WO2002048521A1

WO2002048521A1 - Gas control apparatus

Info

Publication number: WO2002048521A1
Application number: PCT/AU2001/001610
Authority: WO
Inventors: Scott Edward Lacey
Original assignee: Gasresearch Australia Pty. Ltd.
Priority date: 2000-12-13
Filing date: 2001-12-13
Publication date: 2002-06-20
Also published as: AU2002221334A1; AUPR205600A0

Abstract

Apparatus for control of gas to an internal combustion engine having an intake manifold (4), the apparatus including: a control valve (64) having a first inlet (86) for receiving gas at a relatively low pressure; a second inlet (92) for receiving gas at a relatively high pressure; an outlet (88) which, in use, can be coupled to the intake manifold (4) of an internal combustion engine; a first metering device (90) for controlling the maximum rate of gas supply from the first inlet (86) to the outlet (88); and a second metering device (96) for controlling the maximum rate of gas supply from the second inlet (92) to the outlet (88) whereby the metering devices (90, 96) can independently control maximum gas supplies to the intake manifold (4) of the internal combustion engine under power conditions and at idle respectively. The apparatus further including a converter (30) provided with an outlet port (116) connected to a supplementary fuel supply line (118) connected to a port (99) in the intake manifold (4), whereby additional gas is supplied at a rate which depends on the level of vacuum in the intake manifold (4).

Description

GAS CONTROL APPARATUS

This invention relates to gas control apparatus.

More particularly, the invention relates to gas control apparatus for controlling the supply of gas such as liquid petroleum gas (LPG), compressed natural gas (CNG), or other combustible gases to an internal combustion engine.

Most internal combustion engines are designed to run on liquid fuels. Internal combustion engines can, however, be modified so that they will operate on fuel such as LPG or CNG. Normally, however, when the engines are running on gas the performance is inferior to that of liquid fuels.

The object of the present invention is to provide gas control apparatus which enhances the performance of internal combustion engines when operating on gas fuels.

According to a first aspect of the invention there is provided apparatus for control of gas to an internal combustion engine having an intake manifold, said apparatus including: a control valve having a first inlet for receiving gas at a relatively low pressure; a second inlet for receiving gas at a relatively high pressure; an outlet which, in use, can be coupled to the intake of an internal combustion engine; a first metering device for controlling the maximum rate of gas supply from the first inlet to said outlet; and a second metering device for controlling the maximum rate of gas supply from the second inlet to said outlet whereby the metering devices can independently control maximum gas supplies to the intake manifold of the internal combustion engine under power conditions and at idle respectively. The invention also provides an internal combustion engine having an intake manifold for supplying operating air to combustion chambers thereof, the engine including gas control apparatus for supplying a combustible gas to said intake manifold, said gas control apparatus including a converter which in use is coupled to a pressurised source of said gas, the converter including a high pressure chamber and a low pressure chamber and a gas control valve, the gas control valve having: a first inlet for receiving gas from said low pressure chamber; a second inlet for receiving gas from the high pressure chamber; an outlet which is coupled to said intake manifold; a first metering device for controlling the maximum rate of gas supply from the first inlet to said outlet; a second metering device for controlling the maximum rate of gas supply from the second inlet to said outlet whereby the metering devices can independently control maximum gas supplies to the intake manifold when the engine is operated at higher power output levels and at idle respectively.

