AU2016204033A1 - Dual Fuel Knock Control Strategy - Google Patents

Abstract

A system (10) operable for knock detection and control in an internal combustion engine, particularly a compression ignition engine configured for operation as a dual fuel engine. The system (10) comprises a knock sensor system (71) for monitoring the engine cylinders for a threshold knock condition and operable to provide an input to a control system (25) indicative of the threshold knock condition. The control system (25) is operable to control an engine operating variable; such as ignition (injection) timing, in response to generation of a threshold number of knock indication signals in a prescribed interval. ------- ---- ----- ----------------------- -------- ----- ! g Z~rj~t----------- - -- ------- ----- - -- -- -- -------- -- ------ ---- <Cj a 00 14) LL1 P.. M fh > -C 11t5

Description

1 2016204033 16 Jun2016

DUAL FUEL KNOCK CONTROL STRATEGY TECHNICAL FIELD

[0001] This invention relates to strategies for knock control in an internal combustion engine.

[0002] The invention has been devised particularly, although not necessarily exclusively, in relation to an internal combustion engine configured for operation as a dual fuel engine. More particularly, the invention concerns a compression ignition engine configured for operation as a dual fuel engine.

[0003] The invention also relates to a system for, and method of, operation of a dual fuel engine.

[0004] The invention also relates to a compression ignition engine configured for operation as a dual fuel engine with provision for knock control.

[0005] The invention also relates to monitoring the performance of a gaseous fuel in a multi-cylinder compression ignition engine configured for dual fuel operation involving the gaseous fuel and also a liquid fuel.

BACKGROUND ART

[0006] 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.

[0007] There is a trend towards use of gaseous fuels in internal combustion engines.

[0008] 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).

[0009] There are a number of potential advantages in using gaseous fuels in engines instead of, or together with, the more commonly used liquid fuels. One 2 2016204033 16 Jun2016 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.

[0010] It is also advantageous that conventional compression ignition engine designs can be readily configured to operate with gaseous fuels.

[0011] When gaseous fuels are used in compression ignition internal combustion engines, there is a need for a mechanism to ignite the gaseous fuel - air mixture in the combustion chamber, as the temperature of the mixture during compression is usually not sufficient to ignite the mixture automatically. It is known to address this problem by injecting a pilot fuel into the combustion chamber which can be ignited by known compression ignition means and thereby initiate combustion of the gaseous fuel - air mixture.

[0012] Accordingly, a compression ignition internal combustion engine can be configured for dual fuel operation, involving 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 for ignition (hereinafter referred to as the pilot fuel mode).

[0013] Typically, the liquid fuel comprises diesel fuel, although of course other appropriate liquid fuels can be used as would be understood by a person skilled in the art.

[0014] When a dual fuel compression ignition internal combustion engine is operating in pilot fuel mode, it is desirable to reduce the proportion of liquid fuel to a minimum amount required for stable ignition.

[0015] When operating in pilot fuel mode, the proportion of liquid fuel, such as diesel fuel, is typically in the range of about 5% to 20%.

[0016] With such a dual fuel engine, engine knock is a condition that needs to be monitored and controlled during operation of the engine, particularly when operating in pilot fuel mode. 3 2016204033 16 Jun2016 [0017] Accordingly, it is an object of an embodiment of the present invention to seek to provide a strategy for controlling engine knock during operation of a dual fuel compression ignition internal combustion engine in pilot fuel mode.

SUMMARY OF INVENTION

[0018] According to a first aspect of the invention there is provided a system for controlling knock in an internal combustion engine, the system comprising: sensor means for determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and a control means for controlling an engine operating variable; the control means being operable in response to generation of a threshold number of knock indication signals in a prescribed interval.

[0019] The sensor means may comprise a knock sensor for generating a knock signal responsive to the intensity of engine knock; that is, the knock signal is proportional to the intensity of knock.

[0020] The sensor means may further comprise a comparator for comparing the knock signal against a threshold value and generating the knock indication signal if the knock signal exceeds the threshold value.

[0021] With this arrangement, the sensor means is operable to generate the knock indication signal only when the knock signal exceeds the threshold value. In other words, the knock indication signal is merely indicative of the presence of a knock condition; it is neither indicative of the intensity of knock nor proportional to the knock signal itself.

[0022] In other words, the knock signal is proportional to the intensity of knock, while the knock indication signal is merely indicative of the presence of a knock condition and is neither indicative of the intensity of knock nor proportional to the knock signal. 4 2016204033 16 Jun2016 [0023] The knock sensor may be of any appropriate type for generating a knock signal responsive to the intensity of engine knock, as would be understood by a person skilled in the art.

[0024] The knock sensor may be operable to generate two signals at different stages of a combustion cycle of the engine. One stage may, for example, be where there is no combustion within the cylinder of the engine being sensed, and the other stage may be during combustion within the cylinder of the engine being sensed. The two signals may be processed, such as for example by subtraction, to provide the knock signal. In other words, the knock signal is acquired through processing of the two signals generated by the knock sensor.

[0025] With this arrangement, the sensor means may further comprise a knock processor for performing band pass filtering of signals from the knock sensor and for processing the respective two signals, the signals being processed by subtraction to provide a resultant knock signal proportional to the intensity of engine knock.

[0026] The knock signal generated at the stage where there is no combustion may be representative of background noise and vibration in the engine; that is, noise and vibration which are not from the specific cylinder being monitored. With this arrangement, the resultant knock signal is filtered to exclude, to at least some extent, background noise and vibration so as to be more reflective of the actual knock occurring within the cylinder.

[0027] In a multi-cylinder engine, the knock sensor may be configured to monitor several cylinders, with each cylinder being sensed separately. In an arrangement involving a six cylinder engine, for example, there may be two knock sensors, each monitoring three cylinders.

[0028] The system may further comprise a counter responsive to each knock indication signal of the respective engine cylinder, the counter being incremented in response to each said signal. The counter is configured to count the number of knock indication signals generated within said prescribed interval which will hereinafter be referred to as the prescribed count interval.

[0029] The prescribed count interval may correspond to a prescribed time interval or a prescribed number of events. The events may, for example, comprise a prescribed number of engine operating events such as, for example, engine cycles. In 5 2016204033 16 Jun2016 other words, the prescribed interval may comprise a prescribed number of engine cycles.

