WO2013000030A1

WO2013000030A1 - Compensation for gas composition

Info

Publication number: WO2013000030A1
Application number: PCT/AU2012/000771
Authority: WO
Inventors: Tyron Dean Utley
Original assignee: Orbital Australia Pty Ltd
Priority date: 2011-06-30
Filing date: 2012-06-29
Publication date: 2013-01-03
Also published as: AU2012276289A1; AU2012276289B2

Abstract

An engine control system (81) for inferring the composition of the gaseous fuel to compensate variations in the gas composition comprises an exhaust gas sensor (83) in communication with a flow path of the exhaust gas for measuring NOx emissions and Lambda in the exhaust gas at typically a full load condition. An engine control module (23) receives an indication of NOx emissions and Lambda in the exhaust gas from the exhaust gas sensor and references a suitable function, look-up table, or map to identify gaseous fuel composition from the parameters relating to the NOx emissions and Lambda in the exhaust gas received from the exhaust gas sensor. The measurement of each parameter (NOx emissions and Lambda in the exhaust gas) is then compared with the respective allowable range for that load condition and the timing of ignition of a combustible charge adjusted accordingly to provide compensation.

Description

Compensation for Gas Composition

Field of the Invention

This invention relates to an internal combustion engine fuelled with gaseous fuel.

The invention also relates to a method of operation of such an engine to compensate for variations in the composition of gaseous fuel.

The invention also relates to an engine having provision for compensation for variations in the composition of a gaseous fuel.

Background Art

The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

There is a trend towards use of gaseous fuels in internal combustion engines.

The term "gaseous fuels" as used herein refers to compressed gas fuels such as compressed natural gas (CNG) and hydrogen (H₂), and liquefied gaseous fuels such as liquefied petroleum gas (LPG) and liquefied natural gas (LNG).

There are a number of potential advantages in using gaseous fuels in engines of motor vehicles instead of, or together with, the more commonly used liquid fuels. One advantage is that the undesirable exhaust emissions from an engine using a gaseous fuel can be lower than for a comparable engine using liquid fuel. Further, gaseous fuels are generally less expensive than liquid fuels and accordingly the use of gaseous fuels can, in certain cases, translate to a significant cost saving for the user. Natural gas is a gaseous fuel that has particular benefits in certain applications because of its economical and environmental advantages.

Natural gas comprises a complex mixture of hydrocarbons, typically comprising primarily methane, with a component of a higher hydrocarbon such as ethane. The respective proportions of the gas components of the mixture can vary for a variety of reasons, including the diversity of sources of natural gas, the refining process and weathering in the case of liquefied natural gas (in which the proportion of methane depletes through venting).

The performance and emissions of an internal combustion engine fuelled with natural gas can be affected by the varying composition of natural gas. Variations in natural gas composition may particularly affect the combustion quality at lean operating limit conditions.

The variations in the composition of natural gas can adversely affect satisfactory performance of a motor vehicle to the extent that it may be noticeable to the vehicle driver for various weather and road surface conditions, as well as driving demands.

It is against this background, and the problems and difficulties associated therewith, that the present invention has been developed.

Disclosure of the Invention The present invention stems from the realisation that exhaust gas from an internal combustion engine fuelled with gaseous fuel can be used to infer the composition of the gaseous fuel to permit implementation of a compensation strategy to accommodate variations in the gas composition.

According to a first aspect of the invention there is provided a strategy for compensating an internal combustion engine fuelled with gaseous fuel for a variation in the composition of the gaseous fuel, the strategy comprising obtaining an indication of the composition and adjusting the timing of ignition of a combustible charge comprising the gaseous fuel based on the indication obtained to accommodate the variation in the gaseous fuel composition, wherein the indication of the composition of the gaseous fuel comprises an association with NOx emissions in exhaust gas arising from ignition of the combustible charge..

Preferably, the association comprises a relationship between the composition of the gaseous fuel and NOx emissions in the exhaust gas.

