Diaphragm carburetor with air purge system

A diaphragm carburetor-based fuel supply system is equipped with an air purge system that vents trapped air from the fuel supply system. The air purge system preferably includes a vent tube having an outlet opening into the upper portion of the system's fuel tank and an inlet located as close as practical to the diaphragm chamber of the carburetor, preferably within an internal passage of the carburetor or at least in a fitting or fuel supply tube portion located closely adjacent the fuel inlet of the carburetor. The resulting system requires only a few pull strokes to start a freshly fueled engine, as opposed to about 15 strokes in a system lacking such an air purge system. It also permits the use of a choke that is incapable of fully closing, thereby negating the need for a “false hit” during cold engine start.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to fueling systems and, more particularly, relates to a fueling system utilizing a diaphragm carburetor to form an air/fuel mixture and to supply the mixture to an engine. The invention additionally relates to an engine fueled with such a system and a method of its use.

2. Discussion of the Related Art

Diaphragm carburetors are widely used to supply fuel to relatively small two-stroke and four-stroke utility engines. A diaphragm carburetor has a diaphragm chamber which opens to main jet and idling jet orifices. Fuel flow through the carburetor is controlled by a regulator located in the diaphragm chamber. The regulator continually opens and closes an inlet needle in response to the vacuum created in the carburetor's venturi. Fuel is supplied to the inlet needle via either a diaphragm pump or by gravity. In the case of the diaphragm pump, suction pulses of the engine are used force fuel through the pump and a series of check valves. The resultant volume of pressurized trapped fuel then flows from the regulator chamber to the fuel jet orifices at a rate that depends on the velocity of the air flow through the venturi which depends on the setting of the throttle valve and the speed of the engine.

Unlike float carburetors, diaphragm carburetors do not have to be vented, and do not rely on the position of a float to maintain a desired volume of fuel in the carburetor. Fuel therefore cannot leak out of the carburetor, even if the carburetor is used on a machine that is subject to severe vibrations and/or that is often operated while inverted or lying on its side. Machines of this type include weed trimmers, chain saws, snow blowers, rammers, and breakers.

A relative disadvantage of diaphragm carburetors is that engines fueled by them can be difficult to start, particularly when the engine has run out of fuel. This is because air can be trapped in the carburetor passage upstream of the diaphragm and in the fuel supply tube leading from the fuel tank to the carburetor. This air must be purged and the diaphragm chamber filled with fuel before the engine can start and run. Depending on the length and diameter of the fuel supply tube, this purging requirement can necessitate 15-20 starting pull cord strokes to purge all of the trapped air. This can be very fatiguing to operators.

Many components have been made and mechanisms implemented for improving the startability of small engines. The most common device used today is a so-called “prime bulb.” A prime bulb is a cap or bulb mounted on or adjacent to the engine and manually activated by an operator to draw fuel into the carburetor and purge air from it. Prime bulbs can be very effective, but they require manual operation apart from the usual starting operation. Operation of a prime bulb may result in the injection of fuel into the throat of the carburetor. Moreover, activation of a prime bulb when the engine is warm, or when the engine fails to start on the first attempt, can flood the engine so that the engine will not start. Moreover, prime bulbs usually are made of rubber or another resilient material that may become brittle with age and with contact with fuel. They therefore have a limited life. This life is further limited by the imposition of shocks and vibrations on the engine during operation of some implements, such as rammers and breakers.

Another technique that is sometimes employed to improve the cold startability of a diaphragm carburetor-equipped engine is a so-called “closed choke,” which is capable of completely or nearly completely closing a choke plate to minimize airflow through the carburetor during a starting operation so as to maximize the richness of the air/fuel mixture. An engine equipped with a closed choke cannot run with the choke fully closed. Instead, the operator must operate the pull cord with the choke closed until he or she detects what is known as a “false hit” in which the engine begins to run but then dies. The operator must then partially or fully open the choke and pull the cord again to start the engine. Closed chokes require even more complex operator interaction than is required for actuation of a prime bulb. They also increase the risk of engine flooding.