[0030] The prescribed count interval may be assessed within a sliding window.

[0031] The criterion for operation of the control means may be the occurrence of a threshold number of knock indication signals within the sliding window reflecting the count interval.

[0032] With this arrangement, upon the occurrence of the threshold number of knock indication signals within the sliding window, the control means is operated to effect control over the engine operating variable in response to the knock condition.

[0033] The engine operating variable may take any appropriate form, according to the type of internal combustion engine involved, as would be well understood by a person skilled in the art.

[0034] In a spark ignition engine, the engine operating variable may, for example, comprise ignition timing.

[0035] In a compression ignition engine configured for pilot fuel mode involving a gaseous fuel and a liquid fuel as the pilot fuel, the engine operating variable may, for example, comprise a variation in the relative proportions of the gaseous fuel and pilot fuel.

[0036] In a compression ignition engine which is configured for dual fuel operation involving a gaseous fuel and a liquid fuel and which is operable in liquid fuel mode or pilot fuel mode, the engine operating variable may, for example, comprise substitution of at least some of the liquid fuel with gaseous fuel to provide a substitution mixture for combustion.

[0037] The substitution of at least some of the liquid fuel with gaseous fuel to provide a substitution mixture will, for ease of reference, be hereinafter referred to as gas substitution.

[0038] The engine operating variable may, for example, alternatively or additionally comprise ignition (injection) timing for the compression ignition engine. 6 2016204033 16 Jun2016 [0039] The control means may be operable to control several engine operating variables. The various engine operating variables may be controlled simultaneously, sequentially, in overlapping relationship, or in some other relationship. Further, the control of one engine operating variable may be independent of one or more of the other engine operating variables, or the engine operating variables may be interrelated in some way.

[0040] In circumstances where the engine operating variable comprises both ignition (injection) timing and gas substitution, the variation may first comprise ignition timing; typically, retardation of ignition timing. Whether or not the variation later extends to gas substitution may be conditional upon the outcome of the initial variation to ignition timing.

[0041] Where the control means is operable to control several engine operating variables, the respective threshold number of knock indication signals in the prescribed interval at which the control means effects control of the respective engine operating variables may be different.

[0042] In circumstances where the engine operating variable comprises both ignition timing and gas substitution, the threshold number of knock indication signals in the prescribed interval may, for example, comprise a first (lower) level at which control of ignition timing is to be implemented. There may then be a second level of the number of knock indication signals in the prescribed interval at which gas substitution is to be implemented, the second level being higher than the first level.

[0043] By way of example, if the number of combustion events (as reflected by the number of knock indication signals) considered to be knocking is deemed to be a threshold of say 5 in an sliding window interval of say 20 engine cycles, attainment of the threshold of 5 represents the first level at which ignition timing is retarded. If the engine continues to knock such that the second threshold of say 10 knock indication signals within the 20 cycle window is attained, then gas substitution is implemented.

[0044] Accordingly, there are two potential reactions in response to the generation of a threshold number of knock indication signals in a prescribed interval, one being retardation of ignition timing and the other being gas substitution.

[0045] In an embodiment, the two reactions are mutually exclusive and act independently of each other. 7 2016204033 16 Jun2016 [0046] The reaction involving retardation of ignition timing may be applied as a lump sum offset from a base (standard) ignition timing condition to establish a retarded ignition timing condition.

[0047] If upon implementation of this reaction, the engine is subsequently deemed to no longer be in knock (as a consequence of the number of knock indication signals falling below the threshold), the control means may be operable to advance the ignition timing from the retarded ignition timing condition towards the base ignition timing condition. This may be done over a period of time, by ramping the ignition timing from the retarded condition towards the base condition. Typically, the ignition timing would be returned to the base ignition timing condition. If engine knock is detected again, the cycle is repeated.

[0048] The reaction involving gas substitution may involve an initial reduction in the amount of gaseous fuel substituted for the liquid fuel and a corresponding increase in the amount of liquid fuel in the substitution mixture.

[0049] If the initial reduction in gas substitution is not effective in resolving the knock condition, there may be one or more further reductions in gas substitution. The initial reduction in gas substitution, and any subsequent further reductions in gas substitution, are each preferably imposed for a minimum period of time before allowing the next reduction in gas substitution in order to allow the engine behaviour to stabilise to the new substitution level.

[0050] If engine knock persists after the initial reduction in gas substitution, and any further reductions in gas substitution which might be implemented, the gas substitution may be terminated such that the gaseous fuel content of the substitution mixture for the engine is zero; that is, the engine is effectively fuelled not by a substitution mixture but rather only by liquid fuel.

[0051] After a certain time of operation on liquid fuel only, within which the engine behaviour can stabilise, the gas substitution may be implemented again; for example, the gas substitution may be allowed to return to a normal level. Monitoring for knock conditions within the cylinder continues and the cycle may be repeated as necessary.

[0052] The number of times that gas substitution can be terminated while the engine is in operation at any one time may be limited so as to avoid the presence of persistent engine knock conditions, potentially leading to engine damage. 8 2016204033 16 Jun2016 [0053] The system may have provision for adaptation of engine ignition retardation over time. Each time ignition timing is retarded for a particular cylinder, a long term adaption may be gradually implemented, preserving a long term retard offset for that cylinder and thereby enhancing performance of the cylinder.

[0054] The system may have provision for identifying performance issues of the engine in response to the presence of knock conditions in a plurality of engine cylinders. If, for example, a certain number of cylinders experience simultaneous knock conditions which lead to reductions in gas substitution levels with respect to the cylinders, this may be interpreted as an indication of an engine problem or an issue with the quality of the gaseous fuel.

[0055] If all cylinders experience simultaneous knock conditions, this may also be interpreted as an indication of an engine problem or an issue with the quality of the gaseous fuel.

[0056] Accordingly, the system may be implemented to monitor the performance of the gaseous fuel. More particularly, the system may be configured to detect, and where appropriate compensate for, degradation in the quality of the gaseous fuel. Any consequential action arising from detection of degradation in the quality of the gaseous fuel may be implemented globally; that is, on all cylinders. In the first instance, the consequential action may comprise a global change to the timing of ignition (injection) for all cylinders. If the issue persists, the consequential action may be extended to a global reduction in gas substitution; that is, a common reduction in gas substitution for all cylinders.

