THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
&
The Patents Rules, 2003
As amended by the Patents (Amendment) Rules, 2005
COMPLETE SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
Method of controlling idling speed in vehicles with dual fuel supply
APPLICANTS
Name : Bajaj Auto Limited
Nationality : Indian Company
Address : Akurdi, Pune 411035, Maharashtra, India






PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the nature of this invention and the manner in which it is to be performed:

This invention relates to a method for controlling idling speed in vehicles, such as two wheel or three wheel automotive vehicles, with gaseous fuel and liquid fuel operating modes.
Background of the Invention
Two wheel vehicles - such as motorcycles and motor scooters - are an important mode of personal transport, for example being used for commuting in and around cities. Such vehicles have engines operating on the two or four stroke cycle, employing petrol as a fuel. As environmental laws become more stringent, emissions from such vehicles are required to reduce. In addition, petrol costs may form a significant part of the vehicle operator's budget so a rising price of petrol consequently hinders the convenience of owning such a vehicle. One way of achieving environmental benefits in terms of better fuel combustion and lesser emissions while reducing fuel costs is to operate the vehicle through gas power. Gaseous fuels such as liquefied petroleum gas (LPG) and compressed natural gas (CNG) may be used with environmental and economic benefits.
There are, as yet, relatively few LPG refueling stations in India. Thus, it is currently desirable to have a "back up" fuel supply system in the event that a vehicle operator is caught in a situation where the gaseous fuel exhausts in a location where there is no refueling station, yet there may be relatively ready availability of liquid fuels, especially petroleum fuels. Such petroleum fuels are therefore convenient for the back-up fuel supply. In that case, the vehicle could include the gaseous fuel supply, optionally the petrol fuel supply, and the lubricant supply circuit (in case of vehicles operating on two stroke cycle).
In such a vehicle, a suitable switch is provided at a convenient location for the driver to switch between gas and petrol supply by actuating a switch. This switch operates a valve enabling the engine to switch to operate on the second fuel.
Typically, dual fuelled two and three wheel vehicles are provided with carbureted engines with the carburetor located in the air intake passage.
One of the methods for setting the idling speed in such a vehicle involves:
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a) adjusting the air screw / mixture control screw and/or idling speed screw of the carburetor while operating on liquid fuel like petrol; and
b) adjusting the screw of a gas regulator and power valve to achieve desired engine performance. For example, it is desired to minimise carbon monoxide (CO) emission levels in idling and Wide Open Throttle (WOT) conditions respectively.
Tuning to maintain sustainable idling speed and idling CO levels whether the engine operates on gaseous fuel or liquid fuel presents a problem. For example, the engine may be tuned for sustainable idling speed and idling CO levels while operating on gaseous fuel. If the vehicle operator switches to operating on liquid fuel or petrol, the idling speed and idling CO increases to an undesirably higher value hampering fuel economy and increasing emissions. On the other hand, if the engine is tuned for a lower sustainable idling speed and idling CO level while operating on petrol and the driver switches over to operating on gaseous fuel, idling speed may fall below a sustainable threshold limit causing stopping of the engine. Additionally, it may be difficult to start such an engine in gaseous fuel mode.
Summary of the invention
It is therefore an object of the invention to provide method of controlling idling in which idling speed is maintained within controlled limits whether the engine operates on liquid or gaseous fuel and without affecting engine and vehicle performance, particularly from driveability and emissions points of view.
With this object in view, the present invention provides a method of controlling idle speed in a carbureted engine having a dual fuel supply comprising:
a) a liquid fuel supply including a carburetor enabling operation of the engine in a liquid fuel operating mode; and
b) a gaseous fuel supply
wherein the gaseous fuel supply delivers an amount of gaseous fuel to the engine under idle conditions in gaseous fuel operating mode, the delivered amount of gaseous fuel being calculated to achieve a threshold idling speed for operating the engine at idle. The threshold idling speed may be in an idling speed range set for operating the engine in liquid fuel mode.
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Where the engine includes a throttle, the throttle may be in the same operating position during idling in either gaseous fuel operating mode or liquid fuel operating mode. That is to say, that the throttle is not opened or closed to compensate between operating on liquid fuel or gaseous fuel. Conveniently, the carburetor may be of a sliding throttle type with a passage or aperture being provided in the throttle slide. This passage or aperture allows supply of an additional quantity of gas -air mixture to the engine sufficient to sustain the idling speed while operating on gaseous fuel while the throttle is in the idle or closed position. The size of the aperture, varying with engine configuration, is designed in a way to allow sufficient volume of gaseous fuel and air to be delivered to the engine for a sustainable engine speed at idling. Nominally, the area of the passage through the said aperture is in the range of 0.1% to 0.6% vis-a vis the area of a venturi located in an air intake duct of the engine. The conventional carburetor includes a pilot jet for supply of liquid fuel in liquid fuel mode. The carburetor used in the method of the invention may allow supply of air through this duct, or another duct available for supply of liquid fuel in liquid fuel mode, when the engine is operated in gaseous fuel mode. Accordingly, when operating in gaseous mode, an additional airflow into the air intake duct of the engine may occur through the pilot jet from a drained or empty float chamber of the carburettor.
The liquid fuel supply circuit of the carburetor is tuned, taking the additional quantity of a gaseous fuel-air mixture into account, so that desired engine operation may be achieved whichever mode of fuel operation is adopted. Typically, the liquid fuel will be petrol, so the petrol supply circuit is tuned appropriately to optimize the air fuel ratio to a desired level of performance under idling conditions while providing desired starting and driving performance in both gaseous fuel and petrol fuel modes.
In a further aspect, the invention provides a carbureted engine having a dual fuel supply comprising:
c) a liquid fuel supply including a carburetor; and
d) a gaseous fuel supply
wherein the carburetor includes fuel delivery means to deliver an amount of gaseous fuel to the engine under idle conditions in gaseous fuel operating mode, the delivered amount of gaseous fuel being calculated to achieve a threshold idling speed for operating the engine at idle.