Preferably/the indication of the gaseous fuel composition is obtained by inference from a sense of NOx emissions in the exhaust gas Preferably, the sense of NOx emissions in the exhaust gas comprises a measure of NOx emissions in the exhaust gas. Preferably, the sense of NOx emissions is obtained from a sensor exposed to the exhaust gas.

Preferably, the NOx emissions in the exhaust gas are measured at a specified load condition for the engine. The load condition is typically a full load condition. In certain circumstances, the indication of the composition of the gaseous fuel is enhanced through consideration of Lambda in the exhaust gas.

Accordingly, the indication of the composition of the gaseous fuel may further comprise an association with Lambda in the exhaust gas.

Preferably, the association comprises a relationship between the composition of the gaseous fuel and Lambda in the exhaust gas.

Preferably, the indication of the gaseous-fuel composition is obtained by inference from a sense of Lambda in the exhaust gas in conjunction with the sense of NOx emissions in the exhaust gas. Preferably, the sense of Lambda in the exhaust gas comprises a measure of Lambda in the exhaust gas. Preferably, the sense of Lambda is obtained from a sensor exposed to the exhaust gas.

Preferably, both the NOx emissions and Lambda in the exhaust gas are measured at a specified load condition of the engine. The load condition is typically a full load condition.

A control system may reference a suitable function, look-up table, or map to identify gaseous fuel composition from a given parameter representative of the NOx emissions, or given parameters representative of the NOx emissions and Lambda of the exhaust gas.

The measurement of each parameter is then compared with the respective allowable range for that load condition.

If a condition exists where NOx emissions are lower than the allowable range but the Lambda is within the allowable range, it is inferred that there has been a decrease in peak combustion temperature. This is because the quantity of NOx decreases with decreasing peak combustion temperature. The most likely source of lower combustion temperature in an engine fuelled with gaseous fuel is a variation in the gas composition, leading to a slower burn rate of the fuel. In relation to natural gas, this is typically a change in the ratio between methane and ethane.

If a condition exists where the NOx emissions are above the allowable range, it is inferred that there has been an increase in peak combustion temperature. This is because the quantity of NOx increases with increasing peak combustion temperature. The most likely source of higher combustion temperature in an engine fuelled with gaseous fuel is a variation in the gas composition, leading to a faster burn rate of the fuel. In relation to natural gas, this is typically a change in the ratio between methane and ethane. If the NOx emissions are above the allowable range and the injection event is occurring in advance of the normal timing setting, the injection timing is retarded to account for the faster burn rate. In this condition, no consideration need be given to Lambda. Preferably, the NOx emissions, as well as Lambda when considered, are identified by an exhaust gas sensor in communication with a flow path for the exhaust gas. By using the exhaust gas sensor, the composition of the exhaust gas can be understood, which allows the variation in the gaseous fuel composition to be identified. Where the engine comprises a plurality of cylinders each defining a combustion chamber in which exhaust gas is generated, there is preferably a confluence of the exhaust gases and it is the confluence of exhaust gases from which the inference is obtained.

According to a second aspect of the invention there is provided a method of operating an internal combustion engine delivering metered quantities of a gaseous fuel to a combustion chamber of the engine and combusting the gaseous fuel in timed relation to generate exhaust gas, the method comprising a strategy for compensating the internal combustion engine for a variation in the composition of the gaseous fuel, the strategy being in accordance with the first aspect of the invention.

According to a third aspect of the invention there is provided a method of operating an internal combustion engine delivering metered quantities of a gaseous fuel to a combustion chamber of the engine and combusting the gaseous fuel in timed relation to generate exhaust gas, obtaining an indication of the composition of the gaseous fuel, and adjusting the timing of ignition of a combustible charge comprising the gaseous fuel based on the indication obtained to accommodate the variation in the gaseous fuel composition, wherein the indication of the composition of the gaseous fuel comprises an association with NOx emissions in exhaust gas arising from ignition of the combustible charge.