[0057] According to a second aspect of the invention there is provided a system for controlling knock in a compression ignition engine configured for dual fuel operation involving a gaseous fuel and a liquid fuel, in liquid fuel mode or pilot fuel mode, the system comprising: sensor means for determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and a control means for controlling ignition timing in pilot mode operation; 9 2016204033 16 Jun2016 the control means being operable in response to generation of a threshold number of knock indication signals in a prescribed interval.

[0058] According to a third aspect of the invention there is provided a system for controlling knock in a compression ignition engine configured for dual fuel operation involving a gaseous fuel and a liquid fuel, in liquid fuel mode or pilot fuel mode, the system comprising: sensor means for determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and a control means for controlling gas substitution in pilot mode operation; the control means being operable in response to generation of a threshold number of knock indication signals in a prescribed interval.

[0059] According to a fourth aspect of the invention there is provided a system for controlling knock in a compression ignition engine configured for dual fuel operation involving a gaseous fuel and a liquid fuel, in liquid fuel mode or pilot fuel mode, the system comprising: sensor means for determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and a control means for ignition timing and gas substitution in pilot mode operation; the control means being operable in response to generation of a threshold number of knock indication signals in a prescribed interval, the threshold number representing a first level at which control of ignition timing is to be implemented; and wherein there is a second level of the number of knock indication signals in the prescribed interval at which gas substitution is to be implemented, the second level being higher than the first level. 10 2016204033 16 Jun2016 [0060] The control of ignition timing may represent a first reaction to the knocking condition and the gas substitution may represent a second reaction upon persistence of the knocking condition.

[0061] Preferably, the two reactions are mutually exclusive and act independently of each other.

[0062] The systems according to the second, third and fourth aspects of the invention may, as appropriate, have any one or more of various features described above in relation to the first aspect of the invention.

[0063] According to a fifth aspect of the invention there is provided an internal combustion engine having a system for controlling knock according to any one of the preceding aspects of the invention.

[0064] According to a sixth aspect of the invention there is provided a method of knock control in an internal combustion engine, the method comprising using a system for controlling knock according to the first, second or third aspect of the invention.

[0065] According to a seventh aspect of the invention there is provided a method of knock control in an internal combustion engine, the method comprising: determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and controlling an engine operating variable in response to generation of a threshold number of knock indication signals in a prescribed interval.

[0066] According to an eighth aspect of the invention there is provided a method of operating an internal combustion engine, the method comprising: determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and controlling an engine operating variable in response to generation of a threshold number of knock indication signals in a prescribed interval. 11 2016204033 16 Jun2016 [0067] In the case of a compression ignition engine configured for dual fuel operation involving a gaseous fuel and a liquid fuel, in liquid fuel mode or pilot fuel mode, there may be two engine operating variables comprising ignition timing and gas substitution in pilot mode operation. In such a case, the method according to the seventh or eighth aspect of the invention may further comprise controlling ignition timing in response to the generation of a threshold number of knock indication signals in a prescribed interval, the threshold number representing a first level at which control of ignition timing is to be implemented, and controlling gas substitution in response to a second level of the number of knock indication signals in the prescribed interval, the second level being higher than the first level.

[0068] The method according to the sixth, seventh and eight aspects of the invention may, as appropriate, be implemented using any one or more of various features described above in relation to the first aspect of the invention.

[0069] According to a ninth aspect of the invention there is provided a system for monitoring the performance of a gaseous fuel in a multi-cylinder compression ignition engine configured for dual fuel operation involving the gaseous fuel and also a liquid fuel, in liquid fuel mode or pilot fuel mode, the system comprising: sensor means for determining the presence of a prescribed knock condition in each cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; a control means for controlling ignition timing in pilot mode operation; the control means being operable in response to generation of a threshold number of knock indication signals in a prescribed interval; wherein upon all cylinders generating respective threshold numbers of knock indication signals in prescribed intervals, the control means is operable to effect consequential action to compensate for the performance of the gaseous fuel, the consequential action comprising control of ignition timing of all cylinders in pilot mode operation. 12 2016204033 16 Jun2016 [0070] With this arrangement, the system is operable to infer that the knock condition in all cylinders is a consequence of the performance of the gaseous fuel and attribute this to degradation in the quality of the gaseous fuel.

[0071] Accordingly, the system is operable to detect, and where appropriate compensate for, degradation in the quality of the gaseous fuel.

[0072] In circumstances where the control means is operable to also control gas substitution in pilot mode operation, the consequential action may further comprise a reduction in gas substitution for all cylinders.

[0073] According to a tenth aspect of the invention there is provided a method of monitoring the performance of a gaseous fuel in a multi-cylinder compression ignition engine configured for dual fuel operation involving the gaseous fuel and also a liquid fuel, in liquid fuel mode or pilot fuel mode, the method comprising: determining the presence of a prescribed knock condition in each cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; assessing the prescribed knock condition with respect to a baseline knock condition; and implementing a compensation strategy in response to a variance in the prescribed knock condition with respect to the baseline knock condition in all cylinders, the compensation strategy comprising controlling an engine operating variable with respect to all cylinders.

[0074] The extent of variance of the knock condition with respect to the baseline knock condition in all cylinders may be prescribed, such as by way of an input entered into a software program or instruction forming part of the system.

[0075] The control of the engine operating variable may comprise controlling ignition timing of all cylinders during pilot mode operation.

[0076] The control of the engine operating variable may further comprise a reduction in gas substitution for all cylinders. 13 2016204033 16 Jun2016 [0077] The system and method may further comprise a determination as to whether one or more other prescribed operating parameters of the engine are not the likely cause of the variance of the prescribed knock condition with respect to the baseline knock condition. This determination may be made prior to implementation of the compensation strategy. The other operating parameters of the engine prescribed for investigation as part of the determination may, for example, include inlet air temperature, diagnostics from the diesel side of the engine (if available), and engine age. Various other parameters may also be prescribed, as would be understood by a person skilled in the art.

