219 shows a cup employed to produce the injection of the main fuel charge. The operation of this device is based on the fact that every oil, no matter how heavy it may be, contains some light hydrocarbons that will vaporize or distil at a fairly low temperature. The explosion of these lighter parts of the fuel provides the propellant whereby the remainder of the fuel is in jected in a finely atomized condition. It is a further matter of common knowledge that the temperature of ignition of an oil is dependent upon the degree of atomization.
In Fig. 219 the cup or primary cylinder A has ports B and C that communicate with the outside air at D and with the fuel supply at E. As the piston F starts on its suction stroke, the pressure inside the cup A decreases, as the joint between the cup and the cylinder casting is not tight. The reduced pressure in the cup causes a small charge of air and oil to he drawn into the cup. The oil port G is closed by the action of the governor, there by regulating the amount of the fuel charge. At the proper point, at the end of the suction stroke, the cup moves upward and closes the fuel and air ports B and C, and at the same time covers the communicating port H in the side of the cup. As the piston returns on its compression stroke, the air leaking past the joint between the cup and the cylinder raises the pressure in the cup to from 350 to 400 lbs. per sq. inch. The resulting temperature is sufficient to vaporize and ignite some of the light hydrocarbons, causing a maximum ignition pressure inside the cup of about 700 lbs. per sq. inch. Just before the crank reaches dead-center, the cup is rotated a slight amount, uncover ing the port H. The extremely high pressure in the cup immedi ately forces the heavy oil charge out through the port H into the cylinder space. In passing through the port into the low pressure existing in the cylinder, the is atomized suffi ciently to unite with the air charge in the cylinder. It will be observed that the character of the oil determines in a great measure the time of ignition and the shape of the combustion line on the indicator card. If the oil is extremely heavy, with the lightest hydrocarbons of fairly low gravity, then the light portion will not ignite in the cup until practically dead-center is reached. The pressure in the cup, due to this primary combustion, will not reach a high value before the port H is opened. The pressure difference existing between the cup and the cylinder will not be great, and consequently the injection through the port H will be slower, and the combustion line on the card will be practically horizontal.
In the card shown in Fig. 242 the oil used was around 26° Baum6 and contained a considerable percentage of light oils. The explosion in the cup was, consequently, intense, causing a high pressure. This high pressure combined with the explosion of the remainder of the lighter percentage of oil as it entered the cylinder was sufficient to cause the peak on the card at dead center. The heavy particles of oil burned more slowly, as indicated by the sloping combustion line.
Operation.—In operation much depends on the adjustment' of the cup-valve stem. If the movement of the cup uncovers the port H too early, preignition will occur. Since the pressure carried is much higher than in the low-compression engine, pre ignition has a greater detrimental effect. Ordinarily, the bear ings show great wear, and they, as well as the shaft, should be made heavier than is customary.
The cup has a tendency to become fouled with carbon and should be inspected regularly. Attention should be paid to the beveled seat between the cup and the engine casting, as the slight est leakage at this point will prevent the engine from operating satisfactorily. The seat may be ground with coarse emery com pound, finishing with a mixture of pumice flour and oil. Cyl inder • oil such as is used on ammonia compressors makes the best paste.
This ignition device was employed on the St. Mary's oil engine of the vertical type.
A cup device on the same lines is shown in Fig. 220. This embodies the same principle of primary ignition, but the cup does not possess the angular movement for the purpose of opening the port II. This cup is equipped with a fuel valve which is me chanically operated from the camshaft. In operation a charge is drawn into this cup by atmospheric pressure when the engine is on the suction stroke. The fuel valve controls the timing of the period of fuel admission. To regulate the amount of fuel admitted, the needle valve E is connected to the governor. The movement of the governor, through a suitable linkage not shown in Fig. 220, rotates the needle valve stem; this, in turn, alters the area of the fuel port or passage C. It is to be observed that the fuel charge enters the cup on the suction stroke of the engine.