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In a combustion control system, a control unit determines, based on an operating condition of an exhaust purifying device, whether a request for an exhaust temperature rise or a rich A/F ratio engine operating mode is present. The control unit executes, by way of fuel injection control in presence o

In a combustion control system, a control unit determines, based on an operating condition of an exhaust purifying device, whether a request for an exhaust temperature rise or a rich A/F ratio engine operating mode is present. The control unit executes, by way of fuel injection control in presence of the request of the exhaust temperature rise or the rich operating mode, a split retard combustion mode in which a main combustion needed to produce a main engine torque and at least one preliminary combustion occurring prior to the main combustion are both achieved, and the preliminary combustion occurs near top dead center on a compression stroke, and the main combustion initiates after completion of the preliminary combustion. The control unit simultaneously executes an exhaust-emission reduction control that reduces HC and CO emissions, while keeping the excess air factor at a desired value in the catalyst deactivated state.

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What is claimed is: 1. A combustion control system of an internal combustion engine employing an exhaust purifying device in an exhaust passage, comprising: sensors that detect operating conditions of the engine; a control unit being configured to be electronically connected to the sensors, for co

What is claimed is: 1. A combustion control system of an internal combustion engine employing an exhaust purifying device in an exhaust passage, comprising: sensors that detect operating conditions of the engine; a control unit being configured to be electronically connected to the sensors, for combustion control and exhaust emission control purposes; wherein the control unit comprises a processor programmed to perform the following, (a) estimating an operating condition of the exhaust purifying device; (b) determining, based on the operating condition of the exhaust purifying device, whether a predetermined condition, including at least one of a request for a rise in an exhaust temperature and a request for an engine operating mode at an excess air factor corresponding to a rich air-fuel mixture ratio, is satisfied; (c) executing, by way of fuel injection control when the predetermined condition is satisfied, a split retard combustion mode in which a main combustion needed to produce a main engine torque and at least one preliminary combustion occurring prior to the main combustion are both achieved, wherein the preliminary combustion takes place near top dead center on a compression stroke, and wherein the main combustion initiates after the preliminary combustion has been completed; (d) determining whether the exhaust purifying device is in a catalyst deactivated state or in a catalyst activated state; and (e) simultaneously executing an exhaust-emission reduction control that reduces hydrocarbons (HC) and carbon monoxide (CO) emissions exhausted from a combustion chamber of the engine, while keeping the excess air factor at a desired value, during the split retard combustion mode in the catalyst deactivated state. 2. The combustion control system as claimed in claim 1, wherein: the exhaust-emission reduction control, in which the HC and CO emissions are reduced, while keeping the excess air factor at the desired value, during the split retard combustion mode in the catalyst deactivated state, comprises exhaust gas recirculated (EGR) decrease control by which an EGR rate is reduced. 3. The combustion control system as claimed in claim 2, wherein the processor is further programmed for: (f) initiating EGR increase control by which the EGR rate is increased, responsively to a transition from the catalyst deactivated state to the catalyst activated state. 4. The combustion control system as claimed in claim 2, wherein the processor is further programmed for: (g) advancing an injection timing of main fuel injection for the main combustion simultaneously with the EGR decrease control. 5. The combustion control system as claimed in claim 3, wherein the processor is further programmed for: (h) retarding an injection timing of main fuel injection for the main combustion simultaneously with the EGR increase control. 6. The combustion control system as claimed in claim 1, wherein: a catalyst temperature of the exhaust purifying device is used as a threshold value needed to determine whether the exhaust purifying device is in the catalyst deactivated state or in the catalyst activated state. 7. The combustion control system as claimed in claim 1, wherein: at least one of a concentration of the HC emissions at a catalyst outlet of the emission purifying device and a concentration of the CO emissions at the catalyst outlet is used as a threshold value needed to determine whether the exhaust purifying device is in the catalyst deactivated state or in the catalyst activated state. 8. The combustion control system as claimed in claim 2, wherein: a basic value of the EGR rate is determined based on engine speed and engine load. 9. The combustion control system as claimed in claim 1, wherein: an injection quantity of preliminary fuel injection for the preliminary combustion is set to a fuel injection quantity needed in order for an in-cylinder temperature obtained during a main fuel injection period for the main combustion to exceed a self-ignitable temperature value. 