US1844106A - Exhaust muffler - Google Patents

Description

Feb. 9, 1932. F. H. SCHNELL EXHAUST MUFFLER Original Filed Ma "Mill/kill) NVENTOR E 56/7/72 WW w 634W ATTORNEYS Patented Feb. 9, 1932 UNITED STATES FFICE FREDERICK H. SGHNELL, 0F MADISON, WISCONSIN, ASSIGNOR T0 Q. 15. BURGESS LABORA- TORIES, INC., 015 MADISON, WISCONSIN, A CORPORATION 01' DELAWARE EXHAUST MUFFLER Original application filed May-8, 1929, Serial No. 361,376. Divided and this application filed May 29, 1929. Serial No. 366,892.

My invention relates to mufliers for noiseproducing gases and more particularly to exhaust mufllers for expanding and noise-producing pulsating gases such as are discharged from internal combustion engines.

This application is a division of my copending application, Serial No. 361,37 6 filed May 8, 1929, (now Patent No. 1,811,762, granted June 23, 1931).

My improved exhaust mufiler departs very decidedly in construction from the exhaustv mufilers heretofore employed.

,The explosion within the cylinder of an internal combustion engine creates tremendous pressure therein. The steam or vapor in the cylinder of a steam engine is also under very high pressure. Upon the opening of the exhaust port, the gas emerges at high velocity, very quickly reducing the pressure in the cylinder. By the word gas, I mean to embrace both gas and vapor. The result is a succession of pulses of gas at high pressure traveling through the exhaust manifold or pipe or any other type of exhaust system at a high velocity. Upon the emergence of the gas pulse into the atmosphere an additional spurt in velocity is probably acquired by reason of expansion, and a sharp sound results. The greater the pressure gradient between the pulse of gas and the atmosphere the greater the velocity and the sharper the sound. In order to successfully silence exhaust noises the muffler must suppress the pressure peaks and thus even the flow of the escaping gas pulses to the point where a sound is not created. In addition, I have found that sound communicated to or generated within the muflier should be silenced.

The mufflers of the present day accomplish the desired result by any or all of the following methods: cooling the exhaust gases and diminishing their volume; allowing the gases to expand and reducing their'pressure while confined; creating eddy currents and internal friction within the gases; causing friction between the gases and the walls and passages within the muflier; and/or impeding the forward progress of the gases by the interposition of baflies in their path. The typical muflier of todayis a metal shell connected in the exhaust system of an engine which may incorporate one or more of the following features: an expansion and mixing chamber followed by a small outlet; a tortuous passageway; a series of obstacles or baflies in the passageway; and/or a jacket in which a cooling medium is circulated.

Dnly a limited sphere of application can at best be found for amuflier with a cooling jacket. The expansion chamber alone is not successful since it approaches the condition of the atmosphere and sound is created therein. The types embodying tortuous paths, obstacles, and baffles, retard the flow of gas, and in so doing, build up a back pressure within the muflier and reduce the efficiency of the engine. There is a muffler being produced which actually assists in the scavenging of the cylinder but the scope of its application is limited to special types of engines of known, constant speed.

It is an object of my invention to provide an exhaust muffler whose scope of application extends to practically all of the common types of explosion and steam engines, to fire arms, and to air intakes and exhausts as in air compressors.

It is a further object of my invention to provide a mulher which will not build up sufficient back pressure to appreciably reduce the efliciency of the engine.

It is a further object of my invention to provide a mufiler which will silence explosion noises more elfectively than do the mufilers at present employed.

It is a still further object of my invention to provide a muffler which is more simple in construction than are the present mufflers of commerce.

Uther and further objects of my invention will become apparent as the following description progresses, which is to be taken in conjunction with the accompanying drawings, wherein:

Fig. l is a longitudinal, sectional view of one form of the invention; and

Figs. 2 and 3 are charts which illustrate the effect of my mufiiing device upon exhaust gases.

In the form of mufier for internal combus- .tion engines shown in Fig. 1 of the drawtake and exhaust noises, and noises produced by gases at room temperature, flammable gaspressure absorbing and sound-absorbing materials such as wool, cotton, or other cellulosic fibers may be used.

