Gas Engine Magazine

Hot Tube Ignition

By Staff

115C Audino Lane Rochester, New York 14624

A while back I bought a vertical, tank cooled 2 HP Bovaird
engine at an auction. It is a very simple engine, and it was
mistaken for an air compressor by many people because it had no
apparent means of ignition at all. After a bit of examination in
the form of making the whole thing portable enough to fit in the
trunk of the car, we (my friend, Grunch, and I) decided that (1) it
had hot tube ignition, and (2) the before mentioned ignition system
was completely missing from the engine. So I was confronted with
the task of rebuilding this system from scratch.

Now, I’ve been collecting engines for quite some time, and I
understand completely the more traditional forms of ignition, e.g.
buzz coils, ignitors and the like. Hot tube engines don’t find
their way up in the woods of the Tug Hill plateau region of New
York very often, however, and this relic from the Pennsylvania oil
fields was completely unknown to me. So I talked to collectors from
that area, and many others from all over, and have rebuilt it. I
would like to share some of my discoveries with fellow collectors
who may have run across a similar situation. I have since purchased
a 4 HP Eclipse (Olean, New York), which also needed some hot tube
ignition work.

Hot tube ignition consists of a hollow tube about ?’
diameter which is closed at one end, and threaded at the other with
standard pipe thread. This is screwed into the combustion chamber
of the engine, and surrounded by an asbestos lined chimney, about
1?’ to 2′ diameter. The chimney is cast iron, and the tube
itself was originally made of an alloy of nickel and silver.

The chimney has an opening on the side, very near the bottom,
which has pipe threads inside to support a burner. This burner,
when lit, plays a very hot flame on the side of the small tube,
which makes the tube red hot. When a charge of natural gas is taken
in by the engine and compressed, the mixture will soon compress the
air/-burnt gasses in the tube. When it reaches the hot spot on the
tube, it is ignited. The point at which ignition takes place is
adjusted by the height of the flame on the tube: the higher the
flame, the later the ignition.

I have found that tubes made of ?’ stainless steel, about
6′ long and closed with a cap, work quite well. I have used
ungalvanized steel pipe, but this doesn’t last as long.
Ideally, the pipe should be forged closed for a more reliable tube.
For the chimney on the Bovaird I used a piece of 1?’ pipe about
12′ long. I have not found a substitute for asbestos for the
lining, but if someone knows of something else, please let me know.
Asbestos is dangerous, as we all know, and fiberglass can’t
handle the high heat generated during an extended run, it

The burner consists of 3/8‘ x 6′
steel pipe screwed into the chimney. The other end has a coupling
with four air holes drilled in the middle. To this, add a gas
shutoff valve. This valve is modified so that the end inside the
coupling is reduced to a gas jet with a .025’ hole, which comes
to just where the air holes in the coupling start (see fig. 2). The
other end of the valve goes to the low pressure gas supply (I use
propane) before it enters the expansion chamber for the engine gas
supply (see fig. 3).

The height of the burner on the tube depends on the engine. An
engine with bad compression will need to have the flame lower on
the tube than one with good compression, for example. The height
must be determined by trial and error. A good place to start is
with the flame about 1′ to 1?’ on the tube. The two engines
I tried this on have a boss in the head into which the tube screws.
If your engine doesn’t have this boss, then the flame will need
to be higher.

The flame itself must also be adjusted properly. The tube should
glow red hot in about 5 minutes. The flame should be blue, and not
extend far beyond the end of the chimney. The flame, when adjusted
properly, should emit a roaring noise. If the gas mixture is too
rich, a match held above the chimney while the burner is lit will
cause the excess gas to burn off. If it is too lean, the flame will
not be blue, and it won’t roar like it should. In either of the
above cases, the tube will not get hot enough for proper

That is about the extent of what I know about hot tube ignition.
Of course, a note of caution must be issued at this point: Never
operate a hot tube engine in an enclosed space, and always have a
fire extinguisher handy. The air/gas inlet, on the two engines I
have, is very close to the tube, and I have had a couple of
fireballs because of a leaky intake valve. In my cases this was
more startling than disasterous, but a very real clanger

I have only outlined natural gas fired engines in this story,
because that is the only thing I have worked with. Gasoline fired
hot tubes are not unknown, and I would suggest using a modified
blow torch like plumbers used to use, with a gasoline supply
elevated on a pipe, like the originals, instead of the air pump.
Once again, BE CAREFUL, especially with gasoline leaks. A propane
tank, which should always be located some distance from the engine,
can be shut off at the source, and allowed to burn off. A supply of
gasoline may not be so easy to handle.

I would appreciate any comments or questions on hot tube
ignition. If anyone has any better ideas, recommendations, etc.,
etc., please drop me a line. Good luck, and be careful!

  • Published on Oct 1, 1989
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