Practice and Principles of Hot Tube Ignition

By Staff

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

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.

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