In view of the fact that every material which is not a perfect reflector of sound is, to a degree, an absorber of sound, it would probably be well to draw some sort of a specification for the term sound-absorbing material as that term is used in the present case. In all acoustic data the sound transmitted by an open window is used as the standard of comparison. The ratio of the sound absorbed by an area of material to that transmitted by an equal area of open window is called the absorption factor or value of that material. If a material one square foot in area absorbs one-fourth the sound transmitted by one square foot of open w-idow,-that material is said to have an absorption factor or value of 25 percent. In this specification whenever the term soundabsorption factor or value is used in conjunction with an absorbing materialor construction in a muffler, it is tobe understood that this factor or value is obtained by measuring the absorption of such material or construction in the usual Way by means of fiat r pad-s and the like of identical material and construction. This is necessary since it is very difiicult or impossible to obtain the factor after'th'e mufller is assembled. In the mufflers of my invent-ion as the sound absorption factor of the sound absorbing material decreases, the size of the mufller must increase and a material with a factor of 10 percent or less would probably necessitate an unwieldly size. Hence, in the present consideration, by sound-absorbing material I do not contemplate a material having an absorption factor of less than 10 percent at 1024 double vibrations per second. .Throughout this specification all absorption values are based on 1024 double vibrations per second. For most situations an excellent muffler may be constructed with a material having The absorbing material should be of appreciable thickness for best results. It should be at least one-quarter of an inch thick and preferabl one-half inch or more. By mineral fibre contemplate natural or artificial mineral wool, shredded asbestos, or any other mineral material of the same general nature.

' An inner cylinder 3 of foraminous rigid material such as perforated sheet metal provides a straight, open, unobstructed central duct 4 extending from -end to end of the muffier. Because of the high temperature of in,-

ternal combustion engine exhaust gases, cylinder 3 expands and therefore it preferably is not fastened to either metal disc 5 or 6 or both, thereby forming a slip joint and preventing buckling, since cylinderl which is also fastened to discs 5 and 6 does not expand. Disc 5 may be provided with a threaded central opening to receive pipe 7 which conducts the exhaust gases into the mutfler. It is usual in automobile mufflers to provide a slip joint so that pipe 7 is free to move in disc 5 and such construction may be employed in place of the threaded connection shown. The arrows in the drawings illustrate the direction of gas flow. The gases are finally exhausted through pipe 9 in opening 8 of disc 6. It is preferred to have the internal diameter of cylinder 30f the same size as exhaust pipe 7. If it is smaller it lncreases the back pressure. As will be discussed further hereinafter, if cylinder 3 is larger than pipe 7 the acoustic absorption decreases.

As shown the larger end of the truncated cone shell 1 is arranged at the inlet end of the mufiier. As the gas pressure and sound absorbing material 2 fills the space between this shell and inner cylinder 2, it deer-eases ifln thickness toward the outlet end of the muf- As hereinbefore explained, a, series of gas pressure peaks emerges at high velocity from pipe 7. Each peak tends to expand in all directions. The gas and pressure waves freely pass. through perforated cylinder 3 and enter annular space 2. The property of the perforated sheet metal of permitting the free transmission of gas and sound therethrough, together with its other advantages, makes it the preferred foraminous material for a device of this character. The perforations may vary in size and shape. Because of low- 1ngs.-- Holes suitable may vary from approximately 0.070 to 0.125 inches in diameter but 1 do not wish tobe limited to these dimensions. For most purposes the holes are evenly distributed throughout the area of the sheet metal although such even distribution is not necessary. The holes are present in sufficient number so that their total area represents from 2 percent up to 35 percent of the area of the sheetmetal though fair results may be obtained with holes having an area as low as 1 percent of the area of the sheet. The shape, size and distribution of the perforations all have an influence on the ease with which gases and sound pass through the sheet of metal. The holes should be small enough so that the absorbing material will not work its way out through them when the muffler is in service. Excellent results are obtained when the ratio of the unperforated portion of the metal forming such facing to the openings therein is such that a substantially continuous surface is exposed to the sound waves and gas pressure waves. In such cases the average dimensions of the individual openings are usually less than the distance between the edges thereof. Tests show that a muffler containing absorbing material faced with a stiff perforated sheet, the perforated area of which is as low as 2 percent of the area of the sheet with holes about .075 inch in diameter, absorbs as much sound as and has a mufiiing efliciency equal to a similar muffler in which the perforated facing is omitted. It is possible to omit the perforated metal facing or other foraminous facing by construct-ing a molded annular or other suitably shaped absorbing material as by bonding sized crushed mineral particles together at their points of contact to form a porous mass. Such a molded gas-pressure absorbing and sound absorbing material has an opening therethrough corresponding to the foraminous tube 3. In another construction metallic fibers, like steel wool, are formed into a mattress by stitching the fibres together with a metallic thread such as steel wire. This mattress is used to line the inside of the muffler shell 1. The foraminous duct may be omitted when such a mattress is used.

