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A control system and method for operating an engine includes a threshold determination module that determines a plurality of combustion mode thresholds based on the engine speed and engine temperature. The control module also includes a transition module that compares the engine load and the plurali

A control system and method for operating an engine includes a threshold determination module that determines a plurality of combustion mode thresholds based on the engine speed and engine temperature. The control module also includes a transition module that compares the engine load and the plurality of combustion mode thresholds and changes a combustion mode of the engine in response to comparing the engine load and the plurality of combustion mode thresholds.

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1. A method of controlling an engine comprising: determining a first engine speed;determining a first engine temperature;determining a first plurality of combustion mode (CM) threshold curves based on the first engine temperature, wherein each of the first plurality of CM threshold curves is a respe

1. A method of controlling an engine comprising: determining a first engine speed;determining a first engine temperature;determining a first plurality of combustion mode (CM) threshold curves based on the first engine temperature, wherein each of the first plurality of CM threshold curves is a respective load line having a respective engine load versus engine speed relationship;determining a first engine load;based on the first engine speed, comparing the first engine load to each of the first plurality of CM thresholds;based on the comparisons of the first engine load to each of the first plurality of CM threshold curves, changing a combustion mode of the engine from a first homogeneous charge compression (HCCI) mode to a second HCCI mode;determining a second engine temperature;determining a second plurality of CM threshold curves based on the second engine temperature, wherein each of the second plurality of CM threshold curves is a respective load line having a respective engine load versus engine speed relationship;based on the first engine speed, comparing the first engine load to each of the second plurality of CM threshold curves; andbased on the comparisons of the first engine load to each of the second plurality of CM threshold curves, changing the combustion mode of the engine from the second HCCI mode to the first HCCI mode or to a third HCCI mode. 2. A method as recited in claim 1, wherein the first plurality of CM threshold curves and the second plurality of CM threshold curves are determined based on a transmission gear, a brake vacuum or a barometric pressure. 3. A method as recited in claim 1, wherein the first plurality of CM threshold curves and the second plurality of CM threshold curves are determined based on more than three of the engine temperature, a barometric pressure, a transmission gear, and a brake vacuum. 4. A method as recited claim 1, further comprising changing the combustion mode from the second HCCI mode to the first HCCI mode if the first engine load is greater than a first threshold curve for the first engine speed, wherein: the first plurality of CM threshold curves comprise the first threshold curve and a second threshold curve;values of the first threshold curve are greater than corresponding values of the second threshold curve; andthe changing of the combustion mode from the first HCCI mode to the second HCCI mode is performed if the first engine load is less than the second threshold curve for the first engine speed. 5. A method as recited in claim 1, further comprising: determining a second engine load after the determining of the first engine load;comparing the second engine load to each of the plurality of CM threshold curves, wherein the first plurality of CM threshold curves comprise a first threshold curve and a second threshold curve, and wherein values of the first threshold curve are greater than corresponding values of the second threshold curve; andwhile in the second HCCI mode and if, for the first engine speed, (i) the second engine load is greater than the first threshold curve and (ii) the first engine load is less than the first threshold curve or the second threshold curve, changing the combustion mode from the second HCCI mode to the first HCCI mode,wherein the changing of the combustion mode from the second HCCI mode to the first HCCI mode is not performed if the second engine load is less than the first threshold curve for the first engine speed. 6. A method as recited in claim 5, wherein the changing of the combustion mode from the first HCCI mode to the second HCCI mode is performed when the first engine load is less than the second threshold curve for the first engine speed and is not performed when the first engine load is greater than the second threshold curve for the first engine speed. 7. A method as recited in claim 6, further comprising: while in the first HCCI mode, determining a third engine load prior to the determining of the first engine load;comparing the third engine load to the first plurality of CM threshold curves; andremaining in the first HCCI mode if, for the first engine speed, the third engine load is greater than the first threshold curve or the second threshold curve. 8. A method as recited in claim 1, further comprising: determining a second engine load subsequent to the determining of the first engine load;based on the first engine speed, comparing the second engine load to each of the second plurality of CM threshold curves, wherein load values of each of the second plurality of CM threshold curves is less than corresponding load values of each of the first plurality of CM threshold curves; andtransitioning from the second HCCI mode to the third HCCI mode based on the comparisons of the second engine load to each of the second plurality of CM threshold curves. 9. The method of claim 1, wherein: the first plurality of CM threshold curves have first respective slopes; andthe second plurality of CM threshold curves have second respective slopes. 10. The method of claim 1, further comprising: determining a second engine speed;determining a second engine load;based on the second engine speed, comparing the second engine load to each of the second plurality of CM threshold curves; andbased on the comparisons of the second engine load to each of the second plurality of CM threshold curves, changing the combustion mode of the engine (i) from the first HCCI mode to the second HCCI mode, or (ii) from the third HCCI mode to the second HCCI mode or a fourth HCCI mode. 11. The method of claim 1, wherein the second plurality of CM threshold curves are shifted up or down from the first plurality of CM threshold curves. 