DESC:FIELD OF INVENTION:
[001] The present invention relates to precisely controlling the fuel flow in engines thereby controlling the air-fuel ratio in an engine, preferably in gaseous fuel engines for ex LPG or CNG powered engines.
BACKGROUND OF INVENTION:
[002] Precise control of Air-fuel ratio is an important aspect in the calibration or setting of the engines, as air-fuel ratio has various implications. The air-fuel ratio is generally controlled by controlling the amount of fuel flowing towards engine. In case of engines operated by gaseous fuel such as LPG or CNG the amount of gas flow needs to be controlled.

[003] The amount of gas to be allowed to flow towards engine is set using a power screw type arrangement in the fuel flow path. The power screw position is adjusted to control the gas flow at optimum level which helps to achieve better performance. Therefore, precise control of the power screw is a crucial and important aspect of gas/fuel flow adjustment in CNG or LPG engines.

[004] As per the existing construction the fuel flow path is provided with the power screw fitted inside the flow path. The position of the power screw is adjusted to achieve the optimum gas flow towards engine. The front portion of the power screw is provided with a taper, taper may be positive or negative. The fuel flow path is provided with a corresponding opposite taper on its inner wall. In a closed position the inner wall of the fuel flow path and tapered surface of power screw get abutted to stop any gas flow towards engine. Both inner wall and the power screw being of tapered surface there always remains a possibility of any openings as two tapered surfaces cannot be effectively sealed against each other. Any manufacturing variability may lead to ineffective sealing and requires very tight tolerances. This can cause the gas flow even in closed position which is highly undesirable. Achieving fully close position of the power screw is important. The setting activity of the power screw is started from fully close position of the power screw, if the fully closed position is not achieved correctly then it becomes more difficult to calibrate and correctly set the fuel flow. Hence, this problem is more prone in prior art system as the surfaces of power screw and fuel flow path are not effectively sealed against each other.

[005] Currently, the adjustment of power screw is done manually, hence, there is a high possibility of errors while deciding the appropriate position of power screw especially in mass production of the vehicles this becomes a challenge. Any error in the correct setting or calibrating the position can lead to loss of engine efficiency. Therefore, during precise calibration of the power screw many iterations are done before finalizing the position. This process takes enormous time which hampers productivity.

[006] While setting the gas flow for an engine, the power screw is rotated which opens a narrow flow path for gas between the inner wall of flow path and tapered surface of the power screw. Higher the opening created higher is a gas flow. The opening is set in order to get optimum air-fuel ratio. The major drawback of currently available power screw is it is highly sensitive. Sensitivity is a rate of change in the opening of fuel flow path w.r.t. distance of power screw travelled by number of turns. Higher sensitivity means higher opening of the fuel flow path by a single turn of rotation of the power screw. As per conventional design any small amount of rotation of the power screw leads to considerable change in the flow of the gas. Therefore, while rotating the power screw it has to be done with high accuracy and attention. Any small error can lead to higher changes in air-fuel ratio and thereby deteriorating the engine performance. For setting Air-fuel ratio the Lower Control Limit (LCL) and upper Control Unit (UCL) of air-fuel ratio are pre-determined and the position of screw is adjusted such that the desired air fuel ratio falls within LCL and UCL. According to the currently available designs there is very limited scope for power screw to rotate to adjust and achieve the desired air-fuel ratio. One of the major reason for higher sensitivity being design of the power screw. The taper provided on the screw and inner wall of fuel flow path is also higher which leads to higher opening even with a small rotation of power screw.

[007] Another disadvantage of existing system is that the threads provided in the power screw comes in the flow path of the gas. The flow of incoming gas gets disturbed and creates turbulence in the flow due to the uneven surface of the power screw threads which leads to performance related issues. In case of spring operated valves the spring part comes in the flow path and disturbs the flow which is not desirable.
[008] Further, any manufacturing errors in the dimensions of power screw or flow path can lead to amplification of error in the amount of fuel flow. Conventionally, the design of the power screw and its interaction/assembly with associated components is also one of the major cause for the errors which leads to the above mentioned consequences. The currently available design does not accommodate any manufacturing errors. Therefore, very high precision is required in manufacturing said system which leads to increase in cost and time.