Preferably, the indication of the composition of the gaseous fuel may further comprise an association with Lambda In the exhaust gas. In relation to a spark-ignition engine, the step of adjusting the timing of ignition of a combustible charge may comprise adjusting the timing of the spark ignition event of the combustible charge.

In relation to a compression-ignition engine, the step of adjusting the timing of ignition of a combustible charge may comprise adjusting the timing of injection of a pilot fuel for the ignition of the combustible charge.

According to a fourth aspect of the invention there is provided an engine adapted to operate with a strategy according to the first aspect of the invention.

According to a fifth aspect of the invention there is provided an engine adapted to operate in accordance with a method according to the second or third aspect of the invention.

According to a sixth aspect of the invention there is provided an internal combustion engine and exhaust system comprising a combustion chamber, a gaseous fuel delivery system for delivery of a metered quantity of gaseous fuel to the combustion chamber to establish a combustible charge whereby ignition of the combustible charge generates exhaust gas, means for obtaining an indication of the composition of the gaseous fuel, and means for adjusting the timing of ignition of the combustible charge based on the indication of the composition of the gaseous fuel obtained to accommodate the variation in the composition thereof, wherein the indication of the composition of the gaseous fuel comprises an association with NOx emissions in exhaust gas arising from ignition of the combustible charge . Preferably, the indication of the composition of the gaseous fuel may further comprise an association with Lambda in the exhaust gas.

Preferably, the means for adjusting the timing of ignition of the combustible charge comprises a control means. Preferably, the control means comprises as an electronic control unit (ECU).

Preferably, the engine and exhaust system further comprises an exhaust gas sensor in communication with a flow path for the exhaust gas for identifying the NOx emissions in the exhaust gas.

In circumstances where Lambda is also taken into consideration, the exhaust gas sensor may also identify Lambda.

Preferably, the ECU is configured to receive input from the exhaust gas sensor and to control the timing of ignition of the combustible charge accordingly.

Preferably, the engine and exhaust system according to the invention comprises a plurality of cylinders each defining a combustion chamber in which exhaust gas is generated, wherein there is a confluence of the exhaust gases from the cylinders and wherein the indication is obtained from the confluence of exhaust gases.

Preferably, the engine and exhaust system further comprises an exhaust system to receive the confluence of the exhaust gases from the cylinders, the gas sensor being disposed in the exhaust system for communication with the confluence of the exhaust gases.

Where the engine and exhaust system comprises a spark-ignition engine, the engine may further comprise an ignition system for generating a spark ignition event step, and the means for adjusting the timing of ignition of the combustible charge may comprise means for adjusting the timing of the ignition event. Where the engine and exhaust system comprises a compression-ignition engine, the engine may further comprise a liquid fuel delivery system for delivery of a metered quantity of liquid fuel to the combustion chamber to provide a pilot fuel for ignition under compression, and the means for adjusting the timing of ignition of the combustible charge may comprise means for adjusting the timing of the delivery of the pilot fuel.

The engine and exhaust system according to the invention may be configured for bi-fuel fuel operation.

Where the engine and exhaust system comprises a compression-ignition engine configured for bi-fuel fuel operation, the engine may be configured for fuelling with either liquid fuel alone (hereinafter referred to as the liquid fuel mode) or a combination of liquid fuel and gaseous fuel, with the liquid fuel providing the pilot fuel (hereinafter referred to as the pilot fuel mode).

Preferably, the engine and exhaust system further comprises a control means for selectively controlling operation of the liquid fuel delivery system and the gaseous fuel delivery system to control the quantity of the liquid fuel and the corresponding quantity of gaseous fuel delivered to the combustion chamber during operation in the pilot fuel mode.

Preferably, the control means is responsive to a threshold knock condition in the combustion chamber to effect substitution of some of the gaseous fuel with additional pilot fuel to eliminate engine knock or at least reduce engine knock below the threshold knock condition. Such a knock control strategy may of course also be used in conjunction with other knock control strategies, for example, one which varies the ignition or injection timing of the fluid delivered to the combustion chamber. Where required, the compensation strategy can permit the knock control strategy to override any timing modifications from the NOx sensor and retard the timings in the event that knock is detected. Preferably, the function of the control means for selectively controlling operation of the liquid fuel delivery system and the gaseous fuel delivery system is performed by the ECU.