The invention also provides a method of operating an internal combustion engine having an intake manifold by supplying gas from a gas supply tank, the method including the steps of: supplying high pressure gas to a converter having high and low pressure chambers therein; controlling the rate of flow of low pressure gas from the low pressure chamber to the intake manifold so as to vary the output power of the engine; providing a first metering device and adjusting the first metering device so as to control the maximum rate of flow of low pressure gas to the intake manifold; supplying gas from the high pressure chamber to the intake manifold so as to provide an idle supply of gas for the engine at idle; providing a second metering device and adjusting the second metering device so as to control the maximum rate of flow of said idle supply of gas independently of the adjustment of said first metering device . The invention also provides apparatus for control of an internal combustion engine which can selectively operate from petrol or pressurised gas from a gas supply tank , the engine having an intake manifold, an electronic control unit (ECU) and a sensor which senses oxygen in exhaust products from the engine and provides control signals to the ECU in order to adjust fuel supply to the engine in order to achieve a desired air fuel ratio, said apparatus including: low pressure gas supply means for providing a low pressure gas supply to the intake manifold, said low pressure gas supply means being coupled to flow control means for controlling rate of flow of said low pressure gas supply to thereby vary output power of the engine; supplementary gas supply means for providing a supplementary gas supply to the intake manifold including valve means which is coupled, in use, to the ECU and arranged to provide increased rate of flow of said supplementary gas supply when said sensor detects, in use, higher levels of oxygen in said exhaust products.

The invention also provides an internal combustion engine which can operate from pressurised gas from a gas supply tank , the engine having an intake manifold, an electronic control unit (ECU) and a sensor which senses oxygen in exhaust products from the engine and provides control signals to the ECU in order to adjust fuel supply to the engine in order to achieve a desired air fuel ratio and gas control apparatus for controlling supply of gas from the gas supply tank to the inlet manifold, said apparatus including: low pressure gas supply means for providing a low pressure gas supply to the intake manifold, said low pressure gas supply means being coupled to flow control means for controlling rate of flow of said low pressure gas supply to thereby vary output power of the engine; supplementary gas supply means for providing a supplementary gas supply to the intake manifold including valve means which is coupled to the ECU and arranged to provide increased rate of flow of said supplementary gas supply when said sensor detects higher levels of oxygen in said exhaust products. The invention also provides a method of operating an internal combustion engine selectively on petrol or gas, the engine including: an intake manifold, an electronic control unit (ECU) and a sensor which senses oxygen in exhaust products from the engine and provides control signals to the ECU in order to adjust fuel supply to fuel injectors thereof in order to achieve a desired air fuel ratio when operating on petrol, said method including the steps of: providing low pressure gas supply means for providing a low pressure gas supply to the intake manifold, said low pressure gas supply means being coupled to flow control means for controlling rate of flow of said low pressure gas supply to thereby vary output power of the engine; providing supplementary gas supply means for providing a supplementary gas supply to the intake manifold including valve means; and coupling the valve means to the ECU to thereby provide increased rate of flow of said supplementary gas supply when said sensor detects, in use, higher levels of oxygen in said exhaust products.

The invention also provides an internal combustion engine having an intake manifold having a butterfly valve therein, the engine being arranged to operate selectively on petrol or gas, the engine including gas control apparatus including a converter including a housing having first and second diaphragms which define first, second and third chambers therein, the second chamber being located between the first and third chambers, the housing including: an inlet port which is coupled to gas supply tank; an outlet port which communicates with the second chamber for supplying low pressure gas to the inlet manifold and wherein converter is coupled to a supplementary fuel supply line which is coupled to said intake manifold downstream of said butterfly valve, whereby additional gas is supplied through the supplementary gas supply line at a rate which depends, in use, on the level of vacuum created in the intake manifold caused by throttling of the butterfly valve. The invention also provides a method of operating an internal combustion engine with a gas supply, the method including the steps of providing first and second metering supplies for independently controlling the maximum rates of gas supply at idle and under power.

The invention also provides a method of operating an internal combustion engine using a gas supply including the steps of monitoring combustion products from the engine and controlling a supplementary supply of gas at idle when relatively high levels of 0₂ are detected.

The invention also provides a method of operating an internal combustion engine using a gas fuel supply which is normally introduced into an intake having a butterfly valve therein, the method including the steps of providing a bypass passage which permits flow of air through the intake bypassing the butterfly valve, the method including the steps of introducing additional supply of gas into said bypass line under conditions of increased loads on the engine at idle.

