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HOW GAS IS EXPLODED IN CYLINDER TO PRODUCE
Q. How can explosive gas in cylinder be exploded?
A. The compressed gas in the motor cylinder may be exploded by setting fire to it by the application of heat.
Q. When is the charge fired?
A. The charge is exploded when the piston reaches the end of its compression stroke or at top position and just prior to the inception of the power stroke.
Q. Name some early methods of exploding the gas.
A. The gas charge was exploded on early, slow speed, low compression motors by means of a naked flame, hot tube or hot head.
Q. Describe objections to open flame method.
A. The open flame method was suitable only for use on the very early engines that did not compress the charge before ignition. The flame burned outside of the combustion chamber, and at the proper time for firing the charge, a small slide or valve permitted the flame to communicate with the contents of the combustion chamber and explode them. The chief objections were uncertainty of ignition, as many explosions were missed, blowing out of the flame and difficulty of using the flame with even small degrees of compression.
Q. Outline hot tube method.
A. The hot tube was an improvement on the naked flame, and consisted of a simple porcelain or metal tube which was heated by a flame burning outside of the cylinders. Two types of hot tubes are indicated at Fig. 75, while the method of installation can be readily ascertained by examining the diagram at Fig. 75-A. As the gas was exploded by means of the heat of the tube and the naked flame did not communicate directly with the interior of the combustion
chamber, the objections advanced against the simpler open flame ignition were eliminated to a degree.
Q. Where is this method still used?
A. The hot tube method of ignition is still utilized in some forms of stationary engines using crude oil as fuel.
Q. What is the hot head method of ignition?
Fig. 75.-Showing Construction of Hot Tube Formerly Used in Ignit
ing Gases in Automobile Cylinders But Now Confined to Large Stationary Engines Employing Low Grade Fuel.
Fig. 75-A and also at Fig. 76, is a modification of the hot tube method, inasmuch as a certain portion of the cylinder head, usually in the form of a hollow ball, is heated by the flame of a torch in order to get hot enough to start the engine. After the engine had been running for a few minutes the external source of heat could be re
moved and enough heating effect would be obtained from the high temperature of the exploding charges to maintain the heat of the head.
Q. What is the main objection to the ignition methods previously named?
A. One of the main disadvantages of the open flame, hot tube, and hot head methods of ignition which they share in common, was that considerable difficulty obtained in attempting to vary the time of ignition and obtain some degree of engine control by this means.
Fig. 75A.-Diagrams Outlining Principal Methods of Exploding
Charge by Heat. A—Hot Tube System. B-Hot Bulb Method.
Q. How can gas charge be exploded by electrical means?
A. Three methods of producing heat by electric current have been used. These are outlined at Fig. 76-A. The diagram at A shows the first electrical ignition system in which a coil of wire was inserted in the cylinder and heated by passing a current of electricity through it until it was hot enough to explode the compressed gas. A defect of this method of ignition was the rapid deterioration of the ignition coil and the necessity for frequent replacement of these members. In the methods outlined at B and C, an electric spark is produced in the cylinder at just the time it is desired to explode the compressed gas.
Q. How is spark produced in cylinder?
A. There are two methods of causing an electric spark to occur in the combustion chamber. That outlined at B in Fig. 76-A involves a separation of two contact members, through which a current of electricity is flowing. The spark is produced when the points separate. That outlined at C is known as the high tension system and a spark overcomes the air space between the points of the spark plug because it has sufficient power or pressure to overcome the resistance at that point.
Fig. 76.—Section Through Vaporizer and Valve Chamber of Station
ary Engine Utilizing Hot Tube Ignition.
Q. What are the important advantages of the electrical ignition systems?
A. Electrical ignition systems, as a rule, are easily controlled, ignite the charge unfailingly and exactly when desired, and are the most reliable of the ignition methods that permit flexible engine operation.
Q. Why is variable ignition desirable?
automobile motors, it is necessary to fire the charge earlier when the engine is running fast than when it is running slow.
Q. Why should spark be retarded when starting an engine?
A. When an engine is started by hand it is desirable to have the ignition late or retarded in order that the gas will not explode until the piston has started down on its power stroke. If the explosion occurred before the piston started down it might be driven around in the opposite direction to that it should go and the operator would be injured by the reversal of motion of the crank or flywheel.
Fig. 76A.—Diagrams Outlining Possible Methods of Electrical Igni
tion. A-By Incandescent Platinum Wire. B-By Make and Break or Low Tension Spark. C—By High Tension or Jump Spark.
Q. Why is it necessary to advance spark?
A. When the engine speed is increased faster than a normal number of revolutions, a greater number of explosions occur in a given space of time than when the engine is running slowly. The gas requires a certain amount of time to become ignited, and when an engine is running faster and more gas charges must be exploded, it is necessary to start the ignition earlier, which means that the spark time in relation to the crankshaft or piston travel must be advanced.
Q. Why is throttle control alone inadequate?
A. Controlling the engine speed by varying the amount of gas does not permit exploding the gas charges fast enough in order to speed up the engine.