12. The method of claim 1, further comprising, for each of a plurality of temperatures, transitioning between a plurality of HCCI modes based on a plurality of engine loads, wherein: the plurality of HCCI modes, for the first temperature, include the first HCCI mode and the second HCCI mode;the plurality of HCCI modes, for the second temperature, includes the first HCCI mode or the third HCCI mode;the plurality of HCCI modes, for each of the plurality of temperatures, includes a lean HCCI mode, a mixed HCCI mode, and a stoichiometric HCCI mode;the mixed HCCI mode includes performing fuel injection during a beginning portion of a recompression cycle of the engine; andan amount of load on the engine is less during the mixed HCCI mode than during the stoichiometric HCCI mode. 13. The method of claim 1, wherein, for the first engine speed, the comparing of the first engine load to each of the first plurality of CM threshold curves or the comparing of the first engine load to the second plurality of CM threshold curves comprises: setting a first mode value equal to a maximum number of HCCI modes;setting a previous mode value equal to a current mode value, wherein the previous mode value identifies a previous HCCI mode, and wherein the current mode value identifies a current HCCI mode;setting a second mode value and a first predetermined value equal to a same initial value;setting a second predetermined value to be equal to the first predetermined value plus one;determining, for the first engine speed, (i) whether the first engine load is less than a second CM threshold curve, or (ii) whether the first engine load is greater than a first CM threshold curve and the previous mode is greater than the first predetermined value; andif (i) the first engine load is less than the second CM threshold curve, or (ii) the first engine load is greater than the first CM threshold curve and the previous mode is greater than the first predetermined value, setting the second mode value equal to the second predetermined value. 14. The method of claim 13, wherein, for the first engine speed, the comparing of the first engine load to each of the first plurality of CM threshold curves or the comparing of the first engine load to the second plurality of CM threshold curves comprises: if (i) the first engine load is greater than or equal to the second CM threshold curve, or (ii) the first engine load is less than or equal to the first CM threshold curve and the previous mode is less than or equal to the first predetermined value, determining whether the first predetermined value is equal to the first mode value;if the first predetermined value is equal to the first mode value, setting the current mode value equal to the second mode value; andif the first predetermined value is not equal to the first mode value, incrementing the first predetermined value. 15. A control system for an engine comprising: a threshold determination module that determines a first plurality of combustion mode (CM) threshold curves based on a first engine speed and a first engine temperature, wherein each of the first plurality of CM threshold curves is a respective load line having a respective engine load versus engine speed relationship; anda transition module that compares a first engine load to each of the first plurality of CM threshold curves based on the first engine speed and changes a combustion mode of the engine from a first homogeneous charge compression (HCCI) mode to a second HCCI mode based on the comparisons of the first engine load to each of the first plurality of CM threshold curves,wherein the threshold determination module determines a second plurality of CM threshold curves based on (i) the first engine speed or a second engine speed, and (ii) a second engine temperature, wherein each of the second plurality of CM threshold curves is a respective load line having a respective engine load versus engine speed relationship, andthe transition module based on the first engine speed or the second engine speed, compares the first engine load or a second engine load to each of the second plurality of CM threshold curves, andbased on the comparisons of the first engine load or the second engine load to each of the second plurality of CM threshold curves, changes the combustion mode of the engine from the second HCCI mode to the first HCCI mode or to a third HCCI mode. 16. A control system as recited in claim 15, wherein: the transition module changes the combustion mode from the second HCCI mode to the first HCCI mode if the first engine load is greater than a first threshold curve for the first engine speed;the first plurality of CM threshold curves comprise the first threshold curve and a second threshold curve;values of the first threshold curve are greater than corresponding values of the second threshold curve; andthe transition module changes the combustion mode from the first HCCI mode to the second HCCI mode if the first engine load is less than the second threshold curve for the first engine speed. 17. A control system as recited in claim 15, further comprising a control module that determines a second engine load after determining the first engine load, wherein: the transition module compares the second engine load to each of the first plurality of CM threshold curves, wherein the first plurality of CM threshold curves comprise a first threshold curve and a second threshold curve, and wherein values of the first threshold curve are greater than corresponding values of the second threshold curve;the transition module, while in the second HCCI mode and if, for the first engine speed, (i) the second engine load is greater than the first threshold curve and (ii) the first engine load is less than the first threshold curve or the second threshold curve, changes the combustion mode from the second HCCI mode to the first HCCI mode; andthe transition module does not change the combustion mode from the second HCCI mode to the first HCCI mode if the second engine load is less than the first threshold curve for the first engine speed. 18. A control system as recited in claim 17, wherein the transition module changes the combustion mode from the first HCCI mode to the second HCCI mode if the first engine load is less than the second threshold curve for the first engine speed and does not change the HCCI mode from the first HCCI mode to the second HCCI mode if the first engine load is greater than the second threshold curve for the first engine speed. 19. A control system as recited in claim 18, wherein: while in the first HCCI mode, the control module determines a third engine load prior to the determining of the first engine load; andthe transition module compares the third engine load to the first plurality of CM threshold curves, and remains in the first HCCI mode if the third engine load is greater than the first threshold curve or the second threshold curve for the first engine speed. 20. A control system as recited in claim 15, further comprising: determining a second engine load subsequent to determining the first engine load;comparing the second engine load to each of a second plurality of CM threshold curves, wherein values of each of the second plurality of CM threshold curves are less than corresponding values of each of the first plurality of CM threshold curves; andtransitioning from the second HCCI mode to a third HCCI mode based on the comparisons of the second engine load to each of the second plurality of CM thresholds.