US3361227A

US3361227A - Mufflers and exhaust systems

Info

Publication number: US3361227A
Application number: US405086A
Authority: US
Inventors: Kaari Kauko
Original assignee: MEKES Oy
Current assignee: MEKES Oy
Priority date: 1963-10-24
Filing date: 1964-10-20
Publication date: 1968-01-02
Anticipated expiration: 1985-01-02
Also published as: DE1476543A1

Description

Jan. 2, 1968 KAUKO KAARI MUFFLERS AND EXHAUST SYSTEMS Filed oct. 20, V1964 2 Sheets-Sheet l O AU o m o o o 0 mh.

0 I lllll Jan- 2, 1968 KAUKO KAARI MUFFLERS AND EXHAUST SYSTEMS 2 Sheets-Sheet 2 Filed Oct. 20, 1964 rroeA/E Ys United States Patent O 3,361,227 MUFFLERS AND EXHAUST SYSTEMS Kauko Kaal-i, Helsinki, Finland, assignor to Mekes Oy, Helsinki, Finland Filed Oct. 20, 1964, Ser. No. 405,086 Claims priority, application Finland, Oct. 24, 1963, 2,071/63 4 Claims. (Cl. 181-44) The present invention relates to muffler .and exhaust systems and has for its primary object the provision of a new and improved construction and arrangement increasing the operating efficiency of motor cars with which they may be used.

Exhaust systems and mufflers have become an important factor of cost to car owners. One reason is the relative short life because of corrosion. Another is the higher price brought about by the considerable variety of constructions which increase the cost of storage, etc. A reason for the variety of constructions required resides primarily in the fact that mufflers as built to date are comparatively large and bulky and have to be designed in accordance with the available space in which they are to 4be used. Cars have recently been constructed with lower and lower ground clearance with the result that greater difficulties have been encountered. Thus, as a result of the great variety of constructions and dimensional requirements of components of exhaust systems, together with the rapidly increasing number of cars in use and the short life of the components, the matter of cost is becoming increasingly important.

It has been believed quite widely that the influence of the muier on the performance of a four-stroke engine is, as a practical matter, Very small and that an effective muffler should have a volume approximately 3 to 5 times the total cylinder displacement of the engine. On the other hand, it has been verified, especially as a result of studies of racing cars, that the construction and dimensions of the exhaust system is an important factor in the performance of the engine. It can be shown by tests that faulty construction or dimensioning can cause losses as high as in engine performance, which losses can be avoided, in accordance with the present invention, by the construction and arrangement of an exhaust system with a mufler so as to utilize the kinetic energy of the exhaust gases for improving engine suction and, thus, the cylinder filling or intake.

Tests have shown that utilization of the kinetic energy of the exhaust gases can produce an increase in operating efficiency. By and large, a primary consideration in the design of exhaust systems has been the so-called back pressure, which is the average pressure in the manifold. Much of the existing literature indicates that it is essential for the effective working of the engine that the back pressure be as low as possible. However, this is not the entire answer. The average back pressure in the exhaust system depends on the load-rate of the engine and it increases almost linearly when the load is increased. Back pressure itself is not the determining factor, the latter actually being the amount of residual gases left in the cylinder as a result of the back pressure. The present invention contemplates changes in the exhaust system equipped with a `muffler so as to utilize the kinetic energy of the exhaust gases to improve suction .and cylinder filling, thereby to improve performance of the engine, even though the average back pressure may be increased.

Tests made by others have shown that the maximum power output bears a certain relation to the length of the exhaust pipe, which relationship is believed to depend on the kinetic energy of the gases flowing in the exhaust system, although it has not been accurately explained in 3,36 1,227 Patented Jan. 2, 1968 the literature. The results of tests, indicating the foregoing, are set forth and will be described later herein in connection with FIGS. 1 to 3. These tests indicate that utilization of a proper length of exhaust pipe produces pressure minima in the manifold at the required moments to increase the engine suction and thereby increase the operating efficiency of the motor.

