Although it is not essential that motor trucks operate as quietly as pleasure cars, it is quite essential that they operate without disagreeable noise. For this reason the exhaust must be muffled, which is accomplished by passing the spent gases through a muffler before they are discharged into the atmosphere. This muffler is sometimes referred to as a silencer. The object of the muffler is to permit the gases to expand and to cool, thereby reducing the pressure, which is the cause of the noise when they are discharged into the atmosphere directly. It is quite simple to obstruct the passages of the gases from the engine to atmosphere, however, in order to discharge them without disagreeable noise, they must be permitted to escape very freely, so that they will not create any back pressure on the pistons during the exhaust stroke.
In most cases the muffler is mounted as far away from the engine as the general chassis design permits. It is usually mounted under the frame of the vehicle. This arrangement permits the engine to exhaust into an exhaust pipe of considerable length, the capacity of which is sometimes as much as four times the piston displacement of one cylinder. This long pipe gives the gases a chance to cool before they reach the muffler, while the latter should be arranged in such a manner that heat may rapidly be abstracted from the gases.
Mufflers generally consist of a series of expansion chambers which communicate by means of fine and sometimes tortuous passages, and after passing through these chambers the gases are finally permitted to escape. Mufflers should possess certain features, a construction which permits cleaning, strength to withstand pressures of gasoline - air vapor explosions at atmospheric pressure and ability to resist vibration. Cleaning is necessary, as lubricating oil and solid carbon particles held in suspension by the gases will clog the fine passages and increase the back pressure. Some mufflers are so constructed that they can be dis-mantled for cleaning. Explosions in the muffler are frequent, and it is essential to have enough strength to resist this pressure without bursting. The trails of the muffler may be so designed as to prevent ringing which is really a harmonious vibration of suc-oeading exhausts. This is sometimes accomplished by lining the outer shell with sheet asbestos, but this may nut seem very desirable, as it possesses certain heal insulating qualities.
The final outlet is generally through a pipe of considerably less cross section than the exhaust pipe. Tin's is sometimes flattened so that the gases escape in a continuous sheet. In soma cases this is accomplished by a series of small holes in the final outlet or the outer shell.
In order to relieve the so-called back pressure of the muffler, cut-outs are sometimes provided, which are placed forward of the muffler or in the exhaust, pipe, and permit ex-hausting the gases directly into the atmosphere, instead of having them pass through the muffler. The general claim for this cut-out is thai it adds to the efficiency and power of the engine. However, it has been proven that with a well designed muffler a cut-out is of little value, except that the operator may occasionally listen to the action of the engine.
The Pierce muffler (Fig. 247) consists of two adjacent cylindrical chambers, which communicate through a number of holes in the inner chamber. The outer chamber is divided into three expansion chambers, while the inner member is also divided into three pads, the partitions of each member being pressed steel discs. The gases first enter the inner chamber and pass through perforations into the outer chamber and from this chamber they return to the second portion of the inner cham bar and thence into the second expansion chamber, theme through the central member into the third expansion chamber and back into the central member from which they are discharged. This muffler is mounted parallel to the side members of the frame and has east end plates which have integral brackets for frame mounting. The tultubular members and end plates an held together by long tie rods which extend the full length of the muffler.
Fig. 247. The Pierce Muffler.
The Packard muffler (Fig. 248) also consists of an inner and outer chamber with cast end plates, retained by tie rods. However, the necessary volume is obtained by increasing the diameter instead of making the muffler of considerable length. The inner chamber has a series of small holes, through which the gases pass after expanding in the inner chamber. After expanding in the outer chamber they escape at the rear end. This outer chamber is insulated with sheet asbestos retained by plates and small screws to deaden the vibration. The supporting brackets are made of pressed steel and fit into the side members of the frame.
Fig. 248. The Packard Muffler.