The need has therefore arisen to provide a simple, yet reliable mechanism for purging air from a diaphragm carburetor-based fuel supply system in order to facilitate starting of an engine.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, the need identified above is satisfied by providing a fuel system with a vented diaphragm carburetor. Specifically, the engine's fuel supply passage opens into a vent passage that is configured to vent trapped vapor from the fuel supply passage. The fuel supply passage supplies fuel to a metering chamber of the carburetor from the fuel tank. It typically comprises 1) a fuel supply tube that supplies fuel to the fuel inlet of the carburetor from the fuel tank and 2) internal passage(s) supplying fuel to the metering chamber from the fuel inlet of the carburetor. The vent passage preferably comprises a vent tube having an inlet that opens into the fuel supply passage and having an outlet configured to open into an upper portion of the fuel tank. The vent tube inlet preferably opens into either an internal passage in the carburetor, such as into a pump diaphragm chamber of the internal passage, or a downstream portion of the fuel supply tube. The vent passage reduces the number of pull cord actuating strokes required to start a typical two-stroke or four-stroke engine after the engine has run out of fuel and has been refueled. This reduction is from at about 15 pull cord strokes to no more than 5, and even to 3 or less if the vent tube opens into an internal passage of the carburetor. It also can improve steady state operation of the engine by purging fuel vapor from a hot carburetor.

Another benefit of the inventive air purge system is that permits the use of a choke plate that is incapable of being fully closed. For instance, if the choke plate comprises a butterfly valve, the butterfly valve may have at least one aperture formed therethrough through which air passes when the butterfly valve is fully closed. An engine fueled with such a carburetor can start and idle with the choke fully set, hence negating the need to for the operator to detect a false hit and then back off the choke before starting the engine.

The air purge system may also reduce or avoid vapor lock by venting vaporized fuel from the fuel supply passage during engine operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

1. System Overview

The inventive air purge system is usable with virtually any diaphragm carburetor-equipped two-stroke or four-stroke engine. Applications for these engines are also myriad. Hence, while a preferred embodiment of the inventive air purge system will now be described in conjunction with a reciprocating impact tool powered by such an engine, an engine, specifically a rammer, it is to be understood that it is usable with a variety of other powered devices as well.

Referring now to the drawings and initially toFIGS. 1-3, a rammer (sometimes known as a tamper)20is illustrated that includes an engine22and a rammer subassembly24bolted to one another to form an integral unit. The rammer subassembly24includes a rammer crankcase26and a reciprocating tamping shoe28connected to the rammer crankcase26by a reciprocating piston (not shown) so as to oscillate or reciprocate vertically upon rammer operation. The piston is protected at its lower end by a fixed guard30and at its upper end by a flexible boot32that accommodates movement of the shoe28relative to the rammer crankcase26. The machine is supported and guided by an operator's handle34that also serves as a guard.

Still referring toFIGS. 1-3, the engine22is a spark ignited, single-cylinder, internal combustion engine. It may comprise either a two-stroke engine or a four-stroke engine. The cylinder (not shown) is encased in a crankcase38bolted to a rear surface of the rammer crankcase26. The engine22is started via a pull-cord42mounted on the rear surface of the engine crankcase38.

The engine22is supplied with spark via a spark plug44and with fuel via a fuel supply system46. The engine is not equipped with a prime bulb, although one could be provided, if desired. The fuel supply system46instead is equipped with an air purge system48constructed in accordance with a preferred embodiment of the present invention. The fuel supply system46, and especially its air purge system48, will now be described in greater detail.