The invention also provides a method of operating an internal combustion engine having an electric control unit (ECU) which normally produces signals for control of the engine running on liquid fuel, the method including the steps of switching the engine so as to run on a gas supply and controlling at least part of the supply of gas to the engine utilising control signals from the ECU.

The invention will now be further described with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of an engine having gas control apparatus fitted thereto in accordance with the invention;

Figure 2 is a schematic view showing the mechanical components of the system;

Figure 3 is a schematic view showing the manner in which the components are controlled;

Figure 4 is a simplified schematic view of a regulator which forms part of the system;

Figures 5, 6 and 7 show perspective views of the control valve of the invention; and Figure 8 is a schematic view of a modified form of the invention.

Figure 1 shows an internal combustion engine 2 which includes an inlet manifold 4 and an exhaust manifold 6. The engine has fuel injectors 8 coupled for injecting liquid fuel such as petrol for normal running of the engine. The engine is fitted with gas control apparatus 10 in accordance with the invention to permit the engine to be selectively operated on liquid fuel or gas such as LPG or CNG from a gas supply tank 12 (Figure 3). The driver of the vehicle can operate a changeover switch 14 for selectively operating the engine on liquid fuel or gas fuel.

The engine 2 also includes a sensor 16 for analysing the oxygen content in the combustion products in the exhaust manifold 6. The sensor 16 produces control signals which are supplied to the vehicle electronic control unit (ECU) 18, in the usual way when operating on liquid fuel. The engine also includes a butterfly valve 20 which is located in the inlet manifold 4. The butterfly valve 20 is coupled by means of coupling means (not shown) to the accelerator of the vehicle and the arrangement is such that the valve 20 is opened when it is desired to increase power output from the engine. The engine also includes an idle air control valve motor 22 which operates to control the position of a bypass air valve 23. The valve 23 is located in a bypass passage 120 (see Figure 2) which is arranged to allow a controlled amount of air through the intake, bypassing the butterfly valve, for correct operation of the engine at idle. The motor 22 is controlled by control signals from the ECU which are applied via control line 25. When the engine is operating on liquid fuel at idle, additional loads at idle can be provided for by injecting increased fuel from the injectors 8 and controlling the motor and valve 23 to allow sufficient air in the inlet manifold 4 without changing the position of the butterfly valve 20. Sometimes when engines are converted so as to run on gas, the motor 22 is connected out of circuit. The inlet to the inlet manifold 4 is provided with an air filter 24, in the usual way. As will be described in more detail below, when the engine 2 is operating under liquid fuel, fuel is supplied through the injectors 8. The gas control apparatus 10 is essentially isolated by means of closure of a number of isolating valves which prevent supply of gas from the gas tank 12 to the inlet manifold 4. When, however, the driver selects gas, the switch 14 is operated. This causes the control signals to the fuel injectors 8 to be disabled, opens the shut-off valves for the gas thereby permitting supply of gas from the tank 12. Gas is supplied from a gas line 26 through a shutoff valve/filter unit 28 to a converter 30. The apparatus includes a shut off valve 29 which is located near the outlet of the tank 12 and a shut off valve 31 located near the high pressure outlet of the converter 30. The valves 29 and 31 operate in parallel with the unit 28 under control of the switch 14. The converter 30 ensures that gas from the tank 12 is in the gaseous phase (i.e. does not include any liquid) and is at the right pressure. Gas from the converter 30 is supplied, via a line 33, to a supply jet 32 which is located in a venturi 34 which is mounted in the inlet 4 between the butterfly valve 20 and the filter 24, as shown.

In the illustrated arrangement, the converter 30 is coupled to the cooling system of the internal combustion engine 2 by means of coolant lines 36 so that coolant is circulated through the converter 30. This helps to ensure correct operation of the converter 30, in the usual way.

