Abstract:
Engine safety features include an intake silencer defining an internal volume sized to help attenuate air pressure fluctuations generated within the carburetor and engine and transmitted back through the entering combustion air, and also functions as a flame arrester. An internal drip well collects fuel droplets to be siphoned through a hose back to the engine intake system for combustion. The carburetor bowl operates to allow gas and relatively safe vapor out of the carburetor during venting, but disallows liquid fuel from flowing out of the carburetor through the vent. A ball check valve drain disposed in an air intake enables suction of liquid fuel from the silencer into the intake manifold but inhibits suction of air. These features are useful modifications of commercially available engines to improve their suitability for marine applications such as on board electrical power generation, for example.

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. provisional application No. 60/206,051 filed on May 22, 2000, U.S. provisional application No. 60/197,831 filed Apr. 14, 2000, and is a continuation-in-part U.S. application Ser. No. 09/835,277 filed Apr. 13, 2001, the contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to combustion engines, and more particularly to increasing the safety and performance of engines used for marine applications. 
     BACKGROUND 
     Many types and models of engines are commercially available for integration into power systems for a variety of applications. One line of such engines is the DM700G/DM950G series available from Briggs&amp;Stratton-Daihatsu. For some applications, such as providing propulsion or secondary power generation on boats, particularly stringent safety requirements are demanded by law. In the case of marine engines, the performance of safety systems is specified in the United States by the U.S. Coast Guard. For example, 33 C.F.R. §183.526 specifies a fuel leakage test for carburetors of marine engines. To pass such tests, commercially available engines are typically modified to run any carburetor bowl vent to the intake manifold to avoid leaking any liquid fuel such as, for example, when the throttle is opened with the carburetor float stuck. 
     In most applications, it is also desirable that engines perform with as little audible noise as possible. This is especially true for electrical power generation on boats, as such generator systems frequently operate in an absence of background noise. 
     Other desirable engine attributes include low emission of pollutants and high fuel efficiency. 
     SUMMARY 
     The invention features several improvements in the design and structure of engines that, for example, improve their suitability for marine applications such as on board electrical power generation. 
     One such improvement is an intake silencer for use with side-draft carburetors. The intake silencer of the invention defines an internal volume sized to help attenuate air pressure fluctuations generated within the carburetor and engine and transmitted back through the entering combustion air, and also functions as a flame arrester to inhibit the propagation of sparks or flame from the carburetor to the surrounding atmosphere. 
     In some embodiments, the intake silencer also defines an internal drip well at the bottom of its internal volume to collect fuel droplets and condensing vapors during, for example, the carburetor leak test specified in 33 C.F.R. §183.526. Preferably, these collected liquids are siphoned through a hose back to the engine intake system for combustion. 
     In the present embodiment, the intake silencer defines an attachment at one end for a tuned pipe or “zip tube”, in hydraulic communication with the internal volume of the silencer, to provide a quarter-wavelength attenuator. 
     Another such improvement deals with carburetor bowl venting. Instead of suctioning carburetor bowl gasses into the intake manifold at negative gage pressure, as has been done in marine applications employing commercially available engines, in the improved engine the carburetor bowl is vented to the atmosphere through a check valve that operates to allow gas and relatively safe vapor out of the carburetor during venting, but disallows liquid fuel from flowing out of the carburetor through the vent. In early tests, this improvement was able to significantly reduce the emissions of the engine under some operating conditions, as compared with the same engine with the carburetor bowl vented into the carburetor inlet. 
     In another aspect, the invention features an air intake component, such as a silencer or air filter housing, adapted to be attached to a carburetor and including a check drain valve hydraulically connecting the component to the intake manifold of the engine. The drain valve enables liquid fuel to be sucked into the engine from the component but inhibits the transfer of air. In one embodiment the valve includes a check ball that normally seats against the inner surface of the drain to prevent the flow of air into the manifold under manifold vacuum pressure. The ball is constructed to float on liquid fuel, such that in the presence of a sufficient quantity of liquid fuel the ball will rise from its seat and permit the liquid fuel to be sucked into the manifold. This valve is particularly effective for use with engines equipped with side draft carburetors attached to intake components, upstream of the carburetor, with inner cavities that extend to a lower elevation than the carburetor inlet and can therefore tend to collect liquid fuel. 
     Another engine improvement features a damper applied to an engine speed control linkage to dampen linkage oscillation and improve engine speed stability. We have found that connecting the governor speed control lever of certain commercially available engines to the associated engine block with a dashpot can dampen the speed oscillations experienced during a rapid load change without significantly impairing engine speed response time. This improvement is particularly useful for engines coupled to electrical generators or otherwise intended to operate at a fairly constant speed for an extended period of time. For driving AC generators, engine speed stability is extremely necessary for maintaining generator frequency, as well as output voltage stability. We have found, for instance, that proper dashpot and spring selection can improve the suitability of Daihatsu DM700G/DM950G series engines for driving AC generators. 
     The details of one or more embodiments of these improvements are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is an internal combustion engine equipped with an intake silencer. 
     FIG. 2 is a perspective view of the intake silencer. 