Brief Description of the Drawings The invention will be better understood by reference to the following description of one specific embodiment thereof in which:

Figure 1 is a schematic view of an engine system according to the embodiment; and

Figure 2 is a flowchart depicting the process implemented by an engine control module within the engine system for compensation for variation in the composition of gaseous fuel for the engine.

Best Mode(s) for Carrying Out the Invention

The embodiment is directed to an internal combustion engine system 10 configured for bi-fuel fuel operation. The engine system 10 comprises an internal combustion engine 11 and an associated fuel delivery system 13 for delivering fuel to the engine 11.

In this embodiment, the engine 11 comprises a compression ignition engine configured for bi-fuel fuel operation using diesel fuel ignitable by compression ignition and also a gaseous fuel such as CNG or LNG. The engine is operable in either liquid fuel mode or pilot fuel mode. The invention is, however, also applicable to a spark ignition engine configured for fuelling with a gaseous fuel such as CNG or LNG, or bi-fuel operation using a liquid fuel such as gasoline and also a gaseous fuel such as CNG or LNG. ln the arrangement illustrated, the engine 11 comprises a multi-cylinder engine comprising an engine structure 15 incorporating a plurality of cylinders 16 within each of which there is defined a combustion chamber 18. The front of the engine structure 15 is identified by reference numeral 19. The engine system 10 is, however, also applicable to a single-cylinder engine.

The fuel delivery system 13 comprises a liquid fuel delivery system 21 for delivery of a metered quantity of liquid fuel directly to the combustion chambers and a gaseous fuel delivery system 23 for delivery of a metered quantity of gaseous fuel indirectly to the combustion chambers. A control system 25 is provided to respond to information received by various sensors by adjusting one or more operating parameters of the engine 11 and the associated fuel delivery system 13. The control system 25 comprises an electronic control unit 27 (ECU) comprising an engine control module 28 and a direct injector driver 29. The engine control module 28 selectively controls operation of the liquid fuel delivery system 21 and the gaseous fuel delivery system 23 to control the quantity of the liquid fuel and the corresponding quantity of gaseous fuel delivered to the combustion chamber during operation in the pilot fuel mode.

The engine system 10 further comprises an air induction system 31 for supplying air to the cylinders of the engine and an exhaust system 32 which is depicted schematically in the drawings. The exhaust system is of known kind.

The air induction system 31 includes an inlet manifold 33 which is adapted to receive intake air and which communicates with the combustion chambers through respective inlet valves. The air induction system 31 also includes a vent system 35 for venting excess air. The vent system 35 comprises a vent line 37 in which there is incorporated an excess air control valve 38 operable by the engine control module 28. The air induction system 31 further comprises an air pressure sensor 39 and an air temperature sensor 40. The liquid fuel delivery system 21 comprises a liquid fuel supply line 41 adapted to receive liquid fuel from a fuel tank (not shown) and to return excess liquid in known manner. The liquid fuel delivery system 21 further comprises a plurality of liquid fuel direct injectors 43 operable to deliver metered quantities of liquid fuel to the combustion chambers within the cylinders in timed sequence. Typically, the fuel injectors 43 are incorporated in a fuel rail assembly (not shown).

The fuel injectors 43 are operable by the direct injector driver 29 in response to control signals received from the engine control module 28. The operation of the fuel injectors 43 is controlled in terms of the timing of opening, and the duration of opening, of the injectors.

The gaseous fuel delivery system 23 is adapted to deliver gaseous fuel indirectly to the combustion chambers within the cylinders. Specifically, the gaseous fuel delivery system 23 is adapted to deliver gaseous fuel into the inlet manifold 33.

