At the moment the valve a is raised the valve b, which con nects the second cavity with the cylinder, opens. The movement of this valve allows part of the cylinder air charge to rush into the cavity, reducing the cylinder pressure. The pressure now existing within the cylinder is lower than the pressure of the air which is trapped in the fuel chamber. This pressure differ ence forces the oil charge through the valve opening into the cylinder. The velocity of the oil through the passage atomizes it sufficiently for ignition purposes.
A modification of this idea is seen in Fig. 232. In this design the valves are dispensed with and only one chamber is needed. In operation the fuel charge is deposited during the compression stroke in the fuel compartment. Between this and the cylinder is interposed a disk with a number of very small openings. The fuel rests upon the disk and is prevented from flowing into the cylinder both by the minuteness of the openings and the sur face tension of the oil.
When the piston reaches dead-center, the existing pressures in the cylinder and the fuel chamber are of equal value while the oil rests in a layer between these two air charges. As the piston moves past dead-center on the outward stroke, the increase in the engine clearance volume causes the compression in the cylinder to drop rapidly. The original pressure in the fuel then injects the fuel through the atomizer holes into the cylinder where the temperature is high enough to ignite the charge when finely divided. A card from an engine with this ignition would probably appear as shown in Fig. 233.
Neither of these designs has been commercially successful. Attention is called to the similarity of the card from the Nordberg engine, Fig. 226, and the "constructed" card of the Muller engine. They follow the same general outline at the ignition point. It is probable, with heavy oils, that there is no ignition in the cup, the compression pressure running up to the peak d. The piston in starting outward no doubt causes the air charge in the cylinder to drop in pressure to the point b. Due to the absorption of heat from the hot piston head, the air in the cup then naturally experiences a rise in pressure. At the point b the air pressure in the cup, which would have remained above the value d, injects the oil from the cup into the cylinder where ignition occurs. Nothing, of course, is known positively of the
actual events taking place.
De La Vergne F.H. Oil Engine.—Another type of the semi Diesel engine combines the Diesel principle of atomizing the oil charge by means of a stream of high-pressure air, and the hot bulb principle, similar to that used on the low-pressure engine. A cross-section of the De La Vergne type F.H. engine with this combination ignition is shown in Fig. 234. In general outline and in many details it closely conforms to standard horizontal Diesel practice. It has a two-stage compressor A for the supply of the injection and starting air. The fuel valve of the multi-cylinder engines closely follows standard Diesel-engine design; the single-cylinder engine valve is somewhat different in action.
In operation the bulb B is first heated by a torch until fairly hot. The fuel pump is then operated by hand until the line to the injection valve is filled. The engine, which has already been placed in the starting position, is turned over by the manipula tion of the air-starting valve. After two air charges the engine will begin firing. The fuel from the pump enters the body of the injection valve or atomizerwhere it comes in contact with the charge of high-pressure air from the air compressor, which is mounted on the engine frame. At the moment the needle valve is opened, and the combined charge of air and fuel is blown into the cylinder in a highly nebulized condition. The fuel charge is injected slightly before dead-center. The charge, as it blows across the combustion chamber into the vaporizer, burns very rapidly, producing a sharp peak in the line on the indicator card. If the oil is light in gravity, the entire charge is consumed instantaneously since it is injected at a high velocity, the injection occupying only an infinitesimal period of time. On the indicator card, this action produces a decided sharp combustion peak; the card closely resembles ,a card from a low-compression or constant-volume engine. If the fuel is heavy, 24° Baume crude oil for example, the heavier parts are • not ignited immediately and strike the hot vaporizer walls. The hot surfaces vaporize these more complex hydrocarbons with a resultant combustion. It then follows that the heavier the oil the nearer the F.H. card approaches a card from a true Diesel.