     FIGS. 3-5 are top, front and side views, respectively, of the intake silencer. 
     FIG. 6 is a cross-sectional view, taken along line  6 — 6  in FIG.  3 . 
     FIG. 7 is a cross-sectional view, taken along line  7 — 7  in FIG.  6 . 
     FIG. 8 is a bottom view of the intake silencer. 
     FIG. 9 illustrates a governor speed control linkage equipped with a dashpot. 
     FIG. 10 documents a test of engine speed stability without the dashpot. 
     FIG. 11 documents a test of engine speed stability with the dashpot. 
     FIG. 12 schematically illustrates a prior art method of venting a carburetor float bowl. 
     FIG. 13 schematically illustrates venting a carburetor float bowl through a check valve. 
     FIGS. 14A and 14B illustrate examples of applicable check valves. 
     FIGS. 15 and 16 are perspective and side views, respectively, of another intake silencer, equipped with a cylindrical spark arrester and a drain check valve. 
     FIG. 17 is a cross-sectional view. taken along line  17 — 17  in FIG.  16 . 
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a gasoline engine  10  has a carburetor  12  for supplying a predetermined air/fuel mixture for combustion. Air is provided through an intake silencer  14 . 
     Referring also to FIGS. 2-8, silencer  14  is a one-piece cast aluminum housing defining an internal volume  16  of roughly 2.5 inches by 1.5 inches by 5.5 inches. A metal screen  18  is provided in the outlet  20  of the silencer, where the silencer attaches to the side draft inlet of the carburetor (not shown). The metal screen  18  functions as a spark arrester for inhibiting flame propagation from the carburetor back into the silencer. Screen  18  is mounted in a rim  22  that, once installed to the carburetor, is clamped between an inner lip of the silencer housing and the inlet nozzle of the carburetor. 
     A particular advantage of this embodiment is that the spark arrester (screen  18 ) is completely enclosed within the silencer housing. Thus, in one single casting silencer  14  performs both a noise reduction function and a fire safety function. 
     Of course, the optimal size and shape of the internal volume of the silencer will vary with different engines and different operating speeds. For fixed speed applications, such as for driving synchronous generators, the size of the internal cavity should be selected for a satisfactory level of airborne noise at the known operating speed. We have found that this illustrated embodiment works well for the Daihatsu engines mentioned above, when run at a speed of about 1800 RPM. 
     Extending from the bottom side of silencer  14  is an internal drip well  24  disposed immediately below outlet  20  to catch any fuel droplets entering the silencer from the carburetor. A recirculation port  26  provides a connection for a recirculation hose  28  (FIG. 1) for siphoning caught fuel to the intake manifold. We have found only a minor reduction in manifold vacuum, even with a recirculation port of as much as 0.25 inch in diameter. 
     Silencer  14  also defines an air inlet  29  to which a “zip tube” can be readily attached for supplying fresh air for combustion. Neither the volume of the inlet  29 , nor the volume of the outlet  20 , is included in the internal volume of the silencer when discussed in terms of noise attenuation. 
     Referring to FIG. 9, an engine speed control linkage  30  includes a pivot lever  32  fixed to a governor control shaft  34  and adapted to rotate shaft  34  when pivoted about the centerline of the shaft, to adjust the rotational position of the shaft and thereby adjust the speed setting for the mechanical engine governor (not shown). The speed setting is controlled through the positioning of lever  36 , which is attached to lever  32  by extension spring  38 . Thus, a clockwise control torque is applied to lever  32  by spring  38  as a function of the relative positions of the two levers. Counter-acting, counter-clockwise torque is provided by the centripetal acceleration of the flyweights of the governor. Levers  32  and  36  and spring  38  are members of an engine speed control linkage of the line of Daihatsu engines noted above. We have found that adding a dashpot  40  between lever  32  and a fixed point on the engine block improves the stability of engine speed, particularly during load changes. In effect, dashpot  40  acts to dampen the higher frequency content of oscillation of lever  32 , resulting in more speed stability and less ‘hunting’. This is particularly important for applications in which the engine is intended to operate at fairly constant speeds and in which a fast speed change response characteristic is therefore of less importance. In the arrangement illustrated and tested, spring  38  has an extension spring rate of about 1.1 pounds per inch, and dashpot  40  is as purchased from Airpot Corporation, Norwalk, Conn., as their part number 2KS95. Spring  38  was attached to lever  32  at a point about 3.5 inches from the center of shaft  34 , and dashpot  40  was attached at a point about 1.75 inches from the center of shaft  34 . 
     FIG. 10 shows a load response test conducted on a Daihatsu engine connected to an electrical generator, without the control linkage as shown in FIG. 9 but without dashpot  40 . The upper time trace is of generator output frequency, representative of engine speed. The lower trace is of generator output voltage. As indicated above the traces, the first time period of the test was with the generator at effectively no load. At approximately one minute intervals, the load was increased in 25% increments up to effectively 100% rated load, then returned to no load. Of particular interest are the underdamped speed fluctuations occurring at the load transition points, resulting in oscillations in both frequency and voltage. While some speed variation is expected with rapid load changes, it is desirable to minimize such speed fluctuation as much as possible, such that engine speed is as independent of load as possible. 
