TYPES. ADJUSTMENTS. REPAIRS Valves.—The four-stroke-cycle Diesel engines are provided with air admission and exhaust valves as are all four-stroke cycle gas engines. In fact, a number of Diesels are built with cylinder heads and valve mechanisms that are but slight devia tions from the similar parts of a gas engine. This applies par ticularly to the horizontal Diesel of which a number possess what might well be termed "gas engine heads." In the vertical engines the entire valve equipment is distinctly a Diesel feature, differing in many respects from any of the gas engine designs.
In all Diesels the admission valves seat in valve cages along the lines of Fig. 77; the builders are not so unanimous as to the exhaust valve. This latter valve, in some engines, is provided with a cage; in other makes the valve seats directly on the cylinder casting; in others a removable seat which is fastened to the head casting is supplied. Strictly speaking, the exhaust rather than the admission valve should be fitted into a cage. The hot exhaust gases, in passing through the valve opening, wear the ground seats very rapidly, while the admission valve is not subject to such an erosive action. From the operator's viewpoint the exhaust valve should be caged, as well as the admission valve. Engine builders, in a desire to eliminate the difficulty of lowering the exhaust cage under the cylinder head, place the admission valve in a cage and design the exhaust valve with a diameter to allow the latter to be lifted up through the admission valve cavity after the admission valve assembly has been removed. This enables the engineer to grind the exhaust valve by inserting the pin wrench through the admission opening.
In the horizontal engines where the valves are placed with the stems in a horizontal position, as well as in the vertical engines, usually both valves are caged.
Valve Camshafts and Levers.—The present-day vertical Die sels of American manufacture have camshafts along the cylinder heads and are driven by a vertical shaft from off the crank shaft. The drive is invariably by helical gearing. The cam
shaft carries the cams for the admission, exhaust, injection and starting valves; the cams are usually of cast steel, with a tool steel nose on the fuel-injection cam. The lever mechanisms are quite similar for all makes of vertical engines, the variation being mainly in the arrangement for operating the starting valve.
The horizontal engines are all fitted with a layshaft along the engine frame which is driven on a 2 to 1 ratio from the main shaft. The valves receive their motion from cams or eccentrics mounted on the layshaft.
American Diesel or Busch-Sulzer Type A.—The former engine, which was the forerunner of the Busch Type A engine, is of interest since many are still in operation. The positions of the admission and exhaust valves are shown in Fig. 7. The cam shaft is mounted within the enclosed crank-case and is driven from the main shaft, through an idle or intermediate gear, and operates at half engine speed. The exhaust valve seats vertically in the lower face of the head and is driven directly by a ver tical push-rod pinned to the valve lever B. The lever is closed by a spring which is held in place by a washer and lock-nuts as shown in Fig. 78. This valve has two tapped holes in the top which allow a wrench to be used, but the size of the cap screws are such that it is impossible to hold the valve with the wrench when disassembling. The most satisfactory pro cedure to follow is outlined in the sketch to the left of Fig. 78. The spring is compressed by a pinch-bar, allowing the engineer to grasp the valve stem with a pipe wrench. By placing an open-end wrench on the lock-nuts the nuts can be removed, whereupon the spring is released and the valve withdrawn. The valve has a 45-degree angle to its face, seating directly on the cast-iron base of the cylinder head. If the engineer desires to regrind, the valve is reinserted on the stem housing and a light spring set below the valve as indicated by the dotted outline.