2. Construction and Operation of Fuel Supply System

Still referring toFIGS. 1 and 2, the fuel supply system46comprises an air purge system48, a fuel tank50, a carburetor52, and a fuel supply line54. The fuel tank50is mounted on the frame/handle34above the engine crankcase38. It includes an upper fill port56, a lower outlet58, and a hollow interior configured to be filled with fuel to a maximum fill line60spaced from the top of the tank50. The fuel supply line54comprises a flexible tube having an inlet connected to the lower outlet58of the fuel tank50by a first fitting62and an outlet coupled to a fuel inlet68of the carburetor52by a second fitting64.

Referring toFIGS. 2-5, the carburetor52includes a generally rectangular body66having the fuel inlet68, an air inlet70, and a mixture outlet74(FIGS. 6 and 7) which typically takes the form of one or more jets. Airflow into the carburetor52is controlled by a throttle76that is actuated by a throttle cable78in a manner which is, per se, well-known. The air inlet70can be selectively partially closed by a choke plate. In the illustrated example, the choke plate takes the form of a butterfly valve80operated by a manual choke lever82. Pursuant to the invention, however, the butterfly valve80is not fully closable for reasons detailed below. Air and fuel are drawn through the carburetor52from the respective inlets70and64, mixed with one another, and discharged from the outlet74under operation of an internal diaphragm pump84(FIGS. 6 and 7) located in a diaphragm chamber (not shown). Except for the fact that its choke is not fully closable, the carburetor52as thus far described may be of a type commercially available from various manufacturers such as Walbro Corporation of Cass City, Mich. or Tillotson, Ltd. of Ireland. As a point of fact, one of the advantages of the air purge system48as it will now be described is that it can be easily incorporated into an existing carburetor design and even retrofitted into a pre-manufactured carburetor. As a further point of fact, the illustrated carburetor52is a Tillotson carburetor modified only 1) so that its choke is not fully closable and 2) to mate with the air purge system48.

The air purge system48comprises a vent passage and related couplings that vent fuel from a downstream portion of the fuel passage (formed by the fuel line54and the internal passages of the carburetor52leading from the fuel inlet68to the diaphragm chamber) to a location remote from that portion. A variety of different structures could perform this function. In a particularly preferred embodiment, the vent passage takes the form of a simple flexible vent tube90having an inlet92and an outlet94. The vent tube outlet94is disposed so as to safely direct vented air, which may be heavy laden with vaporized fuel, to a remote location, preferably the interior of the fuel tank50. Towards this end, the vent tube outlet94preferably opens into the fuel tank50at a location above the maximum fill line60. This effect is achieved most conveniently by running the vent tube90up into the fuel tank50from a lower vent tube inlet port95.

Since the vent tube90only effectively purges portions of the fuel delivery stream upstream of the vent tube inlet92, the vent tube inlet92is preferably located as close as practical to the diaphragm chamber of the carburetor52. In the embodiment illustrated inFIGS. 1-6, this effect is obtained by connecting the vent tube inlet92to an internal fuel passage in the carburetor52. Hence, in addition to incorporating the above-described fuel inlet port68, air inlet port70, and mixture outlet74, the carburetor body66incorporates a vent port72that opens into an internal fuel passage of the carburetor52.

In the illustrated embodiment in which the carburetor52is a Tillotson carburetor, a convenient location for vent port72is one in which it opens into an auxiliary or pump diaphragm chamber96located on the side of the carburetor body66. As best seen inFIGS. 4 and 5, chamber96can be accessed by removing a cover98from the side of the carburetor body66. The thus-exposed chamber96is bounded at one side by a recess in the carburetor body66and at another side by a facing recess in the cover98. The chamber96has a fuel inlet100connected to the fuel inlet port68of the carburetor52via a first internal passage102in the body66and an outlet104at least indirectly connected to a main diaphragm chamber, more commonly referred to as a metering chamber, via a second internal passage106in the body66. In the stock carburetor, the outer portion of the chamber96typically is separated from the inner portion containing the fuel inlet100and fuel outlet104by a diaphragm (seen in phantom at108). However, diaphragm108can be removed to provide unrestricted airflow from the inner portion of that chamber96to the outer portion thereof when the vent port72is drilled into the outer portion of chamber96by drilling a hole through the cover98. When the cover98is reattached to the body66of the thus-modified carburetor52, the vent tube inlet92can be coupled to the vent port72by a suitable fitting110. Air is now free to flow to the fuel tank50from the chamber96and all upstream portions of the fuel supply passage via the vent port72and the vent tube90.

