Standard Fuel Oil engine, being of the two stroke-cycle type, possesses no air admission or exhaust valves.
The piston performs the duty of both valves, uncovering air scavenging ports and exhaust ports at the forward end of its stroke. The piston action, along with the functioning of the scavenging air valve, should properly fall within the scope of this discussion of engine valves. In Chapter VI a few words were devoted to the scavenging piston which acts as a compressor to supply the scavenging air to the power cylinder, although the means of this accomplishment were not mentioned. Figure 63 represents the engine cross-section, showing both the power pis ton and the scavenging piston, which is bolted to the former. Figure 98 is a view of the injection air compressor. The low pressure air piston is of an hour-glass shape and acts as a valve in directing the proper flow of the scavenging air. The pipe a is the air suction line open to the atmosphere. The air, flowing in along the pipe a, passes through the port 1, allowing this air to enter the scavenging air cylinder H, shown in Fig. 63. As the engine turns over this air is compressed, and the continued movement of the plug piston g, Fig. 98, uncovers the -port d, which permits the air to flow out through the pipe into a low pressure air receiver, not shown. This air is at a pressure of 8 to 10 lbs. gage. After the fuel charge in the power cylinder fires and the piston moves outward, the exhaust ports K are un covered. At the same time the scavenging air ports e are also opened. During this interval the plug piston g has moved, shutting off the connection between the scavenging cylinder and the air-receiver line d, and has placed d in communication with the scavenging air ports e. The air, stored in the receiver, rushes through the ports e, scavenging the power cylinder of the exhaust gases. It is apparent that the successful functioning of the entire air scavenging system depends on the condition of the air-compressor piston or plug valve g.
Valve Timing.—The timing of the fuel valve, as well as the points of opening and closure of the exhaust ports, is indicated in Fig. 99.
De La Vergne Type FD Diesel Valves.—This horizontal Diesel has the valves located horizontally in the head. They are driven by rockers which are actuated by a camshaft 'bolted to the front of the cylinder head, as appears in Fig. 15.
Valve Timing.—Figure 100 gives the valve timing of this Diesel. Individual units may vary slightly from these values.
McIntosh & Seymour Marine Diesel Valves and Valve Cages.—The admission and exhaust valve cages are made of cast iron and do not have separate valve seats. The valves proper are also of cast iron with steel stems cast in, and are guided in the lower part of the valve cage and do not have an upper guide as is used in the stationary Diesel for the guiding of the valve.
Rocker Arms.—The various rocker arms are operated from the cams by means of vertical push-rods, including the air starter and fuel valve rockers, Fig. 101.
Camshaft.—The camshaft is driven by a set of spur gears from the crankshaft at the after end and runs at half the engine speed. A double set of cams is arranged side by side in the following order: exhaust, fuel, starting, and admission. They are keyed to the shaft and bolted together. Cams and rollers are made of cast iron and are chilled at the running edges.
Reversing and Operating Gear.—This gear is located on the forward end of the engine. Two turns of the reversing wheel shift the camshaft in a horizontal direction for either ahead or astern rotation. This mechanism is designed as follows: The handwheel turns a shaft to which a bevel gear is keyed, Fig. 102, which in turn rotates a large bevel gear D mounted on another shaft, at right angles to the first one, carrying a pinion which operates a rack L up and down. This rack has an extension which is provided with a slot in which a roller moves, which roller is fastened to a rocker arm M pivoted around a fixed point.
The other end of the lever is forked and its arms fit in a groove in a sleeve fastened to the camshaft, thus moving this shaft in either direction at will. The rollers for exhaust, fuel, and ad mission are moved away from the respective cams by the rods I and R at the same time the camshaft is shifted longitudinally. Starting is performed by moving the lever 0, which controls fuel and starting, all the way downward, in which event the starting air roller is brought in contact with its respective cam, admitting air As soon as the engine has turned over a few times control handle 0 is moved upward again, bringing the fuel admission at the full-load position. It will be un derstood that the starting air was automatically cut off again as soon as the control lever reached the "full-load" mark on the segment.