The fibrous gas-pressure absorbing and sound absorbing material comprises nonflammable fibres distributed in heterogeneous arrangement to form tiny interstices, pores orcells of more or less uniform size and distribution. It is packed in such a manner that usually only from 1 to 20 percent of annular space 2 is occupied by the actual fibres of the material and therefore 80 to 99 percent is free space. The packed fibres, depending on their physical properties, weigh after packing in the muffler, from 4 to 100 pounds per cubic foot. The packed mineral wool weighs from 9 to 36 pounds per cubic foot, the fibres occupying from 5 to 20 percent of the space. Steel wool weighs from 4 to 100 pounds per cubic foot, the fibres occupying from 1 to 20 percent of the space. Especially good results are obtained with steel wool packed to occupy 2 to 5 percent of the space. If the fibres are packed too loosely they jar down into a more compact mass after the muffler is put into use, and if they are packed too tightly the gas-pressure absorbing and acoustic absorption are cut down, thereby decreasing the muflier efficiency. The absorbing material is subjected to violent pounding and vibration by the oscillating influence of the exhaust gases and must not disintegrate readily under those conditions. Certain types of mineral wool resist this disintegrating action much better than others.

However, a metallic wool like steel wool is highly resistant to this action. The soundabsorbing portion of the mufller, therefore, provides an expansion space for gas-pressure peaks and the maximum pressure of the pulse of gas is decreased as a result of the expansion, the pressure wave being longer and lower in intensity. Fig. 2 shows in solid lines a hypothetical diagrammatic representation of several gas pulses as they enter the muflier and in dotted lines is a similar representation of the same pulses after expansion has taken place. The pressure wave characteristic has become flattened. I also believe that a sec- 0nd phenomenon takes place within annular form of a succession of rapid compressions and rarefactions. The compressions are suppressed and merged together by the choking effect of the walls of the tiny pores or cells with a resulting reduction of the intervening rarefactions. Fig. 3 shows in solid lines the pressure wave characteristic corresponding to the dotted curve of Fig. 2. The dotted lines of Fig. 3 show the pressure wave characteristic after the gas has undergone the choking action of the porous sound-absorbing material. The porous material offers practically no resistance to the fairly steady flow of the gas stream through the straight and unobstructed duct 4, and especially if duct 4 is made of smooth perforated, sheet metal. it facilitates the flow with the net result that there is practically no resistance to the gas flow and little or no back pressure is built up aside from that due to the surface friction. The absorption of the gas-pressure peaks by the absorbing material to smooth out the gas flow and eliminate the noise made by the gas slugs when emerging into the atmosphere is an important feature of my invention. I use the sound-absorbing material for this double function without appreciably increasing the of the mufiler within reasonable limits produced no appreciable change in ultimate quantity of sound, measured by means of an acoustimeter, WlllCll emerged from the muffler when the latter was connected in the ordinary manner in the exhaust system of an internal combustion engine. However, when the unmufiled exhaust noises were acoustically separated from the gases and conducted to the muffler through anintake pipe similar to that used in the prior tests, it was found that the quantity of sound emerging from the muffler varied with the diameter of the mufiier. From such data it appears that a muffler ,of small diameter, while it does not permit much ex pansion and does not act as efliciently upon the gas pulses, acts more efficiently upon the noise created therein with ultimate mufl'lin equal to or better than that of the larger device which allows the gas to expand more and thus acts more efficiently upon it but acts less efliciently upon the noises created therein. The muffler absorption for any one absorber may be increased by increasing the thickness of the absorber within limits, or by increasing the outside diameter of the mufiier within limits, or by increasing the length of the absorber without changing its thickness, or by decreasing the gas duct diameter within limits.

