Abstract:
The present invention discloses a compact efficient exhaust handling device that is particularly advantageous for use with small two-stroke, piston-type internal combustion engines, which device provides both exhaust scavenging and charge densification in the cylinder of the engine by utilizing a first muffler section formed in a helical or wrapped configuration, and a muffler expansion chamber also formed in a wrapped configuration, and axially displaced from the first section.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a device for receiving the exhaust gas output of a reciprocating piston internal combustion engine, and more particularly discloses a muffler assembly that is particularly advantageous for use with a two-stroke gasoline engine, which assembly provides both an exhaust scavenging function and a supercharging function, and in addition, comprises a low profile and compact design.  
         BACKGROUND OF THE INVENTION  
         [0002]    This invention discloses an exhaust gas handling assembly for an internal combustion engine, which is especially useful in small two-stroke gasoline engines such as in radio-controlled airplanes and wheeled vehicles for ground travel, such as motorcycles and all terrain vehicles. The device is commonly referred to as a muffler, but this is a term that is too restrictive for all the functions performed by the device. While the device does serve to muffle or dampen the noise of combustion in such internal combustion engines, it also serves at least two other critical functions, exhaust scavenging and fuel-charge densification.  
           [0003]    The present invention has been found to be particularly advantageous when used on a two-stroke, internal combustion, piston and crankshaft type engine which burns a volatile fuel such as gasoline and/or alcohol, and which utilizes valving consisting of ports formed through the wall of the piston cylinder, controlled by movement of the piston within the cylinder to alternately expose and cover up said ports.  
           [0004]    A typical two-stroke engine has one or more intake ports formed through each cylinder wall and one or more exhaust ports formed through the cylinder wall, usually located on the opposite side of the cylinder from the intake ports. These ports are positioned such that the piston opens and closes them in a carefully controlled sequential manner to allow intake and exhaust of the fuel/air mixture and the products of combustion, respectively. Many such engines pump the fuel/air mixture through the crankcase of the engine into the intake port in the cylinder wall.  
           [0005]    During a normal intake/compression/combustion/exhaust cycle of the two-stroke piston-cylinder combination, when the exhaust port is opened by movement of the piston away from its blocking position over the port, a high-pressure exhaust gas pulse starts down the exhaust tube. The piston continues down and the exhaust pressure bleeds off into the tube. This occurs at around 90-110 degrees from piston Top Dead Center (TDC). At about 15-25 degrees later, the intake ports on the other side of the cylinder are exposed by the piston, and, because of crankcase compression, a fuel/air mixture begins to flow through the intake ports and into the cylinder while exhaust gas is still moving out the exhaust ports. After a small fraction of a second, the pressure pulse moving down the exhaust tube reaches an open area, or expansion chamber, and this starts an expansion wave back toward the exhaust ports. This expansion wave creates an action at the exhaust ports, which serves to draw additional flow of exhaust from the cylinder, including a portion of the new fuel/air charge entering through the intake ports.  
           [0006]    As the expanding exhaust pulse reaches the end of the expansion chamber, it impinges the narrowed end of the tube at the downstream end of the chamber and is compressed, thereby creating a strong compression wave that moves back up the tube to the exhaust port. This results in some of the escaped fuel/air charge being pushed back into the cylinder before the piston closes the exhaust ports, thus achieving the desired charge-densification effect in the cylinder.  
           [0007]    The “tuning” of the muffler is dependent upon the length and volume of the expansion chamber and its distance down the tube from the exhaust ports. This chamber effectively locates the positions of the expansion part of the tube, and the compression portion. The remaining portion of the exhaust tube downstream from the expansion chamber has little effect on the “tuning” of the exhaust.  