The components which have been broadly described above are the same as or similar to those which are conventionally used with internal combustion engines which have been converted so as to run on petrol and/or gas. In accordance with the invention, a number of modifications are included in the gas control apparatus 10 in order to enhance the performance of the engine when running on gas. These modifications will be described below with reference to Figures 2 to 7.

Figure 4 diagrammatically illustrates the basic internal structure for the converter

30. It is very similar to known converters but it does have a modification in order to enable enhanced operation in accordance with the invention. The converter 30 includes a housing 40 including first and second diaphragms 42 and 44. The diaphragms define a high pressure chamber 46 and a low pressure chamber 48. The high pressure chamber 46 receives gas from the line 26 via the unit 28 and this is inputed at inlet port 50. The extent of opening of the port 50 is controlled by means of a needle valve 52 which is coupled to the first diaphragm 42 and is movable therewith by means of a linkage 51. Gas can flow into the second chamber 48 through a port 54 which is controlled by a second needle valve 56 which is coupled for movement with the second diaphragm 44 by means of a linkage 57. The converter 30 includes a third chamber 58 above the second diaphragm 44. The third chamber 58 is normally at atmospheric pressure. The pressure in the high pressure chamber 46 is typically about 3psi to 6psi whereas the pressure in the low pressure chamber 48 is typically about 0.25 to 0.5 in. H 0 (vacuum). The chamber 46 has a high pressure outlet port 60 and the chamber 48 has a low pressure outlet port 62. The ports 60 and 62 are coupled to a control valve 64 of the invention by supply lines 66 and 68, as diagrammatically shown in Figure 2.

In the illustrated arrangement, the third chamber 58 can be open to the atmosphere at port 70 or can be coupled to an intake tube 69 to the air filter 24 by means of a balancing line 72 fitted with an intake barb 73, as shown in Figure 2.

The converter 30 may include a limiting screw 74, the lower end of which engages the second diaphragm 44 to effectively control the degree of upward displacement of the second diagram 44. This effectively controls the extent to which the needle valve 56 can be closed, thereby controlling the amount of fuel which can pass from the chamber 46 into the chamber 48 through the port 54. Normally the main gas supply for the jet 32 can be coupled to the line 68 for supplying gas to the engine when the engine is operating under power. Higher pressure gas is supplied to the jet 32 via the line 66 at idle. In the arrangement of the invention the converter 30 is modified by the provision of a second outlet port 116 which will be described below.

In accordance with the invention, the control valve 64 is coupled between the lines 66 and 68 and the jet 32 so as to provide superior engine control, particularly at idle. The control valve 64 preferably takes the form of a cast body 80 having main and idle gas supply passages 82 and 84 therethrough, as diagrammatically illustrated in Figure 2. The main gas supply passage 82 has an inlet port 86 and outlet port 88. The valve 64 includes a main metering valve 90 for controlling the maximum flow rate through the passage 82. The valve 90 preferably comprises a metering screw which can be rotated in order to restrict flow through the passage 82 in accordance with requirements. Normally the valve 90 is factory preset or adjusted when the vehicle is serviced.

The idle gas supply passage 84 includes an inlet port 92 and an idle supply line 94. Flow of gas into the idle supply line 94 is controlled by means of an idle metering valve 96. Again the metering valve 96 preferably takes the form of an adjusting screw which terminates in a valve needle 93 which is juxtaposed to a valve seat 95 in the idle supply line 94, the position of which is factory preset to control the amount of gas flowing into the idle supply line 94. The other end of the line 94 opens to the passage 82 and fuel supply therethrough constitutes the normal fuel required by the engine at idle. It will be appreciated by those skilled in the art that the valve 64 of the invention provides enhanced control because separate metering valves 90 and 96 are provided for the main and idle gas supplies. In the illustrated arrangement the idle line 94 connects onto the passage 82 but it could be coupled to the inlet manifold 4 by a separate line (not shown).

