While I’m a diesel modeler and dieselhead in general (my motto is ''If It Don’t Smoke Black, Take It Back) ,I always wondered what kind of horsepower any given steamer has, say a 2-8-2 Mikado for instance. Or better yet how about a 2-6-6-4 or a 2-8-8-2 like the big ones on the N&W ? Or a Big Boy ?
How is hp determined? boiler size/pressure?
Google can be your best friend.
Rich
Back in the steam era, locomotives were usually defined by Tractive Effort in thousands of pounds, rather than ‘horsepower’ (which is actually a term used to define early automobiles and carried over to diesel locomotives). It really wasn’t until the transition from steam to diesel that the term ‘horsepower’ was used to define a steam locomotive. So, instead of saying a locomotive like the Big Boy had x amount of ‘horsepower’, the railroad would usually define it as having 140,000 lbs of tractive effort starting a train.
For instance, the Denver and Rio Grande classified their locomotives both by letter type and tractive power. M for Mountain (or Northern), F for a 2-10-2 (the Rio Grande sure wasn’t going to call it a Santa Fe type, LOL!), C for Consolidation, P for Pacific, L for articulated, etc. For instance, a 2-8-8-2 L-131 had 131,000 pounds of tractive effort, a C-48 2-8-0 had 48,000 pounds, and so on.
The term ‘horsepower’ per se was very late coming to steamers, and not used very much at all.
Tom
While Tom is generally correct, there was some discussion of horsepower relative to steam engines. The problem is that the horsepower output of any given steam engine varies relative to the speed the engine is going, unlike a diesel which has all its horsepower available at any time. Starting a train the steamer is at the bottom of its “power curve”. As it gains speed power output increases until the engine reaches its optimum speed in relationship to horsepower output, usually about thirty MPH for a freight engine and fifty MPH for a passenger locomotive. After that speed may be increased but horsepower output will actually decline as the engine is now “ahead of the power curve”. The trick in designing a steamer was to make it so it’s normal operating speed given its intended use was just about at the peak of its power curve, a very difficult thing to accomplish. The Pennsylvania Railroad did take horsepower seriously and tried to measure it precisely at its locomotive test plant at Altoona. On its last steam design, the Q2 freight duplexes, they attained a reading of approximately 8,000 HP which was just about the maximum possible for a steamer.
Horsepower is a rather hard number to quantify. Torque is what does the work of moving an object. Diesel engines, with their large amounts of compression and long piston stroke are very high torque engines at low RPM’s compared to gasoline engines. Horsepower is, in it’s basic form, the speed at which torque is applied. I don’t know how torque converts to tractive effort so I can not comment on how it would compare to a modern diesel engine. What intriegues me is what is the torque rating on a modern diesel engine both the engine itself and the traction motors.
Don’t know if I helped here or muddied the water
Steam locomotive horsepower wasn’t the consideration that diesel horsepower is, as shown by this transition-era truism:
On level track, a steam locomotive can pull more train than it can start. A diesel can start more train than it can pull.
The reason lies in the fact that the diesel’s electric drive allows almost all of the prime mover’s power to be applied to producing forward motion, even at very low speed (yes I know about short-time and one-hour motor ratings. Let’s not go there.) OTOH, a steam loco’s power output (HP) is quite low at low speed, rising to a peak at the low end of its normal running speed, then tapering off as the boiler loses its ability to supply enough steam to the faster-moving pistons.
Speaking only of the N&W big three, the A could deliver 5300-5500 HP at a speed of 55MPH, the Y-6 had a similar power output at 18-22mph, and the J allegedly achieved 6000 HP at its maximum (legal) road speed of 90mph. The NYC Niagara was also designed to provide 6000hp at maximum road speed.
The C&O/Lima 2-6-6-6 was allegedly designed to produce 7000 HP. In drag coal service it never achieved anything like that - the speed was too far down the front side of its power curve, and nobody was interested in rolling hoppers at 65-70mph.
Chuck (Modeling Central Japan in September, 1964)
p62:
Well, yes and no. Horsepower is a unit of power. Power is force multiplied by speed, or torque multiplied by speed. Suppose you have a switch engine which is pulling a train with 10,000 pounds of drawbar force at 10 MPH. A road engine picks up the same train, and pulls it at 40 MPH with an equal drawbar force of 10,000 pounds. While the force is the same, the speed is higher. More work is done in the same given time interval, so the road engine must develop four times the horsepower.
Simply put, a steam locomotive is a constant torque machine and a diesel locomotive is a constant power machine. However, a steam locomotive’s power is limited by its boiler’s steam-generating capacity, and a diesel’s tractive force is limited by the current capacity of its traction motors. There are other limiting factors for both, but this is basically why tractive effort is everything for steam, and horsepower is everything for a diesel. It’s also why you hear the rule of thumb that a steam loco can pull anything it can start (starting is the hard part). It’s probably why diesel switchers caught on so early.
Both are limited in tractive force by their weight on drive wheels.
Horsepower has been calculated or measured for many steam engines, of course. You can find some figures here:
http://www.steamlocomotive.com/misc/largest.shtml
The PRR Q2 could develop just shy of 8000 horsepower. What a monster!
The interesting thing is that the modern ultra-high-horsepower diesels are emulating the steam loco, in a way. They can’t put their full power to the track at low speeds without slipping. It’s used at high speeds. Of course this is all taken care of with control syst