The gaseous fuel delivery system 23 comprises a mixer 51 communicating with the inlet manifold 33 for mixing metered quantities of the gaseous fuel with intake air, with the metered gaseous fuel being transported to the combustion chambers with the intake air.

The gaseous fuel delivery system 23 further comprises a gaseous fuel supply line 53 adapted to receive gaseous fuel from a source such as a tank (not shown) and metered quantities of gaseous fuel are delivered to the mixer 51 through a metering system 57 communicating with the gaseous fuel supply line 53. In the arrangement shown, the metering system 57 comprises a plurality of gaseous fuel delivery injectors 58 assembled into a gas injector bank 59.

The delivery injectors 58 are operable in response to control signals received from the engine control module 28. The operation of each delivery injector 58 is controlled in terms of the timing of opening, and the duration of opening, of the injectors. Typically, each delivery injector 58 is operable individually and in a regime determined by the engine control module 28, whereby the injector bank 59 can deliver the required quantity of gaseous fuel according to operational requirements.

The gaseous fuel delivery system 23 further comprises a gas temperature sensor 61 and a gas pressure sensor 62. The engine 11 further comprises knock sensor system 71 for monitoring the engine cylinders for a threshold knock condition, the knock sensor system being adapted to provide an input to the engine control module 28 indicative of the threshold knock condition.

In this embodiment, the knock sensor system 71 comprises two knock sensors 73 installed on the engine structure 15 to measure vibrations transmitted through the engine structure arising from a knock condition. The sensors 73 are sampled in the crankshaft domain to distinguish between the various cylinders in accordance with known practice. In the arrangement shown, the knock sensors 73 comprise a front knock sensor 73a and a rear knock sensor 73b.

The engine control module 28 has a strategy available for knock control. The strategy involves retarding ignition timing.

The engine 11 further comprises a system 81 for inferring the composition of the gaseous fuel to permit implementation of a compensation strategy to accommodate variations in the gas composition. The system 81 comprises an exhaust gas sensor 83 in communication with a flow path for the exhaust gas in the exhaust system 32 for measuring NOx emissions and Lambda in the exhaust gas at a specified load condition. The load condition is typically a full load condition. The flow path for the exhaust gas is depicted schematically in Figure 1 and identified by reference numberal 84. The engine control module 28 receives an indication of NOx emissions and Lambda in the exhaust gas from the exhaust gas sensor 83. The engine control module 28 references a suitable function, look-up table, or map to identify gaseous fuel composition from the parameters relating to the NOx emissions and Lambda in the exhaust gas received from the exhaust gas sensor 83. The measurement of each parameter (NOx emissions and Lambda in the exhaust gas) is then compared with the respective allowable range for that load condition.

If the NOx emissions are lower than the allowable range but the Lambda is within the allowable range, it is inferred that there has been a decrease in peak combustion temperature. This is because the quantity of NOx decreases with decreasing peak combustion temperature. The most likely source of lower combustion temperature in an engine fuelled with gaseous fuel is a variation in the gas composition, leading to a slower burn rate of the fuel. In relation to natural gas, this is typically a change in the ratio between methane and ethane.

Compensation for the slower burn rate can be provided by advancing the ignition of the combustible charge in each cylinder. In the present embodiment, where the engine 11 is a compression ignition engine, this is achieved by advancing the timing of injection of the pilot fuel. The timing of injection of the pilot fuel is controlled by the fuel injectors 43 under the control of the direct injector driver 29 which in turn is controlled by the engine control module 28.

If the NOx emissions are lower than the allowable range but the Lambda is outside of the allowable range, no compensatory action is taken.

If the NOx emissions are within the allowable range, no compensatory action is taken.

If the NOx emissions are above the allowable range and the injection event is occurring in accordance with the normal timing setting, no compensatory action is taken. In this condition, no consideration need be given to Lambda.