In accordance with the present invention the exhaust system with a muffler is constructed so that it includes no substantial expansion chambers and the total length of the exhaust system or pipes is a whole number times the distance between the pressure peaks travelling in the pipes under normal loading conditions, which distance is referred to as the wave length, whereby it is possible to produce in the manifold pressure minima at the correct moments to increase the suction and cylinder intake. Furth-er, in accordance with the present invention, the exhaust system additionally includes a muffler or silencer. The latter can be included in the system under the following conditions: (1) that there be included no chamber or enlargements in the system (including the silencer or silencers or other chambers filled with absorption materials), which could act as substantial expansion chambers; (2) that the gas flow velocity remain substantially constant throughout the system; (3) that the distance from the exhaust manifold to the first silencer be some whole number (preferably 2) times the wave length, i.e., the distance between the pressure peaks travelling in the pipes under normal loading conditions; (4) that the distance from the first silencer to the second silencer, if there is one, or to the end of the exhaust pipe follow the same rule; and, (5) that the muffler have a length corresponding to the above mentioned whole number times the wave length.

Accordingly, it is an object of the present invention to provide a new and improved exhaust system including one or more mufflers constructed and arranged to produce pressure minima at correct times to increase engine suctions and cylinder filling and to provide adequate muffling or sound attenuation.

An object of the invention is to develop a muffler, which can be made of small size and which, because of its small size, both eliminates the large variety of types heretofore required and reduces corrosion and at the same time effectively dampens the sound `and increases the efficiency of the engine.

A further object of the present invention is the provision of a muffler in which silencing is effected by dividing the gas fiow into several streams having different length fiow paths and then recombining them to effect muffiing and in which the muffler is made of relatively small size by arranging the flow paths in what may be defined as concentric relation. Due to the small cross section, the use of the muffler can be increased, because a small muffler can be mounted under nearly every car. Also, the small size of the muffler reduces the corrosion because small size means a smaller .amount of gas at each unit otsurface of the outer casing. rlihe amount of acid solution formed by condensed exhaust gases at each unit of surface is substantially smaller than in -muflers commonly used and, thus, the corrosion is reduced.

Another object of the invention is the provision of a muier comprising a easing, an inlet for said casing and an outlet from said casing in which said inlet, casing and outlet are generally longitudinally aligned, and also having passage defining means in the casing providing communication between the inlet and outlet and dividing the gas entering through said inlet into two generally longitudinally disposed streams and then reuniting the streams for discharge through said outlet, and wherein said passage means maintains the sum or difference and the direc- 3 tion of the kinetic energy of the gas contained in the two streams substantiaily identical with the kinetic energy of the gas ow at the inlet and outlet.

the straight through type and which curves are based `on the following experimental results, also set forth in the aforesaid book:

In brief, the muier of the present invention includes an outer casing with inlet and outlet pipes aligned axially with the casing and which casing, by means of inside partitions, is divided into several gas ow passages. The passages divide the incoming gas stream into at least two longitudinal or axial streams having iiow paths of different lengths and then reuniting these into one gas stream for discharge :from the muffler. The arrangement is such that the difference between the shortest and the longest fiow paths is almost the same as the interval between the pressure peaks in the inlet pipe and such that the velocities of flow of the different part streams Iare almost identical. The difference between the shortest and the longest paths being almost the same as the pressure peaks is utilized to dampen or silence the gas flow. The dampening is made more effective by dividing the gas stream several times into part streams and again reuniting. By forcing the part streams to ow concentrically, the cross section of the mufer can be reduced considerably and at the same time the natural vibrations are aiso reduced. In case it is desired to effect silencing with one muer, it is possible to use mutiiers of small size, the total volume of which is not greater than the total cylinder volume. Increased silencing can be effected with two or more mufflers, in which event the total volume of the muiflers should not be more than the total cylinder volume. The muffler and the exhaust connections are such that the mufer is located a distance equal to a whole number times the distance between exhaust gas pressure peaks at normal loading, the muier having a length equal to the distance between pressure peaks.

Other objects and advantages of the present invention will become apparent from the accompanying description of embodiments thereof in the course yof which reference will be had to the accompanying drawings, in which:

FIGS. l, 2 and 3 are curves illustrating instantaneous pressure minima obtained with an open pipe exhaust systern of the prior art;

FIG. 4 is a longitudinal cross sectional View of a muier constructed in accordance with the present invention;

FIG. 5 is a transverse cross sectional view taken along the lline 5--5 of FIG. 4;

FIG. 6 is an illustration of an exhaust system constructed in accordance with the present invention;

FIG. 7 is a longitudinal cross sectional view of another embodiment of the muiier of the present invention arid taken along the line 7-7 of FIG. 8;

FIG. 8 is a transverse cross sectional View taken along the line 8 8 of FIG. 7; and

FIG. 9 is a perspective view comparing the size of a mui-lier of the present invention with a comparable known type it can replace.

