CARBURETOR AND METHODS THEREFOR

BACKGROUND

Carburetors are reliable, robust mechanisms for efficiently metering fuel to an internal combustion engine. A carburetor meters the appropriate amount of fuel according to engine demand based on intake airflow to the engine. Generally, carburetors operate on the principle that as the velocity of airflow through a restriction increases, its pressure decreases. Carburetors are configured to take advantage of the pressure differential created between atmospheric pressure surrounding the carburetor and a low pressure region created inside the carburetor, usually by way of a venturi. As an engine draws air through the venture, the low pressure region created by the increasing air velocity meters a proportional amount of fuel into the intake airflow stream. As passive devices, carburetors are both reliable and robust, while thoroughly mixing fuel with incoming airflow which enhances efficient combustion.

While carburetors are simple and cost effective fuel delivery systems, modern emission requirements have limited the application of carburetors on newer products. Many applications have implemented electronic fuel injection in order to maintain precise control of fuel delivery, which allows catalytic converters to be used in an emissions reduction strategy. The introduction of electronic fuel injection has added complexity, cost, weight, and increased electronic bad to modern engines. Fuel injection systems rely on a sensor network. The failure of any single sensor can drastically reduce the emissions performance of the fuel system.

In order to continue to benefit from the carburetor's advantages, improvements to traditional carburetor design are needed in order to ensure the carburetor's ability to meet emission requirements for modern engines.

SUMMARY

Provided herein is a carburetor for an internal combustion engine, comprising a body having an air inlet opening portion, an air outlet opening portion, and a throat portion extending therebetween. A fuel reservoir is in fluid communication with the throat portion and a slide assembly is movably disposed in the body for movement across the throat portion. The slide assembly includes a throttle slide and a metering rod extending across the throat portion and into the fuel reservoir. The air inlet opening includes a pair of concavities operative to direct airflow toward the metering rod. The concavities begin near a peripheral margin of the inlet opening portion and extend inward as the concavities approach the throat portion. The throat portion includes upper and lower portions and the concavities are adjacent the upper portion.

Also contemplated herein is a carburetor having an air inlet opening that includes a manifold, which may be in the form of an arcuate scoop, adjacent to and extending along a portion of a peripheral margin of the inlet opening portion. The manifold is in fluid communication with the fuel reservoir. The manifold has a volume that is proportional to the cross-sectional area of the throat portion. The throat portion includes upper and lower portions, and the manifold is adjacent the upper portion. This carburetor may also include an air inlet opening that includes a pair of concavities operative to direct airflow toward the metering rod that are located proximate either end of the manifold. Wherein the concavities begin near a peripheral margin of the inlet opening portion and extend inward as the concavities approach the throat portion.

In another embodiment, a carburetor for an internal combustion engine is contemplated that includes a slide assembly movably disposed in the body for movement across the throat portion. The slide assembly includes a throttle slide having a metering rod bore and a positioner bore. A metering rod extends through the metering rod bore and across the throat portion into the fuel reservoir. The slide assembly includes a positioning mechanism operative to adjust the position of the metering rod relative to the throttle slide. The positioning mechanism includes a barrel rotatably disposed in the positioner bore. The barrel is threadably engaged with the metering rod such that rotation of the barrel adjusts the position of the metering rod.

The barrel includes a detent for selectively indexing the barrel in one of a plurality of rotational positions. The deter is operative to engage one of a plurality of indentations located at the bottom of the positioner bore. The indentations may be formed in the bottom of the positioner bore or formed in a detent washer disposed in the bottom of the positioner bore, as examples.

In yet another embodiment, a carburetor for an internal combustion engine is contemplated that includes a throttle slide having an outlet gate and an inlet gate including a flow guide disposed on the inlet gate in alignment with the metering rod. The flow guide bisects an arcuate relief on an underside of the inlet gate thereby forming a pair of funnel-shaped grooves. The arcuate relief may be frusto-conical in configuration and the flow guide may be in the form of a pyramid shaped point. Furthermore, the throttle slide may include a stepped portion disposed on the inlet gate for accelerating an airflow past a lower end of the throttle slide.

