Figure 251 shows a view of the Muncie Oil Engine, which makes use of the hot-bulb igniter device.
Hot-tube Ignition.—A second ignition device is that shown in Fig. 248. Here the head is completely water-cooled and has an ignition tube projecting into the cylinder space. This tube may be either fixed or removable. If fixed in place, a part ex tends beyond the outside wall of the head and is heated by some manner of torch. In the drawing shown, the tube is removable and is first heated by the operator at some convenient flame and is then placed in its recess and clamped into place. The engine is started, and the fuel is then injected in a manner similar to that of the hot-bulb engine. The oil, on leaving the nozzle, strikes the red-hot tube and of course "cracks" and vaporizes. This oil vapor mixes with the air charge, and, as soon as the ad vancing piston has raised the compression temperature high enough, the charge is fired and burns.
In a hot-tube design the scavenging air frees the cylinder from almost all of the burnt gases since there is no pocket or bulb. Therefore the opportunity for stratification of the vaporized oil and fresh air is not so good since there is no intervening stratum of inert gases to separate the two; as a result, they doubtless mix immediately. The only way, then, to prevent preignition of the charge is to keep the temperature down until the piston is practically on dead-center. Since the hot tube contains but a small amount of heat, the compression temperature and pressure do not increase as rapidly on the heavy loads as they do in an engine having a large heating element, such as the bulb. Never theless, to keep down preignition recourse is had to water injec tion even at fairly low loads.
It is the general experience that all engines using the hot-tube arrangement will operate better on lighter oils than on the heavier fuel oils—quite the opposite of the hot-bulb design in this respect. The hot-tube engine scavenges better than the hot-ball engine and consequently shows a slightly superior economy at the heavy loads since the cylinder is charged with virtually pure air, insuring a better combustion. The tube is simple, easy to replace if defective, and the design allows the head to be symmetrical and free from casting strains. On the other hand, it does not present a large surface to the injected oils and often does not give off enough heat to vaporize all the charge when using heavy fuel oils.
The operator should be cautious about keeping in service a tube after it shows any considerable amount of corrosion. The tube will burn in two eventually. If part drops into the cylinder, it is liable to cut the walls. Some engineers have a practice of heating the tube at a fire placed some distance from the engine. They often wonder why the engine fails to fire after they hur riedly put the tube into place. It should be remembered that the radiation of heat from a red-hot bolt is very rapid, and the tube quite likely is fairly cold by the time it is locked into place. It is best to use a kerosene or gasolene blow-torch placed not more than 3 feet from the engine. The Primm Engine makes use of a device along these lines. See Fig. 330.
builders have adopted modifi cations of the hot-plate design, as in Fig. 249. In this design a bell-shaped iron casting is bolted to the piston head: To start the engine, the cylinder head is provided with a small hot tube, which is heated externally by a torch. The fuel is injected through the nozzle A, which is located in the center of the cyl inder head. On starting, this oil will drip onto the starting tube B and ignite. After a few revolutions, the bell casting or hot plate attains a sufficient temperature to allow the torch to be removed.
In this design the cylinder is practically free from obstructions, and the scavenging effect of the air charge is good. On the com pression stroke the air charge is compressed to 90 to 120 lb., and at approximately 20 degrees from dead-center the fuel is in jetted through the nozzle. The hot plate remains at a high temperature, due to the heat absorbed from the burning fuel. Using light distillates, the temperature existing in the cylinder is sufficient to vaporize it before any strikes the plate. There is no attempt at stratification since the fuel charge is intermixed with the air immediately upon vaporizing. Where the oil used carries much lighter portions there is a considerable tendency toward preignition at all loads above one-half engine rating. This preignition is especially noticeable on heavy loads where the fuel injection begins very early in the stroke. Consequently, water injection is used at any load above or N rating.