[009] With a wide variations across engine design the amount of fuel flow requirement changes based on engine parameters such as engine CC, output power etc. The engines with higher CC has higher intake vacuum as compared to the engines with lower CC. Therefore, the higher CC engines may draw higher amount of fuel through the same opening as compare to lower CC engine which makes power screw more sensitive to the opening. Therefore, the control of power screw is more difficult in higher CC engines. The amount of fuel flow also changes based on the type of fuel such as LPG or CNG. Hence there are various factors which needs to be considered while designing the power screw. From manufacturing perspective it would be undesirable to have different fuel path and power screw designs for different parameters and ideally it is preferred to have a single fuel flow and power screw design which cater all needs and may be applicable for all sort of engines. The currently available designs do not accommodate the variations in the fuel flow due to the above mentioned parameters.

[010] With the above cited drawbacks of the existing gas flow control system there is need to have a better system to precisely control the gas flow rate to achieve optimum flow of the gas towards the engine.

[011] Hence an object of the present invention is to provide an effective system for precisely controlling the flow of fuel for e.g. gas and thereby controlling air-fuel ratio in order to achieve better performance of engine.
SUMMARY OF INVENTION:
[012] With this object in view, the present invention provides a system for controlling air-fuel ratio for an engine comprising a fuel flow path for delivering fuel towards engine; a controlling member located inside the fuel flow path such that the position of the controlling member is adjusted using an adjusting means for opening, closing or controlling the fuel flow rate; characterized in that the controlling member has a face surface; and at least a portion of the controlling member has a varying cross-section along its length.

[013] The portion of inner wall of the fuel flow path is provided in the form of step protruding from the inner wall such that at least two protruding portions form an aperture there between.

[014] The protruding step gets abutted with the face surface of the controlling member for completely closing the fuel flow path according to one of the controlling position. Additionally, the controlling member may also close the fuel flow path using a combination of face surface and peripheral surface of the controlling member. The face surface is perpendicular to the axis of the controlling member which is used as a sealing surface to close the fuel flow. The protruding step provided on the inner wall of the fuel flow path is substantially perpendicular to the axis of fuel flow path.

[015] The adjusting means for adjusting the position of controlling member is provided such that the adjusting means are substantially outside the fuel flow path. The adjusting means may be in the form of threads or may be a spring or a combination of both. The adjusting means is provided as an integral part of the controlling member

[016] The portion of varying cross section on the controlling member is preferably in the form of a taper of gradually varying cross-section from highest diameter to lowest diameter towards the direction of fuel flow at and said angle is an angle of taper with axis of the controlling member. The taper may not be essentially continuous but may be provided in the form of multiple steps. The different shapes of taper are possible.

[017] The amount of axial travel of the controlling member per rotation and the amount of opening created by the controlling member is majorly governed by the aperture size, the length and angle of taper on the controlling member. This decides the sensitivity of the controlling member.

[018] According to the preferred embodiment of the present invention, the fuel flow path is in the form of a tubular member and the controlling member is a power screw fitted inside the tubular fuel flow path. The adjusting means i.e. screw threads used to adjust the position of the controlling member i.e. power screw. The fuel flow path has a fuel inlet and outlet such that the fuel, preferably a gas CNG or LPG enters from inlet and is delivered towards engine from the outlet. The power screw or controlling member is fitted in between inlet and outlet of the fuel flow path such that its position may be adjusted using the threads preferably provided on the distal part of the power screw/ controlling member.

[019] The controlling member is preferably cylindrical with the front portion is a portion of varying cross-section preferably, a gradually reducing tapered surface. The middle section of the controlling member is smooth cylindrical section forming an annular space or gap with the inner wall of the fuel flow path. Preferably, the area around an annular space of the controlling member in closed position is kept substantially same or higher than the area of an inlet of the fuel flow path. This helps in maintaining the constant fuel flow and to compensate any variation in the flow due to manufacturing variabilities of the associated components thereby help in reducing sensitivity of the controlling member towards gas flow. The threads are provided at the end section of the controlling member to adjust the position which are substantially placed outside the fuel flow path.

