In the majority of cases the two stroke cycle type of Semi-Diesel engines compress the scavenging air in the crank chamber in the same way that a two stroke cycle gasoline motor performs the initial compression, although there are several makes that compress the air in an enlarged portion of the cylinder bore by what is known as a “trunk” piston. This initial compression determines the speed of the engine, the pressure limiting the time in which the air traverses the cylinder bore and sweeps out the burnt gases of the previous explosion. (68) De La Vergne Oil Engines. Two types of four stroke cycle oil engines are built by the De La Vergne Machine Company, which differ principally in the method and period of injecting the fuel into the cylinder. While both types compress only pure air in the working cylinder, the oil is injected in a heated vaporizer during the suction stroke in the smaller engine (type HA), and is injected directly into the combustion chamber of the larger engine (type FH) at the point of greatest compression. This fuel timing classifies the type FH as a semi-Diesel, while type HA comes under the head of that class of engines known as aspirators. [Illustration: 76-a. Elevation of De La Vergne Oil Engine, Semi-Diesel Type. Class F H. ] Semi-Diesel (Type FH) During the suction stroke, air is drawn into the cylinder through the inlet valve located on the top of the cylinder head, and on the return, or compression stroke, the air is compressed to about 300 pounds per square inch in the combustion chamber. The compression heats the air to a high temperature which is still further increased by contact with the hot walls of a cast iron vaporizer D, shown by Fig. 76-b. At the completion of the compression, the fuel is injected in a highly atomized state by compressed air through the spray nozzle F, the spray being thrown into the vaporizer. [Illustration: 76-b. Cross-Section of Type F H, De La Vergne Oil Engine. ] The vapor formed by the contact of the spray with the walls of the vaporizer mixes with the compressed air in the combustion chamber and is ignited at the instant of fuel admission by the combined temperatures of the vaporizer and compression pressure. As the fuel is not injected until the proper instant for ignition, it is possible to obtain a relatively high compression without danger of the charge preigniting. The oil is supplied to the nozzle by a fuel pump under pressure. The atomizing air takes the oil at pump pressure and performs the actual injection. Details of the spray valve are shown by Fig. 76, in which the oil and air are entered at a pressure of about 600 pounds per square inch. [Illustration: Fig. 76. De La Vergne Spray Nozzle. ]