US2046193A - Muffler - Google Patents

Description

INVENTOR (QL/du n ATTORNEYS M .20/'Cer' @AMI/HWY June 30, 1936. w. E. sPlcER MUFFLER Filed Jan. 5, 1931 Patented June 30, 1936 UNITED STATES PATENT OFFICE MUFFLER Delaware Application January 3,

4 Claims.

tion over that shown in the F. H. Schnell application, Serial No. 361,376, led May 8, 1929 (now Patent No. 1,811,762, granted June 23, 1931).

It is the object of this construction to secure better mufiiing than has heretofore beenobtained throughout the entire range of frequencies of sound generated by the explosion of and the expansion of gases.

The explosion within the cylinder of an internal combustion engine creates tremendous pressure therein. 'I'he steam or vapor in the cylinder in 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 gasI mean to embrace both gas and vapor. The result is a succession of pulses or slugs 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 pulsating exhaust noises the muiiiers must suppress the pressure peaks and thus even the ilow of the escaping gas pulses to the point where the volume of sound generated is below the desired minimum. In addition the sound communicated to or generated within the muiier should be silenced.

The muiller disclosed in the Schnell patent does not employ the principles employed in other present day muffiers. Those muiilers rely on the use of tortuous paths, obstacles and baiiies which retard the ow of the gas, but in so doing,v they build up a back pressure within the muffler which reduces the eiiiciency of the engine. In the Schnell invention the noise producing gas passes along the surface of gas-pressure absorbing and sound absorbing material, the passageway being free of bailles and being straight or of such shape that it does not substantially increase the back pressure of the flowing gas.

'Ihe muiiler of this application may be applied to practically all the common types of explosion 1931, Serial No. 506,361

and steam engines, to rearms and to air intakes and to exhausts, as in air compressors.

Other and further objects of my invention will become apparent as the followingidescription progresses, which is to be taken in conjunction 5 with the accompanying drawing wherein:

Fig. 1 is a longitudinal, sectional view of one form of my invention;

Fig. 2 is a transverse, sectional view on line 2-2 of Fig. 1;

Figs. 3 to 7 are transverse, sectional views 'of other forms of my invention;

Fig. 8 is a longitudinal, sectional view of another embodiment of my invention; and

Figs. 9 and 10 are charts which illustrate the 15 effects of my muiiiing device upon exhaust gases.

In one form of muiiier for internal combustion engines shown in Figs. 1 and 2 of the drawing, an outer sheet metal oval shaped cylinder Il is welded or otherwise fastened at its ends to sheet metal end closure discs I2 and I3. Cylinder Il encloses an intermediate space I4 packed with a porous, gas-prssure absorbing and sound absorbing material which may be a mineral fiber, steel, copper, or other metallic wool or mixtures thereof, or any other porous gas-pressure absorbing and sound absorbing, non-dammable material such as sized crushed mineral matter, mica,

-exfoliated vermiculite such as Zonolite, ground slag, coke, pumice, or other porous aggregate such as Haydite and built up units of such aggregate that may be made by cementing the particles together at their points of contact. Porous burned clay products may be used. For silencing blow-oil? gases, air compressor intakes and exhaust noises, and noises produced by gases at room temperature, flammable gas-pressure absorbing and -sound absorbing'material such as wool, cotton, or other cellulosic bers may be used. In some instances the muffler may be used under circumstanceswhich would tend to cause the deterioration of steel wool and in such instances a metallic wool formed of stainless steel or other steel, or rust resisting alloys may be used.

In view oi' the fact that every material which is not a perfect reiiector 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 50 sound transmitted by anopen window is used as the standard of comparison. The ratio of the sound absorbed by an Iarea. oi' material to that transmitted by an equal area of an open window is called the absorption factor or value of that 55 material. lf a material one square foot in area absorbs one-fourth the sound transmitted by one' square foot of open window that material is said to have an absorption factor or value of 25 percent. In this specification whenever the term sound absorption factor or value is used in conjunction with an absorbing material or construction ln a muiiier, it is to be understood that this factor or value is obtained by measuring the absorption of such material or construction in the usual way by means of nat pads and the like of identical material and construction. rlhis is necessary, since it is very diicult or impossible to obtain the factor after the muler is constructed. In the routiers of this invention, as the sound absorption factor of the sound absorbing material decreases, the size oi the muffler must increase and a material with a factor of lll percent or less would probably necessitate an un- Wieldly size. Hence, in the present consideration for sound absorbing material l do not conteniplate a material having an absorption factor oi' less than l percent at 1024. double vibrations per second. Throughout this specification all absorp-m tion values are based on i624 double vibrations per second. For most situations an excellent muiller may be constructed with a material having an absorption factor of 25 percent or more. For exceptional results l prefer to use a material having a factor greater than 45 percent. Because of the present state of the ait it is impossible to express the gas-pressure absorbing qualities of the filler similarly. Since the sound absorbing' and gas-pressure absorbing properties apparently are closely related numerical data are conned to the former and are usually assumed to apply to the latter. rEhe combined absorbing effect may be expressed in 'terms of muffler eiiciency as is explained hereinafter. By mineral ber contemplate natural or artilcial mineral wool, shredded asbestos, or any other mineral material of the same general nature.

