On a hill like Cumbres, the throttle is wide open all the way up. Speed/power is adjusted with the reverse lever (Johnson Bar). Where that is makes a big difference in how the fireman's day goes. The further down you run, the more water you use, the more water you inject in the boiler, the more fuel you have to use to heat that water into steam. If you get to where you have to force-fire the engine to keep up, you make more smoke.
Smoke is incomplete combustion. On a oil burner, it's too much oil for the amount of air being drawn through the fire. Without enough to burn the fuel properly, the fire is cooler than with proper combustion. So the more smoke you make with an oil burner, the worse it steams. Adding to that is all the oil soot you coat the firebox with, which acts as an insulator, which makes it steam worse, unless you sand out all the time. Ugh....bad deal
Coal is a bit different as even with sufficient draft, coal can smoke. Coal is made up of compounds that burn at different temperatures. When you add fresh coal, the coal added has to heat up to the point where all these compounds are burned completely, until that happens, you get smoke. The more coal you put in at once, the more "cold" coal you have in the fire to heat up, so the more smoke you make. As the smoke is wasted fuel going up the stack, there is less heat created by the fire, and pressure lags. Once the new coal heats up and burns clean, more heat is created, and the steam pressure climbs. The trick is to put in just enough coal at a time not to knock the pressure back too far. The other trick is not to put so much in that when it does burn clean and hot, the safety valves don't lift.
A fairly typical way to fire is about 5 scoops at a time. This would lower the pressure 2-3 pounds, when it burned clean, it climbs back up again. If you are energetic, you could do 2-3 at a time and the steam pressure would pretty much stay put. Each scoop of coal would send a gray plume up the stack for about 5 seconds. You had to pay attention and spend much of the trip on the deck. If you waited 5 seconds too long to put in more coal, you'd lose ground and have to catch up.
I could only do the smokeless firing bit with the engine working hard. Shuffling up the east side did not get the fire burning hot enough to make it work right. I ended up making smoke all the time and standing up all the way 50 miles got real old, real fast.
You can put too much coal in an engine. If you don't let the smoke clear and the fire burn hot, adding fresh coal simply deadens the fire and eventually you get so much coal on the grates, you get the fire choked and the air does not get through the grates. Then you smoke all the time, without making the heat you need. The worst thing you can do if you get behind is to bail in a bunch coal. The stack goes black, the steam goes down, so you put in more coal. More black smoke, even less heat, steam goes down more. More than once a told a new fireman "sit down and don't touch that scoop". He would fidgit on the seat while the black smoke poured out, then slowly turned gray and the steam would begin to creep up.