Referring first to FIGS. 1, 2 and 3, these represent findings set forth in a book by Philip H. Smith: The Scientific Design of Exhaust and Intake Systems, 1962- Robert Bently, Inc. indicating pressure minima occurring at the end of the stroke and with open exhaust pipes of As earlier indicated herein, it is seen that with open exhaust pipes of a particular length, the pressure minima are considerably lower than with open pipes of other lengths. As heretofore explained, it is the kinetic energy of the exhaust gases which produce the properly timed minima in the manifold or back pressure and thereby improve the suction and cylinder filling to provide the improved results. Based on studies on Mr. Smiths work, it appears that the wave length in his experiments` was about 22il".

In order to provide a system useable in modern automobiles, the exhaust system of the present invention utilizes the kinetic energy to improve operation and at the same time it provides a mufiling, silencing or attenuation of the exhaust. The system utilizes a muffler indicated as a whole Iby reference character 10, referring now to FIGS. 4, 5 and 6. The muffler has an outer casing 12 which, in the illustrated embodiment, is cylindrical. The casing has a tapered reduced inlet end 14 providing an entrance for an inlet pipe 16 having an outwardly projecting portion 16A by means 0f which the muflier may be connected by a pipe 17 to the manifold 20 of an automobile engine, not shown. Similarly, the muifier has a tapered reduced outlet end 22 provided with a disc-harge pipe 18, the outwardly projecting portion 18A of which is connected as by pipe 24 to atmosphere, or a second muier, or, as shown, to a resonator 26. This is connected to atmosphere by tailpipe 28, see FIG. 6. The resonator is not required but is advantageous.

The interior of the mutller casing 12 is divided into sequential chambers or

compartments

30, 32 and 34 by the

transverse partitions

36 and 38 in casing 12, and through which the inner portion 18B of the outlet pipe 18 extends. The compartment 30 provides muffling in accordance with the present invention while

compartments

32 and 34 dampen intermediate and high frequencies in known manner.

The inlet pipe 16 has an inner portion 16B having an exit end 16C open into the first chamber 30, which is constructed in accordance with the present invention, through an intermediate cylinder 40 provided with an internal central imperforate partition 42 which reflects or reverses the exhaust gases striking it and with a plurality of indentations 40A at one end whereby it is -attached to the end 16C of pipe 16. The circumferential spaces between the indentations 40A provide a plurality of channels 44 extending from the region 46 inside the intermediate cylinder to a forward region 30A of the compartment or chamber 30. The chamber 30 is connected by an annular passageway 30B to a rear portion 30C of the chamber 30 surrounding the inner portion 18B of the `outlet pipe 18. The chamber portion 30C is connected through a `second .series of passageways 44B defined by a second series of indentations 40A to a region 48 within the inner cylinder 40 at the inlet end 18C iof the outlet pipe 18. The intermediate cylinder is thus conveniently supported by the opposite ends of the

pipes

16 and 18.

The inlet pipe 16 is provided with a plurality of apertures 50 through which some of the exhaust gases flow to chamber portion A. Some of the exhaust gases flow through the exit end 16C into the region 46 of the inner cylinder from whence they flow forwardly through the passageway 44 to combine with the gas ll-owing through the apertures S0. The reunited streams ow through the annular passageway 30B. In flowing through this passageway a small portion of gas in the inner cylinder region 46 ows outwardly through the apertures 53 to combine with the major portion of the gas ow.

The -gas then ows into chamber portion 30C where some of the `gas flows through the passageway 44B to region d8 of the inner cylinder where it combines with a small amount of gas to that region 48 through the apertures 54 in the inner cylinder. The gas from region 48 then enters the inlet pipe 18 at inlet end 18C where it is combined with gas owing into the pipe through apertures 56 in the pipe ahead of the partition 36. All the exhaust gas then flows through the pipe 18 where the chamber 32 dampens intermediate frequencies of the gas by virtue of the apertures 58l in the pipe between partitions 36 and 3S. The gas then flows through the length of pipe associated with compartment 34 which is lled with glass wool or the :like 34A and whereat the pipe is provided with apertures 60 to dampen high frequency sounds.