If the NOx emissions are above the allowable range and the injection event is occurring in advance of the normal timing setting, the injection timing is retarded. ln this condition, no consideration need be given to Lambda. More particularly, as the NOx emissions are above the allowable range, it is inferred that there has been an increase in peak combustion temperature. This is because the quantity of NOx increases with increasing peak combustion temperature. The most likely source of higher combustion temperature in an engine fuelled with gaseous fuel is a variation in the gas composition, leading to a faster burn rate of the fuel. In relation to natural gas, this is typically a change in the ratio between methane and ethane.

Compensation for the varying burn rate can be provided by modifying the ignition of the combustible charge in each cylinder. In the present embodiment, where the engine 11 is a compression ignition engine, this is achieved by modifying the timing of injection of the pilot fuel. The timing of injection of the pilot fuel is controlled by the fuel injectors 43 under the control of the direct injector driver 29 which in turn is controlled by the engine control module 28. The gas compensation strategy as described above is outlined in the flowchart in Figure 2 depicting the process implemented by the engine control module 28.

The gas compensation strategy as described above needs to operate in conjunction with the knock control strategy implemented by the knock sensor system 71. The knock control strategy may comprise substitution of some of the gaseous-fuel with additional pilot fuel to eliminate engine knock or at least reduce engine knock below the threshold knock condition. The knock control strategy may also involve retarding ignition timing. In the event of a conflict between the two strategies, the knock control strategy would typically prevail.

From the foregoing, it is evident that the present embodiment provides a simple yet highly effective arrangement for compensating for variations in the composition of a gaseous fuel in operation of an internal combustion engine.

It should be appreciated that the scope of the invention is not limited to the scope of the embodiment described. By way of example, while the embodiment has been described in relation to a bi- fuel fuel compression ignition engine, the invention is also applicable to a spark ignition engine configured for fuelling with a gaseous fuel such as CNG or LNG, or bi-fuel fuel operation using a liquid fuel such as gasoline and also a gaseous fuel such as CNG or LNG. Further, the invention is also applicable to an internal combustion engine configured for operation on a dedicated gaseous fuel; that is, the invention is applicable to an internal combustion an engine which is not configured for bi-fuel fuel operation.

Further, the embodiment has been described in relation to compensating an internal combustion engine fuelled with gaseous fuel for a variation in the composition of the gaseous fuel wherein an indication of the composition is based on an assessment of both NOx and Lambda in the exhaust gas.

There may be certain engine systems which can perform satisfactorily with compensation for a variation in the composition of the gaseous fuel without requiring an assessment of Lambda in the exhaust gas. In such circumstances, the indication of the composition may be based only on an assessment of NOx in the exhaust gas, or alternatively based on an assessment of NOx in the exhaust gas in association with an assessment of some other characteristic of the exhaust gas. Throughout the specification and claims, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Claims

The Claim Defining the Invention is as Follows:

1. A strategy for compensating an internal combustion engine fuelled with gaseous fuel for a variation in the composition of the gaseous fuel, the strategy comprising obtaining an indication of the composition and adjusting the timing of ignition of a combustible charge comprising the gaseous fuel based on the indication obtained to accommodate the variation in the gaseous fuel composition, wherein the indication of the composition of the gaseous fuel comprises an association with NOx emissions in exhaust gas arising from ignition of the combustible charge.

2. The strategy according to claim 1 wherein the association comprises a relationship between the composition of the gaseous fuel and NOx emissions in the exhaust gas.

3. The strategy according to claim 1 or 2 wherein the indication of the gaseous fuel composition is obtained by inference from a sense of NOx emissions in the exhaust gas.

4. The strategy according to claim 3 wherein the sense of NOx emissions in the exhaust gas comprises a measure of NOx emissions in the exhaust gas.

5. The strategy according to claim 4 wherein the sense of NOx emissions is obtained from a sensor exposed to the exhaust gas.

6. The strategy according to any one of the preceding claims wherein the NOx emissions in the exhaust gas are measured at a specified load condition for the engine.

7. The strategy according to claim 6 wherein load condition is a full load condition.

8. The strategy according to any one of the preceding claims wherein the indication of the composition of the gaseous fuel further comprises an association with Lambda in the exhaust gas.

