Concrete Storage every viewpoint the concrete storage tank is the most advisable to install. It is cheaper than a steel tank of a corresponding capacity and has an indefinite life with no danger of leakage if the concrete is well reinforced.
Figure 183 sketches a circular concrete tank that can be made of any desired capacity. The bottom must be well reinforced with steel rods or hog wire fencing. The vertical walls should have %- to %-inch iron rods every 8 to 12 inches. To form the top, concrete beams can be placed on the vertical walls and covered with concrete slabs; these should be reinforced with wire fencing.
Coal Tar Oils.—Coal tar, as a Diesel fuel, has received no attention in the United States. This is due to the low price of the petroleum products. While it will be years before the American Diesel operator will be confronted with the problem of coal tar as fuel, this fuel has its future possibilities. There are several American Diesel engines operating on Mexican crude that can be easily adapted to tar oils, providing a fairly constant load is carried. Under such conditions the atomizer will nebulize the tar oil sufficiently for ignition without the employment of a primary ignition oil.
Methods of Burning Tar Oils.—A number of methods have been devised to successfully burn the tar oil. The following list comprises those that have given fairly satisfactory results.
1. Mixture of tar oil and petroleum oil.
2. Tar oil for full load and petroleum for light loads.
3. Raising thermal range of the engine cycle.
4. Increasing the compression.
5. Application of a light oil for primary ignition.
6. Catalytical action.
1. Mixture of Tar Oil and Petroleum Oil.—Among the first plans advocated was a mixture of tar oil and a lighter petroleum oil. Various experiments have been made with a varying pro portion of coal tar. With a constant load the coal tar percentage can be as high as 75 per cent. without affecting the functioning of the engine. The problem that so far has not been solved is the thorough mixing of the two oils. If a mechanical agitator is placed in the engine fuel tank, a separation of the oils will occur in their passage to the fuel valve. The consequence is an irregu
larity in the ignition of the charges. This method can hardly be termed successful at the present time.
2. Tar Oil for Full Load and Petroleum Oil for Light Loads.— There is no condition existing within the engine cylinder that would prevent this method from being successful. However, the design of the fuel injection system presents many mechanical difficulties. The injection valve as well as the fuel pump must be under governor control. Figure 134 illustrates an injection nozzle having a piston valve under control of the governor. A tar-oil line and an ignition-oil line lead to this piston valve. If the load is light, the governor shifts the valve, causing the full charge of ignition oil to enter the fuel nozzle; the tar-oil line is placed in connection with the tar-oil return line. If the engine is carrying full load, the movement of the piston valve uncovers the tar-oil line, allowing the tar oil to enter the fuel nozzle while the ignition oil flows back through its return line. The ignition-oil to the nozzle enters the nozzle body below the tar-oil entrance. On partial loads when both tar oil and ignition oil enter the fuel nozzle, the ignition oil enters below the tar oil, thereby being blown into the cylinder and igniting before the tar oil enters.
3. Raising Thermal Range of the Engine Cycle.—This method includes the raising of the temperature of the injection air, the fuel charge and the cylinder air charge; in fact, it contemplates the increase of the temperature of the entire working parts. Starting with a higher temperature of the cylinder charge of air, the combustion of the injected fuel takes place at an increased temperature. This insures a positive ignition of the tar oil. The thermal efficiency of the engine depends on the temperature range — t2. Although the actual temperatures and t2 are higher than usual, the range — is identical with the normal Diesel temperature range. Consequently the efficiency would not be changed. The objection lies in the increased stresses occurring in the engine, and the method has never met with commercial success.