     FIG. 11 is from a similar test of the same engine tested in FIG. 10, but with the engine speed control linkage equipped with dashpot  40  as shown in FIG.  9 . Comparing FIGS. 10 and 11, it can be seen that the presence of the dashpot dampened the speed oscillations occurring at rapid load changes, without significantly increasing response time. For example, at point  42 , the increase from no load to 25% load, the standard engine speed went through two complete oscillations before settling, whereas the dashpot-equipped engine settled after only one speed oscillation. 
     Referring now to FIG. 12, a float chamber  52  of a typical carburetion system, as previously modified for marine applications, is schematically represented. A float  54  moves to operate a valve  56  to regulate the level of liquid fuel  58  within the chamber, which is fed to the mixing chamber through port  60 . Air displaced from chamber  52  is vented into the engine&#39;s intake manifold  62  or carburetor inlet. While this venting method keeps liquid fuel from leaking from the system when float chamber  52  is inadvertently flooded, the vacuum pressure in manifold  62  can also draw excess fuel vapor or even unregulated liquid fuel into the intake of the engine, thus causing an undesirably “rich” fuel mixture that can negatively affect performance and emissions. 
     As shown in FIG. 13, we have found that safe float chamber venting may be accomplished by venting the float chamber  52 ′ to the atmosphere through a ball check valve  64 . The function of valve  64  is to inhibit the release of liquid fuel from chamber  52 ′ under all conditions, but to enable the venting of harmless amounts of air and vapor. We have found that, under normal operating conditions, the same Daihatsu engine vented as shown in FIG. 13 produced significantly less emissions than when equipped as shown in FIG.  12 . 
     Ball check valve  64  may be, for example, a “WEATHERHEAD” brass male ball check connector, available from Dana Corporation, Cleveland, Ohio, as catalog number 43X4 and as shown in FIG. 14A, for example. In such a valve, oriented such that the ball end is up, a ball  66  is urged toward a fixed seat  68  by a compression spring  70 . With air or vapor at the interface between seat  68  and ball  66 , there is sufficient leakage about the ball to permit venting of the carburetor bowl through the valve. However, if the level of liquid fuel rises to the interface between seat  68  and ball  66 , the ball/seat interface provides sufficient restriction against upward flow to enable fuel pressure to build under the ball and effectively seal the ball/seat interface against liquid fuel flow. When the liquid fuel level subsequently drops, air and vapor venting is resumed. 
     The valve  64  shown in FIG. 14A may be modified to replace the solid metal ball  66  and spring  70  with a ball  66  adapted to float upward on a rising level of liquid fuel to engage seat  68  to close the valve, but otherwise to remain generally spaced away from seat  68  to allow venting of air and vapor at reasonable air and vapor flow rates. Such characteristics may be obtained by designing such a ball to have an appropriate combination of buoyancy and weight, for example. 
     Alternatively, a membrane valve  64 ′ may be provided as shown in FIG.  14 B. Instead of a ball and spring, a porous membrane  72  is constructed to permit air and some amount of vapor to pass while blocking liquid fuel in droplet or flow form. It is important that membrane  72  be constructed to withstand fuel pump pressures without rupturing. 
     Referring now to FIGS. 15-17, intake silencer  74  provides intake air to a carburetor of a gasoline engine, and defines an internal cavity  76  sized to dampen pressure vibrations that can result in undesirable, air-borne noise. The silencer has a cast aluminum housing  78  with an intake port  80  and a flange  82  for mounting the silencer to a carburetor (not shown). A cylindrical flame arrester screen  84  is held in place within the silencer by flange  82  and a cast protrusion  86  extending inward from the outer side of the silencer, as shown in FIG.  17 . Screen  84  serves to inhibit the propagation of flame or sparks from the carburetor throat to atmosphere through the silencer. Housing  78  is also equipped with a drain  88  at the low point of cavity  76  for draining any condensed or otherwise accumulated fuel from the silencer. The drain is hydraulically connected, through an appropriate hose, to the intake manifold of the engine (not shown), such that under normal operating conditions the output of drain  88  is at negative (i.e., vacuum) pressure. 
     To keep atmospheric air from being sucked directly into the intake manifold from drain  88 , a check ball  90 , of a diameter larger than the smallest diameter of the drain opening of housing  78 , normally rests against a tapered surface  92  within the drain. Thus, drain  88  acts as a check valve against the flow of air from the silencer into the engine manifold. However, ball  90  is constructed to float on the liquid fuel that accumulates in drain  88 , such that in the presence of liquid fuel ball  90  will rise from surface  92  and allow the fuel to be sucked into the engine manifold. When the fuel is no longer present, ball  90  returns to its seat to inhibit flow of air. One presently preferred embodiment employs a ¼ inch diameter hollow ball of polypropylene. Thus, drain  88  prevents the undesirable release of liquid fuel from the engine due to prolonged accumulation within the intake silencer. As mentioned above, this is particularly important for engines operating in regulated environments, such as aboard boats. It will be understood from this disclosure that drain  88  is also useful on other air intake components other than silencers, such as air filter housings. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.