The valve 64 further includes a supplementary supply line 97 which is essentially a continuation of the idle gas supply passage 84. The supply line 97 provides for supplementary flow of gas from the idle gas supply passage 84 to a supplementary gas inlet port 99 in the intake manifold 4, the port being located downstream of the butterfly valve, as shown. The line 97 is coupled at an outlet port 98 to which is fitted a supply hose 100 which communicates with the inlet port 99, as shown. The line 97 includes a valve 102 which controls flow of gas through the line 97 and therefore controls supply of supplementary gas at various operating conditions of the engine from idle to full throttle, as will be described below. The flow of gas through the valve 102 is operable to effectively try to maintain the correct air fuel ratio throughout all operating ranges of the invention, i.e. from idle to full throttle, but it is more effective at lower throttle settings when the flow rate of gas from the supply jet 32 is lower.

The valve 102 is preferably controlled by means of a solenoid 104. The solenoid 104 is, when the engine is running on gas, controlled by the ECU 18. This is accomplished by provision of a relay 106 which is coupled to the ECU 18, as schematically shown in Figure 3. When the engine is to be converted to run on dual fuel, the negative injector pulse conductor 108 is cut at the point shown in broken lines and the circuit to the conductor 108 includes contacts 110 and 112 of the relay 106. The engine includes a 12 volt line 109 to the ECU 18 and this is also cut at the point shown in broken lines in order to insert the changeover switch 14, as shown. The contacts 110 and 112 of the relay 106 are closed when the engine operates on petrol. When the switch 14 changes so as to operate the engine on gas, the contacts 110 and 112 are open circuit thereby effectively preventing operation of the injectors 8. When the engine is operating on gas, the contact 110 is connected to contact 114 and this provides a circuit to the solenoid 104. As mentioned above, the ECU 18 is arranged to receive input signals from the sensor 16 and when excess O is detected, the ECU 18 would normally provide signals for additional petrol to be supplied by the injectors. This signal is, however, effectively coupled to the solenoid 104 in order to open the valve 102 and thereby provide supplementary flow of gas through the supplementary supply line 97. The supplementary gas is supplied to the engine through the inlet port 99 so as to supply a richer mixture thereby ensuring more correct combustion and less pollutants in the exhaust. The hose 100 may include a jet 101 which effectively controls or stabilises the flow of gas through the hose 100 to the port 99. The jet includes an orifice which is normally in the range of 0.8mm to 2.5mm and typically about 1.2mm. The optimum size of orifice for a particular type of engine can be determined by testing different size orifices and analysing exhaust gases under idle conditions in order to determine which size orifices are indicative of a required air fuel ratio, which is normally in the range 13.5 to 15.5:1 (by weight).

The response time of the gas control apparatus 10 needs to be fast in order to respond quickly to changing engine loads. In the embodiments of the invention, the additional gas supplied in the line 97 is in communication with the high pressure chamber 46 of the converter 30. The high pressure gas enables a fast response time to be achieved which would not be achievable if the additional gas required for the additional load were attempted to be supplied from the low pressure chamber 48.

It will be appreciated by those skilled in the art that the ability to achieve better combustion, particularly at idle, by coupling the control valve 64 to the ECU 18 constitutes a significant improvement in operation of internal combustion engines operating on gas. In some internal combustion engines, the ECU 18 may be such that it is not readily adapted for controlling the solenoid 104. In the circumstances, a supplementary ECU (not shown) can be provided which is coupled to the sensor 16 and arranged to provide control signals to the solenoid 104 so that the engine will function as described above.

