The gas burner on the modelTrevithick dredger engine has been more problematic than anticipated. In the absence of published information about gas jet sizes, air hole sizes and numbers, the effect of a ceramic cover, and firebox size and shape, I have resorted to trial and error. Having read many of the comments on the subject in Model Engineering web sites, I can see that most other builders adopt the same approach.
I have lost count of the number of changes which I have tried. These are the ones which I can remember.
- Burner surround made from brass, as per the blog on Feb 5,6 8. Ceramic top. Burner difficult to light, and only the end distal from the jet would light.
- Fitted a tube with about 40 2mm holes beneath the ceramic plate, to better distribute the gas-air mixture. The entire ceramic plate lit up, but the heat output was poor, and the the time to boiler steaming was very slow at 25″. Also the flame contained lots of yellow, indicating inadequate air.
- Drilled holes in the end of the burner surround, to increase the air flow. Some improvement.
- Tried 3-4 different gas jets. Problems with obtaining jets. Lack of published info. Different threads and jet body sizes. Used the CNC lathe to recut some threads to fit the Primus jet enclosure. (0.5mm pitch, 4.5mm diameter). This was all trial and error. Air is admitted through the 4 holes next to the jet, using a venturi effect, which relies on the velocity of the propane coming out of the jet. Important factors are the diameter of the air holes, the number of the air holes, the velocity of the propane, and probably the diameter of the propane cone of gas.
- In order to further increase the supply of air, I drilled the air holes progressively from 4mm to 6.5mm. 6.5mm was the limit due to available metal. In the video which follows, you will see the effect of reducing and increasing the number of air holes.
- Drilled an extra 20 holes in the distributor tube, but the flames from the initial 2mm holes were too big. Then made another distributor tube, with 60 holes, about 1.5mm diameter. Better size flames. See the video.
- Each variation has been tried with and without the ceramic plate. I have decided to not use the ceramic. It seems to restrict the flow of gas-air mixture, and causes flames to shoot out backwards through the burner surround air holes, sometimes setting the jet alight.
Concurrently while experimenting with the burner, I have been applying some paint. SWMBO insists that I am a hopeless painter, but I think that it is going on OK.