It will be noted that the total of the exhaust gases indicated by III flows into the inlet pipe 16 at entrance 16A. This total tiow is divided into two streams indicated by the reference characters I and II. The portion I enters the chamber 30 at region 30A through the apertures S0 and from region 30A it flows through annular passageway 30B to the region 30C. A second portion of the gases indicated by reference character II ilows out through the discharge end 16C of the inlet pipe into the region d6. From this region the greater portion, indicated by IIA, returns through passageway 44 to be united with stream I for How through the annular passageway 30B. A small portion IIB of stream II flows through the passage 52 into the annular passageway 30B. From the annular passageway 3dB the total flow goes into the chamber portion toward tlC. A small part IIB of the flow enters region 48 through the apertures S4. The major flow goes to region 30C where part of it enters the outlet pipe through apertures 56 and part returns to intermediate cylinder region 48 and then returns through the inlet end 18C of the output pipe to be united with stream I. The total flow then goes through the pipe at compartment 32 where noises of the middle frequency are dampened and past chamber 3ft where highest frequency noises are dampened. It should be understood that the

chambers

32 and 34 need not be utilized.

The cross section areas of the flow passages are so dimensioned that the velocity of the gases at the different cross sections are nearly identical and cross section of the muffler corresponds to that of the inlet pipe so that the kinetic energy remains constant. Also, the amount of gases which are flowing along the two paths I and II are substantially equal. The difference between the longer path II-IIA and the shorter path I is substantially equal to the distance between the pressure peaks at the inlet whereby effective muiing or silencing of the noises is effected in both halves of the muier.

The apertures 52 and 54 need not be used and the pipe ends 16C and l18C can have another diameter than the pipe itself. Also, the holes 50 can be located quite near the front end of the casing 12.

In FIGS. 7 and 8 there is illustrated a modilication of the mufer enabling it more readily to be installed under the car to the exhaust system at the point where it works most effectively. This advantage is obtained by constructing the muffler in a substantially flat form and without the high and intermediate frequency mutliing compartments 32 and 34 of the first described embodiment.

The muiiler, indicated as a whole by reference character 100, comprises a substantially attened outer casing 102 having tapered inlet and outlet ends 104 and 106 adapted to receive inlet and outlet pipes and 112, respectively. The casing 102 includes a pair of longitudinally extending and spaced apart interior partitions 108 secured to the sides of the casing and centrally of which is disposed an imperforate baffle 114. The inlet pipe 110 has a portion 110A projecting outside of the casing, an intermediate portion 110B with a series of apertures 116 therein and an exit portion 110C that is spaced from the partition 114 and opens into an inner chamber 11-8. Similarly, the outlet pipe has an external projecting portion 112A, an intermediate portion 112B with a series of apertures and an inlet portion 112C spaced from the partition 114 and opening into an inner region 122.

It will be noted that the construction provides the spaced passageways 124 between the end 110C of the inlet pipe and the adjacent longitudinal partition 108 and corresponding passageways 126 between the partitions 108 and the portion 112C of the outlet pipe` Likewise, the 1ongitudinally extending passageways 128 Vare provided between the outer sides of partitions 108 and the semi-circular portions 1G2B of the casing.

As in the previously described embodiment, the incoming gases enter the muffler through the pipe 110. A first portion I enters compartment region 131i through apertures 116 and a second portion II enters the region 1158 through the end 1C of the inlet pipe. From region 118 the gases II flow back through the passageway 124 and are reunited with the gas stream I and the entire ow is then through the passageway 128 to the chamber portion 132. Here gas stream I enters the outlet pipe 112 through the apertures 120 and gas stream II flows to chamber 122 through the passageway 126 and thence into pipe 112 where the two streams are reunited and ow out through the outlet portion of the pipe.

In this embodiment the cross sections again are such that the gas flows into substantially two streams of equal sizes and velocity. The flow paths of streams I and II are of lengths differing in magnitude by the wave length, i.e., the pressure peaks, so as to provide effective dampenmg.

As heretofore indicated, the muiflers should be located properly in the exhaust system. Referring to FIG. 6, the distance from the exhaust manifold to the exhaust valves should be about two wave lengths, the mutlier should be of a length equal to a wave length, and the distance from the mufiier to the end of the tailpipe 28 should be a whole number multiple of the wave length.

A mudier like that of FIGS. 7 and 8 adapted for use on about 30 European cars with exhaust piping of a diameter of 45 mm. has a length of 450 mm., a thickness of 46 mm. and a width of 105 mm. It has been used with a glass wool lled resonator having a length of 450 mm., a thickness of 55 mm. and a width of 105 mm. Accordingly, it appears that the wave length for normal loading ranges can be considered to be 45()l mm. As a result, the central partitions 42 and 114 in the two muflers are located one-half wave length from each end-which location has been found to provide effective sound dampening.