[0078] In this way, the system first excludes other common potential sources of the adverse change in the baseline knock condition before inferring that the source of increased knock is the gaseous fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

Figure 1 is a schematic view of a dual fuel compression ignition internal combustion engine featuring an embodiment of a knock control system according to the invention;

Figure 2 is a schematic flow chart illustrating steps performed in detecting and determining the presence of a knock condition;

Figure 3 is a schematic view illustrating a sampling operation performed in detecting the presence of a knock condition in the respective engine cylinders;

Figure 4 is a schematic flow chart illustrating steps performed in a first strategy involving retardation of ignition (injection) timing;

Figure 5 is a schematic flow chart illustrating steps performed in a second strategy involving a reduction in gas substitution; and 14 2016204033 16 Jun2016

Figure 6 is a schematic flow chart illustrating steps performed in a long term ignition (injection) timing adaptation strategy.

[0080] The drawings shown are not necessarily to scale, with emphasis instead generally being placed upon illustrating the principles of the present invention.

[0081] The figures depict an embodiment of the invention. The embodiment illustrates certain configurations; however, it is to be appreciated that the invention can take the form of many configurations, as would be obvious to a person skilled in the art, whilst still embodying the present invention. These configurations are to be considered within the scope of this invention.

DESCRIPTION OF EMBODIMENTS

[0082] The embodiment is directed to an internal combustion engine system 10 configured for dual fuel operation.

[0083] The engine system 10 comprises an internal combustion engine 11 and an associated fuel delivery system 13 for delivering fuel to the engine.

[0084] The engine 11 comprises a compression ignition engine configured for dual 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.

[0085] In the arrangement illustrated, the engine 11 comprises a multi-cylinder engine comprising an engine structure 15 having an engine block 16 incorporating a plurality of cylinders 18. A combustion chamber is defined within each cylinder 18.

[0086] 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.

[0087] 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 15 2016204033 16 Jun2016 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 chambers during operation of the engine in the pilot fuel mode.

[0088] 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 Figure 1. The exhaust system is of known kind.

[0089] 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.

[0090] 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 fuel 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).

[0091] 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.

[0092] 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.

[0093] 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. 16 2016204033 16 Jun2016 [0094] 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.

[0095] 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.

[0096] The gaseous fuel delivery system 23 further comprises a gas temperature sensor 61 and a gas pressure sensor 62.

[0097] The engine system 10 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. Operation of the knock sensor system 71 will be described below with reference to Figure 2 which is a schematic flow chart illustrating steps performed in detecting and determining the presence of a knock condition.

[0098] In this embodiment, the knock sensor system 71 comprises one or more knock sensors 73 installed on the engine structure 15 to measure noise and vibrations transmitted through the engine structure arising from a knock condition. The number of knock sensors 73 is determined by the size and configuration of the engine block 16. In the arrangement shown in which the multi-cylinder engine 11 has six cylinders 18, there are two knock sensors 73 positioned respectively approximately one-third and two-thirds along the horizontal distance of the engine block 16 and at a vertical distance that aligns closely with the combustion chambers defined within the engine cylinders 18. The front knock sensor 73a senses noise and vibration on cylinders 1,2 and 3, and the rear knock sensor 73b senses noise and vibration on cylinders 4, 5 and 6. 17 2016204033 16 Jun2016 [0099] The knock sensors 73 are sampled in the crankshaft domain to distinguish between the various cylinders 18 in accordance with known practice. This sampling operation is of known type and is depicted schematically in Figure 3.

[00100] Each knock sensor 73 is operable to generate a knock signal responsive to the intensity of engine knock; that is, the knock signal is proportional to the intensity of knock.

[00101] More particularly, each knock sensor 73 is operable to generate two signals at different stages of a combustion cycle of the engine cylinder. One stage is where there is no combustion within the cylinder of the engine being sensed, and the other stage is during combustion within the cylinder of the engine being sensed, as depicted by blocks 101 and 102 in Figure 2.

[00102] The two signals generated by each knock sensor 73 during the combustion cycle of a respective cylinder are transmitted to a knock processor (not shown) which, in this embodiment, is integrated in the electronic control unit 27 (ECU). The knock processor is operable to perform band pass filtering of the signals from the knock sensor 73 (as depicted by block 103 in Figure 2) and processing of the respective two signals, the signals being processed by subtraction to provide a resultant knock signal proportional to the intensity of engine knock (as depicted by block 105 in Figure 2).

[00103] The signal generated by the knock sensor 73 at the stage where there is no combustion may be representative of background noise and vibration in the engine; that is, noise and vibration not arising from the specific cylinder being monitored. With this arrangement, the resultant knock signal is filtered to exclude, to at least some extent, background noise and vibration so as to be more reflective of the actual knock occurring within the cylinder.

[00104] The knock processor (not shown) and also the knock sensors form part of a sensor means (not shown). The sensor means further comprises a comparator (not shown) for comparing the resultant knock signal against a threshold value and generating the knock indication signal if the knock signal exceeds the threshold value (as depicted by block 107 in Figure 2). The knock indication signal is indicative that the cylinder is deemed to be in a knock condition at that particular time. 18 2016204033 16 Jun2016 [00105] The sensor means through the knock processor determines the intensity of the resultant knock signal and provides a signal to a main processor (not shown) in proportion to the intensity of knock. In this embodiment, the main processor is integrated in the electronic control unit 27 (ECU).

[00106] With this arrangement, the sensor means is operable to generate the knock indication signal only when the knock signal exceeds the threshold value (as depicted by block 107 in Figure 2). In other words, the knock indication signal is merely indicative of the presence of a knock condition; it is neither indicative of the intensity of knock nor proportional to the knock signal itself.

[00107] The sensor means has a counter (not shown) responsive to each knock indication signal of the respective engine cylinder, the counter being incremented in response to each said signal (as depicted by block 109 in Figure 2). The counter is configured to count the number of knock indication signals generated for the respective engine cylinder within a prescribed count interval. In this embodiment, the prescribed count interval is represented by a prescribed number of engine cycles.

[00108] The counter is set to zero each time the electronic control unit 27 (ECU) is energised, corresponding to the engine 11 being started.

[00109] The prescribed count interval is assessed within a sliding window.