           [0008]    Some exhaust mufflers, which are also commonly called “tuned pipes” or “tuned exhaust extractors”, which are currently available commercially for small two-stroke engines are sufficiently “tuned” to allow optimum scavenging of exhaust from the cylinder of the engine and a charge-densification of the incoming fuel/air mixture. This occurs by the advantageous utilization of the above-described impulse/compression wave nature of the exhaust muffler. There are also mass effects involved in exhaust processes, i.e., the volume of exhaust gas in a system does not move through the pipe with a smooth, linear velocity. The velocity rises and falls along with the pressure waves, so that being “in tune” with these differences amplifies the pressure differences. The expansion portion of the exhaust gas wave moving out of the cylinder, through the exhaust valve, and down the muffler tube serves to establish a subnormal pressure condition just outside the exhaust valve, which aids in removing additional combustion products from the cylinder while the cylinder interior is exposed to the open exhaust port. Shortly thereafter, the compression wave passing back up the muffler to the cylinder serves to “supercharge” the incoming fuel/air charge that has begun to exit the open exhaust port by forcing the charge back through the exhaust port and into the cylinder, thereby increasing the density of the fuel/air mixture in the cylinder before the compression and combustion cycles are achieved.  
           [0009]    Unfortunately, prior art muffler devices for small two stroke gasoline engines offer chamber designs that are many times longer than the diameter of the cylinder in which the fuel/air mixtures are combusted. The most prevalent of such muffler devices commercially available for two-stroke gasoline engines suffers from having a length as much as 6-30 times the diameter of the cylinder it is attached to. The specific length of the tuned pipe is primarily a function of the RPM at which the engine designer wishes to “tune” the system. Often a particular torque curve is desired for an optimum match-up with the particular airframe chosen, and this can be achieved by designing the system to be longer or shorter. A short length tube will be utilized for a high-RPM, low torque engine, and a long length tube will be used for a low-RPM, high torque engine. This length is used to create the compression/expansion wave actions referred to above which establish the scavenging and densification functions previously described. If such muffler chamber is not properly sized, the two-stroke engine exhaust will not be “tuned” and performance of the engine will suffer drastically.  
           [0010]    However, when the muffler chamber is properly sized for optimum performance, it results in a muffler having a physical presence that is many times larger than the entire engine to which it is attached. In the world of small engines, this is very undesirable for several reasons. One reason that such bulky and cumbersome exhaust device is undesirable is the ugly aesthetics that it presents. The present commercially available muffler is a long, cigar-shaped tube that must extend down the side of the vehicle to which it is attached. For those who desire authenticity in the appearance of their small gasoline-powered vehicles, the presence of such a bulky and obvious attachment, often extending down the full length of the airplane or land vehicle on which it is used, greatly mars the owner&#39;s enjoyment of the vehicle. This is particularly true in the field of radio-controlled (RC) airplanes and cars.  
           [0011]    In addition to the aesthetically unpleasant feature of current muffling devices, they also are very aerodynamically inefficient, causing unbalanced weight and drag on the vehicles, especially on the RC airplane.  
           [0012]    Commercially available “tuned” mufflers for small two-stroke engines generally comprise a long, cigar-shaped tube/chamber combination that begins with a small diameter next to the exhaust port of the engine cylinder. At this point the cross-sectional area of the muffler may be approximately the same size as the exhaust port of the small engine. As you progress down the tube of the muffler, the cross-sectional area increases several-fold to form the expansion chamber of the muffler to create the expansion wave which moves backward down the muffler to the exhaust port and provides the scavenging function mentioned previously. This serves to “suck” the remaining exhaust gases from the cylinder while also creating a low-pressure condition in the cylinder that aids in inducting a greater fuel/air charge through the intake port which is open at the same time.  
           [0013]    Further down the muffler, the cross-sectional area is narrowed significantly to create the compression wave that then moves back up the muffler to the cylinder and serves to push back into the cylinder the portion of unburned fuel/air charge that had managed to flow partly out the exhaust port and into the muffler, thereby accomplishing the densification or “supercharging” effect of the muffler. When these two chamber sections, i.e. the expansion chamber and the compression chamber, are located in tandem along the same axis, the device must by necessity be very long, i.e., many times the diameter of the cylinder to which it is attached. This creates a muffler system that is sometimes longer than the vehicle on which it is used. This prior art exhaust device can thus be characterized as a Total-Axial-Flow muffler system.  
