Here's the short answer, if you see smoke, the fireman's doing it wrong*. Smoke is unburnt carbon, basically unused fuel. With good coal, proper firing should produce no more
than a slight haze at all times
. With bad coal, well, you’re SOL. What’s probably confusing a lot of people is photos and movies almost always show big billowing clouds of black smoke. We will overfire the locomotive ourselves for photo run-bys. That’s 99% hype and unfortunately has become thought of as what a steam
locomotive should look like.
does exit via the stack. After leaving the cylinders it has one last function to perform. Entering the bottom of the smokebox, it passes through a nozzle located just below the stack entrance. Due to a venturi effect, the blast of steam
exiting the smokebox via the stack and creates a vacuum which then pulls the heated air and gasses from the firebox through the boiler tubes (thus imparting the last bit of energy into the water in the boiler, most of the heating comes from the water around the firebox) and up with the steam
out of the stack. To replace this pulled-out air, fresh air (i.e. oxygen) is pulled in from the bottom of the firebox, though the grates and thus fuels the fire. So the more steam
going into the cylinders, and out the stack, pulls more
air out which forces more
air in which causes the fire to burn more
. So in that sense a locomotive is self regulating on airflow (it’s called a forced-draft system). Most of the time you won’t see the steam
, but if the conditions are right, say a foggy morning, you get huge, clouds of white steam
Percentage of steam
used versus speed is a bit trickier. Like an automobile, a steam
engine uses two controls for speed, reverser and throttle. The throttle controls how much steam
is being allowed to go to the cylinders, obvious. The reverser is like a car’s gearshift. It controls the direction the engine will go, but it also controls how much steam
can enter the cylinders. When it’s “in the corner” or at maximum, the steam
can enter the driving cylinder (the one of the bottom) for almost the entire piston travel, as it is ‘hooked up’ the steam
chest (usually a smaller cylinder on top of the driving cylinder or a box-looking bulge on older locomotives) allows less and less steam
into the cylinder. To get the train started, you put the reverser to maximum and open the throttle a bit. The maximum amount of steam
enters the cylinders and can therefore do the most work. You need less energy to keep something moving than you do to get it moving, so as you speed up, you started allowing less steam
into the cylinders. The throttle and reverser work in harmony, although good engineers can open the throttle wide and start and stop the train using the reverser alone.
*There are exceptions as always. We had some bad coal last year that was horrible to work with, clinkers constantly forming and killing our grate surface, and half a scoop would produce chokingly thick smoke. When tested, the lab results (chemical analysis of the coal) were horrid, looked like we had gotten the edge of the coal seam and there were rocks mixed in. This year the coal we have, new supplier, is great, you have to work to get any smoke to show.
So… I guess in summary, if the Locomotion fireman was good and had good coal, the steam
engine would almost always look like it does at full speed. Bad coal and/or bad fireman, and you get smoke all the time.
Having said all that, I like the idea having constant smoke for Locomotion.
So, how many eyes have glazed over now? Heck, how many read
this entire post?