[020] Though the front portion of the controlling member is provided as tapered shape, any other suitable shape may be provided to the front portion such as conical, step or spherical or combination thereof. Similarly, the width surface of the protruding step provided in the inner wall of the fuel flow path may be flat or a tapered which may get abutted with the surface of the controlling member.

[021] Tubular fuel flow path is provided with one open end used to access the controlling member to adjust its position using suitable means. The controlling member is rotated to open a flow path for fuel.

[022] According to one of the embodiment of present invention, the design parameters of the controlling member and fuel flow path and their interrelation are optimized to reduce the sensitivity comprising dimensions of varying cross section of controlling member, dimensions of fuel flow path including aperture.

[023] Alternatively, the sensitivity of the controlling member can also be achieved by another means such as optimizing the pitch of the screw threads or by providing different tapers to different subsequent sections of the controlling member or by any other suitable means such as increasing length of the taper.

[024] The angle of taper of controlling member may also be decided based on the type of fuel for e.g. LPG or CNG. The LPG is denser as compare to CNG gas therefore with the same opening the amount of gas flowing towards engine may differ. The amount of LPG may flow through the same opening may be lesser as compare to the amount of GNG. Hence for LPG a higher taper angle may be provided whereas in case CNG the angle of taper may be lower which results in providing bit higher opening for LPG in one rotation of the controlling member as against the CNG may provide bit lesser opening in a single rotation of the controlling member.

[025] The gas flow setting is done by adjusting the position of the controlling member such that the optimum fuel flow is achieved thereby achieving the optimum air-fuel ratio. This is advantageous in improving the performance of the engine.

[026] According to one of the embodiment of present invention, the adjusting means (220) is operated using an actuator in communication with a controller wherein; the controller is configured to sense the present air-fuel ratio using at least one sensor and compare the sensed present air-fuel ration with pre-stored upper and lower control limits and operate said actuator for adjusting the position of the controlling member such that the present air-fuel ratio falls between said upper and lower control limits.

[027] With the proposed design of the fuel flow control system the fuel quantity may be set such that the air-fuel ratio falls between the upper and lower control limit to achieve the best performance. The sensitivity of the controlling member and related components towards change in fuel flow is reduced to large extent such that a better control may be achieved over adjusting the position of the controlling member using a described system herein above.

[028] The proposed solution is also a cost effective solution as compared to conventional design since a double taper and close tolerances were required in the conventional power screw, whereas a single taper only on power screw surface is provided on the power screw which is much easier to manufacture. This helps in saving cost as well as time.

BRIEF DESCRIPTION OF DRAWINGS:
[029] The fuel flow control system according to the present invention may be fully understood from the following description of preferred embodiments thereof, made with reference to accompanying drawings in which:

[030] Figure 1 represents a side view of the fuel flow system according to the prior art.

[031] Figure 1a represents a side view of the fuel flow path used in the fuel flow system of the figure 1 according to the prior art.

[032] Figure 1b represents a side view of the controlling member (Power screw) used in the fuel flow system of figure 1 according to the prior art.

[033] Figure 2 represents a side view of the fuel flow system according to the preferred embodiment of the present invention.

[034] Figure 2a represents a side view of the fuel flow path used in the fuel flow system of the figure 2 according to the preferred embodiment of the present invention.

[035] Figure 2b represents a side view of the controlling member (Power screw) used in the fuel flow system of figure 2 according to the preferred embodiment of the present invention.

[036] Figure 3a represents dimensions of the fuel flow path used in the fuel flow system of the figure 2 according to the preferred embodiment of the present invention.

[037] Figure 3b represents dimensions of the controlling member (Power screw) used in the fuel flow system of figure 2 according to the preferred embodiment of the present invention.

[038] Figure 4 shows a graphical comparison of Air Fuel Ratio setting sensitivity of fuel flow system of prior art with fuel flow system according to the preferred embodiment of the present invention for CNG Engines.

[039] Figure 5 shows a graphical comparison of Air Fuel Ratio setting sensitivity of fuel flow system of prior art with fuel flow system according to the preferred embodiment of the present invention for LPG Engines.

[040] Figure 6 shows a graphical comparison of flow area versus number of turns of prior art with fuel flow system according to the preferred embodiment of the present invention.

[041] Figure 7 shows different configurations of the shape of controlling member used in the fuel flow system according to the present invention.

