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An engine operating method and a direct injection gaseous fuelled system directly injects a gaseous fuel into a combustion chamber during a compression stroke, commands gaseous fuel injection pressure to a target value that is determined as a function of at least one pre-selected engine parameter, m

An engine operating method and a direct injection gaseous fuelled system directly injects a gaseous fuel into a combustion chamber during a compression stroke, commands gaseous fuel injection pressure to a target value that is determined as a function of at least one pre-selected engine parameter, measures actual gaseous fuel injection pressure and adjusts fuel injection pulsewidth to correct for differences between the target value and the actual gaseous fuel injection pressure to thereby inject the desired mass quantity of gaseous fuel as determined from an engine map.

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What is claimed is: 1. An operating method for an internal combustion engine, said operating method comprising: delivering a gaseous fuel to a fuel injection valve, wherein gaseous fuel pressure within said fuel injection valve is defined as gaseous fuel injection pressure; during an engine's compr

What is claimed is: 1. An operating method for an internal combustion engine, said operating method comprising: delivering a gaseous fuel to a fuel injection valve, wherein gaseous fuel pressure within said fuel injection valve is defined as gaseous fuel injection pressure; during an engine's compression stroke, injecting said gaseous fuel directly into a combustion chamber through said fuel injection valve; determining a value for at least one pre-selected engine parameter from engine operating conditions and commanding gaseous fuel injection pressure to a predetermined target value that is associated with said determined value of said at least one pre-selected engine parameter; and measuring gaseous fuel injection pressure and adjusting fuel injection pulsewidth to correct for differences between said predetermined target values and measured gaseous fuel injection pressure to thereby inject a desired mass quantity of gaseous fuel as determined from an engine map. 2. The operating method of claim 1 wherein at least one of said preselected engine parameters is engine state, engine load, engine speed, fuel rail temperature, or in-cylinder pressure. 3. The operating method of claim 1 further comprising maintaining a ratio of gaseous fuel injection pressure to in-cylinder pressure that is high enough to result in a fuel jet exiting from said fuel injection valve comprising at least some gaseous fuel with a velocity exceeding sonic velocity within said combustion chamber. 4. The operating method of claim 1 further comprising commanding gaseous fuel injection pressure to predetermined target pressures for predetermined operating points on said engine map, and calculating target pressures for operating points between said predetermined operating points by interpolation. 5. The operating method of claim 1 further comprising defining said fuel injection pulsewidth for predetermined operating points on said engine map, and calculating fuel injection pulsewidth for operating points between said predetermined operating points by interpolation. 6. The operating method of claim 1 further comprising maintaining said fuel injection pulsewidth so that it is no less than a predetermined minimum pulsewidth. 7. The operating method of claim 6 further comprising decreasing gaseous fuel injection pressure to lengthen fuel injection pulsewidth. 8. The operating method of claim 1 further comprising injecting a pilot fuel into said combustion chamber to trigger ignition of said gaseous fuel. 9. The operating method of claim 8 wherein said fuel injection valve is operable to introduce both said pilot fuel and said gaseous fuel into said combustion chamber. 10. The operating method of claim 9 further comprising regulating gaseous fuel pressure and pilot fuel pressure to reduce a pressure differential between said gaseous fuel and said pilot fuel before each of said fuels is delivered to said fuel injection valve. 11. The operating method of claim 10 wherein gaseous fuel pressure is negatively biased with respect to pilot fuel pressure. 12. The operating method of claim 1 wherein when gaseous fuel injection pressure is commanded to decrease, gaseous fuel injection pressure is decreased without venting said gaseous fuel to atmosphere. 13. The operating method of claim 12 wherein said method further comprises decreasing gaseous fuel injection pressure using a negative rate limiter. 14. The operating method of claim 13 wherein said negative rate limiter is operative using closed loop control to decrease gaseous fuel injection pressure by predetermined increments until a target pressure is attained. 15. The operating method of claim 13 wherein said negative rate limiter comprises a negative rate limiting algorithm programmable into an electronic engine controller. 16. The operating method of claim 13 wherein said negative rate limiter comprises an electronic circuit. 