           [0014]    A second prior art muffling device that is commercially available is similar to the “tuned pipe” system described above but adds a further element of a concentric annular outer shell which “wraps” around the tuned muffler and goes from the exit end of the first exhaust pipe, forward to the beginning of the inner pipe to obtain a dual concentric pipe effect. This creates an outer chamber around the inner tube, which chamber serves to act as an expansion/compression wave generating chamber. While this has the effect of providing the desired scavenging and densification effects on the engine and is shorter in length, this second device suffers from the disadvantage of being larger in diameter and less efficient than just the tuned pipe style of muffling system, thus detracting from the aesthetics and streamlining of the vehicle it is used on.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention solves the problems of the prior art exhaust systems described above by providing a muffler device that is not total-axial-flow with respect to exhaust gases, but instead folds the several distinct functional aspects of the “tuned” muffler in on one another in a combined axial-circumferential flow, to greatly reduce the length and size of the device and thereby provide a muffler that can be secured entirely inside the cowling, cabin, or cockpit of the vehicle on which it is used. Thus the present invention presents a “tuned” muffler that is aesthetically pleasing and which eliminates aerodynamic drag on the vehicle.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a front schematic view of the muffler device of the present invention;  
         [0017]    [0017]FIG. 2 is side sectional view of the invention taken at line  2 - 2  of FIG. 1;  
         [0018]    [0018]FIG. 3 is a side sectional view of the invention taken at line  3 - 3  of FIG. 1;  
         [0019]    [0019]FIG. 4 is a front schematic view of a second embodiment of the invention;  
         [0020]    [0020]FIG. 5 is a side sectional view of the embodiment of FIG. 4, taken at line  5 - 5 ; and,  
         [0021]    [0021]FIG. 6 is a side sectional view of the second embodiment taken at line  6 - 6  of FIG. 4. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Referring to FIGS.  1 - 3 , and more particularly FIG. 1, the muffler device  101  is illustrated as it appears when viewing from the front of the engine  102  to which it is attached. Muffler  101  consists of three basic modules, the pipe section  103 , the can section  104 , and the cover plate  105 . As illustrated in this embodiment, the muffler  101  is cylindrically shaped, but as illustrated in the second embodiment herebelow, this shape could just as well be any geometrical shape desired, including square and rectangular.  
         [0023]    [0023]FIG. 2 is a sectional end view of the pipe section  102  taken at line  2 - 2  of FIG. 1. The pipe section  102  consists of a cylindrical outer housing shell  106  and a helical divider wall  107  spiraling inward from the outer wall  106  to a central chamber  108 . The axial height of spiral wall  107  is equivalent at all points to the height of shell  106 , so that the addition of the flat inner wall  109  of can section  104  against pipe section  101  serves to seal against the entire length of divider wall  107 , thereby creating a sealed spiral passage  110 . The only openings to passage  110  are the inlet opening  111  communicating with the exhaust valve of engine  102 , and an exhaust outlet opening  112  formed in wall  109  of can section  104 . One or more mounting holes  113  are formed through the left wall  114  of pipe section  103  for mounting the muffler device  101  to the engine  102 , allowing the passing therethrough of mounting screws or bolts from the wall section  114  to the engine block of engine  102 .  