9. The strategy according to claim 8 wherein the association comprises a relationship between the composition of the gaseous fuel and Lambda in the exhaust gas.

10. The strategy according to claim 8 or 9 wherein the indication of the gaseous fuel composition is obtained by inference from a sense of Lambda in the exhaust gas in conjunction with the sense of NOx emissions in the exhaust gas.

11. The strategy according to claim 10 wherein the sense of Lambda in the exhaust gas comprises a measure of Lambda in the exhaust gas.

12. The strategy according to claim 10 or 11 wherein the sense of Lambda is obtained from a sensor exposed to the exhaust gas.

13. The strategy according to claim 10,. 11 or 12 wherein both the NOx emissions and Lambda in the exhaust gas are measured at a specified load condition of the engine.

14. The strategy according to claim 13 wherein the load condition is a full load condition.

15. The strategy according to any one of the preceding claims further comprising reference to a function, look-up table, or map to identify gaseous fuel composition from a given parameter representative of the NOx emissions.

16. The strategy according to any one of claims 8 to 14 further comprising reference to a function, look-up table, or map to identify gaseous fuel composition from given parameters representative the NOx emissions and Lambda of the exhaust gas.

17. The strategy according to claim 15 or 16 further comprising comparing each parameter with the respective allowable range for that load condition.

18. The strategy according to any one of claims 10 to 17 wherein if a condition exists where NOx emissions are lower than the allowable range but the Lambda is within the allowable range, it is inferred that there has been a decrease in peak combustion temperature.

19. The strategy according to any one of claims 10 to 17 wherein if a condition exists where the NOx emissions are above the allowable range, it is inferred that there has been an increase in peak combustion temperature.

20. The strategy according to any one of claims 3 to 17 wherein if the NOx emissions are above the allowable range and the injection event is occurring in advance of the normal timing setting, it is inferred that there has been a faster burn rate and the injection timing is retarded to account for the faster burn rate.

21. The strategy according to any one of the preceding claims wherein the NOx emissions are identified by an exhaust gas sensor in communication with a flow path for the exhaust gas.

22. The strategy according to claim 21 wherein Lambda is also identified by the exhaust gas sensor.

23. The strategy according to claim 21 or 22 wherein the engine comprises a plurality of cylinders each defining a combustion chamber in which exhaust gas is generated, there being a confluence of the exhaust gases and wherein the inference is obtained from the confluence of exhaust gases.

24. A method of operating an internal combustion engine delivering metered quantities of a gaseous fuel to a combustion chamber of the engine and combusting the gaseous fuel in timed relation to generate exhaust gas, the method comprising a strategy for compensating the internal combustion engine for a variation in the composition of the gaseous fuel, the strategy being in accordance with any one of the preceding claims.

25. A method of operating an internal combustion engine delivering metered quantities of a gaseous fuel to a combustion chamber of the engine and combusting the gaseous fuel in timed relation to generate exhaust gas, obtaining an indication of the composition of the gaseous fuel, and adjusting the timing of ignition of a combustible charge comprising the gaseous fuel based on the indication obtained to accommodate the variation in the gaseous fuel composition, wherein the indication of the composition of the gaseous fuel comprises an association with NOx emissions in exhaust gas arising from ignition of the combustible charge.

26 he method according to claim 25 wherein the indication of the composition of the gaseous fuel may further comprises an association with Lambda in the exhaust gas.

27. The method according to claim 25 or 26 wherein the step of adjusting the timing of ignition of a combustible charge comprises adjusting the timing of the spark ignition event of the combustible charge in a spark- ignition engine.

28. The method according to claim 25 or 26 wherein the step of adjusting the timing of ignition of a combustible charge may comprise adjusting the timing of injection of a pilot fuel for the ignition of the combustible charge in a compression-ignition engine.

29.An engine adapted to operate with a strategy according to any one of claims 1 to 23.