17. The operating method of claim 12 wherein gaseous fuel injection pressure is decreased at a rate that is a function of said engine's gaseous fuel consumption rate. 18. The operating method of claim 17 wherein decreasing gaseous fuel injection pressure further comprises stopping delivery of said gaseous fuel to a fuel rail that feeds said fuel injection valve until gaseous fuel injection pressure drops to a desired one of said predetermined values. 19. The operating method of claim 18 wherein said method further comprises closing a valve associated with a fuel supply line to stop delivery of said gaseous fuel to said fuel injection valve. 20. The operating method of claim 18 wherein said method further comprises shutting down a compressor or pump that is employed to supply said gaseous fuel to said fuel injection valve to stop delivery of said gaseous fuel to said fuel injection valve. 21. The operating method of claim 1 wherein said method further comprises commanding said gaseous fuel injection pressure to one of a plurality of discrete predefined pressures responsive to measured engine parameter values, said engine parameters comprising at least one of engine state, engine load, engine speed, fuel rail temperature, and in-cylinder pressure. 22. The operating method of claim 21 wherein one of said plurality of discrete predefined pressures is P(low) which is lower than a second predefined injection pressure P(normal) and said method further comprises commanding gaseous fuel injection pressure to P(low) when said engine is operating in an idling condition, and commanding said gaseous fuel injection pressure to P(normal) when a load is applied to said engine. 23. The operating method of claim 22 further comprising a third predefined injection pressure P(high), which is higher than P(normal), and said method further comprises commanding gaseous fuel injection pressure to P(high) when said engine is operating at a point on said engine map above a predetermined high load threshold. 24. The operating method of claim 22 wherein said pressure P(low) is a pressure at which a desired amount of gaseous fuel can be injected into said combustion chamber through said fuel injection valve with a fuel injection pulsewidth greater than or equal to a predetermined minimum pulsewidth when said engine is operating in an idling condition. 25. The operating method of claim 22 wherein one of said predefined injection pressures is P(low) which is lower than a second predefined injection pressure P(normal) and said method further comprises commanding gaseous fuel injection pressure to P(Io w) when said engine is operating at a point on said engine map that is below a predefined low load threshold, and commanding said gaseous fuel injection pressure to P(normal) when said engine is commanded to an operating point above said low load threshold. 26. The operating method of claim 25 wherein said pressure P(low) is a pressure at which a desired amount of gaseous fuel can be injected into said combustion chamber through said fuel injection valve with said fuel injection pulsewidth being greater than or equal to a predetermined minimum pulsewidth when said engine is operating in an idling condition. 27. The operating method of claim 25 further comprising a third predefined injection pressure P(high), which is higher than P(normal), and said method further comprises commanding gaseous fuel injection pressure to P(high) when said engine is operating at a point on said engine map above a predetermined high load threshold. 28. The operating method of claim 27 further comprising re-circulating a portion of exhaust gases from one engine cycle to return to said combustion chamber for a subsequent engine cycle. 29. The operating method of claim 27 wherein when said engine is operating at a point on said engine map above said predetermined high load threshold, recirculated exhaust gas makes up at least 15% by mass of the gases introduced into said combustion chamber during an intake stroke. 30. An internal combustion engine system comprising components that cooperate with each other to introduce a gaseous fuel into a combustion chamber where it can be combusted, said internal combustion engine system comprising: (a) a fuel delivery subsystem comprising a device for controlling gaseous fuel pressure and a fuel rail for delivering said gaseous fuel; (b) a fuel injection valve operatively connected to said fuel rail whereby said gaseous fuel at a commanded gaseous fuel injection pressure can be directed from said fuel supply subsystem to said fuel injection valve, and introduced into said combustion chamber through a fuel injection valve nozzle that is disposed in said combustion chamber; (c) a controller operative to: (i) determine a value for at least one pre-selected engine parameter from engine operating conditions; (ii) determine a target value for gaseous fuel injection pressure that is associated with said determined value of said at least one pre-selected engine parameter; (iii) command said device for controlling gaseous fuel injection pressure to increase, or decrease gaseous fuel pressure to said target value if gaseous fuel injection pressure is not equal to said target value; and (iv) command opening and closing of said fuel injection valve to adjust fuel injection pulsewidth to correct for differences between measured gaseous fuel injection pressure and said target value. 