         [0024]    [0024]FIG. 3 is a side sectional view of the can section  104  of the muffler device  101 , taken at line  3 - 3  of FIG. 1. Can section  104  consists of cylindrical housing or shell  115  which is machined for tight-fitting sealing engagement with housing  106  of pipe section  103 , a flat wall section  109  adapted to seal off and create the enclosed spiral passage  110  in pipe section  103 , and a chamber forming divider wall  116  which is attached to wall section  109  and is arranged to seal against cover section  105  to form an expansion and compression chamber  117 . An exhaust inlet opening  112  is formed through wall  109  to communicate with passage  110  in pipe section  103 , and is generally located at about the center of section  104 . The opening between pipe section  103  and can section  104  must be at or near the end of the spiral passage  110  for proper operation, i.e., at the center of the pipe section, designated as outlet  112 . The opening from the expansion chamber  117  to the atmosphere can be located through cover plate  105  so that it aligns with either end of expansion chamber  117 . In FIG. 3 this is indicated in phantom at  118  to show the location of the exhaust port with respect to opening  112 . An alternate location for the exhaust port in the cover plate  105  is also indicated in FIG. 3 at  119 . Either location,  118  or  119 , allows exhaust gas to be flowed out of the muffler device while still taking full advantage of the expansion/compression chamber  117 . However, the exact location of this port is not critical to the operation of the invention, because changing the location serves mainly just to change the back pressure that is created within the exhaust assembly and to also vary the temperatures reached within the exhaust assembly. Also, the size of port  118  determines pressures, temperatures and mass flow effects within the entire exhaust system. One skilled in the art, with only a minimum of trial and error, will be able to vary the size and location of port  118  to optimize the particular exhaust effects desired, depending upon the application of the engine on which the system is to be installed, and depending on the RPM range at which the operator wishes power from the engine to be optimized.  
         [0025]    A cover plate  105  engages with can section  104  to enclose the chamber area  117  by sealing with housing  106  and having a flat wall section  109  that engages the top of divider wall  116 . An exhaust port  118  is formed through the wall of cover plate  105  and communicates with compression/expansion chamber  109  to allow spent exhaust gases to exit the muffler assembly into the atmosphere.  
         [0026]    An assembly hole can be formed centrally in all of the three sections,  102 ,  104 , and  105  so that a bolt, screw, pin, or other elongated fastening device may be passed through the separate assembly sections to secure them together into a single assembly  101 . Alternatively, the separate sections could be formed so that they telescope into each other, with telescoping sections along the outer periphery of each section that can be fastened securely by threads formed on each section, by fusion means such as welding, or by fasteners passing through the telescoped outer walls where they overlap.  
         [0027]    In addition to the above described structure of the three elements consisting of the pipe section  103 , the can section  104 , and the cover plate  105 , it is possible to manufacture the assembly by forming the can section and the cover section as a single integral part, by making the flat wall section  109  of the can section as a separate individual divider plate that is inserted between the pipe section and the can section and held there by pressure from these two adjacent sections, and/or by one or more fasteners as described hereinabove.  
         [0028]    Further modifications of the invention from the specific embodiment described above can be achieved without changing the efficiency and operation of the invention. For example, instead of having the pipe section and the can section located with respect to each other so that they are coaxial and concentric, it is possible to have the sections located so that they are still touching each other while being axially displaced from each other, but not concentric, as long as the outlet port from the pipe section still communicates with the inlet opening of the can section. It is possible to slide one section radially outward from the other and still maintain contact between the two sections sufficient to allow communication between the outlet port of one with the inlet port of the other, while maintaining the operation and efficiency of the invention, so long as they are still axially displaced one from the other and their diametral planes are still relatively parallel to each other and displaced axially.  
         [0029]    The muffler assembly  101  may be made of any structural metal which is light, strong, and temperature-resistant, such as aluminum, steel, brass, copper, or alloys of these and other metals. Likewise, the assembly could be manufactured from a strong temperature-resistant thermosetting polymer known to those skilled in the thermosetting plastics art. Or, various parts of the assembly could be made of different metals, alloys, or polymers from other parts of the assembly without going beyond the limits of the herein described invention.  