DETAIL DESCRIPTION OF PREFERRED EMBODIMENT:
[042] The present invention provides a system for controlling air-fuel ratio for an engine preferably powered by gaseous fuel such LPG or CNG. The system according to one of the embodiment of the present invention is herein below explained in detail with the help of accompanying figure 2 to 7.

[043] Figure 1, 1a and 1b represents an air-fuel control system according to prior art. A fuel flow system (100) for engines operated using gaseous fuel preferably CNG or LPG comprises a fuel flow path (115) which a hollow tubular member with inlet (111) and outlet for fuel (112). A controlling member (110) in the form power screw is provided inside the tubular fuel flow path (115) to control the fuel flow. The position of the controlling member (110) may be adjusted using threads (120) provided on the outer surface of the controlling member (110). The inner wall (114) provided on the outlet side (112) of the fuel flow path (115) is tapered and the front portion (125) of the controlling member (110) is provided with opposite taper that of inner wall (114) such that in the closed position both the tapered wall abut against each other to completely close the fuel path as shown in Figure 1. In the closed position the threaded surface (120) comes in the fuel flow path and disturbs the fuel flow. This system (100) is having drawbacks as mention herein above in the background section. In order to overcome said drawbacks a new control system is developed, one of the embodiment of which is explained herein below with the help figure 2 to 7.

[044] The fuel flow system (200) according to one of the embodiment of the present invention comprises a fuel flow path (215) with an inlet (211) and outlet (212) for fuel. A controlling member (210) is provided inside the fuel flow path such that its position can be adjusted using an adjusting means preferably threading (220). The position of controlling member (210) decides the amount of fuel flowing towards engine. The fuel enters from inlet (211) and flows through the opening created around the controlling member (210) and passes towards engine through the outlet (212). The fuel flow path (215) is provided with one open end (280) use to access the controlling member (210) to adjust its position using suitable means.

[045] An inner wall (218) provided on the fuel flow path (215) are substantially straight and the threaded portion (220) is kept substantially outside the fuel flow path such that there is minimum disturbance created to the fuel flow as against the prior art. The controlling member (210) is provided with front portion (225) having varying cross section preferably a tapered surface. A step (250) is provided on the controlling member (210) at the start of taper surface which form a face surface. A middle section (240) of the controlling member (210) is a smooth cylindrical section forming an annular space or gap (237) with the inner wall of the fuel flow path. The threads (220) are provided on the distal end of the controlling member. The middle section (240) is a smooth cylindrical surface. The inner wall (218) of the fuel flow path is provided with protruding portion or step (235). At least two such protruding portions are provided in the fuel flow path though number may change or continuous ring type protruding portion may be provided. The face surface (250) of the controlling member is abutted against the protruding surface (235) of the inner wall (218) thereby completely sealing the fuel flow. Therefore, the face surface (250) is used for closing the fuel flow which provides a better sealing as compare to prior art. Since there is no taper provided on the sealing surfaces good sealing is achieved. Also there remains a minimum possibility of any manufacturing defects as providing close tolerances on tapered surface is difficult as in prior art system. The threading portion (220) is also provided on the distal end of the controlling member (220) such that it does not come in the fuel flow path so that there is minimum disturbance to fuel flow.

[046] As the position of controlling member (210) is adjusted using threads (220) the fuel flow rate changes. For example, as controlling member (210) is rotated in anti-clockwise direction the controlling member (210) starts moving backward and a gap is created between the tapered front surface (225) of the controlling member (210) and the protruding portion (235) of the fuel flow path. As the controlling member starts moving backward said gap goes on gradually increasing due to the tapered surface (225). The position of controlling member (210) is set such that an optimum fuel flow is passes through the gap created in order to have optimum air-fuel ration. This helps in obtaining better performance and may in turn help in reduced pollution as air-fuel ratio is precisely controlled.

[047] One of the major problem with prior art system being high sensitivity towards fuel flow i.e. even with a small rotation of the controlling member (110) a large gap is created which enable higher fuel flow towards engine and makes it difficult to control the air-fuel ratio. The present system is designed to have reduced sensitivity in order to have better control over fuel flow rate. In order to have lower sensitivity the controlling member (210) and the fuel flow path (215) are designed such that the sensitivity of the system is reduced and a better control is achieved over fuel flow.