31. The internal combustion engine system of claim 30 wherein one of said pre-selected engine parameters is in-cylinder pressure, and said target value is selected to maintain a ratio of gaseous fuel injection pressure to in-cylinder pressure that is high enough to result in a fuel jet exiting from said fuel injection valve comprising at least some gaseous fuel with a velocity exceeding sonic velocity within said combustion chamber. 32. The internal combustion engine system of claim 30 wherein said fuel delivery subsystem further comprises a pump or compressor that can be employed for delivering high pressure gaseous fuel to said fuel injection valves, and wherein said controller can be programmed to command said pump or compressor to stop to allow gaseous fuel injection pressure to decrease as gaseous fuel within said fuel rail between said pump or compressor and said fuel injection valve is consumed by said engine. 33. The internal combustion engine system of claim 30 further comprising an exhaust gas recirculation apparatus for directing an exhaust gas stream, which originates from said combustion chamber, back into said combustion chamber, and an EGR valve for regulating the amount of exhaust gas that is recirculated. 34. The internal combustion engine system of claim 30 further comprising a sensor associated with said fuel injection valve or said fuel rail between said device for controlling gaseous fuel pressure and said fuel injection valve, for measuring gaseous fuel pressure. 35. The internal combustion engine system of claim 30 further comprising a sensor for measuring in-cylinder pressure directly or for measuring an operating parameter associated with said engine from which in-cylinder pressure can be determined indirectly. 36. The internal combustion engine system of claim 30 wherein said device for controlling gaseous fuel pressure is a pressure regulating valve associated with said fuel rail. 37. The internal combustion engine system of claim 36 wherein said controller can command said pressure regulating valve to close to allow gaseous fuel injection pressure to decrease as gaseous fuel within said fuel rail between said pressure regulating valve and said fuel injection valve is consumed by said engine. 38. The internal combustion engine system of claim 30 wherein said pre-selected engine parameter(s) define an engine map and a low load threshold divides regions of said engine map, whereby: (a) when said engine is operating at a point on said engine map that is below said low load threshold, said controller is operative to command gaseous fuel injection pressure to a predefined pressure P(low); and (b) when said engine is operating at a point on said engine map that is above said low load threshold, said controller is operative to command gaseous fuel injection pressure to a predefined pressure P(normal), wherein P(normal) is greater than P(low). 39. The internal combustion engine system of claim 38 further comprising a high load threshold whereby said engine map is divided into three regions, and when said engine is operating at a point on said engine map that is above said high load threshold, said controller is operative to command gaseous fuel injection pressure to a predefined pressure P(high), wherein P(high) is greater than P(normal). 40. The internal combustion engine system of claim 30 further comprising a negative rate limiter associated with said device for controlling gaseous fuel pressure, said negative rate limiter restricting decreases in gaseous fuel pressure to prevent venting of said gaseous fuel to atmosphere. 41. The internal combustion engine system of claim 40 wherein said controller is an electronic controller. 42. The internal combustion engine system of claim 41 wherein said negative rate limiter comprises a dedicated electronic circuit. 43. The internal combustion engine system of claim 41 wherein said negative rate limiter comprises a negative rate limiting algorithm programmed into said electronic controller. 44. The internal combustion engine system of claim 30 wherein said fuel injection valve is integrated with a pilot fuel injection valve whereby pilot fuel and said gaseous fuel can each be introduced into said combustion chamber through a single fuel injection valve assembly. 45. The internal combustion engine system of claim 44 wherein for said gaseous fuel and said pilot fuel, said single fuel injection valve assembly comprises separate fuel passages and separate valve members, allowing separate and independent injection into said combustion chamber of said gaseous fuel and said pilot fuel. 46. The internal combustion engine system of claim 45 wherein said device for controlling gaseous fuel pressure is also operative to maintain a pressure differential between said gaseous fuel and said pilot fuel within a predetermined range. 47. The internal combustion engine system of claim 46 wherein said gaseous fuel injection pressure is negatively biased with respect to pilot fuel injection pressure. 48. The internal combustion engine system of claim 46 wherein said device for controlling gaseous fuel pressure comprises a dome-loaded regulator that is in communication with a pilot fuel rail.