lVhile I have illustrated and described but a very few simple forms of my invention, it is understood that I may employ a great many forms since I believe that I have inventedthe broad principle of providing a muffier with gas-pressure absorbing and sound absorbing material adjacent the path 'of the exhaust gases and do not wish to be confined to structural details. For instance, the inner shells may be of wire screen, or other suitable foraminous material, or may be entirely omitted as hereinbefore explained, and the general shape of the device need not be cylindrical. Its cross section may be circular, rectangular, elliptical or of any other form and the manner of assembling the parts may vary considerably from that described hereinbefore. The rectangular section is more effective than the circular section because of the greater area of absorber exposed to the gases in proportion to the cross-sectional area of the duct. My muffler consists essentially of a duct lined with gas-pressure absorbing and sound absorbing construction which ma be a duct lined with absorbing material without a foraminous lining. It may be a fibrous gas-pressure absorbing and sound-absorbing 'jectionable.

preciable back pressure. Many present muf flers assist the silencing action by constricting greatly the cross-sectional area of the gas outlet but this cannot be done without building up back pressure.

Although published data available relative to mechanics of noise production by internal combustion engine exhausts are meagre, I believe the hypotheses and explanations hereinbefore advanced to explain the manner in which my device accomplishes the muffling of exhaust or explosion noises are true. Whatever the theories and explanations may be, the fact remains that, although a substantially straight and unobstructed g path is offered the exhaust gases and noises they do not traverse that path unchanged but undergo a change therein which causes them to emerge almost, if not quite, noiseless.

Mufliers which are considered satisfactory and used by automobile manufacturers, show by acoustimeter measurement, a total muffling efficiency of at least percent when the internal combustion engines, for which the mufflers were designed, are run at full load. A muffler for an automobile engine which has a mufiling efficiency at full load of percent is exceptional. Although the muflling efficiency of satisfactory mufllers for automobiles is only 55 percent at'full load, it must be recognized, that in cities where this efficiency should be high, automobile engines practically are never run at full load. Under light loads, characteristic of city driving, the efficiency may be Well over 95 percent. Even on country roads where high speeds may be attained, automobile engines are seldom subjected to full load. On the other hand, motor boat engines and especially outboard motors, are very often run at full load, so that unless the muffler efficiency is high, that is. above 95 percent, the exhaust noise is excessive. This noise is heard for a long distance over the water and therefore is ob- The ordinary outboard motor muffler, although seemingly inefficient, has an efficiency of from J7 5 to 90 percent at the sacrifice of some power due to back pressure. Stationary internal combustion engines and others which are run so that they are well loaded, usually must be equipped with high efficiency mufflers.

The efficiency of the muffler is determined by measuring the noise generated by the gases escaping into the atmosphere with and without muifiing. The percent efficiency is the ratio of the decrease in measured sound to that measured without muffling. 'l he muffier efliciency should be determined when the muffler is used with the engine for which it is designed. This is essential since the size,

shape, amount of absorbing material, and

other variables of the muffler are determined for each of the various sizes and types of engines.

Using my invention, as hereinbefore described, it is possible to construct mufflers of varying efficiencies, from 55 percent to well over 95 percent by varying the quantity and type of absorbing material and by varying the construction as hereinbefore described. Such mufflers may be constructed so as to cause practically no increase in the back pressure on the engine at full load aside from that due to the surface friction of the duct walls. Furthermore, such mufflers occupy a small space, are light in weight, and may be built at a considerable saving in cost over the present-day mufiler.

I claim:

1. A muffler for noise producing expanding gas comprising the combination with a substantially imperforate casing of a duct arranged in said casing, and forming a substantially straight passage for said gas through said casing, said duct having a gaspressure absorbing and sound absorbing wall structure, said absorbing structure decreasing in thickness from the inlet to the outlet of said duct and consisting of porous packed material in layers of appreciable thickness whereby the noises are effectually silenced.