Also, and speaking generally, the ratio of the combined volume of the manifold 20, pipe 17, the mutiler 10 and pipe 24 relative to the total volume of the exhaust system is such that cooling of the exhaust gases in the exhaust system by the ambient air, when the engine is turned olf, does not cause the air to flow in through the tailpipe farther than pipe 24.

In both the embodiments of the invention described above the kinetic energy of the exhaust gases remains fundamentally unaltered. The flow with the muffler in the system is comparable to the corresponding ow in an open pipe type exhaust without a muiiler and, thus, differs sharply from corresponding properties of muiers used thus far and adapts open pipe principles to exhaust systems equipped with muttlers. Also, the capability of the muiiler of the present invention with respect to sound dampening is at least as good and generally better than the dampening provided by traditional car mufflers. In addition, one muler can be used on many cars with improved gas eiiciency, gas consumption, power output and acceleration and a reduction in exhaust fumes and carbon.

Also, use of the mulers of the present invention has indicated that the wave length, i.e., the distance between pressure peaks within reasonable normal loading ranges does not vary greatly, so that the mufers can be applied to many cars and operate satisfactorily over relatively wide loading ranges.

An important advantage of the present invention is the more effective utilization of the fuel with a consequent reduction in carbon monoxide and partly burned fuel in the exhaust gases, as well as reduction in the presence of propane, butane, etc.

K Another advantage is that the choke need be opened but about half of the time.

Mufllers in use indicate very little inside corrosion and it is expected that they will have a life of at least three years. It is expected also that galvanizing the exterior adds to the life.

The small size of a muffler made in accordance with the present invention can be noted from FG. 9 where such a muflier 10 has been shown alongside a muier 140 of a comparable known type it can replace.

While the present invention has been described in connection with details of illustrative embodiments thereof, it should be understood that these details are not intended to be limitative of the invention except insofar as set forth in the accompanying claims.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A muiiier comprising an elongated flattened casing of generally rectangular cross section, a pair of longitudinally extending interior partitions of lengths shorter than the casing mounted in said casing in spaced relation to each other, said partitions being equally spaced from the adjacent sides of the casing and Kalso equally spaced from the ends of the casing, a transverse imperforate baille located centrally of and extending between the partitions, and opposed inlet and outlet pipes having ends disposed between the interior partitions and spaced from the transverse baille, said inlet and outlet pipes having a plurality of apertures intermediate their ends and opening into the outer casing between the ends of the casing and adjacent ends of the interior longitudinal partitions, said inlet and outlet pipes being of a diameter corresponding substantially to the narrow dimension of the flattened casing, the pipes, casing partitions and apertures being constructed and arranged to divide the exhaust gas ilow into two streams of substantially equal size and velocity and with flow paths differing in lengths approximately in accordance with the exhaust system pressure peak separation in a normal leading range.

2. A muier for an exhaust system of the type having successive pressure peaks comprising an elongated ilattened casing of generally rectangular cross section of which the opposite shorter sides are rounded, a pair of longitudinally extending interior partitions of lengths shorter than the casing mounted in said casing in spaced relation to each other and from the adjacent sides `of the casing to provide end chamber regions and a first pair of passageways from one e-nd chamber region to the other of the casing, a transverse imperforate baille located centrally of and between the partitions, and opposed inlet and outlet pipes having outer ends outside of the casing and inner ends disposed between the interior partitions and spaced from the transverse baille to provide inner chambers at opposite sides of the baffle, said inlet and outlet pipes being of a diameter corresponding substantially to the narrow dimensions of the casing and providing a second pair of passageways at opposite sides of the pipes and between the pipes and the partitions to provide communication between 4the inner and end chambers, the casing having a length corresponding substantially to the distance between the exhaust gas pressure peaks in the exhaust system at normal loading conditions, said inlet and outlet pipes having a plurality of apertures intermediate their ends and opening into said end chambers, and said baiile being located about one half wave length from each end, and said pipes, apertures, and the passageways dividing the gas ilow into two paths both at the inlet and outer ends of the muiiier and maintaining the kinetic energy of the gas iiow in the muffler substantially the same as in the pipes.