[048] The figure 3a and 3b represents the preferred dimensions required in order to eliminate the problems related to sensitivity of the fuel flow system (200). The tapered surface (225), the angle of taper and the length of taper portion are one of the important aspects of the controlling member (210) from sensitivity perspective while the distance between two protruding surfaces is an important aspect in fuel flow path design. However; other dimension also contributes towards the overall sensitivity of the system (200).

[049] The design parameters of the controlling member (210) and fuel flow path (215) and their interrelation are optimized to reduce the sensitivity. The dimensions on the controlling member (210) are defined w.r.t the mean diameter (D) of the controlling member (210) which is a diameter at the start of tapered surface (225). According to one of the embodiment the diameter at the start of tapered surface (225) is an average diameter of the controlling member (210). The length of tapered surface (225) is 0.8 to 2.5 times a mean diameter (D) of the controlling member (210) and said taper is at an angle within 0.5 to 3 times the mean diameter (D) of the controlling member (210) w.r.t an axis of controlling member. While the diameter at the end of tapered surface is preferably between 0.2 to 0.9 times the mean diameter (D) of the controlling member. The few other dimensions of the controlling member (210) is diameter at the middle section (240) which is around 1.2 to 2 times the diameter (D) having length of 1.5 to 4 times the mean diameter (D).

[050] The fuel flow path (215) is also provided with specific design such that there is a minimum variability in the fuel flow and the sensitivity is reduced. The protruding portions or steps (235) of the fuel flow path creates an aperture (227) of 1.02 to 1.1 times the diameter of the mean diameter (D) such that tapered portion (225) of controlling member (210) passes through the said aperture (227). The protruding step (235) has a width of 0.3 to 0.6 times the mean diameter of the controlling member. The protruding step (235) gets abutted with the face surface (250) of the controlling member (210) for completely closing the fuel flow path.

[051] The dimensions of middle section (240) and the inner wall (218) of the controlling member (210) creates an annular space (237) such that the enough intake area is created which helps in maintaining the constant inflow of fuel which contributes towards reduced. Preferably, the area of an annular space (237) of the controlling member in closed position is kept substantially same or higher than the area at inlet (211) of the fuel flow path. This helps in maintaining the constant fuel flow and to compensate any variation in the flow due to manufacturing variabilities of the associated components thereby help in reducing sensitivity of the controlling member towards gas flow.

[052] The dimensions provided above define the range in which the components of fuel flow system must be designed in order to reduce the sensitivity and achieve better control over fuel flow. With these dimensions the axial distance travelled by the controlling member (210) reduces thereby reducing the gap created for fuel flow as compared to the prior art system. Therefore, the rate change of opening created for the fuel flow reduces which helps in achieving better control. The table below summaries all the dimensions and also provides the actual dimensions according to one of the embodiment of the present invention.
Parameters Range Range in Values (Example) Actual Values (Example)
Taper start diameter (mm) D 6 6
Hole in PV body (mm) 1.02 ~ 1.1D 6.12 to 6.6 6.5
Angle of taper (Degree) 0.5 D ~ 3D 3 to 18 8° , 15°
Length of Taper (mm) 0.8D ~2.5D 4.8 TO 15 11.5
Width of Step (235) (mm) 0.3 ~ 0.6D 1.8 ~ 3.6 2.35
taper end diameter (mm) 0.2 ~ 0.9 D 1.2 to 0 5.4 4.39, 2.4
[053] The dimensions of various parts of fuel flow system as explained herein above helps in compensating for any sort of variations in the fuel flow such as manufacturing errors, fuel flow variability, type of fuel, capacity of engine, 2stroke or 4 stroke engine etc. this helps in reducing the sensitivity and to achieve better control over fuel flow rate.

