Practice and Principles of Hot Tube Ignition

Content Tools

Rt. 1. Glouster, Ohio 45732

The use of hot tube ignition must certainly be one of the earliest methods of igniting fuel in gas engines. It must have preceded spark ignition by many years, but was not very practical for engines that moved, such as early car engines. This type ignition found its place on stationary engines, and is still used today. We have two engines working every day with this ignition. There are also several others in this area, all fairly old, one previous to 1898, and all older than 1920.

This ignition system is the ultimate in dependability, is not affected by extreme moisture, and has no moving parts. I have pumped with engines, one of which was nearly covered with water in a flood. After washing off the mud as best as possible, the cylinder still had quite a bit of water in it. The exhaust pipe went up through the roof, and there was no way to drain the water out. I lit the tube, oiled up the engine, rolled the flywheel. The engine fired with a muffled thud. The water in the engine and exhaust pipe blew out, and came down on the roof, sounding like rain. Obviously, there could not have been much water in the cylinder, or it would have been forced up into the tube, and the engine would not have fired. This example, however, does illustrate very poor conditions under which hot tube ignition will still work. In similar conditions, spark ignition would be a problem.


All the tubes in use in this area are made of 'German silver.' This may not be a true description, but the best tubes are made of a nickel alloy, similar to stainless steel so as to be tough when hot. I have seen tubes made of common steel pipe with the top welded shut, but this practice gives me chills. I have also seen tubes made of stainless steel pipe, also welded up, and if done properly, should be safe enough. The best tubes are made of solid rod, bored out. It is necessary to be careful to drill straight down the center, or one side will be thin, and consequently weak. There is obviously a lot of pressure on the tube at the time of ignition. Tubes are made in several sizes, and the most common are ', ', and ?' all with pipe threads. New tubes are, or were, available from the Dependable Pipe and Supply Company, P.O. Box 589, Spencer, West Virginia 25276. Tel: 304-927-1660. I would recommend tubes machined from solid, rather than welded up, if they are available. Hot tubes are located on top of, or on the end of, the cylinder. The distance from the inside of the cylinder to the base of the tube varies quite a bit of different engines, so must not be a factor in proper ignition. The tube is enclosed in a chimney about a foot high, and two inches in diameter. This chimney is packed, or lined with asbestos paper about 3/16' thick, again not critical. We use a thin sheet, all that is now available, and roll the paper to three thicknesses. Lightly wet the paper when you roll it, to avoid breathing loose fibers. Slide the damp paper to the ledge at the bottom of the chimney. Open any air holes in the bottom of the chimney that the paper may cover. Locate the paper as low in the chimney as possible, to permit heating the tube as close to bottom as possible. This is necessary to achieve easy starting. Old operating manuals advise to heat the tube to a cherry red. In actual practice this high heat is not always necessary, and shortens the life of the tube. As a general rule, if the tube shows a low to medium red, this is plenty. My 15 HP Reid show engine will start if the tube shows a small amount of red. I have known engines to start hard if the tube is too hot. Evidently the 'time' of an engine increases as the tube gets hotter. Ignition timing is difficult to explain. Obviously as the compression squeezes the gas mixture into the heated tube, it fires. As the exhaust dumps, there being no circulation of air into the tube, there is a certain amount of exhaust that remains in the tube at all times. This must compress to allow a fresh charge to be squeezed far enough up into the tube to fire. As there is no way to 'time' the ignition, the burned gas remaining in the tube must regulate the 'time.'

To further advance this theory, I looked up the part on hot tubes in an old 1910-1920 operators book that came with an 'Acme' 10 HP engine, made by S.M. Jones, Toledo, Ohio. They advise for a speed of 140 to 150 rpm, to use a tube of 5'. For every increase of 15 rpm use a tube of an inch longer. That is, for speeds of 180 to 200 rpm, use a 7' tube. For 250 to 280, use an 11' tube. In practice, this idea is not necessary, as I know a Reid engine; and probably all or most other engines will run well at speeds over 200 rpm with 6' tubes.

I have a new tube on hand. It's 6' long has an outside diameter of  9/16', has a .328, or 21/64 hole in it, and the top end is solid for .4'. Thread size ' pipe. We use a pipe bushing between this size tube and engines taking a ?' tube.

Safety. Always a necessity.

When operating a hot tube fired engine, it is not a good practice to look down the chimney to check on the heat. It would not be impossible for a tube to blow out, from bad threads, with very unhealthy results. I have never known this to happen, but once is too much! I have known tubes to fail after many years of use. They will develop a slight bend, likely from being bored off center, and the outside bend, being thinner, will develop a crack. When the engine is rolled up on compression, the escaping air will blow into the fire and be obvious. If you attempt to start a hot tube fired engine, keep in the back of your mind that a hole could develop in the tube unknown to you. This hole will let gas into the fire around the tube and the engine will fire at about any position, even when you're not expecting it. Do not place your foot through the spoke, bearing weight on the arch of the foot. Straddle the spoke, keeping the big ball of the foot on one side of the spoke, and the little ball under you little toe on the outside. In this manner, if the engine fires unexpectedly, the foot will slide to the outside and off the spoke.