Driving cross country at the moment (wife currently driving
) so I'll try to be brief (difficult for the engineer in me
Getting propane burners setup correctly can take a lot of fiddling or trial and error so patience is required.
I am returning from Train Mountain where my 4-8-4 Northern fired very well on propane (safeties going while climbing the serpentine). Her firebox is about 12 x 18 and I burn about 9 pounds of propane per hour of Train Mountain type running.
Your problem is very typical of propane-fired locomotives caused by the draft path being fine for normal firing but allows too much air when working up hill. The excess air (over and above that required for complete combustion) adds nothing to the energy produced and the extra (low temperature) air just serves to cool the boiler - exactly what you don't want.
I had the same problem but did not want to interfere with the great "stack talk" by altering the blast nozzle.
First thing to do is arrange a plate at the bottom of the burner to close off the air flow as much as possible while still allowing you to sustain full fire with the blower on. This can be done with a plate below the burner or a plate with holes in it placed over a "bunsen burner" type of burner. You will be surprised how little air area is needed - remember: extra airflow is just cooling the boiler.
Next you have to do one of two things: a) a variable damper in the smoke-box to "dump" excess vacuum or b) a variable damper under the burner to cut down draft when working hard.
"b" is difficult because there is no easy way to arrange things so that the damper is automatic - closed for less air while working hard and open for more air when needed.
On my engine I used a water manometer to measure the smoke-box vacuum required to support "full fire" (max usable gas pressure). In all modes of operation more vacuum than this is wasting energy. Mine was 2" water.
My locomotive has two stacks - the rear was a dummy. I designed a "poppet valve" having a large diameter aluminum seat and an aluminum (light) poppet valve held closed by two ball-point pen springs on a 1/8" stem. I adjusted it so that it opened at 2" water vacuum. It works well, as I climb a hill I can see the valve opening in time with the exhaust blasts.
For people without the extra stack I recommend that they machine a large (depends on locomotive but in general as large as practical) rectangular hole in the bottom of the smoke-box, install a rectangular skirt that extends up into the smoke-box to form a flat surface and fit a damper door on top held closed by a light spring adjusted to whatever vacuum you need for full fire.
Some additional tips I find useful:
- An arch is absolutely required for propane firing. Propane is a slow burning fuel. It is still "burning" after you can't see the flame. This burning is "quenched" as soon as the gasses enter the tubes because its temperature falls below that required to support combustion. The longer the gas path before it passes into the tubes the better. If at any time you smell an 'acid' smell from a propane locomotive you are smelling un-burned hydrocarbons (i.e.: wasted fuel).
My arch starts just below the lower row of tubes, extends back about 1.5" and then bends up to within about 1" of the crown sheet at a steep angle (maybe 30 degrees from vertical) and then extends to the rear to within about 3-4 inches of the rear of the firebox. The arch should fit as close as possible to the firebox sides to prevent gasses from getting around the sides of the arch. My choice of arch shape was designed to expose as much of the firebox as possible to the radiant heat from the burner while providing the longest gas path.
Flames on firebox
- you should not allow the flames to directly impinge on any firebox surface - again, the flame is quenched as soon as it contacts a metal surface below the combustion temperature. This is sometimes difficult to accomplish while trying to get as much flame as possible in the firebox. On my "furnace type" burner (9 tubes fed by one orifice & mixing tube) I relocated the side row of holes to the top of the two side burner tubes to prevent the flames from hitting the firebox sides.
Another approach that I have not yet tried is to arrange a stainless steel strip between the flame and the firebox sides. The idea here is that the SS strip will quickly rise to combustion temp (red hot). It will then not quench the flame because it is red hot and it will radiate the heat energy to the firebox sides.
Flames too high in the firebox
- I see this a lot with the "bunsen burner" type burners like you have. The bottom of the flame should be level with the bottom of the water in the water leg (not the leg itself). Too much higher and you are not exposing all the water to direct heat radiation.
I find that, to support full fire, I do not need the blower if I can hear any stack talk at all so I will sometimes shut off the blower if I am on a long uphill climb. If one does this they must be absolutely sure to remember to turn the blower back on (or burner off) if the throttle is closed at all (stopping, derailment, etc.) or you'll have to repaint the cab sides after the burner flames strip the paint!
Someday I hope to design an automatic blower control that is controlled by smoke-box vacuum.
(I have not tried any of these)
You can add twisted metal strips in the tubes ("turbulators"). This has two effects. They promote heat transfer to the tubes (cooler smoke-box temps) and they add air resistance to the gas path to slow down the gasses and increase smoke-box vacuum (increasing the effectiveness of a smoke-box damper).
Adding a stainless steel grid above the burner - this will glow red hot and increase the radiant energy available to the firebox.
Hope this is of some help ...