(77) Hot Tube Ignition.
A combustible gas may be ignited by bringing it into contact with surface heated to, or above the ignition temperature. It is upon this principle that hot tube ignition is based.
In practice this surface is provided by the bore of a tube which is in communication with the charge in the cylinder, the outer end of the tube being closed or stopped up. Around this tube is an asbestos-lined chimney which causes the flame from the Bunsen burner to come into contact with the tube and also prevents draughts of air from chilling it.
A Bunsen burner is located near the base of the tube and maintains it at bright red heat. The gas for the burner is supplied from a source external to the engine. When the fuel used is gasoline, a gasoline burner is used, which is fed from a small supply tank located five or six feet above the burner.
During the admission stroke, the hot tube is filled with the non-combustible gases remaining from the previous explosion, therefore, the fresh entering gases cannot come into contact with the hot walls of the tube and cause a premature explosion, before the charge is compressed.
As the compression of the new charge proceeds, the fresh gas is forced farther and farther into the tube and at the highest point of compression it has penetrated far enough to come into contact with the hot portion. At this point the explosion occurs.
The tube being of small bore, does not allow of the burnt gases mingling with the fresh within the tube; the waste gases in the tube acting as a regulating cushion. The distance of travel of the new mixture is proportional to the compression, hence the explosion does not occur until a certain degree of compression is attained.
The length of the tube required for a given engine is a matter of experiment, as is also the location of the heated portion. High compression naturally forces the mixture farther into the tube than low, therefore the flame should come into contact with the tube at a point nearer the outer end with high compression than with a low compression.
Shortening the tube causes advanced ignition, as the mixture reaches the heated portion sooner, or earlier in the stroke, because of the decreased cushioning effect of the residue gases in the tube.
The length of tube and location of maximum heat zone should be so proportioned that combustion will take place at the highest compression. Moving flame to outer end of the tube retards ignition. Moving the flame toward the cylinder advances it.
While the hot tube is the acme of simplicity in construction, it is not the easiest thing to properly adjust, as the adjustment depends on compression, temperature of the tube, and the quality of the mixture. Any of these variables may cause improper firing.
The hot tube is rather an expensive type of ignition with high priced fuel, as the burner consumes a considerable amount of gas, and is burning continuously during the idle strokes as well as during the time of firing.
It is practically impossible to obtain satisfactory results from a hot tube on an engine that regulates its speed by varying the mixture or compression, as engines running on a light load will not have sufficient compression to cause the mixture to come into contact with the hot surface, the engine misfiring on light loads.
The tubes are made of porcelain, nickel steel alloy, or common gas pipe, and are of various diameters and lengths.
All of these materials have their faults. Porcelain being very brittle, is liable to breakage. Gas pipe burns out and corrodes rapidly. Nickel alloy is not liable to breakage, is not so susceptible to corrosion as iron, but is far from being a permanent fixture.
Timing valves are a feature of some systems of hot tube ignition, which correct to a certain extent the irregularity of firing of the plain type of tube.
The timing valve is introduced in the passage connecting the cylinder and tube, and prevents the gas in the cylinder from coming into contact with the heated surface until ignition is desired.
The valve is operated by means of mechanism connecting it with the crank shaft. It is evident that with sufficient compression in the cylinder, the time of ignition can be obtained with certainty.
This mechanism is rather complicated, and subject to wear, and the advantage gained by the fixed point of ignition is offset by mechanical complication and consequent trouble.
The action of hot tube igniters is erratic and their use is not advisable unless under unusual conditions. The open flame used in heating the tube is a constant menace, as it is surrounded by inflammable vapors. This feature alone condemns it in the eyes of the insurance underwriters; in many places the use of the hot tube is prohibited both by the underwriters and city ordinances.
The above inherent defects of hot tubes are supplemented by breakage, “blowing,” and clogging of the tube or passage with soot and products of corrosion, each factor of which will cause misfiring.
In case of misfiring, after determining that the tube is not broken or clogged with soot or dirt, see that the engine is being supplied with the proper mixture; that you are obtaining the proper compression; and that the Bunsen burner is delivering a bright blue flame on the tube at the proper point. Never allow the burner to develop a yellow sooty flame. A yellow flame indicates that insufficient air is being admitted to the burner. Remember that an overheated tube is quickly destroyed, and will cause misfiring as surely as an underheated tube. Regulate the gas supply to the burner.
A small leak near the outer end of the tube will destroy the cushioning effect of the burnt gas, and hence will cause premature firing of the charge. Procure a new tube.
Many engines are provided with a sliding burner and chimney which allows of some adjustment of the flame on the tube. In cases of persistent misfiring, move the chimney one way or the other. It may improve the ignition.