Hot Tubes Igniters Low Coils Summary Disclaimer
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With the advent of the internal combustion engine, a means of igniting the compressed fuel mixture at an appropriate time was necessary. First came FLAME IGNITION where the combustion chamber is exposed to an open flame by a sliding portal.

slide valve ignition

Jim Fehl helps me out here by explaining: "There are a few different types of flame ignition but the Crossley at Coolspring works something like this: A sliding plate in the cylinder head has a machined chamber which is used to to transfer the flame to the combustion chamber. The plate is connected by a linkage to the side shaft and as the shaft rotates the plate moves back and forth. As the plate moves to one end of the stroke it connects with a gas delivery port which charges the chamber with gas. The plate starts it's return stroke and midway back it passes another port that exposes the gas filled chamber to a burning flame of a Bunsen burner. This flame ignites the small amount of gas in the moving chamber as it passes by. When the plate reaches the other end of the stroke the chamber is closed off from the outside flame but connects with the port leading to the cylinder and BANG! I understand it's a little more complicated than that with a special bleed port to equalize the pressure in the plate chamber, etc. but this is the general principle."

hot tube ignition

Next came the HOT TUBE ignition, where a narrow capped tube that protrudes from the combustion chamber is heated red hot, blow torch style. The ignition timing depends upon the degree of compression and the spent products of combustion from the previous cycle that remain in the tube. When the compression stroke pushes the fresh fuel mix into the hot tube and it reaches the hot spot, ignition occurs. The outside portion of this tube is enveloped by a chimney to contain the applied heat from escaping to the atmosphere. Different lengths of tubes are used to effect the timing of the ignition. We're talking about the 1890's here, before batteries were common.

Accurate timing is impossible with this method, other than its length from the combustion chamber and the amount of heat applied. I have seen some operators of these engines place a tin can over the outside chimney for added "insulation." The initial start-up must have been very difficult, perhaps instigating the "kitchen match" starting method used by Fairbanks Morse engines. The "reverse-start" method is most likely used, much the same as in Witte engines.

(Fairbanks Match Start Engine)

Click the photo to see "starting" the 32HP Fairbanks Morse at Hunsader Farms. This engine was originally set up with a hot tube AND an igniter so the operator could make a choice of ignition used and it incorporated MATCH starting.

Craig Prucha wrote an email that explains his hot tube engine timing:

"I run a couple of hot tube engines that I own and the timing is set by the length of the hot tube (the hollow portion inside the tube) and the amount of heat. The dead air in the cylinder then goes up the hollow portion of the tube and when a fresh air/gas mixture hits the red hot spot in the tube it ignites under compression. I start the engines just before the tube gets cherry red. This is like retarding the spark and the engine is less violent when cranking it over. The longer the hot tube the more advanced the timing, the dead air goes up the tube faster and the air/gas mixture ignites quick. The shorter the tube the more retarded the ignition is. The piston has to really compress all the dead air and it takes more stroke for the good air/gas mixture to get to the hot spot. The above is why the hot tube length is different for every engine. I know you will think all of the above sounds crazy but the timing of a hot tube engine works just the opposite of the way you think it should."

SmokStak MESSAGE: (#1796) Hot Tube Ignition, by: John C. Neagley ...

For many years I pumped crude oil up here in northern Ohio using S.M.Jones engines with hot tubes on powers. The length of the tube is critical, longer tube for advanced firing and shorter for retarded timing. Another thing to consider is the RPM's desired out of your engine. Sometimes in stripping these old wells, I wanted to run the engines slow. An even shorter tube was needed. When running at a higher RPM, say 250, a longer tube is used so the timing occurs enough before top center for optimum performance without knocking. Yet if the same engine is slowed down, using the same tube, it will knock considerably because more time is available between the firing of the charge and top dead center. My suggestion is to experiment with length of tube at the desired RPM's you wish to operate your engine. Also, the hot tube chimney needs to be lined with some type of insulation to reduce heat transfer and keep the tube red without damaging it. Many a nickel tube has been ruined because of too harsh a gas flame, the inexperienced operator trying to keep his engines tube red hot without insulating between the tube and chimney.

SmokStak MESSAGE: (#1817) Some More Hot Tube Experiences, by: John C. Neagley ...

Operating using a hot tube, I always used a couple of layers of asbestos sheet as the insulator between the tube and chimney. This enables using a lazy gas flame just big enough to keep the tube a nice red without burning it up. Since the asbestos scare, you may need to find a substitute for what works very well - asbestos! If a roaring flame is burning inside or above the top the chimney, something is not right inside the stove. On the stove's inlet pipe just after the gas control valve, between it and the burner, should exist holes or a slot for the purpose of mixing air with the natural gas. There should be a movable collar or sleeve around this pipe to adjust this mixture by sliding to either cover or uncover the air inlets. Adjust air opening only enough to prevent a yellow flame and keep a neutral blue flame down inside the chimney, just above the burner, around the tube. When adjusted properly, this sleeve will also prevent the flame from jumping back and burning inside the inlet pipe before reaching the burner. Watch out for drafts blowing by the bottom of the stove. These will effect the flame inside and can result in a cooling of the tube. It maybe necessary to construct a shield to prevent this. Hot tube stove designs differ, some are more sensitive to drafts than others. Hot tube ignition engines are very easy to start, dependable, and run very well when set up and operated properly. If anyone is interested in discussion about gasometers, wet or dry, and the critical role they play in the operation of natural gas engines, please let me know and the next time I have a headache and time to type we'll discuss them. Thanks for allowing me to share what little I know with you and bearing with me. -- John


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