When steam engines aren't working very hard, their smoke is relatively low and sometimes barely visible. However when a steam engine is working really hard, black smoke may be billowing out of the smoke stack. Why is this? I thought that a harder working engine would create a much stronger draft on the firebox which should mean more oxygen to the fire. Isn't the presence of smoke a sign of not enough oxygen? It would seem as if the engine should smoke less. What am I missing?
I don't think the amount of oxygen is directly connected to the amount of smoke, although the draft can affect the color in that when the engine is working hard, it's creating enough draft that unburned particles of coal are being sucked up thru the stack, making the smoke black. Otherwise, when working hard an engine is burning more fuel faster than it otherwise would, generating more smoke. If you rev the engine of your car it's going to create more exhaust than it does when it's idling.
Diesel engines smoke more when they aren't getting enough oxygen to burn everything off completely. Leaning them out decreases smoke and richening things up increases it.
Since oil burning steamers can smoke badly too, is that just an oily wet cloud in the air? It does make sense that you'd pull sediment out with an increased draft but if this is true then wouldn't something cleaner burning such as alcohol (if it could be used) leave less smoke?
Smoke is definitely a sign of inefficient combustion. Most of the time, in steam locomotives, its caused by feeding fuel into the fire faster than it can be efficiently burned. This can happen even if the engine is working hard and causing a large draft in the fire, if the excess fuel is greater than the increase in oxygen supply.
Does the fireman increase fuel flow to the firebox when the engineer opens the throttle and if so does he just arbitrarily go to full fuel flow if the throttle is set at max? If smoke is a sign of incomplete combustion then why wouldn't the fireman want to decrease fuel flow to decrease smoke output? If this were done would it mean that the fire isn't hot enough to meet with the steam demands? I'm assuming that the fire can keep getting hotter (to a point) after the smoke level is quite excessive. Sorry for the questions. I understand all manners of internal combustion engines but am still learning the technicalities of a steam engine.
My impression taken over a few years of reading about steam engines is that a black tower of smoke above a steamer is either negligence, ignorance, or purposeful. If the fireman isn't aware, or is simply being negligent, a steamer will get too much fuel and it will smoke. A fireman who is new to the task, and not familiar with the engine, say, may cause more smoke than he or observers would like. It is often purposeful when done to impress or please fans trackside during roll-bys. In other words, it was often staged for effect in images you may have seen.
I have seen video and still imagery of steamers working quite hard, and except for steam, their emissions were quite clean...thanks very much. A fireman who is attentive and skilled will not allow more fuel to be consumed than is necessary to perform any amount of work over time. The hogger was meant to keep an eye as well...the emissions from the stack were always a good indicator to the hogger of how good a job his fireman was doing.
Usually the crew knows the route...so they anticipate when they'll need less or more steam as a change of grade is pending. If more steam is going to be needed, the fireman knows to begin to stoke the fire (if it's a coal burner) to ensure sufficient depth and even coverage. This is because if the hogger suddenly draws back on the throttle, the increased stack emission of steam with each venting of the cylinders will draw huge volumes of air through the coal bed. Thin spots will burn through quickly and cool, thus putting stresses on the flue pipe welds of the flues that get the suddently cooler air.
Once again, this is not meant to be a definitive answer, just a synthesis of my understanding to this point. Surely someone with footplate time will be able to give us a little primer?
-Crandell
There may be two questions here: 1) Why do engines sometimes produce a lot of smoke?? and 2) Why do engines sometimes produce black smoke??
It seems normal that an engine when starting produces a lot of smoke and steam - I took some video of UP 3985 starting up on it's way back south from St.Paul MN. From across the river, you got a great view of the huge billows of smoke/steam coming up as the engine started moving and then began getting up to speed. But the smoke was more white/gray than black.
My impression from reading about steam engines and seeing them in action is that at times a steam engine is going to have to produce a lot of smoke, but that black smoke is an indication that the coal wasn't being fully burned before being sucked up the stack, which may be a situation controllable by the fireman...but then, a steam engine can produce a pretty powerful draft, supposedly powerful enough to have sucked the fireman's shovel into the fire (according to railroad mythology).