[00110] If a prescribed number of "counts" are recorded within a calibrated number of engine cycles within the sliding window, the respective cylinder is determined to be knocking. The prescribed number of "counts" represents a threshold within the sliding window at which remedial action is initiated in response to the knock condition. The number of counts corresponds to the number of combustion events (as reflected by the number of knock indication signals) considered to be knocking.

[00111] There are two levels of "counts" within the sliding window at which the engine control module 28 reacts to the knock condition. The two levels of "counts" comprise a first (lower) level and a second (higher) level. The first level corresponds to the prescribed number of "counts" which represent the threshold within the sliding window at which remedial action is initiated.

[00112] In this embodiment, the engine control module 28 has two strategies available for knock control when the engine 11 is operating in pilot fuel mode. 19 2016204033 16 Jun2016 [00113] The first strategy is initiated in response to the first level of "counts" and involves retarding ignition (injection) timing, as it has the effect of lowering the cylinder pressure and therefore the tendency of that cylinder to detonate. This is the fastest way to respond to the detection of a knocking cylinder. This strategy is implemented in the pilot fuel mode by controlling the timing of ignition (injection) of the pilot fuel. This involves appropriate control of the fuel injectors 43. This first strategy is illustrated in Figure 4, which is a schematic flow chart illustrating the steps performed in the strategy.

[00114] The second strategy is initiated in response to the second level of "counts" and involves a reduction in the gas substitution level. This involves appropriate control of the fuel injectors 43 and the gaseous fuel injector bank 59. This second strategy is illustrated in Figure 5, which is a schematic flow chart illustrating the steps performed in the strategy.

[00115] By way of example, if the threshold number of 'counts" is prescribed as 5 (reflecting the number of combustion events considered as knocking) in a sliding window interval of 20 engine cycles, attainment of the threshold 5 "counts" represents the first level at which ignition timing is retarded. If the engine continues to knock such that the second threshold of say 10 "counts" within the 20 engine cycle window is attained, then gas substitution is implemented.

[00116] Accordingly, there are two potential reactions in response to generation of a threshold number of knock indication signals in a prescribed interval, one being retardation of ignition timing and the other being gas substitution.

[00117] In this embodiment, the two reactions are mutually exclusive and act independently of each other.

[00118] The reaction involving retardation of ignition (injection) timing may be applied as a lump sum offset from a base (standard) ignition timing condition to establish a retarded ignition (injection) timing condition, as depicted by output 121a of block 121 in Figure 4. The lump sum offset is typically a fixed angle offset from the standard diesel injection timing lookup map. The angular offset is typically applied for a fixed time period.

[00119] If upon implementation of this reaction, the engine is subsequently deemed to no longer be in knock (as a consequence of the number of "counts" falling 20 2016204033 16 Jun2016 below the threshold within the sliding window), as depicted by output 121b of block 121 in Figure 4, the engine control module 28 may be operated to advance the ignition (injection) timing from the retarded ignition (injection) timing condition towards the base ignition (injection) timing condition. This may be done over a period of time, by ramping the ignition (injection) timing from the retarded condition towards the base condition, as depicted by block 123 in Figure 4. Typically, the ignition (injection) timing would be returned to the base ignition (injection) timing condition. If engine knock is detected again, the cycle is repeated.

[00120] The reaction involving gas substitution may involve an initial reduction in the amount of gaseous fuel substituted for the liquid fuel, as depicted by block 131 in Figure 5, and a corresponding increase in the amount of liquid fuel in the substitution mixture.

[00121] If the initial reduction in gas substitution is not effective in resolving the knock condition, there may be one or more further reductions in gas substitution (as depicted by block 133 in Figure 5). Flowever, the initial reduction in gas substitution, and any subsequent further reductions in gas substitution, are each preferably imposed for a minimum period of time before allowing the next reduction in gas substitution to be effected in order to allow the engine behaviour to stabilise to the new substitution level (as depicted by block 135 in Figure 5).

[00122] If engine knock persists after the initial reduction in gas substitution and any further reductions in gas substitution which might be implemented, the gas substitution may be terminated such that the gaseous fuel content of the substitution mixture for the engine is zero (as depicted by block 137 in Figure 5); that is, the engine is fuelled not by a substitution mixture but rather only by liquid fuel.

[00123] After a certain time of operation on liquid fuel only, within which the engine behaviour can stabilise, gas substitution may be implemented again; for example, the gas substitution may be allowed to return to a normal level. Monitoring for knock conditions within the cylinder continues and the cycle is repeated as necessary.

[00124] While the first strategy can be implemented globally to apply to all cylinders 18 when any knock condition above the threshold is detected, it is preferred that it be implemented to control each cylinder individually as a better outcome may be achieved in terms of drivability of a vehicle powered by the engine 11. 21 2016204033 16 Jun2016 [00125] In operation of the engine 11, the first strategy is implemented initially if an engine knock condition is detected as reflected by the first "count" level.

[00126] If implementation of the first strategy does not eliminate engine knock or at least reduce engine knock below the threshold knock condition, the second "count" level is reached and the second strategy is implemented.

[00127] If implementation of the second strategy does not eliminate engine knock or at least reduce engine knock below the threshold knock condition, the engine 11 is switched to a liquid fuel mode, as mentioned above.

[00128] When the engine 11 is operating in pilot fuel mode, it is desirable to reduce the proportion of liquid fuel to a minimum amount for stable ignition. This has economical and environmental benefits.

[00129] Where the liquid fuel comprises diesel fuel, the proportion of diesel may typically be in the range of about 5% to 20%, but it does vary according to engine speed and load. More particularly, typical proportions might range from (i) 6% diesel fuel and 94% gaseous fuel to (ii) 22% diesel fuel and 78% gaseous fuel, over a range of engine speeds and load.

[00130] Implementation of the second strategy involves increasing the proportion of liquid fuel and correspondingly decreasing the proportion of gaseous fuel. In other words, some liquid fuel is substituted for some gaseous fuel. Typically, the proportion of liquid fuel can be increased to about 40% while still achieving benefits available through dual fuel operation.

[00131] Substitution leading to about 50% or more diesel fuel may present difficulties in distinguishing between diesel fuel knock and gaseous fuel knock.