An inner cylinder l of foraminous rigid material such as perforated sheet metal provides a straight open unobstructed central duct lli extending frorn end to end of the mufier. Because of the high temperature of internal combustion engine exhaust gases, cylinder l5 expands and therefore it preferably is not fastened to either metal discs l2 or i3 or both, thereby forming a slip joint to prevent buckling since cylinder il which is also fastened to discs l2 and i3 does not expand appreciably. Disc i3 may be provided with a threaded central opening to receive pipe l l which conducts the exhaust gases into the muffler. It is usual in automobile mulers to provide a slip joint so that pipe il ls Iiree to move in disc i3 and such construction may be employed in place l of the threaded connection shown. rl'he arrows in the drawings illustrate the direction of gas iiow. The gases are finally exhausted through pipe i8. It is preferred to have the internal diameter of cylinder l5 of at least the same size as exhaust pipe il. If it is smaller it increases the back pressure. If cylinder l5 is larger than pipe l'i the acoustic absorption usually decreases, the exceptions occurring under certain special conditions. The contributing factors which cause these conditions are not fully recognized or understood. Y

As hereinbefore explained, a series of gas-pressure peaks emerges at high velocity from pipe il. Each peak tends to expand in all directions. The gas and pressure waves freely pass through perforated cylinder I5 and enter annular space it.

accesos 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 oi this character. vary in size and shape. Because of lower manufacturing costs I prefer circular openings. Holes suitable may vary from approximately 0.070 to 0125 inch in diameter but I do not wish to be 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 suicient number so that their total area represents 2.5% to 35% of the area oi the sheet metal though fair results may be obtained with holes having an area as lowas l percent of the area of the sheet. A greater area may be perforated. The size, shape and distribution of the perforations all have an influence on the ease with which gases and sound pass through the sheet metal. |The holes should be small enough so that the absorbing material will not Work its way out through lthem when the muiiier is in service but not too small to be plugged by dirt and especially the carbon present in the exhaust gases of internal combustion engines. 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 stii perforated sheet, the perforated area of which is as low as 2.5 percent of the area of the sheet with holes about .075 inch in diameter absorbs as much sound as and has a muiiling efficiency equal to a similar muiiler in which the perforated facing is omitted. It is possible to omit the perforated metal facing or other foraminous facing by constructing a moulded 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 moulded gas-pressure absorbing and sound absorbing material has an opening therethrough corresponding to the foraminous tube i 5. In another construction metallic bers like steel wool are formed into a mattress by stitching the fibers together with a metallic thread such as steel wire. The mattress is used to line the inside of the muffler shell l. Under certain conditions the foraminous duct may be omitted when such a mattress is used.

Fig. 9 shows in solid lines a hypothetical diagrammatic representation of several gas pulses as they enter the muiller and in dotted lines is a similar representation of the same pulses after expansion has taken place. The pressure Wavevcharacteristic has become flattened. also believe that a second phenomenon takes place within annular space it to further flatten the wave characteristic. Relatively speaking the rapid moving gas enters the annular space I 4 in the form of a succession of rapid compressions and rarefactions. The compressions are suppressed and merged together by the choking eects of the walls of the tiny pores or cells with a resulting reduction of the intervening rarefactions. Fig. l0 shows in solid lines the pressure wave characteristics corresponding to the dotted curve of Fig. 9. The dotted lines of Fig. l0 show the pressure wave characteristic after the gas has un- The perforations may dergone the choking action of the porous sound absorbing material. The porous material ofl'ers practically no resistance to the fairly steady flow of the gas stream through the straight and unobstructed duct i5 and especially if duct I5 is made of smooth perforated sheet 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 back pressure. The exact relation between these various factors, in so far as it affects the intensity of sound emitted from the muffler is not known.

I believe that some noise is generated at the end of pipe i1 as explained heretofore in connection with expansion chambers. This noise is silenced by the gas-pressure absorbing and sound absorbing material. It is not projected longitudinally of the duct I6 since sound does not project itself in a single direction to form rays. It emanates equally in all'directions unless it encounters reflecting surfaces. Foraminous cylinder I5 allows this sound to pass freely therethrough and it is thereafter absorbed by the material in space Il.