A method for configuring the throat of a carburetor to optimize airflow to an engine is also contemplated. Where the carburetor includes an upper portion of a first diameter and a lower portion of a second diameter that is offset from the first diameter, the method comprises deriving an optimum size for the first and second diameters and the offset based on mass airflow requirements of an engine. Broadly, the method comprises determining the venturi flow coefficient (C_v) of the carburetor and determining the mass airflow requirements ({dot over (m)}) of the engine. The optimum size for the first and second diameters and the offset are derived based on the mass airflow requirements and venturi flow coefficient. Both the venturi flow coefficient and the mass airflow requirements may be determined experimentally. In addition, determining the mass airflow requirements of the engine may include measuring the pressure differential (ΔP) and the air density (ρ).

The method includes resolving the width (w) as a function of throttle slide position (y) according to the equation

$w (y) = \frac{\partial}{\partial y} [\frac{\dot{m}}{C_{v} \sqrt{2 ρ Δ P}}] .$

The optimum size for the first diameter (Ø₁) is selected to match the width (w_wot) at a wide open throttle slide position (y_wot). The optimum size for the second diameter (Ø₂) is selected to match the width (w_i) at an idle throttle slide position (y_i). The optimum offset (X) is the difference between the wide open throttle slide position (y_wot) and the idle throttle slide position (y_i).

Also contemplated herein is a metering rod for use on a carburetor. The metering rod comprises an elongated cylindrical rod extending along a rod axis and having opposed first and second end portions. A wake generator is formed on the cylindrical rod extending from the first end portion and varying in cross-sectional areas along at least a portion of the length of the cylindrical rod.

In an embodiment, the wake generator comprises a flat region angled with respect to the rod axis and bordered by an elliptical edge. The metering rod may further comprise a plurality of grooves intersecting the elliptical edge.

The wake generator may include grooves that extend parallel to at least a portion of the rod axis and may include an arcuate portion. The wake generator may comprise a concave cross-section, such as, for example, and without limitation a dihedral cross-section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view in elevation of the carburetor illustrating the flow geometry of the inlet opening portion according to an exemplary embodiment;

FIG. 2 is a perspective view of the inlet of the carburetor shown in FIG. 1;

FIG. 3 is a front view of the carburetor illustrating flow characteristics of the inlet opening portion with the throttle slide at partial open throttle.;

FIG. 4 is a front view of the carburetor illustrating flow characteristics of the inlet opening portion similar to FIG. 3 with the throttle slide at a further open position;

FIG. 5 is a perspective view of the throttle slide according to an exemplary embodiment;

FIG. 6 is a side view in elevation of the throttle slide shown in FIG. 5;

FIG. 7 is a front view in elevation of the throttle slide shown in FIGS. 5 and 6;

FIG. 8 is a bottom plan view of the throttle slide shown in FIGS. 5-7;

FIG. 9 is a front view of the throttle slide illustrating the pressure changes as airflow enters the carburetor;

FIG. 10 is a side view of the throttle slide illustrating the pressure changes across the throat of the carburetor;

FIG. 11A is a schematic diagram of the throat portion of the carburetor illustrating the upper and lower portions;

FIG. 11B is a schematic diagram of the throat portion similar to FIG. 11A, illustrating a variation in the offset of the upper and lower portions;

FIG. 12A is a schematic diagram corresponding to FIG. 11A showing an exemplary throat portion profile;

FIG. 12B is a schematic diagram corresponding to FIG. 11B showing an alternate exemplary throat portion profile;

FIG. 13 is a partial perspective view of the metering rod positioning mechanism according to an exemplary embodiment;

FIG. 14 is a front view of a metering rod according to an exemplary embodiment;

FIG. 15 is a side view of the metering rod shown in FIG. 14;

FIG. 16 is a cross-sectional view of the metering rod shown in FIG. 14 taken about line 16-16;

FIG. 17 is a close up view of a portion of the flat on the metering rod shown in FIGS. 14-16;

FIG. 18 is a schematic representation of the grooves formed on the flat portion of the metering rod shown in FIGS. 14-17;

FIG. 19 is a front view of a metering rod according to another exemplary embodiment that schematically represents an alternative groove arrangement;

FIG. 20 is a cross-sectional view of a metering rod according to an alternative embodiment; and

FIG. 21 is a cross-sectional view of a metering rod according to yet another alternative embodiment.

