Patent Publication Number: US-11661873-B2

Title: Compact slip-in spark arrestor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of provisional patent application Ser. No. 63/271,858, filed 2021 Oct. 26 by the present inventor. 
    
    
     PRIOR ART 
     The following is a tabulation of some prior art that presently appears relevant: 
     
       
         
           
               
             
               
                   
               
             
            
               
                 U.S. Patents 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Pat. No. 
                 Kind Code 
                 Issue Date 
                 Patentee 
               
               
                   
                   
               
               
                   
                 5,509,947 
                 A 
                 1996 Apr. 23 
                 Burton 
               
               
                   
                 3,757,892 
                 A 
                 1973 Sep. 11 
                 Raudman, Jr. 
               
               
                   
                 3,407,575 
                 A 
                 1968 Oct. 29 
                 Krizman 
               
               
                   
                 3,009,539 
                 A 
                 1961 Nov. 21 
                 Papp 
               
               
                   
                   
               
            
           
           
               
            
               
                 Foreign Patents 
               
            
           
           
               
               
               
               
               
            
               
                 Foreign 
                   
                   
                 Pub./Issue 
                   
               
               
                 Doc. Nr. 
                 Cntry Code 
                 Kind Code 
                 Dt 
                 Patentee 
               
               
                   
               
               
                 404,722 
                 AU 
                 A 
                 1968 Feb. 1 
                 Davis et al. 
               
               
                 455,032 
                 CA 
                 A 
                 1949 Mar. 8 
                 Bourne et al. 
               
               
                   
               
            
           
         
       
     
     BACKGROUND 
     This invention relates to internal combustion engine exhaust systems for off-road vehicles (ORVs) such as motorcycles, three and four wheel all-terrain vehicles (ATVs), side by side or utility task vehicles (UTVs), two and four wheel drive off-road automobiles and trucks, any of which could have the disclosed invention applied to their exhaust systems, however; for the purposed of discussion, will be described specifically for motorcycles. 
     Currently in the U.S.A. it is required for all federal public lands, in addition to most state and locally owned lands used by the public, that internal combustion engines operated on said lands must possess a functional spark arresting device on the exhaust system to prevent emission of hot sparks in the form of particulate matter or debris such as carbon particles that may start fires. The arresting efficiency of these spark arrestors is tested by measuring the percent of carbon particles retained or destroyed by the spark arrestor under test conditions described by the United States Forest Service (USFS) and Society of Automotive Engineers (SAE), with a passing arresting efficiency of 80% or greater. 
     The two classifications of spark arrestors for small engines as described by the USFS include:
         Centrifugal type: A type of spark arrestor that uses baffles, traps, and/or vanes, to remove particulate matter from exhaust flows.   Screen type: A type of spark arrestor which uses a screen mesh to trap particulate matter.       

