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A technique for fuel system protection for an internal combustion engine includes introducing a directly injected fuel into a combustion chamber through a direct fuel injector, introducing a fumigated fuel upstream of an intake valve, selectively operating the internal combustion engine with at leas

A technique for fuel system protection for an internal combustion engine includes introducing a directly injected fuel into a combustion chamber through a direct fuel injector, introducing a fumigated fuel upstream of an intake valve, selectively operating the internal combustion engine with at least one of the directly injected fuel and the fumigated fuel, determining a temperature of the direct fuel injector as a first function of engine operating parameters, and performing a temperature mitigation technique when the temperature rises above a first predetermined value such that the temperature is maintained below a second predetermined value.

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1. A method of fuel system protection for an internal combustion engine, the method comprising: introducing a directly injected fuel into a combustion chamber through a direct fuel injector;introducing a fumigated fuel upstream of an intake valve;selectively operating said internal combustion engine

1. A method of fuel system protection for an internal combustion engine, the method comprising: introducing a directly injected fuel into a combustion chamber through a direct fuel injector;introducing a fumigated fuel upstream of an intake valve;selectively operating said internal combustion engine with at least one of said directly injected fuel and said fumigated fuel;determining a steady state temperature of said direct fuel injector as a function of engine operating parameters;determining a temperature of said direct fuel injector as a function of said steady state temperature, a history of said temperature and a time constant representative of a thermal characteristic of said direct fuel injector; andperforming a temperature mitigation technique when said temperature rises above a first predetermined value such that said temperature is maintained below a second predetermined value,wherein said time constant is determined as a function of one of a directly injected fuel mass fraction and a fumigated fuel mass fraction. 2. The method of claim 1, wherein said first predetermined value is greater than said second predetermined value. 3. The method of claim 1, wherein said first predetermined value and said second predetermined value are related by one of: said first predetermined value is less than said second predetermined value; andsaid first predetermined value is equal to said second predetermined value. 4. The method of claim 1, wherein said mitigation technique comprises switching operation of said internal combustion engine from only said fumigated fuel to only said directly injected fuel. 5. The method of claim 1, wherein said mitigation technique comprises increasing a quantity of directly injected fuel introduced into said combustion chamber. 6. The method of claim 1, wherein said mitigation technique comprises reducing an equivalence ratio of said internal combustion engine. 7. The method of claim 6, wherein said equivalence ratio is reduced to between 0.88 and 0.92. 8. The method of claim 1, wherein said mitigation technique comprises at least one of retarding spark timing and retarding combustion phasing. 9. The method of claim 1, wherein said engine operating parameters comprise at least one parameter representative of a quantity of fuel introduced into said combustion chamber, and at least one parameter representative of how often said quantity of fuel is introduced into said combustion chamber. 10. The method of claim 1, wherein said engine operating parameters comprise at least two of engine speed, air mass flow, engine coolant temperature, equivalence ratio, spark timing, combustion phasing, manifold air temperature, effective EGR rate, intake charge oxygen concentration, trapped oxygen mass, fumigated fuel mass fraction, intake cam position, exhaust cam position, external EGR temperature and directly injected fuel mass fraction. 11. The method of claim 1, further comprising: determining a base temperature as a third function of said engine operating parameters; anddetermining at least one temperature correction factor as a function of said engine operating parameters;wherein said steady state temperature is determined as a function of said base temperature and said at least one temperature correction factor. 12. The method of claim 1, wherein said directly injected fuel is selected from the group consisting of gasoline, ethanol-gasoline blends and diesel. 13. The method of claim 1, wherein said fumigated fuel is natural gas. 14. The method of claim 1, wherein said fumigated fuel is selected from the group consisting of ethane, methane, propane, biogas, landfill gas, dimethyl ether, hydrogen and mixtures thereof. 15. An apparatus for fuel system protection in an internal combustion engine comprising: a direct fuel injector for introducing a directly injected fuel into a combustion chamber of said internal combustion engine;a fumigation system for introducing a fumigated fuel upstream of an intake valve of said combustion chamber;an electronic controller programmed to:select one of a direct injection mode and a fumigation mode;determine a steady state temperature of said direct fuel injector as a function of engine operating parameters;determine a temperature of said direct fuel injector as a function of said steady state temperature, a history of said temperature and a time constant representative of a thermal characteristic of said direct fuel injector; andperform a temperature mitigation technique when said temperature rises above a first predetermined value such that said temperature is maintained below a second predetermined value,wherein said time constant is determined as a function of one of directly injected fuel mass fraction and a fumigated fuel mass fraction. 16. The apparatus of claim 15, wherein said temperature mitigation technique comprises at least one of: switching operation of said internal combustion engine from said fumigation mode to said direct injection mode;increasing a quantity of directly injected fuel introduced into said combustion chamber;reducing an equivalence ratio of said internal combustion engine; retarding spark timing; andretarding combustion phasing. 17. A method of fuel system protection for an internal combustion engine, the method comprising: introducing a directly injected fuel into a combustion chamber through a direct fuel injector;introducing a fumigated fuel upstream of an intake valve;selectively operating said internal combustion engine with at least one of said directly injected fuel and said fumigated fuel;determining a direct steady state temperature of said direct fuel injector as a function of said engine operating parameters, said direct steady state temperature representative of a first portion of a steady state temperature of said direct fuel injector due to combustion of said directly injected fuel;determining a fumigated steady state temperature as a function of said engine operating parameters, said fumigated steady state temperature representative of a second portion of said steady state temperature of said direct fuel injector due to combustion of said fumigated fuel;determining said steady state temperature of said direct fuel injector as a function of said direct steady state temperature, said fumigated steady state temperature and one of a directly injected fuel mass fraction and a fumigated fuel mass fraction;determining a temperature of said direct fuel injector as a function of said steady state temperature of said direct fuel injector, a history of said temperature and a time constant representative of a thermal characteristic of said direct fuel injector; andperforming a temperature mitigation technique when said temperature rises above a first predetermined value such that said temperature is maintained below a second predetermined value.