I will see if I can make a better scan of the drawing than what I posted earlier. That picture was from an on-line copy of the manual, but I actually have an original hard copy somewhere.

The 1949 PRR Fireman's Examination Book is an excelent introductory manual for the serious novice, BTW. It's explanations are easy to understand and copiuosly illustrated, and it's concise, yet pretty thorough.

Unfortunately for this conversation, the one thing the book doesn't explain too well is this smoke abatement device. One sentence and the drawing is pretty much it.

The reason I even remembered the device was because when I first saw it in the book, I figured it was some sort of supplementry draft device, but not being by any means an expert on the finer points of combustion, I have always been curious about how exactly it is supposed to work myself, and how it works in relation to the blower.

It's also interesting that in the locomotive cross-sectioning thread, the conversation drifted into the Buchanan firebox, which is described as having its own version of steam jets shooting into the firebox.

And of, course, the location of that drawing was another one of those arcane, useless facts I retrieved from the back of my head somehwere. Just don't ever ask me for useful information or what I had for breakfast earlier today ; ).

Just out of curiosity, back when I used to fire 3' gauge equipment, with crummy coal (it was free) we used to get "drumming" which was explained to me as combustion occurring in the flues. This usually happened after adding coal and in heavy smoke conditions. To cure it, we would just crack the fire door for a moment or two and the "drumming" would go away. It could occur siting still or while moving, depending how heavy the smoke was.

Could these tubes have been simply providing air to avoid such "drumming?" By aiding correct combustion?

Just a thought. I was always curious as to the issues of the drumming.
J.R. May

When I was firing the MIDH 91, we also would occasionally experience drumming. My experience was that it would usually occur after a somewhat abrupt change in how hard the engine was working, therefore, how much draft was going through the fire, and how bright the fire was as a result. For examplte, the railroad has a station stop almost immediately after a very steep grade. We would crack the blower to alleviate the drumming problem. Interestingly, in that situation, not enough blower or too much blower would result in drumming. We used the blower because our practice was to avoid opening the firedoor any more than absolutely necessary, in order to avoid admitting cool air into the firebox, and having the resulting temperature differential make for leaky tubes and staybolts. I have heard the CNR E-10a's tended toward leaky staybolts, and 91 in particular has had a history of that, even back in her CNR days.

As I understand it, and again, I'm not an expert on combustion or much of anything, really, drumming is actually little explosions caused by a fire that's too heavy for conditions and/or starved for air. There is a really good explanation (which unfortunately, I don't remember too much of right now) of the causes of drumming, why it's bad, and how to deal with it in Prior's book called Fuel Economy. IMHO, it's probably the best manual on locomotive firing out there. While I think anyone who has done it would agree that you have to actually fire to learn how to do it, the book gives you an easy-to-understand background of what's supposed to be going on in the boiler and firebox rather than just throwing coal in the firedoor and seeing what happens. Little River was doing reprints of them a while back, and that's where I got my copy, but unfortunately, I'm not sure they're in that line of busines anymore.

Getting back to the smoke abatement device, I always wondered why these kinds of things were applied in addition to the blower. I.E., what do they add to the combustion process that the additional draft that the blower creates doesn't do already, or why would one device be more advantageous than the other, or when would you use one and not the other?

Combustion is not a linear process. Temperature and volatiles build to a point, combustion occurs, and then there is a pause to restart.The movie "Backdraft" illustrated the point years ago. Fire in a closed room pushes smoke through a door crack, then the fire chokes, the air cools, and the contraction sucks new air into the room, which refreshes the combustion and leads to an explosion that blows open the door.

Twenty-three years ago on the Strasburg, I would experience drumming on the 90 during a station stop at the Grove if I had just finished applying fresh coal unevenly. The firedoor would rattle with the little explosions. Increasing the airflow disrupts these accumulating clouds of combustion.

On my miniature engine, if I stop hot with little or no blower, and pile on a lot of fine coal, I get little explosive puffs out of the firedoor that singe my hair.

You should read David Wardale's account of an incident on his first modified locomotive in South Africa, 19D 4-8-2 #2644. A dual Lempor exhaust was installed and the firebox was modified to Gas Producer Combustion System (GPCS) configuration. As part of the GPCS the firebox door was altered to have a relatively large air opening, in addition to the combustion air tubes installed on each side of the firebox. After the modifications, they found that the engine did not respond well to the "little and often" firing technique. It was better to apply a fairly heavy dose of coal and wait a while before shoveling in the next batch. Wardale said that one night during a signal stop, the fireman decided to go a little too far with this firing technique. With the engine on minimum blower, the fireman shoveled steadily for about 10 minutes, building up a heavy bed of coal on the fire. IIRC Wardale said the crew was sort of sitting around the cab wondering if that was a good idea, when they heard a slight "thump", followed by a loud "WHOOM!!!" A large jet of flame shot out of the firedoor air opening and all the combustion air holes on the sides of the firebox. Everybody leapt out of the cab but it was a wasted reaction as the firebed had already calmed back down. No damage done to the engine except for one of the turbulence-inducing nozzles having been blown out of its combustion air duct. The fireman didn't try this stunt again.

Mr. Harrod's explanation of the combustion induced drumming is probably one of the better I've heard. I can say without a doubt that it is not combustion in the flues, although I have heard this explanation repeated numerous times. The temperature inside the flues is far too cool to allow combustion, and actually, from my experience the drumming phenomenon is most likely to occur when an engine is being fired up from cold when, until the boiler is up to temperature, the cold firebox sheets tend to retard the combustion of volatile gasses.

To pass on some more interesting information relating to the G5s smoke abatement device, we've been lucky enough to dig up some more information from a closely guarded source. It appears that these devices were not only applied the G class, but also many others as well, notably switchers, but also other road class engines like the I1's. The installation was not a system wide standard, but was to be done at the request of local CMP's, most likely they were installed on engines that frequently operated in urban areas with smoke ordinances, there being no need to abate smoke that no one was complaining about.

The style of device in question was actually the second device installed on this and many other PRR locomotives. Originally, LIRR #39 had a more familiar style of overfire jet arrangement on the sides of the firebox. If you look closely at the sides of the box on this particular boiler you will notice small round flush patches that surround one staybolt. These were installed after the overfire tubes were removed. One reason for the changeover would probably be due to the relative simplicity of the 2nd version over the relatively installation and maintenance of the overfire jets.

The smoke abatement device was probably not intended to add any air to the fire to help consume all the volatile gasses (i.e. black smoke) but instead the jets of steam issued from them were most likely aimed just under the brick arch to create turbulence in the box which would in turn delay the gasses being drawn out of the box unconsumed. How well either of these devices worked, it's hard to say.

Is it possible there were also patent license fees for the overfire jets? That would explain the temptation to tinker and try a home grown replacement, even if it already worked.

There are extant PRR shop drawings of the overfire jets, and although I have not studied the drawings at length, I saw nothing to indicate that they were not a homegrown design as well.

Brendan Zeigler wrote:
...from my experience the drumming phenomenon is most likely to occur when an
engine is being fired up from cold when, until the boiler is up to temperature, the
cold firebox sheets tend to retard the combustion of volatile gasses.

Hi guys. When firing, I've noticed that even with a nicely drafting hot fire, when we'd cross a trestle the locomotive would drum, albeit briefly. I suspect, related to what Brendan says above, that the ambient air temperature is markedly cooler just above the water beneath us, and when drawn into the hot firebox, she'd thrum. Interesting thread. God bless. Seth