     Spark arrestors are typically mounted downstream in the exhaust flow just prior to the exhaust exit and traditionally for motorcycles attached to the end of or internal to the silencer, which is the last component of the exhaust system and functions to attenuate the exhaust noise. Because the spark arrestor is placed inside of the silencer, the size is critical and preferred to be as small as possible so that more of the overall silencer volume may be utilized to perform its primary function of attenuating the exhaust noise. If a spark arrestor is too long and takes up too much of the allocated silencer volume, the resulting exhaust system can be too loud to meet maximum sound restrictions, specifically the 96 decibel maximum as defined by the USFS for off-highway vehicles. 
     The primary design challenge for a spark arrestor is to meet the 80% arresting efficiency requirement while imparting a minimal change to exhaust flow rate. This is a significant design challenge because the two factors of exhaust flow rate and spark arresting efficiency are fundamentally competing mechanisms such that as the spark arrestor design is modified to increase arresting efficiency there is classically a reduction of exhaust flow rate that accompanies this improvement. Specifically for screen type spark arrestors: the addition of a screen mesh to the exhaust flow path disrupts laminar flow which increases back pressure and creates a significant reduction in exhaust flow rate. Additionally, as the screen becomes clogged with particulate matter, back pressure is further increased as the available flow path area is reduced and the reduction in exhaust flow rate compounded until the screen is removed and cleaned. Compared to screen type spark arrestors, centrifugal type spark arrestors typically have less of an effect on exhaust flow rate and do not become clogged because the particulate matter is removed from and trapped outside of the flow path. The increased back pressure and associated flow rate reduction created by centrifugal type spark arrestors is a result of turbulent flow disorders caused by the addition of flow path directors such as fins and annular chambers which serve to accelerate flow, change flow direction, divert, and then trap the exhaust particulate matter. Typically as more flow path directors are added to the system, arresting efficiency goes up along with the number of turbulent flow disorders which causes an increase in back pressure and decrease in exhaust flow rate. 
     Specifically for motorcycle engines, the current state of the art screen and centrifugal spark arresting devices meeting the 80% arresting efficiency requirement produce roughly a 25-55% change in flow rate for a given back pressure. The 25-55% change in flow rate reduces the engines volumetric efficiency and hence performance. Historically this decrease in engine performance has been accepted as a necessary compromise to meet the 80% arresting efficiency requirement and acts as a deterrent to the use of a spark arrestor. 
     The disclosed spark arrestor design is the first that has been shown to reach the goal of no change in flow rate, while at the same time meeting the 80% arresting efficiency requirement. In addition, the disclosed design is compact enough to fit inside of existing original equipment manufacturer (OEM) off-highway vehicle exhaust system silencers without adding additional length to the silencer, and also meeting the goal of not increasing sound levels. This compact size characteristic increases marketable value for the design because an aftermarket spark arrestor slip-in for existing OEM silencers may be produced at a much lower cost compared to a completely new silencer system containing a centrifugal type spark arrestor. 
     Both U.S. Pat. No. 3,407,575 to Krizman (1968) and U.S. Pat. No. 3,009,539 to Papp (1961) disclose centrifugal type spark arrestors but make no claims as to capability of meeting any specific arresting efficiencies, flow rate goals, or size requirements. This prior art does not disclose the novel features of a centrifugal whirling means having an elliptically shaped apex section, compound curvature of fins on two planes, teardrop tail, nor interior shell novel features of both converging and diverging sections with a plurality of passages, or the radiused transition disclosed. It has been demonstrated that the application of these features results in a design which meets the 80% arresting efficiency goal with no change in flow rate. In addition, the compact size of this design has been fit into existing off-highway vehicle silencers and allows for more than sufficient silencing space to meet the 96 decibel requirement. 
     Papp discloses fin curvature but not compound curvature and curvature in one plane not two which is required to reduce turbulent flow and still provide significant directional velocity for particulate matter to be ejected from the exhaust flow stream. Furthermore, Papp discloses the location of these fins to be on the upstream conical component of the centrifugal whirling means as opposed to the disclosed spark arrestor identifying fin placement after the conical component. 
     Both Papp and Krizman disclose conical sections to the leading profile of the centrifugal whirling means with the conical sections having open bodies at the rear, this open body design produces significant turbulent flow post whirling means which reduces flow rate &amp; decreases arresting efficiency. By adding a trailing cylinder and closed section tail cone with a teardrop shape the post turbine flow turbulence is significantly reduced so as not to affect flow rate. The bulb tail cone also serves to deflect impacting particulate matter from the exhaust flow stream increasing arresting efficiency. 
     SUMMARY 
     A compact slip-in spark arrestor with novel features consisting of: a centrifugal whirling means having an elliptically shaped apex section, compound curvature of fins on two planes, teardrop tail, an interior shell of both converging and diverging sections with a plurality of passages, and the radiused transition disclosed. This novel spark arrestor design is compact enough such that it may be inserted into existing OEM exhaust system silencers without increasing sound output and exhibits performance attributes of a minimum of 80% spark arresting efficiency with no change in exhaust flow rate. 
    
    
     
       DRAWINGS—FIGURES 
         FIG.  1    front vertical longitudinal sectional view of a spark arrestor constructed in accordance with the present invention as installed in an existing motorcycle silencer shell. 
         FIG.  2    isometric view of the interior shell. 
         FIG.  3    front view of the centrifugal whirling means. 
         FIG.  3 A  bottom view of the centrifugal whirling means. 
         FIG.  4    Flow comparison chart comparing current state of the art spark arrestors to the disclosed spark arrestor. 
     
    
    
     
       
         
           
               
             
               
                   
               
               
                 Drawings-Reference Numerals 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 100 existing silencer shell 
               
               
                   
                 105 perforated tube 
               
               
                   
                 110 tubular conduit 
               
               
                   
                 115 inlet cap member 
               
               
                   
                 120 silencer packing 
               
               
                   
                 125 interior shell 
               
               
                   
                 130 tubular center section 
               
               
                   
                 135 second tubular section 
               
               
                   
                 140 plurality of passages 
               
               
                   
                 145 outlet cap member 
               
               
                   
                 150 radiused transition 
               
               
                   
                 155 outer chamber 
               
               
                   
                 160 exterior shell 
               
               
                   
                 165 apex section 
               
               
                   
                 170 trailing cylinder 
               
               
                   
                 175 plurality of fins 
               
               
                   
                 180 teardrop tail 
               
               
                   
                 185 small aperture 
               
               
                   
                 200 tabs 
               
               
                   