Not all diaphragm carburetors may have an internal passage that is easily accessible for connection to a vent tube inlet. In this case, it may be necessary to couple the vent tube inlet92to another location in the fuel supply passage. That location should preferably be in the fuel supply tube as close as practical to the carburetor fuel inlet port, such as in the fuel inlet fitting coupling the fuel supply tube to the fuel inlet of the carburetor. An air purge system J148configured in this manner is illustrated schematically inFIG. 7, in conjunction with the same fuel supply system46ofFIGS. 1-6. In this system, the fitting164connecting the fuel supply tube54to the carburetor fuel inlet port68takes the form of a T-fitting having a fuel inlet coupled to the outlet of the fuel supply tube54, a fuel outlet opening to the fuel inlet port68of the carburetor52, and an air outlet coupled to the inlet92of the vent tube.

Experiments have shown that providing an air purge system having a vent tube inlet opening into the carburetor in the location illustrated inFIGS. 1-6can dramatically reduce the average number of pulls required to start an engine after it has run out of fuel and the tank refilled. Specifically, the required number of pull strokes required to start the engine22typically has decreased from the 15 to 17 range to less than 5 when the air purge system is added to the engine's fuel supply system46. In fact, the typical, freshly fueled engine can be started with three pull strokes or even less. These benefits have been established experimentally for a two-stroke engine, and are believed to apply equally or nearly equally to a four-stroke engine. The air purge system148of the embodiment ofFIG. 7is slightly less effective at improving startability, but still dramatically reduces the number of pulls required to start the engine. It is estimated that the system ofFIG. 7requires no more than 5 to 6 pull strokes to start a freshly fueled engine—still a dramatic improvement over the 15 to 17 that might otherwise be required.

The air purge system as described generally above and more specifically with respect to either the embodiment ofFIGS. 1-6or the embodiment ofFIG. 7offers additional advantages to those described above.

For instance, as mentioned briefly above, it permits the use of a choke that is not fully closable. As mentioned in the Background section above, diaphragm carburetors typically employ a choke plate that must be closed fully prior to engine starting to maximize the richness of the fuel charge during a cold start operation. Also as mentioned above, an engine equipped with this type of carburetor cannot run and remain idling with the choke is fully closed but, instead, is subject to a “false hit” in which the engine runs a few revolutions on its own and then dies. The operator must then partially open or “back off” the choke prior to once again attempting to start the engine. It has been discovered that the inventive air purge system is so effective at obtaining rapid fuel delivery to the carburetor that it is unnecessary to fully close the choke to start a cold engine. Hence, the choke plate can be configured to lack the ability to fully close but, instead, to have a minimum airflow passage that it is a relatively small percentage of the maximum airflow passage. The airflow passage available upon choke plate closure is typically on the order of 5% of the maximum area of the airflow passage. This effect could be achieved, for instance, by providing a stop in the vicinity of the choke plate seat and/or adjacent the choke lever to prevent full choke plate closure. In the illustrated embodiment in which the choke plate comprises a butterfly valve80, this effect can be achieved simply by drilling one or more apertures120in the butterfly valve80having a combined area on the order of at least 4%, and preferably about 5% of the total area of the butterfly valve80. The thus equipped choke allows sufficient airflow through the carburetor52to allow the engine to start and run at idle, even when the choke is fully set. The need to obtain a false hit and then open the choke prior to starting the engine therefore is negated.

Still another benefit of the inventive vapor air purge system is that it may prevent vapor lock by venting vaporized fuel from a hot carburetor and thereby preventing the vaporized fuel from backing up into the fuel line.