The gas control apparatus 10 of the invention includes a further modification in order to improve performance of the engine at idle. A problem which can occur with internal combustion engines operating on gas at idle is the need to cope with increased loads at idle where the accelerator or throttle of the vehicle has not been changed. For instance, if the engine is idling and the vehicle's air conditioning and/or lights or other accessories are operated which require increased power, gas operated internal engines have in the past had difficulty coping with these increased loads. This sometimes causes the engine to stall. The additional fuel supplied by the supplementary line 97, whilst improving combustion efficiency, may not be supplied with a sufficiently fast response time to be able to assist with handling rapid increases of engine load at idle. In accordance with the invention, this problem is alleviated by supplying additional gas to the engine when required. In accordance with the invention, the converter 30 is provided with an additional outlet port 116 from the second chamber 48 and this is connected to a supplementary fuel supply line 118, as diagrammatically illustrated in Figure 2. The supply line 118 is again connected to the port 99 in the intake manifold 4.

In the illustrated arrangement, the supply line 118 includes a jet 119 to effectively restrict the amount of gas which can flow from the line 118 through the port 99 to the inlet intake of the engine. The jet normally includes an orifice which would normally be in the range from 0.8mm to 2.5mm in diameter and typically about 1.2mm. For a particular engine, it is possible to select the appropriate orifice size by testing different sized orifices using the technique mentioned above, i.e. by analysing the air/fuel ratio in the source at idle. An air fuel ratio of about 13.5 to 15.5:1 (by weight) is normally appropriate for an internal combustion engine running on gas.

The supplementary fuel line 118 is effectively coupled between the low pressure chamber 48 and the port 99. Accordingly, some fuel will generally flow through the supplementary fuel line 118 through all operating conditions of the engine. More fuel will, however, flow through the line 118 at low throttle settings because of the lower pressure in the inlet manifold 4 downstream of the butterfly valve 20. In addition to supplying additional fuel under lower throttle settings, a significant effect of the line 118 is to increase the sensitivity of operation of the second diaphragm 44 in the converter 30. This is because at lower throttle settings, the vacuum levels in the intake manifold are effectively coupled to the chamber 48 through the line 118 thereby making downward movement of the diaphragm 44 easier and hence it is more responsive to changing fuel demands required by the engine. Accordingly, the gas control apparatus 10 of the invention is better able to manage varying engine loads at idle compared with ^"known internal combustion engines operating on gas.

A bypass passage 120 is also provided to effectively bypass the butterfly valve 20 to enable additional airflow to the engine even though the butterfly valve 20 is closed or nearly closed, subject to the control of the valve 23 located in the bypass passage 120. The valve 23 is controlled by means of the motor 22 which in turn is controlled by the ECU 18. When the engine is running on petrol, the ECU 18 produces signals for controlling the motor 22 so that when increased idle engine loads are produced, the valve 23 is progressively opened to permit greater air flow through the inlet manifold 4 when required for good combustion at increased idle loads. The bypass passage 120 and valve 23 are provided in most fuel injected internal combustion engines and, in the embodiments of the invention, these components function in the usual way. In the illustrated arrangement the downstream end of the bypass passage 120 is connected to the port 99 but a separate port could be provided for this purpose.

Figures 5, 6 and 7 illustrate the preferred structure for the valve 64. The valve 64 is preferably formed from a cast body which has integral bosses for providing the ports 86 and 88 and a further boss for mounting of the metering valve 96. The metering valve 90 is mounted in a tapped hole formed directly in the casting, as shown. In the illustrated arrangement, the port 98 is connected directly to the valve 102. The valve 102 is provided with a mounting spigot 132 to which an end of the hose 100 can be connected. The inlet port 92 is also provided with a mounting spigot 136 for connecting to the high pressure inlet line 66. The drawing shows the casting having a plug 138 which simply covers a hole which is used for forming some of the internal cavities within the casting.

Figure 8 shows a modified form of the invention. Figure 8 is similar to Figure 2 and the same reference numerals have been used to denote parts which are the same as or correspond to those in Figure 2. In the modified arrangement, the hose 100 which extends from the outlet of the valve 102 is connected into the supply line 33 rather than being coupled to the port 99 as in the arrangement of Figure 2. The jet 101 is provided in the hose 100, as before.

Many modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention.