DETAILED DESCRIPTION

Basic carburetor design is generally well known to those of ordinary skill in the art. For example, a suitable carburetor to which the present improvements may be applied is described in U.S. Pat. No. 6,505,821 issued Jan. 14, 2003 to Edmonston, the disclosure of which is hereby incorporated by reference in its entirety.

FIGS. 1 and 2 illustrate flow geometry designed to concentrate flow near the carburetor's metering rod 10 (see FIGS. 3 and 4) and encourage mixing. The entrance 14 to the throat 12 (known as the bell) includes features to direct flow “F” toward the metering rod 10 and induces a set of secondary vortical structures “V” which increase turbulence intensity and promote mixing. The concavities 26 begin near the upper and outer portion of the venturi and extend downward while turning inward as they approach the flow restriction created by the slide assembly 16. Momentum is carried along the primary curvature of the concavity and collides near the metering rod 10. The flow concentration in the center of the bore helps to minimize the buildup of liquid boundary layers, increases vacuum on the fiat (not shown) of the metering rod to draw fuel, and increases shear forces within the flow to force fuel into increasingly smaller droplets. The secondary flow forms two weak, counter-rotating vortices, normal to the primary streamline. The cross-flow momentum helps to mix fuel across streamlines and creates a more uniform mixture.

FIGS. 3 and 4 illustrate the vortical flow “F” of air entering the bell, or inlet portion, at different throttle slide positions. FIG. 3 illustrates vortical flow with a small throttle slide opening, such as would be expected at engine idle speeds. FIG. 4, on the other hand, illustrates vortical flow of air entering the be at a larger throttle slide opening, such as at mid-throttle.

The carburetor,shown in FIGS. 1-4, also includes a manifold 20 designed to maintain a steady atmospheric pressure on the fuel in the float bowl. In this case, manifold 20 is in the form of an arcuate scoop. Steady pressure on the float bowl generates uniform fuel flow and efficient mixing of the fuel with incoming air. The manifold 20 is located in the upper portion of the air inlet adjacent to and extending along a portion of a peripheral margin of the inlet opening portion. The manifold serves to trap the air in a relatively stagnant, non-turbulent state at the entrance to the inlet openings 22 to maintain a constant pressure on the fuel in the float bowl.

The geometry of the manifold 20 may be altered to change some characteristics of the carburetor performance. Turbulent flow enters the manifold and comes to rest. It is this conversion of dynamic pressure into static pressure that applies compensating pressure on top of the fuel reservoir. Both the volume and depth of the manifold are elements that damp oscillations in the flow. The length and diameter of the passages 22 leading to the fuel reservoir are of an appropriate ratio to allow viscosity to dominate the fuel driving pressure. The damping acts only upon the transient pressures encountered by the manifold.

FIGS. 5-8 illustrate the flow-modifying geometry applied to the front gate of the slide assembly, which improve the atomization and metering characteristics of the carburetor. The slide assembly 16 includes a stepped portion 32 upstream of the throat for concentrating and compressing the air entering the throat. The stepped portion 32 forces air entering from the inlet to compress before going under the slide assembly, thereby increasing the velocity of the airflow past the slide and fuel outlet. This is especially effective for the thorough mixing of incoming fuel and air and efficient burning of the fuel-air mixture at low settings of the carburetor.

The underside 34 of the forward gate 36 of the slide includes two funnel-shaped grooves 38 placed directly to either side of the metering rod location 40. The material between the grooves forms a frenulum or flow guide 42, in the for of a pyramid shaped point or chevron, leading into the flow. The flow guide bisects an arcuate relief on the underside of the inlet gate thereby forming a pair of funnel-shaped grooves. The arcuate relief is preferably frusto-conical in configuration. Flow guide 42 causes the metering rod to appear to have a teardrop-shape within the flow at low throttle position. The funnel-shaped grooves 38 allow air to accelerate to their highest velocity more near to the metering portion of the venturi increasing atomization. Flow separation and the orthogonal surface vector of the feature reduce lift on the slide, which may cause undesirable fluctuations in the fuel delivery. This design has been shown to improve function in the form of lower NOx emissions and a resistance to slide float. FIGS. 9 and 10 are computational fluid dynamic (CFD) vector plots illustrating the flow characteristics of the frenulum.