         [0030]    FIGS.  4 - 6  illustrate a second embodiment of the invention in which the overall general shape of the muffler assembly  201  is a rounded-corner rectangular shape rather than that of a right circular cylinder as disclosed in the first embodiment. FIG. 4 illustrates a schematic diagram of the muffler assembly  201  which consists of a rectangular pipe section  203  to which is attached a matching rectangular can section  204 , closed off by a rectangular cover plate  205 . The muffler is attached to the exhaust port of an internal combustion engine  202 .  
         [0031]    [0031]FIG. 5 is a sectional side view of the pipe section  203 , which is the view taken at line  5 - 5  of FIG. 4. In this figure the pipe section  203  is formed in a similar fashion to the pipe section  103  of the first embodiment, in that it consists of an outer housing or shell section  206  extending axially with the exhaust flow from engine  202 . Inside housing  206  is a barrier wall  207  extending down a substantial portion of the vertical length of section  206  and forming an exhaust flow channel  210  which is a U-shaped closed passage created by the sealing of wall  209  of can section  204  against wall section  207  and barrier wall  207 . An inlet port  211  is formed through wall  214  of the pipe section to communicate with the exhaust port of the engine to which the muffler  201  is attached.  
         [0032]    [0032]FIG. 6 illustrates a side sectional view of the can portion  204  of the muffler of the second embodiment, which view is taken at line  6 - 6  of FIG. 4. Can section  204  has an external housing shell  215  which is a constant-height rectangular wall forming the external shape of the can section  204 . Shell  215  is attached to a flat can wall section  209  and forms internal expansion/compression chamber  217  therein. This chamber is made into an ell shape by the addition of internal wall section  216  which is attached to can wall  209  and is of equal height to housing wall  215 .  
         [0033]    A closed chamber or dead space  219  results from the ell shape of chamber  217 . An inlet port  212  is formed through wall  209  of can section  204  and communicates with U-shaped passage  210  of pipe section  203 . Preferably, inlet port  212  is located on the opposite side of barrier wall  207  from inlet port  211  coming from the exhaust valve of the engine. A rectangular cover plate  205  is attached to the can section  204  by sealing engagement of the outer edge of cover plate  205  with housing shell  206  of the can section. Also, cover plate  205  contacts the full length of barrier wall  216  to enclose chambers  217  and  219 . An exhaust port  218  is formed through the wall of cover plate  205  to exhaust spent gases to the atmosphere. As with the first embodiment, the exact location of port  218  is not critical to the operation of the invention, but allows the designer to vary pressures and temperatures within the exhaust system.  
       Typical Operation  
       [0034]    In typical operation, an exhaust gas pulse enters the exhaust entry port  111  of pipe section  103  from the opened exhaust valve of engine  102 . The gas pulse enters the spiral passage  110  of the pipe section and traverses down this passage toward the outlet port  112  formed in the can section. The exhaust gas pulse flows through the port  112  and reacts with the volume of the expansion/compression chamber  117 , with the immediate result that the volume of the gas pulse is rapidly expanded, thus creating an expansion wave that moves back up the spiral passage  110  to provide the needed scavenging of the engine cylinder through the still-open exhaust valve and port  111 .  
         [0035]    After the expansion pulse has moved around the full length and volume of chamber  117  it hits the ends of the chamber, thereby creating a compression wave that then travels back through the chamber  117 , port  112 , up the spiral passage  110 , and into the exhaust valve of the engine cylinder, thus providing the charge-densification effect previously mentioned.  
         [0036]    The entire system is “tuned” according to the engine designer&#39;s desires by altering the length and/or volume of the individual chambers. The volume can be altered by making the pipe section or the can section, or both, wider or narrower in the axial direction. The lengths of the passages can be altered by changing the degree of curvature and length of the spiral wall section  107 , and/or the degree of curvature and length of wall section  116 . Volume and length of all the internal passages can be altered by increasing the radial diameter of the entire device  101 , thereby simultaneously increasing the length of the internal passages while also increasing their volumes.  