Claims

CLAIMS:

1. Apparatus (10) for control of gas to an internal combustion engine having an intake manifold (4), said apparatus including: a control valve (64) having a first inlet (92) for receiving gas at a relatively low pressure; a second inlet (86) for receiving gas at a relatively high pressure; an outlet (88) which, in use, can be coupled to the intake of an internal combustion engine; a first metering device (90) for controlling the maximum rate of gas supply from the first inlet to said outlet; and a second metering device (96) for controlling the maximum rate of gas supply from the second inlet to said outlet whereby the metering devices can independently control maximum gas supplies to the intake manifold of the internal combustion engine under power conditions and at idle respectively.

2. Apparatus as claimed in claim 1 wherein the control valve (64) includes a body (80) having a first gas supply passage (84) which extends from said first inlet to said first outlet.

3. Apparatus as claimed in claim 2 wherein the first metering device includes an adjusting screw, the position of which can be adjusted to partly block said first gas supply passage.

4. Apparatus as claimed in claim 2 wherein the body includes a second gas supply passage (84, 94) which extends from said second inlet to said first gas supply passage downstream of said first metering device.

5. Apparatus as claimed in claim 4 wherein the second metering device includes a valve needle (93), the position of which can be adjusted relative to a valve seat (95) in order to control flow of gas through the second gas supply passage to the first gas supply passage.

6. An internal combustion engine (2) having an intake manifold (4) for supplying operating air to combustion chambers thereof, the engine including gas control apparatus (10) for supplying a combustible gas to said intake manifold (4), said gas control apparatus including a converter (30) which in use is coupled to a pressurised source (12) of said gas, the converter including a high pressure chamber (46) and a low pressure chamber (48) and a gas control valve (64), the gas control valve having: a first inlet (92) for receiving gas from said low pressure chamber; a second inlet (86) for receiving gas from the high pressure chamber (46); an outlet (88) which is coupled to said intake manifold (4); a first metering device (90) for controlling the maximum rate of gas supply from the first inlet to said outlet; a second metering device for controlling the maximum rate of gas supply from the second inlet to said outlet whereby the metering devices can independently control maximum gas supplies to the intake manifold when the engine is operated at higher power output levels and at idle respectively.

7. A method of operating an internal combustion engine (2) having an intake manifold (4) by supplying gas from a gas supply tank (12), the method including the steps of: supplying high pressure gas to a converter (30) having high and low pressure chambers (46, 48) therein; controlling the rate of flow of low pressure gas from the low pressure chamber (48) to the intake manifold (4) so as to vary the output power of the engine; providing a first metering device (90) and adjusting the first metering device so as to control the maximum rate of flow of low pressure gas to the intake manifold (4); supplying gas from the high pressure chamber (46) to the intake manifold (4) so as to provide an idle supply of gas for the engine at idle; providing a second metering device (96) and adjusting the second metering device so as to control the maximum rate of flow of said idle supply of gas independently of the adjustment of said first metering device (90).

8. Apparatus for control of an internal combustion engine which can selectively operate from petrol or pressurised gas from a gas supply tank (12), the engine having an intake manifold (4), an electronic control unit (ECU) (18) and a sensor (16) which senses oxygen in exhaust products from the engine and provides control signals to the ECU in order to adjust fuel supply to the engine in order to achieve a desired air fuel ratio, said apparatus including: low pressure gas supply means (32, 48, 82, 90) for providing a low pressure gas supply to the intake manifold (4), said low pressure gas supply means being coupled to flow control means (30) for controlling rate of flow of said low pressure gas supply to thereby vary output power of the engine; supplementary gas supply means (46, 84, 97, 99) for providing a supplementary gas supply to the intake manifold (4) including valve means (102, 104) which is coupled, in use, to the ECU and arranged to provide increased rate of flow of said supplementary gas supply when said sensor detects, in use, higher levels of oxygen in said exhaust products.