[054] The improvement in setting Air-fuel ratio (AFR) for engine is graphically represented by graphs shown in figures 4 to 7. The figure 4 and 5 represents the AFR against the rotation of the controlling member (210) for CNG and LPG engines respectively. The graph shows an Upper Control Limit (UCL) and Lower Control Limit (LCL). The AFR needs to be set within the band of UCL and LCL. The lines (550, 560) represents the change in AFR w.r.t. the rotation of the controlling member (210) according to the prior art system. While the lines (520, 510) represents the change in AFR w.r.t. the rotation of the controlling member (210) according to the system of present invention. The line (525, 540) represents a mean for LCL and UCL. All the lines meet the UCL and LCL at breaking points. For example, the line (560) comes within LCL and UCL at breaking points (535, 545). The distance x is a rotational distance of the controlling member (210) as per the prior art system. For the same AFR the line (520) comes at breaking points (555, 565) according to the system of present invention. Therefore, to achieve the AFR within LCL and UCL the controlling member (210) is rotated by distance y as shown in figure which is higher than the distance x. This may be observed with every line (520, 525, 510) according to present system. Hence, the system according to the present invention has higher distance for rotation of the controlling member (210) to adjust the AFR within UCL and LCL this means the sensitivity of the present system is reduced as compared to prior art.

[055] The figure 6 illustrates the rate of change in opening of the fuel flow area against number of turns for the system according to prior art and compares it with according to the present invention for four stroke LPG engine. The line (600) is steep line representing quick and higher rate of opening for each 1 turn of rotation of the controlling member (210) using the system of prior art whereas the line (602) represents relatively slow and lower rate of opening for the system according to the present invention as it requires 2.5 turns as against 1 turn of prior art. The point (604) represent higher opening area created by the controlling member for line (600) whereas the point (606) represents lower opening area created by the controlling member for line (602) for same number of turns. This may be observed for any sort of engine operated by any fuel CNG or LPG. Therefore, this clearly suggest reduction in the sensitivity of the controlling member (201).

[056] The angle of taper of controlling member may also be decided based on the based on the density of fuel for e.g. LPG or CNG. However, even though any sort of fuel is used there is no need to design the fuel flow system beyond the range of dimensions specified herein above. Therefore, the given specification is applicable for any sort of fuel or any type of engine.

[057] Though the front portion (225) of the controlling member (210) is provided as tapered shape, any other suitable shape may be provided to the front portion such as conical or spherical. Similarly, the width surface of the protruding step (235) provided in the inner wall (218) of the fuel flow path may be flat or a tapered which may get abutted with the surface of the controlling member (225). Similarly, the taper need not be always continuous and it may be provided in the form of steps as represented by the figure 7. At least a part of the front portion (225) according to any of the design of figure 7 shall fulfill the dimension criteria in order to get the required sensitivity defined herein above.

[058] Alternatively, the sensitivity of the controlling member can also be achieved by another means such as optimizing the design parameters of the controlling member (210) and fuel flow path (215) and their interrelation comprising changing dimensions of varying cross section (225) including its length and angle, dimensions of fuel flow path, the pitch of the screw threads or by providing different tapers to different subsequent sections of the controlling member or by any other suitable means such as increasing length of the taper.

[059] With the proposed design of the fuel flow control system (200) the fuel quantity may be set such that the air-fuel ratio falls between the UCL and LCL to achieve the best performance. The sensitivity of the controlling member (210) and related components towards change in fuel flow is reduced to large extend such that a better control may be achieved over adjusting the position of the controlling member using a described system herein above.

[060] The angle of taper may be decided based on the requirement of rate of change of opening the flow path i.e. sensitivity of the controlling member towards the opening of fuel flow path.

[061] According to one of the embodiment of present invention, the adjusting means (220) for controlling the position of the controlling member (210) is operated using an actuator preferably, an electric motor (not shown). A controller (not shown) is in communication with the electric motor to operate said motor based on present air-fuel ratio. The values of upper and lower permissible control limits for air-fuel ratio are pre-stored in the controller. A sensor preferably, lambda sensor is used to sense the present air-fuel ratio. If the air-fuel ratio is not within said upper and lower control limits then the controller commands the actuator to operate for adjusting the position of the controlling member (210) such that the air-fuel ratio falls with said limits.

