Watching a few of the videos on another thread I realized that I don't quite understand what makes the chugging sound and the synchronized belch of smoke from the stack. Is that the exhaust stroke from the huge pistons and is the steam then mixed with the smoke from the firebox? Are there pistons on either side and do they rotate at the same time and speed or right then left then right etc etc. Seems like that would create quite a vibration and torque. Is there a website someone can recommends, with a good explanation for a less than mechanically invlined person?
Not a poem
Ira
Posted by ehbowen (Member # 4317) on :
Ira,
Ah, but a big steam locomotive is already poetry in motion...
Now for some technicalities:
A steam locomotive is, sorry to say, probably the most inefficient mechanism ever used on a large scale during the 20th century. Steam boats and ships are able to condense and reuse their steam by cooling it with lake, river, or sea water, and by pumping out the air which leaks into a condenser they do so in a vacuum, which increases the power that they can develop from it tremendously. A steam locomotive does not have that reservoir of cool water to reject heat to...some European designs and the stillborn ACE 3000 had an air-cooled condenser, but air is not as efficient a cooling mechanism as water and seldom produces enough of a vacuum to make the extra complexity worthwhile. Therefore, most steam locomotives simply exhaust their steam to the atmosphere (and therefore have to make water stops every 50-75 miles to refill their tenders).
However, you can use that steam for a bit of work on its way up and out. Early locomotive builders found that directing that steam up the stack induced a much stronger draft than just the heat of the fire alone, pulling extra air through the firebox and allowing the engine to develop that much more steam and power. And in the final "Super Power" years the engineers realized that there was a good deal of heat in that steam which could be recovered to preheat the feedwater and increase efficiency...not enough to overcome that lack of a condenser, but enough to stretch an extra 10-15% from each ton of coal.
The pistons are on each side of the engine, yes, and the driving axles are "quartered"...i.e., the left wheels are offset 90 degrees from the right. So if one side of the loco is at top or bottom dead center, where there is the minimum amount of leverage for starting, the other side is in the exact middle of its stroke developing maximum tractive effort.
You are quite correct when you note that this mechanism creates quite a vibration and torque. If you take a close look at steam locomotive driving wheels you will see large counterweights cast into them in an attempt to balance the movement of the siderods and main rods. The problem is that if you balance out the rotating masses exactly then there is nothing to counter the reciprocating mass of the pistons and main rods, but if you increase the weight of the counterweights to try to balance the pistons and rods then you overbalance the rotating mass, which means your whole assembly is out of balance. The best you can do is to try to find a happy medium between the hammer blow of overbalanced wheels and the side-to-side nosing caused by pistons and main rods. I'm certain that George Harris could spend hours enlightening us as to the demands that steam engine action put onto those who designed the bridges and roadbeds that big steamers operated over at speed.
Posted by George Harris (Member # 2077) on :
Well, I won't spend hours. I would say do a search as see what you get. Warning: Not everybody that talks about this stuff knows what they are talking about.
One of the main reasons that we have railroad bridges in this country that are a century plus old carrying loads far beyond their designers worst nightmares is their design for "hammer blow" and "nosing" from steam engines that do not exist with diesels. Another is the use of relatively large safety factors. A third is careful maintenance, meticulous inspection, and replacement and strengthening of components as needed. Do not confuse lack of attention to cosmetic issues with lack of attention to structural integrity.
In the highway world, politicians don't mind neglecting existing bridges as they can have replacements built as monuments to their egos. The replacement of the east part of the San Francisco Bay Bridge is an outstanding example. They may have kept the paint job going, but they apparently did not deal with corrosion that was out of the public eye.
In the railroad world they call the bridge people that do not keep ahead of the game unemployed.
In bridge design there are three parts: 1. Dead Load, the weight of the structure itself 2. Live Load, the design load of the trains 3. Impact, a multiplier on the live load to allow for equipment bounce and sway. Ice loads, wind loads, and such are usually considered part of the live load, but do not have the impact factor added.
For steam, hammer blow was an additional multiplier both up and down added to the load of the driving axles. Also, with steam there was a side to side force added to the weight of the drivers called nosing. Remember, for every action there is an equal and opposite reaction. Thus, when the cylinder pushes the drive rod, and cylinders push in both directions, the reaction tries to push the cylinder in the opposite direction. Since it can't really go forward and backward, the force ends up showing the front of the engine from side to side.
The end of these forces and what they did to track and bridges was also a significant factor in the cost savings due to dieselization.
Any additions or corrections from a real railroad bridge engineer would be appreciated.
Posted by irishchieftain (Member # 1473) on :
quote:A steam locomotive is, sorry to say, probably the most inefficient mechanism ever used on a large scale during the 20th century
That's only compared to other land-based heat engines, and frankly, gasoline engines aren't much better when the average is taken, nor are diesels attached to truck axles; the major comparison is usually diesel-electric locomotive to steam locomotive, and that's purely on the formula Q(in) = W(out) + Q(out). There are other efficiency factors, one of the least not being cost of fuel and type of fuel; you also have other inefficiencies related to engine construction costs, since most (but not all) steamers were custom-built rather than mass-produced, usually to fit the profile of the railroad that ordered them; and typically in the USA, the demand was for locomotives that were not maintenance-intensive, so other improvements beyond superheating and feedwater-heating such as four-cylinder compounding with inside cylinders (a feature that not only improved heat efficiency but also gave the best driving wheel balance and minimized piston hammerblow) was considered an extravagance.
Lest anyone forgets, steam locomotives were one of the things that drove industrial revolutions and were a primary cause of large change in social systems, a lot for the better; they also are a feature of most of the history of railroads thus far, being their primary motive power for over 1½ centuries. And Amtrak is not fond of them, per anecdote.