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Q. What else may be done to use kerosene? A. Some forms of two-cycle motors use kerosene by the method outlined at Fig. 74. When the air is transferred from the engine base where it has been compressed, a portion of the air stream is deflected through a by-pass to the top of the float bowl so that the liquid contained therein is placed under pressure. This pressure causes a certain amount of the fuel to spray through the spray pipe or nozzle placed at the top of the cylinder by-pass passage and a stream of kerosene is injected into the entering air stream and vaporized. With either of these systems it is not possible to start directly on kerosene; it is necessary to run the engine on gasoline until it becomes hot enough to evaporate the less volatile liquid fuel. With the carburetor shown at Fig. 73, it is a relatively simple matter to change from one fuel to the other. With the injection system shown at Fig. 74, the float bowl may be filled with gasoline directly from the gasoline tank with the valve handle in the position shown by the dotted line, and when the valve handle is straight up the gasoline inlet is shut off and the fuel supply is through the kerosene inlet.

Q. What becomes of the burnt gases when they are discharged from the engine?

A. After the burnt gases pass out of the cylinder, they pass through the exhaust manifold and exhaust pipe to the muffler.

Q. What is a muffler?

A. A muffler is a simple device designed to silence the gases as they issue from the engine before they pass to the outer air with as little loss of power due to back pressure as possible.

Q. Why is it imperative that as little back pressure exist in muffler and exhaust piping as possible?

A. When the gases are discharged through the open exhaust valve, their pressure is approximately 40 pounds per square inch, and if the muffler and exhaust piping offers a decided resistance to their passage to the outer air, there will be a negative pressure equivalent to that exerted by the exhaust gases that will tend to prevent the piston coming to the top of its scavenging stroke. This would mean that considerable force or power would be utilized in

expelling the exhaust gases that could be employed to much better advantage in propelling the car.

Q. What is a muffler cut-out valve?

A. The muffler cut-out valve is a fitting interposed between the exhaust manifold and the muffler in such a way that it can be operated to allow the exhaust gases to escape directly to the air from the manifold without passing through the muffler.

Q. How is a muffler cut-out valve operated and what advantage does it offer?

A. The muffler cut-out valve is usually operated by a simple pedal that can be pressed down by the toe or heel of the foot and locked in the open position. A muffler cut-out offers several advantages, the most important being that it permits the motorist to listen to the exposions to see that they follow in regular sequence and also to relieve any back pressure due to the muffler when all the engine power is needed as in high speed work and hill climbing.

Q. Can a muffler be both silent and efficient?

A. Mufflers of modern construction offer but little back pressure and are silent enough so that nothing but a barely perceptible hissing noise is heard when the engine is running.

Q. Describe method of silencing the exhaust in a muffler.

A. A muffler consists of a series of compartments in which the gases are allowed to expand and as they increase in volume their pressure is reduced. The exhaust passes from one compartment of the muffler to the neighboring one through a series of holes which break up the main stream into a number of smaller streams, this cooling the gases and reducing the pressure until it is but slightly greater than that of the air. The ideal muffler would be one that would discharge the gases only when their pressure had been reduced to such a low point that there would only be difference enough between the exhaust gas and atmospheric pressure to insure clearing out the muffler.

LESSON SEVEN

HOW GAS IS EXPLODED IN CYLINDER TO PRODUCE

POWER

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 ?
A. The hot head method of ignition which is outlined at B,

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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 removed 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.

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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 ign-tion 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.

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