9. Apparatus as claimed in claim 8 wherein the flow control means includes a converter (30) having a low pressure chamber (48) and a high pressure chamber (46) which respectively provide the low pressure gas supply and the supplementary gas supply.

10. Apparatus as claimed in claim 9 wherein said valve means (102, 104) includes a solenoid valve.

11. An internal combustion engine which can operate from pressurised gas from a gas supply tank (12), the engine having an intake manifold (4), an electronic control unit

(ECU) (18) and a sensor (16) which senses oxygen in exhaust products from the engine and provides control signals to the ECU in order to adjust fuel supply to the engine in order to achieve a desired air fuel ratio and gas control apparatus (10) for controlling supply of gas from the gas supply tank (12) to the inlet manifold (4), said apparatus including: low pressure gas supply means (32, 48, 82, 90) for providing a low pressure gas supply to the intake manifold (4), said low pressure gas supply means being coupled to flow control means (30) for controlling rate of flow of said low pressure gas supply to thereby vary output power of the engine; supplementary gas supply means (46, 84, 97, 99) for providing a supplementary gas supply to the intake manifold (4) including valve means (102, 104) which is coupled to the ECU and arranged to provide increased rate of flow of said supplementary gas supply when said sensor detects higher levels of oxygen in said exhaust products.

12. An internal combustion engine as claimed in claim 11 wherein the ECU (18) has an output control line (108) for controlling fuel injectors (8) of the engine and wherein said valve means (102, 104) is coupled, in use, to said control line (108).

13. A method of operating an internal combustion engine selectively on petrol or gas, the engine including: an intake manifold (4), an electronic control unit (ECU) (18) and a sensor (16) which senses oxygen in exhaust products from the engine and provides control signals to the ECU in order to adjust fuel supply to fuel injectors thereof in order to achieve a desired air fuel ratio when operating on petrol, said method including the steps of: providing low pressure gas supply means (32, 48, 82, 90) for providing a low pressure gas supply to the intake manifold (4), said low pressure gas supply means being coupled to flow control means (30) for controlling rate of flow of said low pressure gas supply to thereby vary output power of the engine; providing supplementary gas supply means (46, 84, 97, 99) for providing a supplementary gas supply to the intake manifold (4) including valve means (102, 104); and coupling the valve means to the ECU to thereby provide increased rate of flow of said supplementary gas supply when said sensor detects, in use, higher levels of oxygen in said exhaust products.

14. An internal combustion engine having an intake manifold (4) having a butterfly valve (20) therein, the engine being arranged to operate selectively on petrol or gas, the engine including gas control apparatus (10) including a converter (30) including a housing (40) having first and second diaphragms (42 and 44) which define first, second and third chambers (46, 48 and 58) therein, the second chamber being located between the first and third chambers, the housing including: an inlet port (50) which is coupled to gas supply tank (12); an outlet port (62) which communicates with the second chamber for supplying low pressure gas to the inlet manifold and wherein converter (30) is coupled to a supplementary fuel supply line (118) which is coupled to said intake manifold (4) downstream of said butterfly valve, whereby additional gas is supplied through the supplementary gas supply line at a rate which depends, in use, on the level of vacuum created in the intake manifold caused by throttling of the butterfly valve.

15. An internal combustion engine as claimed in claim 14 including wherein the supplementary fuel supply line (118) includes a jet (119) to control the rate of flow of gas through said supplementary gas supply line.

16. An internal combustion engine as claimed in claim 14 wherein the converter includes a port (70) which communicates with the third chamber (58), the third port (70) being open to atmosphere or being coupled to the intake manifold (4) upstream of said butterfly valve (20) whereby, when the butterfly valve is throttled, there is a substantial pressure differential across the second diaphragm (44) thereby enabling more rapid movement thereof whereby, when loads on the engine are rapidly increased at idle, additional fuel can rapidly be supplied through said supplementary fuel supply line to thereby avoid stalling of the engine.