[062] Any Modifications and variations to the fuel flow system described in the present specification may be apparent to skilled readers of this disclosure. Such modifications and variations are deemed within the scope of the present invention.
,CLAIMS:1. A system for controlling air-fuel ratio (200) for an engine comprising:
a fuel flow path (215) for delivering fuel towards engine;
a controlling member (210) located inside the fuel flow path (215) such that the position of the controlling member (210) is adjusted using an adjusting means (220) for opening, closing and controlling fuel flow rate; characterized in that the controlling member (210) has a face surface (250); and
at least a portion of the controlling member (210) has a varying cross-section (225) along its length.

2. A system for controlling air-fuel ratio (200) for an engine as claimed in claim 1 wherein; the fuel flow path (215) is provided with a step (235) protruding from an inner wall to form an aperture (227) such that the portion with varying cross-section (225) of the controlling member (210) goes through the aperture (227) and closes the fuel path (215) by abutting at least a face surface (250) of the controlling member (210) against a portion of an inner wall (235) in one of the controlling position.

3. A system for controlling air-fuel ratio (200) for an engine as claimed in claim 2 wherein; the adjusting means (220) used for adjusting the position of the controlling member (210) to control the fuel flow rate within desired upper and lower control limit by varying a gap created between said portion with varying cross-section (225) of controlling member (210) and said aperture (227) of fuel flow path (215).

4. A system for controlling air-fuel ratio (200) for an engine as claimed in claim 2 wherein; the fuel flow rate decreases as the controlling member moves towards said aperture and fuel flow rate increases as the controlling member moves away from said aperture.

5. A system for controlling air-fuel ratio (200) for an engine as claimed in claim 1 wherein; the adjusting means (220) is in the form of threads or a spring or a combination of both.

6. A system for controlling air-fuel ratio (200) for an engine as claimed in claim 1 wherein; the adjusting means (220) is provided as an integral part of the controlling member (210).

7. A system for controlling air-fuel ratio (200) for an engine as claimed in claim 1 wherein; the adjusting means (220) is provided with plurality of threads such that threads are substantially placed outside the fuel flow path (215) on distal part of the controlling member (210).

8. A system for controlling air-fuel ratio (200) for an engine as claimed in claim 1 wherein; the portion of varying cross section (225) on the controlling member (210) is in the form of a taper of gradually reducing cross-section from highest diameter to lowest diameter towards the direction of fuel flow making an angle (Ø) with axis of the controlling member.

9. A system for controlling air-fuel ratio (200) for an engine as claimed in claim 1 wherein; the portion of varying cross section (225) is provided in the form of multiple sections including tapered, conical, spherical or step or a combination thereof.

10. A system for controlling air-fuel ratio (200) for an engine as claimed in claim 9 wherein; the angle of taper (Ø) of controlling member (210) is decided based on the type of fuel.

11. A system for controlling air-fuel ratio (200) for an engine as claimed in claim 1 wherein; the fuel flow path (215) is provided with one end open to operate the controlling member (210) for opening/closing/adjusting the position of the controlling member (210) using suitable means.

12. A system for controlling air-fuel ratio (200) for an engine as claimed in claim 1 wherein; the controlling member (210) is provided with a smooth cylindrical middle section forming an annular space or gap (237) with the inner wall (235) of the fuel flow path (235) such that the area of said annular space (237) is kept substantially same or higher than the area of an inlet (211) of the fuel flow path (215) in closed position.

13. A system for controlling air-fuel ratio (200) for an engine comprising;
a fuel flow path (215) for delivering fuel towards engine;
a controlling member (210) located inside the fuel flow path such that the position of the controlling member (210) is adjusted using an adjusting means (220) for opening, closing and controlling fuel flow rate; characterized in that the adjusting means (220) is operated using an actuator in communication with a controller wherein; the controller is configured to sense the present air-fuel ratio using at least one sensor and compare the sensed present air-fuel ratio with pre-stored upper and lower control limits and operate said actuator for adjusting the position of the controlling member such that the present air-fuel ratio falls between said upper and lower control limits.

14. A system for controlling air-fuel ratio (200) for an engine as claimed in any of the preceding claims wherein; the design parameters of the controlling member (210) and fuel flow path (215) and their interrelation are optimized to reduce the sensitivity comprising changing dimensions of varying cross section (225) including its length and angle, dimensions of fuel flow path, adjustment of pitch of the screw threads (220) or by providing different portions with different varying cross sections (225) on the controlling member (210).