With reference to FIGS. 11A-12B, throat 12 includes a lower portion 15 that is narrower in width than the upper portion 13. Lower portion 15 is operative to accelerate airflow past the lower end of the throttle slide 16 at part throttle for the purpose of amplifying the signal at the metering rod 10. As the throttle slide 16 is opened further, the larger upper portion 13 is exposed to provide increased airflow to the engine at higher engine speeds and/or loads.

In one embodiment, the geometry of the throat 12 includes an upper portion 13 of a first diameter and a lower portion 15 of a second diameter that is offset a distance “X” from the first diameter. The sizes of the circle(s) determine the throttle bore size.

FIG. 11A illustrates an example of a geometry configuration for throat 12 having a first diameter (Ø₁) equal to 3.40 cm and a second diameter (Ø₂) equal to 2.35 cm with an offset “X” between the first and second diameters. FIG. 11B illustrates another example of geometry configuration for throat 12′. In this example, the first and second diameters are the same as in FIG. 11A; however, the offset distance “X′” has been increased. The larger offset distance “X′” provides a more progressive transition between idle and wide open throttle, which is suitable for a 4-stroke engine, for example. FIG. 11A illustrates geometry that is better suited to a 2-stroke engine and provides a more abrupt transition between idle and wide open throttle or near wide open throttle. As can be appreciated in FIGS. 12A and 12B, the two diameters corresponding to upper and lower portions 13 and 15, respectively, are smoothed together by a radius “R” to provide a smooth air intake surface.

Methods for configuring the throat of a carburetor, such as described above, are also contemplated. The geometry (Ø₁, Ø₂, X) of throat 12 may be optimized to improve airflow to an engine depending on the engine parameters. Several parameters of carburetor design may be optimized in a prescribed fashion to achieve the highest atomization efficiency and flow for improved performance of an internal combustion engine.

Generally, the method uses the mass airflow requirements ({dot over (m)}) for a particular engine to define the carburetor venturi profile. The mass airflow requirements ({dot over (m)}) are obtained by direct measurement and isolation of the air delivery requirements of a particular engine. The airflow requirements are combined with carburetor venturi flow coefficients (C_v) to define the required throat or venturi area (A_v) as a function of throttle slide position.

Regarding measurement of the mass airflow requirements ({dot over (m)}), piston engines, both two-cycle and four-cycle, consume air as part of an unsteady process. Air metering technology is not optimally suited for net mass flow measurement of this unsteady flow. It is advantageous to damp out these perturbations and flow reversions in the case of some two-cycle engines in order to support accurate measurements. Accordingly, the inlet port of the engine is ducted to a vessel of sufficient volume to suppress the effects of unsteady pumping action such that the volume of the vessel is much greater than the displacement of the engine. The vessel is then supplied air at a pressure equivalent to atmospheric or desired conditions by a rotary style blower, for example. Mass flow of air ({dot over (m)}) is measured at the intake of the blower which provides a smooth continuous flow.

Once mass flow ({dot over (m)}) is determined as a function of engine speed and load, the carburetor venturi cross section is calculated. Using the incompressible form of Bernoulli's equation and one-dimensional continuity equation, an equation for ideal mass flow rate can be shown.

{dot over (m)}=A
_v√{square root over (2ρΔp)}

{dot over (m)}=Mass Flow Rate of Air

A_v=Area of Carburetor Venturi, where A_v=f(Slide Position)

ρ=Air Density

ΔP=Static Pressure Differential of Venturi to Atmosphere

Geometry, turbulence, and viscous effects all contribute to reduce the mass flow rate below indicated by the ideal expression. For standard venturi tube profiles, flow coefficients are experimentally determined and included in the mass flow equation. A flow coefficient (C_v) specific to the subject carburetor is similarly determined by experimentation. This coefficient is itself a function of area ratio or slide position, density, and pressure differential. The modified equation is shown below:

{dot over (m)}=A
_v
C
_v√{square root over (2ρΔP)}

C
_v
=f(a,ΔP,slide position)

The mass flow rate ({dot over (m)}), pressure differential (ΔP), and venturi flow coefficient (C_v) are all determined by experimentation as described above, while the density (ρ) is measured directly from the environment. The mass flow equation can then be solved, as described more fully below, to give an expression for area (A_v) as a function of throttle position (y).