Where the carburetor is of slide type, the fuel delivery means may take the form of a passage or aperture through the throttle slide. Diameter and location of the passage or aperture may be optimized to deliver sufficient gas, conveniently in admixture with air, to ensure stable operation of the engine in gaseous fuel mode.
The invention may be more fully understood from the following description of preferred but non-limiting embodiments thereof made with reference to the accompanying drawings in which:
Figure 1 illustrates a schematic layout of a carbureted engine having a dual fuel supply to operate in petrol or gaseous fuel modes;
Figure 2 illustrates a sectional view of a prior art carburetor in idling position;
Figure 3 illustrates a sectional view of a carburetor to be used in a method according to the present invention and illustrating operation in petrol mode in idling position;
Figure 4 is a sectional view of the carburetor of Figure 3 and illustrating operation in gaseous fuel mode;
Figure 5 is a schematic of the carburetor of Figures 3 and 4 viewed from a filter side of the carburetor;
Figure 6 is a schematic of the carburetor of Figures 3 to 5 viewed from the engine side of the carburetor;
Figure 7 is a sectional view of the air intake system of the engine showing the carburetor as illustrated in Figures 3 to 6; and
Figure 8 is a sectional view of the float chamber of the carburetor shown in Figures 3 to 7.

Figure 1 illustrates a typical layout of a prior art engine 16 working on bi-fuel or dual fuelled operation. Gaseous fuel, for example LPG, operation is the preferred mode of operation of engine 16 with petrol fuel being used as a back-up fuel. The gaseous fuel supply system includes a venturi 12 connected to an inlet of carburetor 11. The venturi 12 receives a supply of gas from regulator 13 through power valve 14. The carburetor 11, of slide type, receives petrol supply from petrol solenoid 15 and a petrol tank (not shown). The outlet of carburetor 11 is connected to engine 16 through inlet manifold 17. The venturi 12 and carburetor 11 receive an air supply from the outlet of air filter 19 through air intake duct 18. A vacuum sensing line (20) provides a safety shut-off function to close the gas off at the regulator when no vacuum is present (that is, when the engine is not running). A switch (not shown) for switching over from petrol mode to gaseous mode and vice versa are provided at a convenient location, for the driver, such as on the instrument panel or handle bar of the vehicle.
Figure 2 illustrates a carburetor 11, in which the fuel supply is controlled by operation of a throttle slide 2. Position of the throttle slide 2 may be controlled by an engine operator. Along with the internal circuit of the carburetor 11, needle 3, needle grooves 6, main jet 4 and pilot jet 5 control the fuel supply to the engine 16, engine 16 being tuned in accordance with a conventional tuning procedure for a carbureted engine. In this engine 16, gaseous fuel is supplied from venturi 12 into the air intake duct 18. When throttle slide 2 is closed, or in idle position, insufficient LPG is supplied to the engine 16 such that idling speed may fall below a threshold speed for operating the engine. The engine may consequently stall or stop. In addition, driveability and startability problems may be experienced.
Figures 3 to 7 illustrate a carburetor 111 for use according to the method of the
invention. In this case, fuel supply to the engine 116 is controlled by operation of a
throttle slide 112. Throttle slide 112 is provided with tin additional passage or aperture
140 to supply additional LPG - air mixture when the engine operates at idle in the
gaseous fuel or LPG mode. Even though the throttle is closed in idle position, passage or
aperture 140 of predetermined diameter and location to supply an additional amount of
LPG-air mixture calculated to maintain a threshold idling speed to ensure sustainable
operation of the engine at idle. When the engine is operated in LPG mode, air is
introduced through the pilot jet 5 through which petrol would flow if the engine was being
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operated in petrol mode. Air-fuel ratio remains in a suitable air fuel ratio range for stable operation of the engine.
As shown in Figure 8, the pilot jet 5 may be connected to a float chamber 45 fixed to a body 51 of carburetor 111. When operating in gaseous fuel mode, float chamber 45 is drained of petrol. Air enters float chamber 45 through breather 48 and may be drawn, through a manifold vacuum, into pilot jet 5 therefore entering the engine 116. Air may also be drawn through the pilot jet 5, main jet 4 and/or starter jet 60 as well as air passage 150 and into the engine. Tuning of the engine 116 will take such air flows into account.
As shown in Figure 3, the control of air fuel ratio to maintain efficient engine operation in petrol mode is achieved by tuning main jet 4 size and needle groove position 6 and controlling conventional parameters of a needle 3 eg needle groove position 6 and dimensions of needle 3 to ensure a supply of a petrol-air mixture of appropriate air-fuel ratio when the engine operates in petrol mode. Such tuning takes into account the carburetor configuration enabling stable engine 116 operation in gaseous fuel mode. During petrol operation, petrol is introduced from float chamber 45 and through pilot jet 5, again in the requisite air-fuel ratio range to enable stable idling operation of engine 116. The idling circuit of the engine 116 maintains idling speed in both petrol and gaseous mode within a predetermined idling speed range and above a threshold speed for idling operation of engine 116. This enables fuel economy and reduced emissions in petrol mode while overcoming driveability and startability issues when the engine is operated in gaseous fuel mode. Sustainable idling and driveability of a vehicle with a dual fuelled engine may therefore be more efficiently achieved without excessive effort from rider or maintenance personnel.
Modifications and variations to the method of controlling idling of the present invention may be apparent to the skilled reader of this disclosure. Such modifications and variations are considered to be within the scope of the present invention.

