AIR STARTING SYSTEMS. OPERATING TROUBLES Air Starting Systems.—In engines up to 30 h.p. an air starter is not required. It is not difficult to turn these small engines back against the compression in order to start. In the larger engines some form of starter is imperative.
An air starting system consists of an air storage tank, small air compressor, piping and the engine starter-valve mechanism. The capacity of the tank depends on the engine size: a 24X72 in. tank is ample for engines of 60 h.p. or less. For engines from 60 h.p. to 150 h.p. two tanks 24X60 in. should be used. These tanks should be built for 200 pounds working pressure, and the seams are preferably welded. The air piping should be constructed of extra heavy pipe and malleable fittings. The pipe lines should be provided with a drip or drips at the low points, while the tanks should have drip cocks and safety valves.
The types of air compressors used are quite varied as to size and design. Since the air is used but a few minutes daily, the air compressor can be quite small. A 2 X VA in. compressor is much used in installations below 100 h.p. capacity. For larger installations a 3X3 in. or 3%X3 in. compressor is more suitable. If the compressor is air-cooled, the plant piping is greatly simplified, and the compressor operation is just as satis factory. A motor-driven machine is very attractive, although in the small plants, as a matter of cost, the compressor is best belt-driven from the engine shaft. Where this plan is adopted, a small gasolene engine should be installed to operate the com pressor in case the air tanks lose their charge, similar to Fig. 339. In all installations the discharge from the pump should enter the top of the air tank, as should also the line to the engine starting valve. This eliminates the danger of water settling or condens ing in the pipe line.
A number of arrangements for starting the engine are em ployed. The majority of builders furnish some form of quick opening valve. This valve is manipulated by the operator. The greatest objection to the hand-controlled valve is the difficulty of opening it when the piston is in the proper position. Usually
it is necessary to give the engine several air charges as the fuel charge is slow in igniting. The only way to distinguish the proper point for starting the air injection is by means of a mark on the wheel. As the wheel turns over, it is no easy task to jerk open the valve at the precise moment.
Fairbanks-Morse Air Valve. —The drift of present-day air starting design is to provide some form of mechanical controlled valve. Figure 340 is the starter used on the Fairbanks-Morse vertical en gines. When the engine is in operation, a globe valve in the air line, not shown, is closed, cutting off the air. This removal of air pressure from the starting valve disk allows the spring to move the valve stem A from engagement with the starting cam B. In starting the engine the flywheel is barred over until the piston is about 4 inches past top dead-center. The globe valve is opened and the air, blowing past the starting valve C, enters the cylinder, causing the piston to move. The air pressure on the valve disk C holds the stem A in contact with the cam B, and, as soon as the cam nose is passed, the-air pressure against the valve disk C closes the starting valve. As the engine turns over, the nose again opens the air valve, allowing another charge to blow into the cylinder. This action is repeated until the operator closes the globe valve. Since the starting arrange ment is connected to only one cylinder in the multi-cylinder engines, the other cylinder can begin to fire while the starter is still in operation. In the single-cylinder engines, it is always ad visable to give the engine at least three charges of air before closing the globe valve. This will give sufficient impetus to the flywheel to enable it to turn over in case the first fuel charge fails to ignite. To prevent the products of combustion from blowing back into the air lines, a check valve is mounted on the cylinder.