$A_{v} - \frac{\dot{m}}{C_{v} \sqrt{2 ρ Δ P}}$

For an arbitrary venturi profile, the area of the revealed shape can be described by the following integral:

embedded image

Combining the mass flow rate equation with the area integral, and solving for the width (w) returns the following expression.

$w (y) = \frac{\partial}{\partial y} [\frac{\dot{m}}{C_{v} \sqrt{2 ρ Δ P}}]$

This equation for width (w) as a function of throttle position (y) describes the venturi geometry. As can be appreciated with reference to the integral below, the ideal throat 12 geometry is approximated with two diameters (Ø₁, Ø₂) separated by a distance (X).

embedded image

By matching the throat cross section to the engine's characteristics, combustion is improved by improved flow,increased atomization, and consistent fuel delivery. Furthermore, a carburetor tailored, according to the above defined method, will deliver a fuel mixture that is more uniform and consistent and provides a progressive, linear throttle response to the user.

Turning now to FIG. 13, an exemplary metering rod positioning mechanism 50 is described. As is known in the art, adjusting the position of the metering rod 10 relative to the throttle slide 16 acts to enrich or lean the mixture of air and fuel delivered to an engine. Positioning mechanism 50 actuates the metering rod 10 independently from the slide assembly 16. A cylinder or barrel 52 has a thread 56 through the center to accept the metering rod 10. As the barrel 52 is indexed rotationally, threaded contact alters the axial position of the metering rod 10. Barrel 52 includes a spring plunger 58 that is thread ably engaged with the barrel 52. The spring plunger or detent 58 is operative to engage one of a plurality of indentations or divots 62. Thus, the barrel 58 may be selectively indexed into one of the rotational positions and wherein the detent 58 maintains the barrel position until readjusted. Barrel 52 is received in positioner bore 44 (See FIG. 5). Indentations 62 may be formed in the bottom of bore 44 or may be formed into a separate detent washer 60 disposed in the bottom of bore 44. Detent washer 60 may also include a tab 64 to maintain its angular position relative to the slide assembly. Barrel 52 is retained in bore 44 with a snap ring 66 and a wave washer 68. In this case, barrel 52 includes a slot 54 to allow rotational adjustment of the barrel with a suitable tool, such as a screw driver. Metering rod positioning mechanism 50 may be replaced by or incorporate a motor, such as a small scale servo or steppermotor, to electronically control the positioning of the metering rod 10.

Metering rod 10 is fashioned with a flat 17 to engage a D-shaped washer 63 that is fixed in position by a spring tension from below (spring 65) and a retaining ring 63 from above. The D-shaped washer 63 engages a contour (not shown) within the slide assembly 16 to maintain the angular orientation of the metering rod 10 with respect to the throttle slide 16 and throat portion 12.

With reference to FIGS. 14 and 15, metering rod 110, according to an exemplary embodiment, includes a wake generator, in the form of a flat portion 117, which helps metering rod 110 atomize fuel more effectively when compared to traditional tapered needle valve arrangements. In this embodiment, the wake generator is a flat portion that is formed by grinding the metering rod at an angle. The flat ground portion is oriented at an angle with respect to the metering rod's axis “A” as shown in FIG. 15, for example. As air accelerates through the venturi, a portion of the metering rod within that airstream encounters the round cylinder 114 of metering rod 110. With further reference to FIG. 16, flow accelerates further within this local region near the surface of the metering rod, and reaches a peak velocity at “V_P” downstream along the metering rod approximately equal to one metering rod radius. Flow decelerates slightly beyond this point, until it separates from the surface near the flat portion 117 on the back of metering rod 110, creating a wake region “W” on flat portion 117. The differential pressure between the atmospheric pressure within the float bowl and the low pressure wake region draws liquid fuel up the flat portion 117 of the metering rod 110, where it is sheared off in the higher velocity airstream created by the disturbance of the cylindrical portion of the metering rod. It is the additional increase in shearing force and the distribution along the length of the rod that offers an improvement over a tapered needle valve arrangement.

