When the engine is run on fuel, the reversing gear and the fuel regu lations are interlocked; and only when the control lever is on STOP does possible to turn the reversing wheel as notch is disengaged from the bevel gear rim. The control lever cannot be moved -unless the engine is fully reversed either way.
Nelseco Four-cycle Marine Diesel Valves and Valve Gear.— Figure 103 is a cross-section of this marine Diesel. The admission valve is provided with a cage, and the exhaust valve seats on a removable seat in the combustion chamber S walls. The air and fuel valves are actuated through levers by the starboard camshaft while the exhaust camshaft is on port side. The two camshafts are driven by a 2 to 1 reduction gear from the engine shaft.
Reversing Gear.—The majority of these Diesels are equipped with a gear for reversing, though some have a direct reverse arrangement shown diagrammatically in Fig. 104. The cam shafts, of which there are three, have spiral keyways cut in them. Each shaft has a collar with a spiral key which is shifted by a pneumatic reversing cylinder. A movement of the rocker A throws the collars to one side, thus turning the camshafts through an angle that will set the valve cams correct for reverse running. Due to the varying angle through which the three sets of cams must move, the spiral keyways do not have the same pitch, thus securing proper shifting of the cams.
Valve would appear from the foregoing discussion, all the manufacturers use cages on the admission valves while the practice is not universal as regards the exhaust valves. There can be no doubt that the cage design is superior and is of advantage to the operator. When a valve seat leaks, a spare valve and cage can be used to replace the defective parts. No matter how thrifty the operator must be as to the capital in vested in replacement parts, it is the height of economy to carry one extra admission and one extra exhaust valve with their cages.
Cages and to the severe tempera tures of the exhaust gases as they pass through the exhaust valve cage, water-cooling of the valve stem housing is well-nigh im perative in engines above 12-inch cylinder bore. All manufac turers provide for this stem cooling even though a valve cage is not included in the valve design. It is clear that the circulation
of water must be positive if the sticking of the stem is to be avoided. Where the water carries any considerable percentage of mineral salts, there is a decided tendency toward the scaling up of the cooling space, which is, at best, of small volume. Con sequently, each time the exhaust cage is removed the water cooling cavity should be filled with a 10 per cent. muriatic acid solution and afterward thoroughly flushed out with a stream of water under at least 20 lbs. pressure. Since the engine has already extracted the available portion of the heat of the gases, there is no loss occasioned by a cool valve cage; the cage cooling water discharge should be kept below 120° Fahrenheit as a low temperature improves the valve action. Even at this temperature of the discharge the valve stem is actually much hotter, and the lubricating oil often forms a gumming deposit that makes the valve motion erratic. A little kerosene should be injected along the stem after each shut-down.
Water-cooled Valves.—On some of the larger engines water cooling of valves, especially the exhaust valve, is essential to the successful functioning of the valve. The water is customarily introduced at the side of the stem and flows down to the body, where it enters a central tube which carries it to a connection at the end of valve stem. With both water-cooled valves and water-cooled cages the water must be circulated before the engine starts firing. If this matter is deferred until the engine has warmed up, the sudden chilling of the parts by the cold water will invariably produce a fracture.
Pitting and Corrosion of Valve Seats.—Both chemical and mechanical reactions cause pitting and corrosion of the valve seats, particularly of the exhaust valve. The pounding of the valves, as they close rapidly, produces a series of minute surface fractures that, in time, allow small portions of the metal to flake off, giving the effect of pits. A more prevalent result of the valve hammering is the formation of grooves around the seat. These serve as recesses to hold carbon deposits, which soon cause the valve to leak.