Actually I meant it from the standpoint of why so much smoke? :D To elaborate on my question, I am not asking why the engine smokes at all. I understand that. I also don't consider steam exiting the smoke stack to be "smoke". I am asking why there is so much black smoke when steam engines are working hard as to my understanding more oxygen from the added draft should decrease the smoke level.
The apparently isn't true however and it has been stated that the increased draft pulls soot out of the firebox and expells it. However if this is true, what happens in a clean firebox on an oil fired locomotive? I guarantee it still smokes heavily when working hard so soot can't be all of it although it can be some.
My technical based mind is kicking in here but it my current hunch says that although the added draft does increase oxygen flow to the fire which should decrease smoke levels, that it isn't because air velocity is too high through the firebox and that although more oxygen is being admitted, the added draft actually has a downside. What this would mean is that although there is ample oxygen present, at these power levels so much fuel needs to be added to sustain the fire that some of it doesn't have time to burn and just gets completely drafted out of the engine. In car engines, unburned fuel shows up in the form of high levels of hydrocarbons. This would explain soot in coal fired engines but also the oily cloud that oil fired steamers could send up.
If this is in fact what's happening, I would think that you'd want to have an increased oxygen flow but at the same time not have the draft velocity rate of flow through the firebox. This seems counterproductive but smoothing out the chuffing to a more constant flow could probably accomplish this. I would think that by utilizing the exhausting steam from the cylinders so that their velocity worked with the next chuff, that it could smooth out a little bit. Combine this with a slightly more restrictive smoke stack so airflow can't be as fast through the firebox and flues. Of couse you'd want to set this up so that the velocity of the cylinders can better provide a constant pull and draft. The key to the more restrictive smokestack being that if exiting steam were directed through it properly, it's velocity would really lower firebox pressure allowing more air to flow in. But if these chuffs or pulses were smoothed out, average velocity of exiting gasses could stay the same even though the extremes change. The draft would never get as slow or as fast as current. The max exhaust area needs to be based on the max velocity of whatever is exiting through it. If you kept the average the same but decreased the extremes, you wouldn't need as large of a smokestack. Theoretically.
Now you'd have a firebox that can be drafted with slower airflow speeds that don't pulse as extremely. Balancing this out though you'd want to change the way air gets into the firebox by potentially enlargening or moving the intake locations, wherever they may be. If you balanced all of this properly you should be able to keep a high amount of oxygen flowing into the firebox but with a total of less velocity. This should give the fuel more time to burn which means two things. One is that more of it burns off which is cleaner emissions and less black smoke when working hard, and two is that since you aren't wasting time blowing raw fuel out the smoke stack, you should be able to use less fuel which means a more efficient engine.
Does any of this make any sense to anyone? Sorry if I'm getting too technical. This comes from the mind of a mechanical engineer based nothing more on my knowledge of how internal combustion engines work which admittedly aren't external combustion.
Fredswain: I believe that it has to do with design. The N&W J's did not smoke even when under heavy load and many pictures of 611 in excursion service showed the fireman had to work to give the smoke for pictures.
A drifting steam engine (moving but not loaded) tends to build up soot in the flues and ash in the fire. Opening the blower or the throttle will dislodge both the soot and the fly ash resulting in blacker smoke the normal. Now add the dilution effect of exhaust steam from the cylinders, and it appears that there is more smoke than there really is - what you are seeing black tinted steam.
The engines I fire sit for 1 to 2 hours between scheduled operations and I try to keep as light a fire as possible but still keep the steam pressure in the low operating range. Thus, there is quite a bit of black smoke for the first 100 feet of the run and then I can get the stack to clear up.
One other factor, adding new fuel to a hot fire also generates some soot as the dust and low temperature burning volitiles burn off first. These tend to be sooty. When the carbon content starts to burn - the stack clears up as carbon dioxide and water are both colorless.
dd
ps - We have also been known to add oily rags to the fire to generate smoke for the official photographer.
In an oil-burning locomotive, a great deal of soot will accumulate in the flues, and it could stop the flues up if it is not removed. How to remove it? Introduce sand into the firebox, and the draft will carry the sand through the flues, and the sand will scour the soot out--and black smoke will be seen.
Johnny
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