30. An engine adapted to operate in accordance with a method according to any one of claims 24 to 28.

31. An internal combustion engine and exhaust system comprising a combustion chamber, a gaseous fuel delivery system for delivery of a metered quantity of gaseous fuel to the combustion chamber to establish a combustible charge whereby ignition of the combustible charge generates exhaust gas, means for obtaining an indication of the composition of the gaseous fuel, and means for adjusting the timing of ignition of the combustible charge based on the indication of the composition of the gaseous fuel obtained to accommodate the variation in the composition thereof, wherein the indication of the composition of the gaseous fuel comprises an association with NOx emissions in exhaust gas arising from ignition of the combustible charge .

32. The internal combustion engine and exhaust system according to claim 31 wherein the indication of the composition of the gaseous fuel further comprises an association with Lambda in the exhaust gas.

33. The internal combustion engine and exhaust system according to claim 31 or 32 wherein the means for adjusting the timing of ignition of the combustible charge comprises a control means.

34. The internal combustion engine and exhaust system according to claim 33 wherein the control means comprises as an electronic control unit (ECU).

35.The internal combustion engine and exhaust system according to any one of claims 31 to 34 further comprising an exhaust gas sensor in communication with a flow path for the exhaust gas for identifying the NOx emissions in the exhaust gas.

36.The internal combustion engine and exhaust system according to claim 35 wherein the exhaust gas sensor also identifies Lambda.

37.The internal combustion engine and exhaust system according to claim

35 or 36 wherein the ECU is configured to receive input from the exhaust gas sensor and to control the timing of ignition of the combustible charge accordingly.

38. The internal combustion engine and exhaust system according to any one of claims 31 to 37 wherein the engine comprises a plurality of cylinders each defining a combustion chamber in which exhaust gas is generated and wherein there is a confluence of the exhaust gases, the inference being obtained from the confluence of exhaust gases.

39. The internal combustion engine and exhaust system according to claim 38 wherein the exhaust system is adapted to receive the confluence of the exhaust gases from the cylinders, the gas sensor being disposed in the exhaust system for communication with the confluence of the exhaust gases.

40. The internal combustion engine and exhaust system according to any one of claims 31 to 39 wherein the engine comprises a spark-ignition engine and wherein the engine further comprises an ignition system for generating a spark ignition event step, and the means for adjusting the timing of ignition of the combustible charge comprise means for ~ adjusting the timing of the ignition event.

41. The internal combustion engine and exhaust system according to any one of claims 31 . to 39 wherein the engine comprises a compression- ignition engine and wherein the engine further comprises a liquid fuel delivery system for delivery of a metered quantity of liquid fuel to the combustion chamber to provide a pilot fuel for ignition under compression, and the means for adjusting the timing of ignition of the combustible charge may comprise means for adjusting the timing of the delivery of the pilot fuel.

42. The internal engine and exhaust system according to any one of claims 31 to 41 configured for bi-fuel fuel operation.

43. The internal combustion engine and exhaust system according to claim 41 configured for bi-fuel fuel operation, wherein the engine is configured for fuelling with either liquid fuel alone (hereinafter referred to as the liquid fuel mode) or a combination of liquid fuel and gaseous fuel, with the liquid fuel providing the pilot fuel (hereinafter referred to as the pilot fuel mode).

44. The internal combustion engine and exhaust system according to claim

43 further comprises a control means for selectively controlling operation of the liquid fuel delivery system and the gaseous fuel delivery system to control the quantity of the liquid fuel and the corresponding quantity of gaseous fuel delivered to the combustion chamber during operation in the pilot fuel mode.

45. The internal combustion engine and exhaust system according to claim

44 wherein the control means is responsive to a threshold knock condition in the combustion chamber to effect substitution of some of the gaseous fuel with additional pilot fuel to eliminate engine knock or at least reduce engine knock below the threshold knock condition.

46. The internal combustion engine and exhaust system according to claim

45 wherein the function of the control means for selectively controlling operation of the liquid fuel delivery system and the gaseous fuel delivery system is performed by the ECU.