functions are performed by the firebox. Burning of the fuel and liberation of the heat contained is first and most import ant. The second function is to permit the transmission of the heat through the firebox heating surfaces to the water. Under normal conditidns only a part of the fuel burns on the grates. A considerable portion, approxi mately one-half, of the heat is liberated by the burning of the gases above the fuel bed and in the tubes. Sufficient air supply, com plete mixing of the gases and ample combus tion space are necessary to prevent a large amount of combustible gases from passing out the smoke stack.
Combustion The firebox fur nishes seldom more than 10 per cent of the total heating surface, but is responsible for more than 25 per cent, and sometimes as high as 50 per cent of the total evaporation. It is very essential that sufficient combustion space be provided in which the combustible gases given off by the fuel bed can burn and give off their heat. The carbon in the fuel next to the grates may be completely burned to carbon dioxide, but in passing up through the fuel bed freshly placed will leave largely as carbon monoxide. If no place is provided for this gas to burn it will pass up the stack and lower efficiency will result.
Mechanical Stokers.— Mechanical stokers have been applied to a limited extent within recent years. On account of the increased size of the •locomotive the ultimate capacity of the heating surface cannot be realized by hand firing and relief is sought in the application of mechanical firing. Many experiments are being conducted and the use of stokers is be coming more general.
Steam Action.— To understand the action of the steam in a modern locomotive let us take for example a locomotive about to start on its trip. A good fire is burning on the grates, the steam pressure is up to maximum and a suffi cient coal, and water supply is on the tender. On opening the throttle, after setting the shift ing lever, the steam enters the steam chest above the engine cylinders, having passed through the superheater, if the locomotive is so equipped, and through the steam pipes con necting the superheater or boiler with the steam chest.
The steam then enters whichever end of cylinder is opened through the steam chest or valve chamber and due to its pressure causes the piston to move. The piston being con nected to the piston rod forces the crosshead and connecting rod and crank to perform their function of turning the axles upon which are mounted the driving wheels and valve mechan ism. A movement of the valve mechanism causes a movement of the valve in the valve chest which admits the steam alternately to opposite ends of the cylinder and at the proper time permits the steam in the cylinder that has done its work to escape through the exhaust pipe and pass up the stack, thus producing a suction and creating a draft in the firebox.
The valve and its movement are so designed that four events take place in each end of the cylinder as the piston moves from one end to the other and back again, which requires one revolution of the drivers. One revolution of the drivers causes the valve to move from one end of its travel to the other and back again. When the valve is at one end of its stroke steam can enter the cylinder and when it is at the other end the steam can leave the cylinder. The opening and closing by the valve is a proc ess continually, kept up and the opening varies from zero to its maximum. The valve starts to uncover the steam port slightly before the piston reaches the end of its stroke and we say admission takes place. Steam continues its flow into the cylinder and the piston starts on its working stroke. Before the stroke is com plete the valve shuts off the steam and we have the cut off. The steam in the cylinder then ex pands, doing work until the valve moving in the opposite direction opens the exhaust port and allows the steam to escape. This last event is called release, which as a rule occurs slightly before the piston reaches the end of its working stroke. The piston completes its working stroke and starts its return stroke, forcing the steam out of the cylinder. Before the piston completes its return stroke the valve has started to close the exhaust port and upon closing we have the fourth event or com pression. Some steam is caught in the cylin der and compressed until admission occurs and a new cycle begins.
A similar cycle is performed in the other end of the cylinder and so timed that When ex haust and compression are taking place in one end admission and expansion are taking place in the other. The above-described events are changed at the will of the engineer by setting the shifting link for different cut-offs. When the shifting link is moved forward from mid position the cut-off is increased and the loco motive will run ahead. If he pulls the lever back of mid-position the locomotive will be re versed. The nearer mid-position, the earlier the cut-off occurs and the less steam is ad mitted. The events of the stroke are ex pressed in per cent of the stroke completed in which they occur. For instance 25 per cent cut-off means the steam is cut off when the piston completes 25 per cent of its working stroke.