2. A muflier for noise producing gas comprising the combination with a substantially imperforate casing, of an open-ended duct arranged in said casing and forming a substantially straight passage for the gas through said casing, said duct having a gaspressure absorbing and sound absorbing wall structure which decreases in thigkness from the inlet to the out-let of said duct, said wall structure being of such efficiency that its average sound absorbing value is at least 25 per cent.

3. An exhaust muffler for internal combustion engines comprising an inner, open-ended, perforated, cylindrical, metal shell and an outer co-axial open-ended truncated cone shaped substantially imperforate metal shell surrounding said inner shell and spaced therefrom, the larger end of the cone being toward the intake end, the annular space between said shells being filled with porous packed metallic fiber,

4. A muffler for the exhaust gas of an internal combustion engine comprising the combination with a substantially imperforate casing, of a cylindrical duct arranged in said casing and forming a passage for the gas through said casing, said duct having a gaspressure absorbing and sound absorbing wall structure comprising porous packed nonflammable fibers, said wall structure decreasing in thickness from the inlet to the outlet of said duct, said muffler having a muliling efficiency of at least 55 per cent when used with a fully loaded engine.

5. An exhaust mufller for internal combustion engines comprising an inner, openended, perforated metal cylinder, an outer coaxial, open-ended, truncated cone shaped substantially imperforate metal shell surrounding said cylinder and forming an annular space between said cylinder and said shell, the larger end of the cone being toward the intake end, and a filling of porous packed non-flammable fibers arranged in the annular space between said cylinder and said shell. said fibers presenting surfaces to the exhaust gas passing through the muffler and being of such efiiciency as a gas-pressure absorbing and sound absorbing material that the average sound absorbing value is at least 25 per cent.

6. A mufiler for noise producing gas comprising the combination with a substantially imperforate casing, of a foraminous duct arranged in said casing and forming a passage for said gas through said casing, said duct having a gas-pressure absorbing and sound absorbing backing, said backing decreasing in thickness from the inlet to the outlet of said duct, said backing comprising porous packed non-flammable fibers, the individual fibers occupying from 1 to 20 per cent of the space occupied by said packed fibers.

7. A muffler for the exhaust gas of an internal combustion engine comprising the combination with a substantially imperforate casing, of a foraminous duct arranged in said casing and forming a substantially straight passage for the gas through said casing, said duct having a gaspressure absorbing and sound absorbing backing, said backing decreasing in thickness from the inlet to the out let of said duct.

8. A muffler for the exhaust gas of an internal combustion engine comprising the combination with a substantially imperforate casing, of a perforated metal duct arranged in said casing and forming a substantially straight passage for the gas through said casing, said duct having a gas-pressure absorbing and sound absorbing backing, said backing decreasing in thickness from the inlet to the outlet of said duct, said backing comprising porous packed steel wool, the fibers thereof occupying from 1 to 20 per cent of the space occupied by said packed steel wool.

9. A muffler for the exhaust gas of an in ternal combustion engine comprising the combination with a substantially imperforate casing. of a foraminous duct arranged in the casing and forming a substantially straight passage for the gas through said casing,-and

a gas-pressure absorbing and sound absorbing backing for said duct, said backing varying in thickness.

10. A mufiier for noise producing gas comprising the combination with a substantially imperforate casing, of a duct arranged in said casing and forming a substanitally straight passage for said gas through said 1 casing, said duct having a gas-pressure absorbing and sound absorbing wall structure, said absorbing structure decreasing in thick ness from the inlet to the outlet of said duct and consisting of porous packed material in layers of appreciable thickness, said material weighing from 4 to 100 pounds per cubic foot.

11. A muflier for theexhaust gas of an internal combustion engine comprising the combination with a substantially imperforate casing, of a duct arranged in said casing and forming a substantially straight passage for the gas through said casing, said duct having a gas-pressure absorbing and sound absorbing wall structure, said structure varying in thickness and weighing from 4 to 100 pounds per cubic foot.

In testimony whereof I afiix my signature.

' FREDERICK H. SCHNELL.

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