[00132] While the substitution of liquid fuel for gaseous fuel may be done progressively until the detected engine knock condition is eliminated, or at least until it is below the threshold knock condition, the second strategy may involve substitution to a predetermined proportion of diesel fuel. By way of example, the predetermined proportion may comprise 40% diesel fuel and 60% gaseous fuel. In other words the substitution may result in a fuelling arrangement involving from 40% diesel fuel and 60% gaseous fuel when any adverse knock condition at or above the threshold is 22 2016204033 16 Jun2016 detected. The proportions would not vary according to the extent of the adverse knock condition.

[00133] The first strategy involving retarding ignition timing may also be implemented if an adverse engine knock condition is detected when the engine is operating in liquid fuel mode. This strategy is implemented in liquid fuel mode by controlling the timing of ignition (injection) of the liquid fuel.

[00134] The number of times that gas substitution can be terminated while the engine is in operation at any one time may be limited so as to avoid the presence of persistent engine knock conditions leading to potential engine damage.

[00135] The system may have provision for adaptation of engine ignition (injection) retardation over time. This is illustrated in Figure 6, which is a schematic flow chart illustrating the steps performed in the adaptation strategy. Each time ignition (injection) timing is retarded for a particular cylinder, a long term adaption may be gradually implemented, preserving a long term retard offset for that cylinder and thereby enhancing performance of the cylinder. If there has been a previous timing adaptation it is stored in a non-volatile memory within the electronic control unit 27 (ECU), as depicted by block 141 in Figure 6. Upon the ECU 27 being energised at engine start-up, the previous timing adaptation is recalled from memory and adopted, as depicted by block 143 in Figure 6.

[00136] The adaptation of engine ignition (injection) retardation is used to offset the diesel ignition (injection) timing from the nominal start of the ignition (injection) value, which is normally calibrated for the best quality fuel and moderate ambient conditions, to a value more appropriate to lower quality fuel and/or hotter ambient conditions. This may be accomplished through a diesel ignition (injection) timing offset map, the contents of which are weighted by the value of an adaptive knock control factor.

[00137] The system may have provision for identifying performance issues of the engine in response to the presence of knock conditions in a plurality of engine cylinders. If, for example, a certain number of cylinders experience simultaneous knock conditions which lead to reductions in gas substitution levels with respect to the cylinders, this may be interpreted as an indication of an engine problem or an issue with the quality of the gaseous fuel. 23 2016204033 16 Jun2016 [00138] The system can optionally also be used to monitor the performance of the gaseous fuel, particularly to detect/compensate for degradation in gaseous fuel quality. Such an implementation of the system may be based on the premise that a knock count would typically be expected to be higher the poorer the quality of the gaseous fuel. In other words, a knock count would typically be inversely proportional to the quality of the gaseous fuel. As an example, a lower quality gaseous fuel would be expected to have a higher knock count than that of better quality gaseous fuel.

[00139] By way of background, LNG usually contains some impurities, these being larger molecule (higher fraction) hydrocarbons such as ethane, propane, and the like. Whilst these impurity components have a higher calorific value, they also boil off at a different temperature than the bulk gas energy source, which is primarily methane-based. While LNG is in storage within a cryogenic container, any pressure that builds as the container heats up results in the lighter fractions in the gas (typically methane with the lowest boiling point) being vented. This results in the concentration of the impurities within the gas remaining within the container effectively increasing. This is a known process commonly referred to as “weathering”.

[00140] Methane has a greater resistance to knock when combusted than do the higher fractions such as ethane, propane and the like. Accordingly, as LNG weathers, it also becomes more susceptible to knocking when combusted under the same given conditions, owing to the higher concentration of impurities. This represents degradation in the quality of the LNG as a combustible fuel.

[00141] In the present system, there are two levels of "counts" comprising the first (lower) level and the second (higher) level. The first level corresponds to the prescribed number of "counts" which represent the threshold within the sliding window at which remedial action is initiated, as previously described.

[00142] The system may establish a baseline knock condition, typically representative of a good quality fuel. In one arrangement, the system may store data related to occurrences where the first level threshold is reached and develop a historical average which represents the baseline knock condition. A variation from the baseline knock condition to the extent of representing an adverse change may be indicative of a deterioration of fuel quality, although not necessarily so. 24 2016204033 16 Jun2016 [00143] The extent of variance of the knock condition with respect to the baseline knock condition in all cylinders to represent an adverse change may be prescribed, such as by way of an input entered into a software program or instruction forming part of the system.

[00144] Because a change from the baseline knock condition can be detected, the system can also be configured to detect, and where appropriate compensate for, degradation in the quality of the gaseous fuel.

[00145] More particularly, the system may determine the presence of a prescribed knock condition in each cylinder of the engine and generate a knock indication signal indicative of the prescribed knock condition. The system may then assess occurrences of the prescribed knock condition with respect to a baseline knock condition. In response a prescribed variance of the prescribed knock condition with respect to the baseline knock condition in all cylinders, the system may implement a compensation strategy involving consequential action. Any consequential action arising from detection of degradation in the quality of the gaseous fuel would be implemented globally; that is, on all cylinders. In the first instance, the consequential action may comprise a global change to the timing of ignition (injection) for all cylinders. If the issue persists, the consequential action may be extended to a global reduction in gas substitution; that is, a common reduction in gas substitution for all cylinders.

[00146] If there is a prescribed change in the baseline knock condition, and all other conditions are determined to be equal, the system can infer that the source of increased knock is the gaseous fuel (i.e. the weathered LNG).

[00147] Before implementing the compensation strategy, the system may first determine whether one or more other prescribed operating parameters of the engine are not likely to be the cause of the variance of the prescribed knock condition with respect to the baseline knock condition. These other operating parameters are hereinafter referred to as baseline operating parameters. The baseline operating parameters may, for example, include inlet air temperature, diagnostics from the diesel side of the engine (if available), and engine age. With respect to engine age (as reflected in terms of hours of operation or distance travelled), there may be different triggering thresholds at which an abnormal event is deemed to exist in order to accommodate for factors such as engine wear. 25 2016204033 16 Jun2016 [00148] In other words, the system first excludes other common potential sources of the adverse change in the baseline knock condition before inferring that the source of increased knock is the gaseous fuel.

