In the water connections to the cooling ring it is imperative to have an atmospheric vent. If this is not provided, steam will be trapped in the water space and all cooling effect prevented. The cap continues to become more heated and ultimately will give way.
As there is good stratification, the builders have eliminated the water injection feature. Without water injection the engine operates, on the partial loads, up to about 80 per cent. of normal rating, without preignition; at full load the oil charge is great enough to allow some of it to escape into the cylinder, resulting in preignition. This, of course, applies to engines where the fuel used is of high gravity, since with such oil the compression pressure is sufficient to cause ignition, being about 160 pounds. This type of engine operates best on distillates above 32° Baum& The temperature of the combustion chamber is not sufficient to completely vaporize the heavy oils, such as 24° fuel oil Consequently, using this latter fuel, there is always a heavy deposit of carbon in the combustion chamber, requiring frequent cleaning. The fuel economy, compared with other low-pressure engines, is good. This is due, in part, to the fact that there is not the thermal loss which occurs with water injection. It might be well to call attention to the fact that an engine with a given cylinder volume will not develop as 'great a horsepower without water injection as it will with water injection. This is based solely on the fact that water injection will prevent preignition, which occurs in any engine pulling a heavy load. It is customary to find an engine given a lower rating when the water feature is abandoned. An English firm reduced the ratings of their engines 25 per cent. when they gave up the water injection. An American engine builder, when first going into the crude-oil engine business, used the water feature. This was abandoned as a result of a conclusion that it possessed certain objectionable characteristics. On changing the design to eliminate the water, the ratings were changed approximately 20 per cent. The engine then developed only the normal rated output without preignition, while the first design easily developed 30 to 40 per cent. overload with the same size cylinder and the same speed.
This design is used on several English engines, notably the Petter, as well as on American engines. Figure 253 is a view of the Fairbanks-Morse vertical engine, which makes use of this device. The Bolinder engine also uses a device along the same lines.
As mentioned previously, these engines, using the combustion chamber, employ a nickel starting tube. This tube heats more than the cast-iron head. The oil charge is so directed as to strike the tube and vaporize readily. A second nickel tube is sometimes used in order to positively vaporize and fire the heavy crude oils. This tube has a cavity opening into the chamber, and the oil enters this opening and is vaporized by the hot walls of the tube. This tube has a tendency to coke up and will soon lose its usefulness. Each time the cap is removed, the tube should be cleaned out. In case of a burned tube, it can be removed by unscrewing it from the inside of the cap, using a chisel driven into the tube cavity. If an extra tube is not available, an iron bolt about 2 inches long will be about as ser viceable since it will project into the chamber and the fuel will strike it, or at least in its close vicinity.
A modification of this design is used on the Fairbanks-Morse vertical engines. In this construction the entire combustion chamber is water-cooled with the exception of a flat cover-plate. By this means there is no overheating at full load, as the water cooling keeps the temperature within reasonable limits; in fact, on partial loads, the cooling is too successful, resulting in some of the fuel charges not igniting at all. This is especially notice able when using heavy oils. As a result, if the engine runs on partial loads very much of the time there is a decided tendency toward heavy carbon deposits both on the combustion chamber walls and on the flat cover-plate. If this carbon is not cleaned out, the engine will run very hot on heavy loads and will pre ignite very pronouncedly, and the exhaust will show very smoky. The carbon will work loose and drop into the cylinder cavity. When it is recalled that this deposit becomes as hard as iron, it is easy to see how quickly a little carbon will ruin the cylinder walls.