WE CLAIM:
1. A method of controlling idle speed in a carbureted engine having a dual fuel
supply comprising:
a) a liquid fuel supply including a carburetor enabling operation of the engine in a liquid fuel operating mode; and
b) a gaseous fuel supply
wherein the gaseous fuel supply delivers an amount of gaseous fuel to the engine under idle conditions in gaseous fuel operating mode, the delivered amount of gaseous fuel being calculated to achieve a threshold idling speed for operating the engine at idle.
2. The method of claim 1 wherein the threshold idling speed is in an idling speed range set for operating the engine in liquid fuel mode.
3. The method of claim 1 or 2 wherein the engine includes a throttle and the throttle is maintained in the same operating position during idling in either gaseous fuel operating mode or liquid fuel operating mode.
4. The method of claim 3 wherein the throttle is not opened or closed to compensate between operating on liquid fuel or gaseous fuel.
5. The method of any one of the preceding claims wherein the carburetor is of a sliding throttle type with a passage or aperture being provided in the throttle slide, this passage or aperture allowing supply of an additional quantity of gaseous fuel -air mixture to the engine sufficient to sustain the idling speed while operating on gaseous fuel while the throttle is in the idle or closed position.
6. The method of claim 5 wherein the size of the passage is selected to allow sufficient volume of gaseous fuel and air for a sustainable engine speed at idling.
7. The method of claim 6 wherein the area of the passage through the said aperture is in the range of 0.1% to 0.6% of the area of a venture located in an air intake duct of the engine.

8. The method of any one of the preceding claims wherein air is supplied through a duct for supply of liquid fuel in liquid fuel operating mode when the engine is operated in gaseous fuel mode.
9. The method of claim 8 wherein, when the engine is operating in gaseous mode, an additional airflow into an air intake duct of the engine occurs through the duct from a drained or empty float chamber of the carburettor.
10. The method of any one of claims 5 to 8 wherein the liquid fuel supply circuit of the carburetor is tuned, taking the additional quantity of a gas-air mixture into account, so that desired engine idle operation is achieved whichever mode of fuel operation is adopted.
11. A carbureted engine having a dual fuel supply comprising:

a) a liquid fuel supply including a carburetor; and
b) a gaseous fuel supply;
wherein the carburetor includes fuel delivery means to deliver an amount of gaseous fuel to the engine under idle conditions in gaseous fuel operating mode, the delivered amount of gaseous fuel being calculated to achieve a threshold idling speed for operating the engine at idle.
12. The engine of claim 11 wherein the carburetor is of sliding throttle type and the fuel delivery means is a passage or aperture extending through the throttle slide.
13. The engine of claim 12 wherein at least one of diameter, flow area and location of the passage or aperture is optimized to deliver sufficient gaseous fuel to ensure stable operation of the engine in gaseous fuel operating mode.
14. The engine of any one of claims 11 to 13 wherein the area of the passage through the throttle slide is in the range of 0.1% to 0.6% of the area of a venture located in the air intake duct of the engine.
15. The method of controlling idle speed substantially as hereinbefore described with reference to Figures 3 to 8.

The engine substantially as hereinbefore described with reference to Figures 3


Dated this 7th day of March 2007