Hot Tube Engine Basics

By Staff
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The same engine but with the chimney removed to show the hot tube itself.
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Hot tube ignition on a circa 1898 40 HP Climax.
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A diagram of a hot tube ignition.

Editor’s note: This is the first in a series of articles by Andrew K. Mackey examining antique engine fuel and ignition system basics.

There are quite a few ignition and fuel systems for antique internal combustion engines, and I will attempt to explain some of the most commonly encountered. Most of them are seen at engine shows, but some are only seen in museums. Some are integral with the speed regulation of the engine, others are independent, so some of my explanations will intertwine the two.

Hot tube engines

Hot tube engines are engines that use a separate heat source to ignite the fuel/air charge in the combustion chamber. The heat source uses a fuel supply independent to the combustion fuel operation of the engine. Hot tube engines were built before reliable magnetos were built, and they needed virtually no maintenance in the field. If you have back issues of Gas Engine Magazine, take a look at Oil City Boiler Works/South Penn Cross-Breed Engine. This is part of an article I wrote explaining the restoration of an Oil City/South Penn half-breed engine that I helped restore for my engine club, and this section explains the operation of that engine’s hot tube. For those of you who do not have that back issue, I will give a brief explanation of the general operation of a hot tube ignition.

A hot tube is just that: A closed, thin-walled tube that is heated with an external flame. This tube is usually mounted on a 90 degree L, mounted on the head of the engine, with the open female end facing up. The hot tube is mounted in the upright position. Most often the L is mounted in the center of the cylinder diameter, both top to bottom and side to side. The hot tube is surrounded by a chimney that is insulated to prevent heat loss. On the old chimneys, the insulation material was probably asbestos. Nowadays, a material not unlike fiberglass has replaced the dangerous asbestos. The chimney also has a small burner mounted in its side that aims the flame at the hot tube itself. On the oil field, or where engines were run on city gas, the fuel was drawn directly from the fuel source through a regulator. The flame was adjusted to make the hot tube anywhere from a barely visible dull red to a bright orange heat at the lower third of the tube. Usually the tube was heated to the near yellow/orange heat, the engine started, then the hot tube flame turned down as low as possible to maintain a steady, reliable, operational temperature. This was done to save wear and tear on the metal of the hot tube, and in cases where the source was city gas to save fuel consumption.

How a hot tube operates

The hot tube burner is lit, and the hot tube brought to bright orange heat. When setting, the flame was observed through a mirror to prevent heat injuries to the eyes and also prevent injury if the hot tube failed. As metals back then (1860s through 1900s) were not well-made, hot tube failure was not unknown, but it was rare. If a tube failed, the top would be blown out of the chimney assembly at gunshot velocity, often with explosive force! NEVER look directly into a hot tube chimney!

After the tube was heated, the engine fuel was turned on, and the engine rolled over on the compression stroke, opposite the direction that was wanted for operation. As the fuel/air charged in the combustion chamber was compressed, the mixture was pushed into the hot tube. When the right fuel/air mix reached the orange heated section of the tube, it would explode, sending a flame front into the center of the fuel/air mixture in the combustion chamber, thus igniting it. The burning fuel/air mix would then drive the piston downward, thus starting the engine in the direction you wanted it to go. This operation was called bump starting.

Once the engine warmed enough, the fuel supply to the hot tube could be reduced, and the engine would still run on the red hot tube. To stop the engine, all that was needed was to cut off the fuel supply to the hot tube. The tube would cool to the point where it could no longer ignite the engine fuel/air supply in the combustion chamber, and the engine would stop.

Basic facts about hot tube operation

The engine cannot have the ignition timing retarded to a point after Top Dead Center — Compression. The fuel/air charge cannot go any further into the hot tube beyond the highest compression point — TDC. The advance or retard of ignition is controlled by three factors: 1. Length of the hot tube itself (usually between 4 inches and 6 inches in height) 2. Compression pressure (lower pressure retarded ignition) 3. Heat of the tube itself (hotter tube makes for more advance). A hot tube MUST have a heat source applied at all time of engine operation. If heat is removed, the tube will cool, shutting off the ignition.

Most new hot tubes are made of stainless steel. They are less prone to the corrosive heat deterioration seen in plain black steel tubes made when these engines were built. Hot tubes are very long lived — some are original to the engines, where they were first installed. Most hot tubes will give little trouble, unless fuel/air mixture is heavily over rich or the engine is burning oil excessively. In this case, carbon and coke will build up inside the tube, and will act like an insulator, preventing reliable ignition. Most times, this coked up material can be removed with a drill, turned inside the tube. In one extreme case, a hot tube fired 25 HP Bessemer engine in Pennsylvania has run for more than 125 years without shut down. The engine was installed in the early 1880s, and the original hot tube failed in 10 years. The well operator, not wanting to lose time pumping, put in a 6-inch-long steel pipe nipple with a pipe cap on it, and restarted the engine. It has not been shut down since!

A hot tube can be used on both 2-stroke and 4-cycle engines, as it uses an independent fuel source for the heat needed to ignite the charge in the cylinder. Fuel for the hot tube usually is either natural gas or propane, although any fuel that will heat the hot tube to red heat will do. In the oil fields, well gas is used as a free fuel source, for both the engine and hot tube. Engine fuel is usually the same as that used in the hot tube assembly (natural or propane), but engine fuel may vary. I have heard of some people who are using gasoline in the engines as the main fuel for the engine itself. Hot tube engines were used in the oil fields to pump oil and in cities to make power turning generators in order to make electricity and to provide motive power for industry.

More Gas Engine Basics

Low-Tension Ignition Basics
High-Tension Ignition Basics
Flame Ignition Basics
Diesel Ignition Basics
Semi-diesel or Hot Head Engines

Andrew K. Mackey, 26 Mott Pl., Rockaway, NJ 07866 •

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