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A method and apparatus determine the fuel injection on-time to accurately meter fuel injected directly into the combustion chamber of a gaseous fuelled internal combustion engine. The fuel injection on-time is determined by interpolating between values retrieved from two of a plurality of predetermi

A method and apparatus determine the fuel injection on-time to accurately meter fuel injected directly into the combustion chamber of a gaseous fuelled internal combustion engine. The fuel injection on-time is determined by interpolating between values retrieved from two of a plurality of predetermined look-up tables, which each define fuel injection on-time as a function of gaseous fuel rail pressure and fuelling amount. Each table corresponds to a fixed value of a third parameter that correlates to in-cylinder pressure.

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1. A method of determining fuel injection on-time for an internal combustion engine fuelled with a gaseous fuel that is injected directly into a combustion chamber through a fuel injection valve, the method comprising: (a) storing predetermined injection on-time values in a plurality of look-up tabl

1. A method of determining fuel injection on-time for an internal combustion engine fuelled with a gaseous fuel that is injected directly into a combustion chamber through a fuel injection valve, the method comprising: (a) storing predetermined injection on-time values in a plurality of look-up tables, with on-time in each look-up table being a function of gaseous fuel rail pressure and fuelling amount and with each table associated with a fixed value for a third parameter, which correlates to in-cylinder pressure;(b) sending input data to a controller, said controller determining from said input data, values for said gaseous fuel rail pressure, said fuelling amount and said third parameter;(c) identifying from said plurality of predetermined look-up tables two look-up tables which are associated with said fixed values for said third parameter that are closest in value to that of said third parameter determined in step (b);(d) retrieving respective injection on-times as a function of gaseous fuel rail pressure and fuelling amount from said two look-up tables identified in step (c); and(e) determining a desired injection on-time by interpolating between injection on-time values retrieved in step (d) based on the value of said third parameter determined in step (b). 2. The method of claim 1 further comprising collecting said input data during a compression stroke and determining said desired injection on-time for a fuel injection event beginning during said compression stroke. 3. The method of claim 1 where said third parameter is calculated as a function of at least one parameter that correlates to in-cylinder pressure. 4. The method of claim 1 wherein said third parameter is a product of two parameters that correlate to in-cylinder pressure. 5. The method of claim 4 wherein said two parameters are intake manifold pressure and timing for start of injection. 6. The method of claim 1 wherein said third parameter is one of: (i) timing for start of injection;(ii) intake manifold pressure; and(iii) compression pressure. 7. The method of claim 1 wherein said third parameter is calculated using a mathematical formula comprising at least two parameters that correlate to in-cylinder pressure and which are sent to said controller as part of said input data, said at least two parameters selected from the following: (i) timing for start of injection;(ii) intake manifold pressure;(iii) compression pressure; and(iv) gaseous fuel rail pressure. 8. The method of claim 1 wherein said third parameter is calculated by said controller as a difference between two parameters at least one of which correlates to in-cylinder pressure. 9. The method of claim 1 wherein said third parameter is calculated by said controller as a difference between said gaseous fuel rail pressure and compression pressure. 10. The method of claim 9 wherein said compression pressure is calculated as a function of intake manifold pressure, the engine's compression ratio and a polytropic index. 11. The method of claim 1 wherein said third parameter also correlates to a pressure differential between in-cylinder pressure and injection pressure. 12. The method of claim 1 wherein said input data further comprises an engine parameter that correlates to an engine operational state, said method then further comprising correcting said injection on-time by applying a correction factor which is retrieved from a look-up table that determines said correction factor as a function of said fuelling amount and said engine parameter. 13. The method of claim 12 wherein said engine parameter is engine speed. 14. The method of claim 1 wherein said plurality of look-up tables comprises: (i) a first look-up table associated with a fixed value for said third parameter, which is equal to or near an empirically determined minimum value for said third parameter;(ii) a second look-up table associated with a fixed value for said third parameter, which is equal to or near an empirically determined maximum value for said third parameter; and(iii) a third look-up table associated with a fixed value for said third parameter, which is between said empirically determined minimum and maximum values for said third parameter. 15. The method of claim 14 wherein for a range of values for said third parameter between said minimum value and said maximum value, said fixed value for said third parameter that is associated with said third look-up table corresponds to an inflection point, a local minimum or a local maximum of a curve of a plot of injection on-time across said range. 16. The method of claim 15 wherein said third parameter is the product of timing for start of injection and intake manifold pressure and said desired injection on-time is determined by linear interpolation. 17. The method of claim 1 further comprising: (i) storing in a plurality of predetermined correction factor look-up tables correction factor values retrievable as a function of said gaseous fuel rail pressure and said fuelling amount with each table corresponding to a value for a fourth parameter that is one of said parameters that correlates to in-cylinder pressure which is different than said third parameter;(ii) sending to said controller input data indicating values for said fourth parameter;(iii) from said plurality of predetermined correction factor look-up tables identifying two correction factor look-up tables which correspond to fixed values of said fourth parameter that are closest in value to that of said fourth parameter in said input data;(iv) determining a desired correction factor by interpolating between said two correction factor look-up tables; and(v) correcting said injection on-time by applying said desired correction factor. 18. The method of claim 17 wherein said fourth parameter is one of timing for start of injection, intake manifold pressure, and compression pressure.