Gas and liquid contact apparatus – Fluid distribution – Pumping
Reexamination Certificate
2002-08-23
2004-04-06
Chiesa, Richard L. (Department: 1724)
Gas and liquid contact apparatus
Fluid distribution
Pumping
C261S069100, C261SDIG006
Reexamination Certificate
active
06715737
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a fuel metering system, and more particularly to a fuel metering system having a planar diaphragm for an externally-purged-type carburetor.
BACKGROUND OF THE INVENTION
Typically, carburetors have been used to supply a fuel-and-air mixture via an intake passage to both four stroke and two-stroke internal combustion engines. For many applications where small two-stroke engines are utilized, such as hand held power chain saws, weed trimmers, leaf blowers, garden equipment and the like, carburetors with both a diaphragm fuel delivery pump and diaphragm fuel metering system have been utilized. When the engine is operating, the diaphragm fuel delivery pump supplies fuel under pressure to the diaphragm fuel metering system through an inlet or flow control valve of the fuel metering system, which in-turn supplies fuel to a fuel-and-air mixing passage of the carburetor for mixing with air prior to flowing into a combustion cylinder of the engine.
A convoluted flexible diaphragm or membrane of the fuel metering system typically has a peripheral edge sealed to the carburetor body. A metering chamber and an air chamber is thus partitively disposed over and under the diaphragm, respectively. During operation, when the amount of fuel in the chamber decreases and the convoluted diaphragm is moved due to a negative pressure in the fuel-and-air mixing passage, the flow control valve is opened against the force of a spring by a pivoting lever that operates together with the diaphragm and is fixed to a wall of the carburetor body by a support shaft. In this way, the fuel is supplied from the fuel delivery pump to the metering chamber. As a result, the amount of fuel in the metering chamber is kept at about a constant level or volume.
Commonly, the carburetor has an external purge or manually actuated primer or suction pump having a flexible bulb attached to the bottom side of the carburetor body. The bulb internally defines a pump chamber in which a composite valve functions to admit fuel to the pump chamber and deliver fuel to the metering chamber of the fuel metering system. Moreover, before the engine starts for operation, the bulb is repetitively manually pressed and released to suck unwanted fuel vapor and air from the fuel pump and fuel metering system into the pump chamber of the external purge via the composite valve. The fuel vapor and air are transferred back to the fuel tank via the composite valve. At this time, since the metering chamber is under a negative pressure, the fuel in the fuel tank is supplied to the metering chamber through a fuel chamber of the fuel delivery pump and the flow control valve.
The diaphragm of the fuel metering system typically has five basic functions: (1) maintain a seal between the air and the metering chambers, (2) respond instantly to differential pressure (engine manifold pressure referenced to atmospheric), (3) open the flow control valve when the engine needs fuel, (4) close the flow control valve when the engine has enough fuel, and (5) perform consistently over the life of the engine (i.e., no loss of elastomeric flexibility of the convoluted diaphragm from age or fuel exposure).
The convoluted metering diaphragm is typically made of an elastomeric membrane and molded to form convolutions to achieve flexibility and a pre-established total travel distance necessary to open and close the flow control valve. This total travel distance commonly ranges from about 0.020 to 0.065 of an inch, and includes a degree of free-play before a head of the flow control valve actually moves to open and close the valve. During engine operation, from idle to wide open throttle conditions, the convoluted diaphragm typically moves approximately within a range of 0.001 to 0.015 of an inch and thus the head proportionately moves accordingly. This range depends upon the carburetor and its application.
FIGS. 8-10
, illustrated as prior art, show such a metering diaphragm
20
having a molded convolution
22
. Under normal engine/carburetor operating conditions, a center or circular section
24
of the diaphragm, circumscribed by the convolution
22
, provides the primary movement for operation of the flow control valve
26
. The convolution itself has little contribution to achieving the required fuel delivery pressure balance in the metering chamber (not shown). The metering diaphragm
20
transmits a relative movement to a pivoting lever
28
which transmits opposite movement to a head
30
of the flow control valve
26
based on a pressure differential formed across the diaphragm. The differential is initiated from the sub-atmospheric pressure exposed to the metering chamber by the fuel-and-air mixing passage of the carburetor and the reference atmospheric pressure of the air chamber of the metering system.
FIGS. 8 and 9
illustrate the common convoluted metering diaphragm
20
having a central rigid plate
32
, a washer
34
and a rivet button
36
for transmitting this force to the pivoting and spring biased lever
28
of the flow control valve
26
, which in turn moves the valve head
30
away from a valve seat
38
carried by the carburetor body to open, and against the valve seat
38
via the resilience of the spring (not shown) to close the valve. The diaphragm must have sufficient resilience for transmitting displacement in proportion to the pressure differential, yet remain flexible enough to respond to sudden changes in pressure such as for engine acceleration and engine starting. Unfortunately, the cost of manufacturing a flexible diaphragm having rigid hardware which is engaged sealably to the diaphragm is expensive, and the diaphragm penetration required to secure the hardware creates a source of potential leakage between the metering chamber and the reference chamber.
Aside from the rigid hardware, there are several reasons for the additional diaphragm travel afforded by the convolution in a standard diaphragm carburetor design. The convolution provides extra material for maintaining diaphragm flexibility should the fabric or elastomer coating shrink (typically made of woven silk and nitrile material) upon exposure to hydrocarbon fuels or aging effect. This extra material measured or extending perpendicular to the general plan of the diaphragm itself also maintains necessary operating clearances or free-play travel distance between the pivoting lever and diaphragm if this shrinkage occurs. The extra convolution material also allows more diaphragm travel (increased metering fork leverage) to “uncork” a stuck head of the flow control valve, particularly for carburetors which do not have a manual external purge or bulb device to create a strong vacuum. In-other-words, the convolution assists to release stuck heads for those carburetors which utilize the weaker engine manifold vacuum in combination with a choke valve to generate the metering chamber vacuum for opening the flow control valve for purging the carburetor of air or vapor to better start the engine.
However, there are also inherent problems associated with the metering diaphragm convolution which have adverse impact on carburetor performance. Such problems include the inadvertent changes in baseline carburetor fuel flow settings, inconsistent fuel delivery and exhaust emission variation, poor acceleration response, and the potential for leaking/dripping from the carburetor main nozzle. For instance, a distorted convoluted diaphragm can change the original or installed operating clearance between the rivet button and the lever so that an adverse shift in idle performance due to vibration or orientation of the engine can cause fuel leakage leading to a rich idling engine. At wide open throttle conditions, such fuel leakage can result in engine stall during deceleration from wide open throttle to idle. For non-running engines, a distorted convolution which eliminates clearance can depress the lever to allow fuel leakage out of the carburetor causing fuel tank drainage.
The process of convolution molding is known to contribute to variations in di
Galka William E.
Hilbig Bradley D.
Roche Ronald H.
Van Allen James E.
Chiesa Richard L.
Reising Ethington Barnes Kisselle P.C.
Walbro Corporation
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