[00149] The system also determines whether all cylinders are in issue. If all cylinders are not in issue, then the source of increased knock would not normally be attributed to the gaseous fuel. In other words, if one or some, but not all, of the cylinders are experiencing the knock condition, then the quality of the gaseous fuel is not normally the issue.

[00150] Further, the degree of increased knock may be correlated to the degree of weathering of the gaseous fuel.

[00151] The system can also be configured to have regard to, and be responsive to, one or more additional factors (such as for example when refuelling occurred, tank pressure, etc) which may enhance the effectiveness of the regimen for detection of, and compensation for, gas quality degradation. These additional factors may be reflected in inputs to the system, with decisions made as appropriate and any consequential action implemented.

[00152] As mentioned above, the system may store data related to occurrences where the first level threshold is reached and develop a historical average which represents the baseline knock condition. Any change from the baseline knock condition can be flagged by the system for subsequent investigation, if desired; for example, as a maintenance issue.

[00153] In the foregoing description, the terms “ignition timing”, “injection timing” and “ignition (injection) timing” are used. In many respects, such terms are interchangeable, as would be understood by a person skilled in the art. Specifically, the term “ignition (injection) timing” is used with reference to compression ignition engines (whether pilot fuel operated or dual fuel operated). With such engines, an ECU actually controls the “injection timing” which in turn directly affects the “ignition timing” (even though it does not strictly control the timing of ignition due to the compression ignition mode of operation), as would be understood by a person skilled in the art. 26 2016204033 16 Jun2016 [00154] From the foregoing it is evident that the present embodiment provides a simple yet highly effective system for knock detection and control in an engine operating in a dual fuel mode.

[00155] Notably, the method whereby the number of knock indicator signals within the sliding window are counted, rather than one which just reacts to the amplitude of the knock signal itself, prevents reactions in a case where only a single combustion event may be exhibiting knock. Furthermore, in one embodiment the method enables engine torque to be maximised when knock is detected by only retarding the cylinders(s) that are exhibiting knock.

[00156] It is also worth noting that knock on a dual fuel, compression ignition engine is different in the way that it occurs compared to knock experienced on a mono fuel, spark ignition engine. Generally speaking, more is known about knock on mono fuel, spark ignition engines, partly because dual fuel, compression ignition engines are not as common in terms of their application to the field of automotive or truck engines.

[00157] More particularly, knock on a mono fuel, spark ignition engine tends to be more predictable in terms of its occurrence and characteristics. For example, an engine calibrator can edge an engine up towards its knock threshold and maintain its operation at or around this point quite comfortably. The engine knock will increase in intensity as the calibrator advances the ignition timing or makes corresponding modifications to the engine air/fuel ratio.

[00158] In contrast, when knock occurs on a dual fuel, compression ignition engine (especially on a manifold injected natural gas engine), the amplitude of the knock can go from nothing to very extreme, and back to nothing, over a small number of subsequent engine cycles (e.g. such as three engine cycles). One potential cause for this high degree of difference in knock amplitude is pulsating airflow within the intake manifold that results in a rich air/gas mixture entering a cylinder on a particular engine cycle; that is, a dual fuel, compression ignition engine can be very sensitive to the air/fuel ratio, where the engine goes from best power operation to knock very quickly. Other causes which affect the knock amplitude or intensity relate to exhaust gas flow and diesel auto-ignition points within the cylinder.

[00159] As alluded to above, the benefit of using the knock count method of the present invention over a more common knock strategy is that the strategy of the 27 2016204033 16 Jun2016 present invention is able to distinguish any ‘outlier occurrences’ of knock from the general trend of knock on a particular cylinder and hence not overreact to detection of knock.

[00160] Typically, reaction to knock results in a lowering of the engine output either through reducing the fuelling to an engine cylinder and/or changing the ignition (injection) timing for the cylinder. A reduction in engine output, if not strictly necessitated, is highly undesirable as the conditions for knock to occur are typically when the engine is highly loaded, so a loss in torque is not what an operator desires during such engine operation. This fact alone highlights the benefit of providing a knock control strategy which only reacts to knock that is likely to lead to engine damage (i.e. sustained high level knock), and the present invention with its ability to deduce multiple counts of knock above a set threshold achieves this.

[00161] It should, however, be understood that the invention need not be limited to knock detection and control in an engine operating in a dual fuel mode. The invention may have application to compression ignition engines fuelled primarily with gaseous fuel. Such an engine would utilise a pilot fuel for ignition, but would not have a liquid fuel mode as such. The system may also have application to spark-ignition engines in which the engine operating variable may comprise ignition timing.

[00162] While the present invention has been described in terms of a preferred embodiment in order to facilitate better understanding of the invention, it should be appreciated that various modifications can be made without departing from the principles of the invention. Therefore, the invention should be understood to include all such modifications within its scope.

[00163] Much of the inventive functionality and many of the inventive principles are best implemented with or in software programs or instructions and integrated circuits (ICs) such as application specific ICs. In the interest of brevity and minimisation of any risk of obscuring the principles and concepts according to the present invention, discussion of such software and ICs, if any, is limited to the essentials with respect to the principles and concepts within the preferred embodiment.

[00164] This disclosure is intended to explain how to fashion and use various embodiments in accordance with the technology rather than to limit the true, intended, and fair scope and spirit thereof. The foregoing description is not intended to be 2016204033 16 Jun2016 28 exhaustive or to be limited to the precise forms disclosed. Modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principle of the described technology and its practical application, and to enable one of ordinary skill in the art to utilise the technology in various embodiments and with various modifications as are suited to the particular use contemplated.

[00165] Reference to any positional descriptions, such as "upper", "lower", "top" and "bottom", are to be taken in context of the embodiment depicted in the drawings, and are not to be taken as limiting the invention to the literal interpretation of the term but rather as would be understood by the skilled addressee.

[00166] Additionally, where the terms “system”, “device”, and “apparatus" are used in the context of the invention, they are to be understood as including reference to any group of functionally related or interacting, interrelated, interdependent or associated components or elements that may be located in proximity to, separate from, integrated with, or discrete from, each other.

[00167] Throughout this specification, 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 (28)

1. A system for controlling knock in an internal combustion engine, the system comprising: sensor means for determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and a control means for controlling an engine operating variable; the control means being operable in response to generation of a threshold number of knock indication signals in a prescribed interval.

