 11:17 AM  
Blogger CPRR Discussion Group said...

Surprised to learn that there apparently was such a disconnect between physics and empirical steam engine engineering in the 19th century. Had wrongly assumed that they were the same subject because the history of thermodynamics arose from steam engines and the work of Sadi Carnot in 1824. Perhaps, the two fields remained apart because even though safer, large ideal Stirling Cycle Engines could not be manufactured in the 19th century.

6/26/2006 11:37 AM  
Blogger CPRR Discussion Group said...


It's not to say master mechanics didn't know about such things in the 19th century, but it just wasn't the frame of reference. Keep in mind most of these guys came up through apprenticeship programs in railroad shops, not through technical schools. It was understood that a certain size locomotive could do a certain level of work. And not only were sizes of locos limited by construction issues, but also by infrastructure (track and bridge) issues. Weight was a big factor – and cylinder sizes and wheel arrangements roughly corresponded with weight. So that became a convenient short hand frame of reference.


6/26/2006 4:33 PM  
Blogger CPRR Discussion Group said...

The articles linked indicate that tractive effort is Force, not Power.

The Wikipedia article says that "Tractive effort is the figure most often quoted when people are comparing the power of different steam locomotives, but the use can be misleading, because tractive effort shows the ability to start a train, not the ability to do work by hauling it."

Power = (Force x Distance)/Time,

so specifying the force does not allow you to compare the power of a modern automobile with the power of an historic locomotive.

That article gives an example of an engine with huge tractive effort that can start moving a huge load, but which had an underpowered engine so it could only get up to a few miles per hour. You can tow any size load with a small engine if you are willing to go slow enough – you just need a low enough gear ratio (or small enough drive wheels).

This is just another version of Archimedes' famous statement: "Give me a lever long enough and a fulcrum on which to place it, and I shall move the world."

6/26/2006 4:43 PM  
Blogger CPRR Discussion Group said...


Electric motors develop maximum power at 0 RPM – thus a diesel-electric's maximum power is at starting and declines from there. A steam loco develops maximum power when already in motion (maybe 5 mph or so – not sure). Thus starter engines used in starting the train and then cut out when started were sometimes designed into the trailing truck or the lead tender truck on steam locos to get the train started.

And as I noted earlier, 300 hp is not the prime factor either. That 300 hp auto would be no match for a steam locomotive developing considerably less than 300 hp. So the original question is based on false assumptions that the horsepower rating is the significant factor – which it is not.

In all cases, some measured or calculated feature is used as a general reference for comparing similar vehicles. Using horsepower on a car is no more accurate than using cylinder size or tractive force on a steam loco. (There are 100 hp auto that will blow the socks off that 300 hp behemith in a race.)


6/26/2006 5:06 PM  
Blogger CPRR Discussion Group said...

Also see:

Discussion of tractive effort versus power at The "Largest" Steam Locomotives webpage:

" ... there are several ways to measure horsepower (drawbar horsepower, indicated or cylinder horsepower, and calculated horsepower). Also, drawbar horsepower is measured with a dynamometer car where the firing rate and track gradient may effect the results. ... the horsepower rating of steam locomotives just wasn't as important as their tractive effort rating. The reason for this is because for most steam locomotives, the limiting factor was how much tonnage a locomotive could move from a standing start (tractive effort). In general, if a steam locomotive can get a train rolling, it could pull it at an efficient speed. ... Railroads preferred ... Tractive effort ... to HP ratings because HP involved a time quantity which was determined, in part, by how well the locomotive was being fired (among many other variables). Tractive effort, on the other hand, was determined strictly by the geometry of the locomotive. ... "

Specifications of Famous Steam Engines.

Horsepower defined.

6/26/2006 6:38 PM  
Anonymous Anonymous said...

Nobody can answer the question, apparently because the force needed to get a train moving from a stand still is typically the limiting requirement for a steam locomotive, so they don't measure or care about the actual power. Tractive effort is cited as the being the "power" of the steam engine, but tractive effort is a force, not power that actually is instead (force x distance)/time. But one of the sources cited above gives an example of an unsuccessful locomotive that had one of the highest tractive effort capabilities ever, but because the engine was underpowered (due to a small boiler) could never operate at more than a few miles per hour.

7/05/2006 4:08 PM  

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