                 300 first compound curvature 
               
               
                   
                 305 second compound curvature 
               
               
                   
                   
               
            
           
         
       
     
     DETAILED DESCRIPTION—FIGS.  1 ,  2 ,  3 ,  3 A 
     One embodiment of the disclosed spark arrestor is shown in  FIG.  1    as it would be fit inside of an existing silencer shell  100  of a motorcycle exhaust system having an elliptical shape. The exhaust flow enters the spark arrestor at the bottom of  FIG.  1    through a perforated tube  105  common to motorcycle silencers, and exits through a tubular conduit  110  to the atmosphere at the top of  FIG.  1   . The perforated tube  105  and entrance to the tubular conduit  110  are aligned axially. An inlet cap member  115  is positioned perpendicular to and in contact with the perforated tube  105  in addition to being in contact with the interior of the existing silencer shell  100  thereby producing a barrier between the silencer packing  120  used in motorcycle silencers, and the spark arrestor. 
     An interior shell  125  is aligned axially to the perforated tube  105  and exhibits a circular cross-section the diameter of which changes along its axial length. The diameter of the interior shell  125  is at a maximum where it is attached to the inlet cap member  115 , this diameter may be truncated at the attachment to the inlet cap member  115  as required by geometrical constraints of the existing silencer shell  100 . The diameter of the interior shell  125  subsequently tapers inward in a linear fashion to a radius transition into a tubular center section  130  having a diameter larger than that of the tubular conduit  110 , thereafter the diameter continues to a second radius transition before increasing in a linear fashion and reaching a third radius transition into the second tubular section  135  of a diameter intermediate to the tubular center section  130  and maximum observed at the connection to the inlet cap member  115 . A plurality of passages  140  are cut out of the second tubular section  135  and third radius transition. The subject embodiment presents  4  of such passages having equivalent shape, size, and equal symmetrical placement about the longitudinal axis of interior shell  125 . 
     The interior shell  125  is also in assembled relation to an outlet cap member  145  by use of tabs  200  visible in  FIG.  2   . Again referring to  FIG.  1   , the outer periphery of the entrance to the tubular conduit  110  is joined to the outlet cap member  145  through a radiused transition  150  while an exterior shell  160  contains the tubular conduit  110  and completes an enclosure about it by forming a juncture with the tubular conduit  110  at the atmosphere communicating end and also with the existing silencer shell  100  and outlet cap member  145  creating an outer chamber  155  to the interior shell  125  such that exhaust flow into the atmosphere is constrained to the tubular conduit  110 . 
     Again referencing  FIG.  1   , a centrifugal whirling means consisting of an apex section  165 , a trailing cylinder  170 , a plurality of fins  175 , and a teardrop tail  180 , is fixed in axial alignment with and interior to the interior shell  125  thereby forming an annular passage through which exhaust flow must pass. The apex section  165  is of a general conical shape having the apex oriented such that it is piercing oncoming exhaust flow and henceforth increasing in diameter to a maximum equal to the trailing cylinder  170  and attached thereto. The subject embodiment identifies six fins spaced evenly about the circumference of the trailing cylinder  170 , and being contiguous to the interior shell  125  and trailing cylinder  170 . Additionally a teardrop tail  180  is attached axially to the trailing cylinder  170  opposite of the apex section  165  having a diameter equal thereto and thereafter progressively increasing before decreasing on approach to the outlet cap member  145  and converging down to a small aperture  185 . 
     Referring to  FIG.  3    and  FIG.  3 A , the plurality of fins  175  of the present embodiment exhibit airfoil shaped surfaces with compound curvatures on two separate planes. In  FIG.  3    a first compound curvature  300  is on a plane parallel to the interior shell  125  axis having the characteristics of a curve made up of two or more circular arcs of successively shorter radii, joined tangentially without reversal of curvature, producing an airfoil mean camber line near parallel at leading edge to the interior shell  125  axis thereafter progressively diverging from alignment until reaching a maximum divergence at the rounded trailing edge of said airfoil shape. As shown in  FIG.  3 A , a second compound curvature  305  in a plane perpendicular to the axis of the interior shell  125  is shown with the second compound curvature  305  having the characteristics of a curve made up of two or more circular arcs of successively longer radii, with respect to proximity to the axis of the trailing cylinder, joined tangentially without reversal. 
     Operation— FIGS.  1 ,  2 ,  3 ,  3 A   
     Depending on the embodiment the disclosed spark arrestor may be installed interior to the existing silencer shell  100  of an internal combustion engine exhaust system as shown in  FIG.  1   , or constructed as an original integral internal component to a new silencer for an internal combustion engine exhaust system. 