         [0037]    The skilled mechanic in the art of muffler or “expansion chamber” design for two-stroke engines, such as those used in motorcycles and airplanes, can design the length and diameter of the internal passages of the muffler to obtain the particular results desired of the particular engine being “tuned” by the exhaust system. This requires that the designer know the speed at which sonic waves travel through the expansion chamber of the muffler device. This in turn depends upon the temperature of the exhaust gas moving down the chamber. Exhaust gases exit the combustion chamber at approximately 1200 degrees F. and drop to around 800 degrees F. at the outlet pipe. Because of the cooling from expansion in the chamber, they can be cooled to as low as 500 degrees or lower before reaching the final outlet pipe. Critical dimensions, besides the length and diameter of the expansion chamber, include the rate or angle of divergence of the expansion chamber wall section, and the cross-sectional shape of the chamber. Also critical is the angle of convergence in the compression section at the end of the expansion chamber.  
         [0038]    These factors are more particularly spelled out for the skilled artisan in publications available commercially, such as the book “TWO STROKE TUNER&#39;S HANDBOOK” by Gordon Jennings, copyright 1973 by H.P. Books, Box 5367, Tucson Ariz.; book number 41-ISBN 0-912656-41-7; the contents of which are incorporated herein by reference. It should be noted that according to this reference, the design of any muffler device for a two-stroke engine is an exercise in compromise, because of the many different end results that can be achieved by the engine designer. For example, some exotic racing engines are tuned to obtain a peak horsepower rating in a very narrow RPM range because of their close-ratio, multiple speed transmissions which are designed to keep the engine revved up and operating continually at a desirable high RPM. Alternatively, other two-stroke engines, because they do not enjoy the advantage of being able to “shift gears” while in operation, may need to utilize a muffler system with an expansion chamber designed to optimize the average power output over a broader range of RPM.  
         [0039]    For example, the angle of divergence of the expansion chamber wall, called the “diffuser angle” determines the width of the engine “power band”. In a conventional “cigar-tube” expansion chamber, a diffuser angle of greater than 8 degrees creates a short-duration wave that results in maximum power at peak RPM. A more gradual taper, less than 8 degrees, spreads the power band out over a broader range of RPM. Likewise, the compression taper at the end of the expansion chamber has a similar but less dramatic effect on the power band of the engine. Thus it can be seen that the dimensions and angles of the expansion chamber section of the muffler of the present invention can be optimized in several different configurations to fit the designer&#39;s power goal, depending upon the desired final result of the engine designer, by using a very small amount of trial and error.  
         [0040]    In the second embodiment of the invention, these same changes can be used to obtain the same variations in dimensions and capacities. In addition, in the second embodiment, the length and volume of the expansion/compression chamber  217  can be altered by increasing or decreasing the volume of the dead space  219 .  
         [0041]    Thus, the present invention has provided both the scavenging and charge-densification effects necessary to have an efficient “tuned” exhaust system. These are the same functions provided by the extensively long and bulky prior art muffling devices; however, the present invention provides these features in a compact muffler that can be completely contained in the cowling, cabin, or cockpit of the airplane or ground vehicle to which the muffler is attached.  
         [0042]    It has been shown how the present invention has solved the problem of the prior art devices by providing a compact and efficient “tuned” exhaust muffler assembly that is concealable within the confines of the vehicle on which the device is being used.  
         [0043]    While multiple embodiments of the invention have been illustrated, it is to be understood that the invention is not confined to the precise disclosure, and it will be apparent to those skilled in the art that various changes and modifications can be made in the invention without departing from the spirit of the invention or from the scope of the appended claims. For example, whereas the device is illustrated in cylindrical and rectangular form, it is clear that other geometric shapes such as triangular or elliptical could also be used advantageously. Also, whereas it is noted that the material of which the device is manufactured is a strong light metal or thermosetting plastic, it is clear that other materials such as carbon fiber material could also be used advantageously.