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A VERY important part of a gasolene motor is the exhaust. As the burnt gases leave the cylinder at a speed of from 6,000 to 12,000 ft. per minute with a pressure of from 25 to 35 lbs. per square inch, it will be seen that it is of the utmost importance that the exhaust opening is of ample size to allow the gases to escape without creating a back pressure in the cylinder. In four-cycle motors the exhaust valve should also be large enough, and with sufficient lift, to allow the

gases to escape quickly and completely during the scavenging stroke. If the exhaust gases were allowed to escape freely into the air there would be little danger of back pressure in the cylinder, but the speed and pressure of the gas would cause loud explosive noises and considerable flame; to overcome the disagreeable noise various devices are used, known as "silencers” or "mufflers." Mufflers and silencing devices are of a great variety of designs and construction, but the object in all is to overcome the noise to the greatest possible extent without creating back pressure. If the exhaust can be quickly cooled after leaving the cylinder, or if the gas can be allowed to fully expand before reaching the air, very little noise will result.

Many motors intended for marine use have an auxiliary exhaust chamber as an integral portion of the engine. In the Gray motors this chamber is merely an enlargement of the exhaust opening, but being water-cooled it serves to allow the gases to expand in addition to cooling them, thus materially reducing their pressure (Fig. 100). In other engines the auxiliary exhaust is in the form of a separate chamber or box attached

Fig. 100.—“Gray” Auxiliary Exhaust

to the cylinder by bolts and is either water-jacketed or arranged with a valve that allows a certain amount of the circulating water to pass directly into the exhaust gases. Such an auxiliary exhaust chamber is illustrated in Fig. 101.

On marine engines, it is customary to lead all or a portion of the circulation water into the exhaust pipe or muffler and thus cool the gas and reduce the pressure. In the case of vehicle or stationary engines this cannot be done to advantage, as the water used for cooling is generally confined to a definite amount which passes through a radiator or cooling device and is used over and over again. In air-cooled motors there is of course

no method of passing water into the exhaust. If the exhaust is merely led into a large cylinder or chamber before reaching the air, the gases will expand and will pass out with but little noise. For stationary use such

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expansion-chambers, if of ample size, will usually prove efficient as a silencing device, an old cask or barrel, or even an inverted box, often being all that is required with small motors. In the case of large motors a cement or brick chamber is often used, and this can easily be

made large enough to permit full expansion of the gases and practically eliminate the sound of the explosions. In marine service there is often an under-water opening to the exhaust, but before leading the exhaust outboard an expansion chamber of ample size must be connected with the engine. With two-cycle motors there are many objections to the use of an under-water exhaust. The exhaust port being open during a considerable portion of the operation of the motor, combined with the fact that the explosive impulse and the inrushing fresh charge are the only methods of carrying off the burnt gases, often results in water or steam working back into the cylinder.

In a four-cycle motor the exhaust valve is closed against any back pressure during the entire intake stroke, and the burnt gases are forced out by the pressure of the piston during the scavenging stroke. For these reasons there is little chance of water or steam getting into the firing chamber. Wherever an under-water exhaust of any kind is used, however, it should be provided with a relief cock at its highest point of piping, as well as a drain cock at its lowest point; and it is also good practice to provide a three-way cock or valve at some point of the pipe in order that the exhaust may be turned off from its under-water connections and deflected through an exhaust pipe leading to the air above the water line. The valve connecting the underwater exhaust should always be turned off when the engine is to be idle for any length of time, for no matter how carefully the exhaust is installed or how far above the water line the motor may be, there is always a chance

that the boat may fill through a leak in the piping or that it may leak sufficiently or be filled with rain to such an extent that the inside connections will be lower

Fig. 102.—“Reid” Underwater Exhaust

than the water line, thus allowing the boat to fill and sink. The relief valve should always be opened when starting the motor to avoid the possibility of steam or water working back into the cylinder, and this valve should always be kept open when the motor is not in use.

Fig. 103.–Exhaust on Boat

The drain cock at the lowest point in the pipe will serve to drain off any water in the pipe in cold weather and also in starting, for if a pocket of water collects in

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