Although a muffler of the Schnell construction, in which the absorbing material thickness does not vary appreciably throughout the annular space I4, is emcient, it apparently absorbs selectively at certain frequencies, its efficiency seemingly being lowest at the low frequencies. These frequencies may be gas-pressure frequencies induced by sounds, explosions or mechanical means. Since the sound absorption and gas-pressure absorption are effected more or less simultaneously it has been impossible to determine whether this selective absorption is caused by an effect either on the sounds reaching or generated in the mufiler or on the gas-pressure Waves or both. I believe also that the gas-pressure peak absorption varies with the absorber thickness for varying exhaust gas speeds. For this reason the noise produced by the individual pressure slugs from any one internal combustion engine exhaust va.- ries with the thickness of absorber, speed and power output of the engine. By providing a gaspressure absorbing and sound absorbing lining which varies substantially in thickness in any transverse plane along the greater part or all of the duct I6 it is possible to obtain a better absorption since all frequencies seem to absorb more uniformly, and if my hypothesis is correct, it absorbs gas-pressure peaks more uniformly at all exhaust gas speeds. There is also less chance for any one frequency or pressure effect due to a certain gas speed to build up with such a varying thickness. It is desirable that the maximum thickness of the lining in-some of the transverse planes should be at least twice the minimum thickness in the same transverse plane.

There are many constructions which allow the thickness of the lining to vary substantially in any transverse plane along the duct length. Some of these are illustrated in the accompanying drawing. Fig. 2 shows the use of a circular duct in an oval shaped container. Fig 3 shows a circular duct off center in a circular container. Fig. 4 shows a circular duct off center in an oval container. Fig. 5 shows a circular duct in an odd shaped container. 'Fia'.G shows a circular duct in a substantially triangular shaped container. Fig. 7 shows an oval duct in a circular container. Fig. 8 illustrates another construction whereby the thickness of the lining varies 5 substantially along transverse planes. In this construction a circular container is used but the duct enters off center at one end and leaves off center at the other end at the side opposite from the first end.

Other methods for obtaining the desired efl'ect will suggest themselves to those illed in the art. In any form of device I may c oose to construct the cross sectional ares. of the outlet should be large enoughA not to build up any appreciable l5 back pressure. Many present muffiers 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 on the me- 20 chanics 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 mufiiing of exhaust explo- 25- sion noises are true., Whatever the theories and explanations may be, the fact remains that although a substantially straight and unobstructed path is offered the exhaust gases and noises, they do not traverse that path unchanged but undergo 30 a decided change therein which causes the gases to emerge quieted in proportion to the amount of effective absorbing material used.

Mufflers which are considered satisfactory and used by automobile manufacturers, show by 35 acoustimeter measurement, a total muflling efllciency of at least 55 percent when the internal combustion engines, for which the muiflers were designed, are run at full load. A muler for an automobile engine which has a muflling efficiency 40 at full load of`75 percent is exceptional. Although the muifling efficiency of satisfactory mufllers for automobiles is only 55 percent at full load, it must be recognized that in cities where thisefliciency should be high, automobile en- 45 gines practically are never run at full load. Under light loads, characteristic of city driving, the efficiency may be weil over 95 percent. Even on country roads where high speeds may be attained, automobile engines are seldom subjected 50 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 muiller efficiency is high, that is, above 95 percent, the exhaust noise is excessive. This noise is heard for 55 a long distance over the water and therefore is objectionable. The ordinary outboard motor muffler, although seemingly inefllcient, has an efficiency of from '75 to 90 percent at the sacrifice of some back pressure. Stationary internal com- 50 bustion engines and others which are run so that they are well loaded, usually must be equipped with high efllciency mufllers.

The efhciency of the muffler is determined by measuring the noise generated by the gases es- 05 caping into the atmosphere with and without muffling. .The percent efficiency is the ratio of the decrease in measured sound to that measured without muffiing. The muffler efficiency 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, itis possible to construct muillers of varying etilciencies, from per cent to well over 95 percent by varying the quantity and type of absorbing material and by varying the construction as hereinbeiore described. Such muiilers 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 muiilers may be made to occupy a small space, may be made light in weight, and may be built at a considerable saving in cost over the present-day muiller.

I claim:

1.'A muilier comprising a substantially imperforate, oval outer casing having an opening in each end, a.. filling of sound-absorbing materialin said casing, a cylindrical foraminous duct through said illling connecting said openings, the distance between said duct and said casing being'at least twice as great along the major transverse axis of said casing as it is along the v minor transverse axis of said casing, whereby the maximum thickness of said absorbing material in any transverse plane along said duct is at least twice the minimum thickness in the same plane.

2. A muilier comprising a substantially imperforate casing having an opening in each end, a lining of sound absorbing material in said casing having a duct therein connecting said openings whereby all of the gases passing through said sacamosA periorate casing having an opening in each end, 10

a lling of sound absorbing material within said casing, a cylindrical passageway through said filling connecting said openings and communicating with said filling, whereby all oi' the gases passing through said casing flowthrough said l5 passageway, said passageway being displaced substantially from the central axis of said casing, whereby the thickness of said lining varies substantlally in any transverse plane along the greater part of its length. 20 n.

4. A muffler comprising a substantially imperforate casing having an opening in each end, a foraminous tube within said casing connecting said openings whereby all of the gases passing' through said casing iiow through said tube, sound 25 absorbing material` in the space surrounding said tube, said tube being displaced substantially from the central axis of said shell whereby the thickness of said lining varies substantially inany transverse plane along the greater part oi' its 30 length.,

WALTER. E. SPICER.

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