     In function, as exhaust flow enters the spark arrestor shown in  FIG.  1    through the perforated tube  105  and into the interior shell  125 , the relatively large initial cross-sectional area therein compared to the perforated tube  105 , serves to reduce the exhaust flow velocity and turbulence as the exhaust flow is re-directed into the annular passage created by the apex section  165  and tubular center section  130 . As the exhaust flow enters the annular passage at the tubular center section  130  the cross-sectional area available for exhaust flow is reduced thereby increasing the exhaust flow velocity, also acting on the exhaust flow in this area are the plurality of fins  175  re-directing the flow into a whirling motion and imparting centrifugal force on the exhaust flow and particulate matter contained therein. The previously disclosed airfoil shape and compound curvature of the plurality of fins  175  are designed such that the typical turbulent flow region caused by the leading edge and body of straight flat fins of prior art is all but eliminated in this region, providing a reduction in back pressure compared to prior art. As the whirling exhaust flow exits the tubular center section  130 , centrifugal force acts on particulate matter in the exhaust flow forcing it to the outside perimeter of the interior shell  125  where it passes through one of the plurality of passages  140  and is captured in the outer chamber  155 . Additional particulate matter capture mechanisms include deflection of particulate matter off of the teardrop tail  180  or the radiused transition  150  of the outlet cap member  145  before entering one of the plurality of passages  140  and being removed from exhaust flow. The disclosed teardrop tail  180  also provides a tapering surface leading into the tubular conduit  110  that supports non-disrupted exhaust flow reducing back pressure and maintaining exhaust flow rate. The small aperture  185  in the teardrop tail  180  allows escape of pressure internal to the centrifugal whirling means if a brazing or welding construction method is used and also during normal operation as hot exhaust flow increases the internal temperature of the centrifugal whirling means. The curved nature of the tubular conduit  110  is common for motorcycle silencers for the purpose of exhaust flow noise attenuation and to prevent entrance of foreign matter when the exhaust flow is not present. The curvature may be adjusted depending on the spark arrestor mounting orientation for both of these purposes. 
     Example 1—FIG.  4   
       FIG.  4    summarizes flow testing performed on multiple aftermarket brands and models of new state of the art silencer systems that included integral centrifugal spark arrestors designed to replace the OEM silencer of 2017 KTM 250SX two-stroke motorcycles (KTM OEM part #554.05.079.000). Flow testing was also performed on the OEM non-spark arrested silencer (KTM OEM part #554.05.079.000) in addition to testing on the OEM silencer with 2 different screen type spark arrestors installed as well as the disclosed spark arrestor installed. The flow testing apparatus consisted of A LAMB brand blower used to generate airflow, a Pitot tube to capture dynamic pressure, and a port in the ducting for static pressure. Both pressures were measured using the same digital manometers and ducting diameter feeding the silencers was controlled in size to allow standard flow calculations using Bernoulli&#39;s equation. The flow rate range tested covers that which the subject engine would produce, 0-80 Cubic feet per minute (CFM), under normal operating conditions. All testing was performed under the same ambient conditions on the same day. As shown in  FIG.  4    the disclosed spark arrestor installed into the OEM silencer produced the same flow rate for all given back pressure conditions compared to the OEM silencer without a spark arrestor. By comparison, all of the other spark arrestors tested produced moderate to severe reductions in flow rate at the subject back pressures compared to the OEM silencer without a spark arrestor. 
     Example 2—Table I 
     The arresting efficiency of the disclosed spark arrestor installed in an existing silencer shell (KTM OEM part #554.05.079.000) was tested by the USFS San Dimas laboratory and passed testing with an arresting efficiency of 80% or greater. Table I summarizes said test results. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                   
                 Flow rate 
                 Carbon size 
                 Back pressure 
                 Arresting 
               
               
                 Run 
                 (CFM) 
                 SAE J997 
                 (psi) 
                 efficiency (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 125 
                 Large 
                 0.99 
                 87.21 
               
               
                 2 
                 125 
                 Small 
                 0.99 
                 87.75 
               
               
                 3 
                 94 
                 Large 
                 0.78 
                 82.38 
               
               
                 4 
                 94 
                 Small 
                 0.68 
                 81.35 
               
               
                 5 
                 69 
                 Large 
                 0.28 
                 89.91 
               
               
                 6 
                 69 
                 Small 
                 0.28 
                 86.56 
               
               
                 7 
                 41 
                 Large 
                 0.5 
                 84.35 
               
               
                 8 
                 41 
                 Small 
                 0.12 
                 87.82 
               
               
                 9 
                 13 
                 Large 
                 0.01 
                 N/A 
               
               
                 10 
                 13 
                 Small 
                 0.02 
                 N/A