2. The system according to claim 1 wherein the sensor means comprises a knock sensor for generating a knock signal responsive to the intensity of engine knock.

3. The system according to claim 1 or 2 wherein the sensor means further comprise a comparator for comparing the knock signal against a threshold value and generating the knock indication signal if the knock signal exceeds the threshold value.

4. The system according to claim 1,2 or 3 wherein the knock sensor is operable to generate two signals at different stages of a combustion cycle of the engine.

5. The system according to claim 4 wherein the sensor means further comprise a knock processor for performing band pass filtering of signals from the knock sensor and for processing the respective two signals, the signals being processed by subtraction to provide a resultant knock signal proportional to the intensity of engine knock.

6. The system according to any one of the preceding claims further comprising a counter responsive to each knock indication signal of the respective engine cylinder, the counter being incremented in response to each said signal.

7. The system according to claim 6 wherein the counter is configured to count the number of knock indication signals generated within a prescribed count interval, said prescribed count interval constituting said prescribed interval.

8. The system according to claim 7 wherein said prescribed count interval corresponds to a prescribed time interval or a prescribed number of events.

9. The system according to claim 7 or 8 wherein said prescribed count interval is assessed within a sliding window.

10. The system according to claim 9 wherein the control means is operable upon occurrence of a threshold number of knock indication signals within the sliding window reflecting the count interval.

11. The system according to any one of the preceding claims wherein the engine operating variable comprises ignition timing in a spark ignition engine.

12. The system according to any one of claims 1 to 10 wherein the engine operating variable comprises a variation in the relative proportions of the gaseous fuel and pilot fuel in a compression ignition engine configured for pilot fuel mode involving a gaseous fuel and a liquid fuel as the pilot fuel.

13. The system according to any one of claims 1 to 10 wherein the engine operating variable comprises substitution of at least some of the liquid fuel with gaseous fuel to provide a substitution mixture for combustion in a compression ignition engine which is configured for dual fuel operation involving a gaseous fuel and a liquid fuel and which is operable in liquid fuel mode or pilot fuel mode.

14. The system according to any one of claims 1 to 10 wherein the engine operating variable comprises ignition (injection) timing for the compression ignition engine.

15. The system according to any one of claims 1 to 10 wherein the engine operating variable comprises both ignition (injection) timing and gas substitution, and wherein the variation first comprises ignition timing.

16. The system according to any one of claims 1 to 10 wherein the engine operating variable comprises both ignition (injection) timing and gas substitution, and wherein the threshold number of knock indication signals in the prescribed interval comprise a first (lower) level at which control of ignition timing is to be implemented and a second level of the number of knock indication signals in the prescribed interval at which gas substitution is to be implemented, the second level being higher than the first level.

17. The system according to any one of the preceding claims wherein the control means is operable to control several engine operating variables.

18. The system according to claim 17 wherein the several engine operating variables are controllable simultaneously, sequentially, in overlapping relationship, or in some other relationship.

19. The system according to any one of the preceding claims further comprising provision for adaptation of engine ignition retardation over time.

20. A system for controlling knock in a compression ignition engine configured for dual fuel operation involving a gaseous fuel and a liquid fuel, in liquid fuel mode or pilot fuel mode, the system comprising: sensor means for determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and a control means for controlling ignition timing in pilot mode operation; the control means being operable in response to generation of a threshold number of knock indication signals in a prescribed interval.

21. A system for controlling knock in a compression ignition engine configured for dual fuel operation involving a gaseous fuel and a liquid fuel, in liquid fuel mode or pilot fuel mode, the system comprising: sensor means for determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and a control means for controlling gas substitution in pilot mode operation; the control means being operable in response to generation of a threshold number of knock indication signals in a prescribed interval.

22. A system for controlling knock in a compression ignition engine configured for dual fuel operation involving a gaseous fuel and a liquid fuel, in liquid fuel mode or pilot fuel mode, the system comprising: sensor means for determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and a control means for ignition timing and gas substitution in pilot mode operation; the control means being operable in response to generation of a threshold number of knock indication signals in a prescribed interval, the threshold number representing a first level at which control of ignition timing is to be implemented; and wherein there is a second level of the number of knock indication signals in the prescribed interval at which gas substitution is to be implemented, the second level being higher than the first level.

23. An internal combustion engine having a system for controlling knock according to any one of the preceding claims.

24. A method of knock control in an internal combustion engine, the method comprising using a system for controlling knock according to any one of claims 1 to 22.

25. A method of knock control in an internal combustion engine, the method comprising: determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and controlling an engine operating variable in response to generation of a threshold number of knock indication signals in a prescribed interval.

26 A method of operating an internal combustion engine, the method comprising: determining the presence of a prescribed knock condition in a cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; and controlling an engine operating variable in response to generation of a threshold number of knock indication signals in a prescribed interval.

27 A system for monitoring the performance of a gaseous fuel in a multi-cylinder compression ignition engine configured for dual fuel operation involving the gaseous fuel and also a liquid fuel, in liquid fuel mode or pilot fuel mode, the system comprising: sensor means for determining the presence of a prescribed knock condition in each cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; a control means for controlling ignition timing in pilot mode operation; the control means being operable in response to generation of a threshold number of knock indication signals in a prescribed interval; wherein upon all cylinders generating respective threshold numbers of knock indication signals in prescribed intervals, the control means is operable to effect consequential action to compensate for the performance of the gaseous fuel, the consequential action comprising control of ignition timing of all cylinders in pilot mode operation.

28 A method of monitoring the performance of a gaseous fuel in a multi-cylinder compression ignition engine configured for dual fuel operation involving the gaseous fuel and also a liquid fuel, in liquid fuel mode or pilot fuel mode, the method comprising: determining the presence of a prescribed knock condition in each cylinder of the engine and generating a knock indication signal indicative of the prescribed knock condition; assessing the prescribed knock condition with respect to a baseline knock condition; and implementing a compensation strategy in response to a variance in the prescribed knock condition with respect to the baseline knock condition in all cylinders, the compensation strategy comprising controlling an engine operating variable with respect to all cylinders.