A combination of features creates a system where liquid fuel is ordered and delivered directly into the region of airflow with the highest shear force. Fuel is directed to the corners formed where the cylindrical surface is interrupted by the flat surface. Droplets are then sheared into much finer particles than when they are simply lifted from the flat into the wake region. Finer atomization allows for more efficient combustion and reduces the production of harmful emissions.

The surface finish of the rod must be sufficiently fine to accurately meter fuel at the metering rod and nozzle interface, yet coarse enough to reduce surface tension effects and allow the fuel to wet into the flat surface of the rod. The cylindrical portion 114 of the rod 110 may be polished to as fine a finish as is economically feasible to reduce wear against the nozzle. A suitable surface finish in may be approximately 25 to 50 micro centimeters and, in at least one embodiment approximately 40-41 micro centimeters. In order to encourage fuel to wick into the rod, large surface discontinuities s should be sufficiently reduced. Pockets, pores, or damage from manufacturing processes may all work against the smooth surface adhesion of fuel and discourage flow up the rod.

As seen in FIG. 17, the flat surface 117 of the metering rod 110 is comprised of a series of very small, non-intersecting grooves, for example, representative grooves 118. These grooves are also referred to as channels or microchannels The primary orientation of the grooves is parallel with the slender axis “A” of the rod. Surface tension wetting and aerodynamic pressure forces guide fuel into the grooves, which direct It along the metering rod. The cross-wise scale of the grooves is quite small, on the order of hundreds of molecular lengths. These grooves may be formed by a variety of abrasive methods including, but not limited to grinding, honing, electrolytic grinding, lapping, or the like, to name but a few examples. As fuel is forced into the grooves, liquid is grouped into many small channels 118. As each channel intersects the cylindrical surface of the metering rod along edge 120 (see FIG. 18), the top of an individual channel acts as its own nozzle ejecting fuel into the free stream. Fuel sheared from the tops of these micro channels enters the flow at a much smaller dimension than those sheared from an ordinary surface. These drops which are smaller at their origin at the metering rod are then sheared into even finer droplets by the velocity gradients and turbulence within the carburetor venturi and engine intake tract.

Linear grooves 118 provide good atomization for those grooves which terminate near the maximum cord length of the rod. However, many grooves would terminate near the peak of the ellipse (i.e. edge 120) in the wake region far from the high gradients near the outside edges. In another embodiment shown in FIG. 19, an additional advantage is then available by terminating as many grooves as possible near the outside regions of high velocity gradient. Thus, grooves 218 have a chevron or curved shape that follows along the long axis “A” of the metering rod before turning an arc toward the edge 220.

The low pressure inside the wake region behind the metering rod is a primary component in the driving pressure associated with moving liquid fuel into the venturi of the carburetor. The wake generator, such as flat portion 117, of the metering rod may be modified to enhance the formation of the wake and then also the fuel driving pressure. The wake generator of the metering rod can be augmented by a variety of shapes to enhance the wake. For example and without limitation, the wake generator may be in the form of a dihedral section 317 or concave conical section 417 as shown in FIGS. 20 and 21, respectively.

Returning briefly to FIG. 15, the wake generator may be formed at an angle with respect to rod axis “A” thereby varying the cross-sectional area “G₁” of the wake generator along the length of the metering rod. The wake generator may otherwise vary in size with respect to its cross-sectional area along the length of the metering rod. With reference to dihedral section 317 shown in FIG. 20, the shape or size of the wake generator's cross section may vary. For example, the angle of dihedron 317 may change along the length of the metering rod thereby changing the cross-sectional area “G₂” of the wake generator along the length of metering rod 310. The above are only examples, and the cross-sectional area of the wake generator may otherwise vary along the length of the metering rod.

Accordingly, the carburetor and methods, therefore, have been described with some degree of particularity directed to the exemplary embodiments. It should be appreciated, though, that the present invention is defined by the following claims construed in light of the prior art so that modifications or changes may be made to the exemplary embodiments without departing from the inventive concepts contained herein.

	Number	Date	Country
Parent	12913629	Oct 2010	US
Child	13807999		US

CARBURETOR AND METHODS THEREFOR

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