3. A muffler comprising an elongated flattened casing of generally rectangular cross section, a pair of longitudinally extending interior partitions of lengths shorter than the casing mounted in said casing in spaced relation to each other, said partitions being equally spaced from the adjacent sides of the casing and also equally spaced from the ends of the casing, a transverse imperforate baille located centrally of and extending between the partitions, and opposed inlet and outlet pipes having ends disposed between the interior partitions and spaced from the transverse baille, said inlet and outlet pipes having a plurality of apertures intermediate their ends and opening into the outer casing between the ends of the casing and adjacent ends of the interior longitudinal partitions, and said inlet and outlet pipes being of a diameter corresponding substantially to the narrow dimension of the ilattened casing.

4. A mu'ler for an exhaust system comprising an elongated flattened casing of generally rectangular cross section of which the opposite shorter sides are rounded, a pair of longitudinally extending interior partitions of lengths shorter than the casing mounted in said casing in spaced relation to each other and from the adjacent sides of the casing to provide end chamber regions and a iirst pair of passageways from oneend chamber region to the other of the casing, a transverse imperforate baille located centrally of and between the partitions, and opposed inlet and outlet pipes having outer ends outside ofthe casing and inner ends disposed between the interior partitions and spaced from the transverse baille to provide inner chambers at opposite sides of the baille, said inlet and outlet pipes being of a diameter corresponding substantially to the narrow dimensions of the -casing and providing a second pair of passageways lat opposite sides of the pipes and between the pipes and the partitions to provide communication between the inner and end chambers, and said inlet and outlet pipes having a plurality of apertures intermediate their ends and opening into said end chambers between the ends of the casing and the adjacent ends of the interior partitions.

References Cited UNIT ED STATES PATENTS 1,338,520 4/1920 Moores 181-53 XR 2,018,084 10/1935 Oldberg. 2,099,887 11/1937 Heath. 2,109,220 2/1938 Noblitt et al. 181-54 2,111,537 3/1938 Noblitt et al. 181-54 2,182,204 12/ 1939 Hector 181-54 2,193,791 3/ 1940 Hollerith et al. 2,305,946 12/1942 Wilson et al. 2,567,568 9/1951 Lievense et al. 2,640,557 6/1953 Gaffney 181-56 XR 2,675,088 4/1954 McLeod. 2,872,998 2/1959' Tinker 181-53 3,194,341 7/1965' Haag 181-54 XR 3,212,603 10/1965 Walker.

FOREIGN PATENTS 1,060,334 11/1953 France.

ROBERT S. WARD, IR., Primary Examiner.

RICHARD B. WILKINSON, Examiner-

Claims

1. A MUFFLER COMPRISING AN ELONGATED FLATTENED CASING OF GENERALLY RETANGULAR CROSS SECTION, A PAIR OF LONGITUDINALLY EXTENDING INTERIOR PARTITIONS OF LENGTHS SHORTER THAN THE CASING MOUNTED IN SAID CASING IN SPACED RELATION TO EACH OTHER, SAID PARTITIONS BEING EQUALLY SPACED FROM THE ADJACENT SIDES OF THE CASING, AND ALSO EQUALLY SPACED FROM THE ENDS OF THE CASING, A TRANSVERSE IMPERFORATE BAFFLE LOCATED CENTRALLY OF AND EXTENDING BETWEEN THE PARTITIONS, AND OPPOSED INLET AND OUTLET PIPES HAVING ENDS DISPOSED BETWEEN THE INTERIOR PARTITIONS AND SPACED FROM THE TRANSVERSE BAFFLE, SAID INLET AND OUTLET PIPES HAVING A PLURALITY OF APERTURES INTERMEDIATE THEIR ENDS AND OPENING INTO THE OUTER CASING BETWEEN THE ENDS OF THE CASING AND ADJACENT ENDS OF THE INTERIOR LONGITUDINAL PARTITIONS, SAID INLET AND OUTLET PIPES BEING OF A DIAMETER CORRESPONDING SUBSTANTIALLY TO THE NARROW DIMENSION OF THE FLATTENED CASING, THE PIPES, CASING PARTITIONS AND APERTURES BEING CONSTRUCTED AND ARRANGED TO DIVIDE THE EXHAUST GAS FLOW INTO TWO STREAMS OF SUBSTANTIALLY EQUAL SIZE AND VELOCITY AND WITH FLOW PATHS DIFFERING IN LENGTHS APPROXIMATELY IN ACCORDANCE WITH THE EXHAUST SYSTEM PRESSURE PEAK SEPARATION IN A NORMAL LEADING RANGE.