10. The combustion control system as claimed in claim 1, wherein: a start of the main combustion is retarded from a start of the preliminary combustion by at least 20 degrees of crankangle, for initiating the main combustion after completion of the preliminary combustion. 11. The combustion control system as claimed in claim 1, wherein: an end of the main combustion is retarded by at least 50 degrees of crankangle from the TDC on the compression stroke. 12. The combustion control system as claimed in claim 1, wherein: the exhaust purifying device comprises a NOx trap catalyst that traps nitrogen oxides contained in exhaust gases during an engine operating mode at the excess air factor corresponding to a lean air-fuel mixture ratio, and a period that the request for the exhaust temperature rise based on the operating condition of the exhaust purifying device is present, is at least a sulfur poisoning release period during which sulfur oxides trapped by the NOx trap catalyst is desorbed from the NOx trap catalyst by rising up the exhaust temperature. 13. The combustion control system as claimed in claim 1, wherein: the exhaust purifying device comprises a NOx trap catalyst that traps nitrogen oxides (NOx) contained in exhaust gases during an engine operating mode at the excess air factor corresponding to a lean air-fuel mixture ratio, and a particulate filter (DPF) that accumulates particulate matter (PM) contained in exhaust gases and is disposed downstream of the NOx trap catalyst, and a period that the request for the exhaust temperature rise based on the operating condition of the exhaust purifying device is present, is at least one of (i) a DPF regeneration period during which the PM accumulated in the DPF is burned and removed from the DPF, (ii) a sulfur poisoning release period during which sulfur oxides trapped by the NOx trap catalyst is desorbed from the NOx trap catalyst by rising up the exhaust temperature, and (iii) a NOx desorption-purification period during which the trapped NOx is desorbed from the NOx trap catalyst. 14. The combustion control system as claimed in claim 13, wherein: during the DPF regeneration period, the excess air factor is set to the desired value ranging from 1 to 1.4, the desired value being determined based on a quantity of the PM accumulated in the DPF; during the sulfur poisoning release period, the excess air factor is set to the desired value corresponding to a stoichiometric air-fuel mixture ratio; and during the NOx desorption-purification period, the excess air factor is set to the desired value corresponding to a rich air-fuel mixture ratio. 15. A combustion control system of an internal combustion engine employing an exhaust purifying device in an exhaust passage, comprising: sensor means for detecting operating conditions of the engine; a control unit being configured to be electronically connected to the sensor means, for combustion control and exhaust emission control purposes; wherein the control unit comprises: (a) means for estimating an operating condition of the exhaust purifying device; (b) means for determining, based on the operating condition of the exhaust purifying device, whether a predetermined condition, including at least one of a request for a rise in an exhaust temperature and a request for an engine operating mode at an excess air factor corresponding to a rich air-fuel mixture ratio, is satisfied; (c) means for executing, by way of fuel injection control when the predetermined condition is satisfied, a split retard combustion mode in which a main combustion needed to produce a main engine torque and at least one preliminary combustion occurring prior to the main combustion are both achieved, wherein the preliminary combustion takes place near top dead center on a compression stroke, and wherein the main combustion initiates after the preliminary combustion has been completed; (d) means for determining whether the exhaust purifying device is in a catalyst deactivated state or in a catalyst activated state; and (e) means for simultaneously executing an exhaust-emission reduction control that reduces hydrocarbons and carbon monoxide emissions exhausted from a combustion chamber of the engine, while keeping the excess air factor at a desired value, during the split retard combustion mode in the catalyst deactivated state. 16. A method of executing an exhaust-emission reduction control function for an exhaust purifying device disposed in an exhaust passage of an internal combustion engine capable of recovering an operating condition of the exhaust purifying device, the method comprising: estimating the operating condition of the exhaust purifying device; determining, based on the operating condition of the exhaust purifying device, whether a predetermined condition, including at least one of a request for a rise in an exhaust temperature and a request for an engine operating mode at an excess air factor corresponding to a rich air-fuel mixture ratio, is satisfied; executing, by way of fuel injection control when the predetermined condition is satisfied, a split retard combustion mode in which a main combustion needed to produce a main engine torque and at least one preliminary combustion occurring prior to the main combustion are both achieved, wherein preliminary combustion takes place near top dead center on a compression stroke, and wherein the main combustion initiates after the preliminary combustion has been completed; determining whether the exhaust purifying device is in a catalyst deactivated state or in a catalyst activated state; and simultaneously executing an exhaust-emission reduction control that reduces hydrocarbons and carbon monoxide emissions exhausted from a combustion chamber of the engine, while keeping the excess air factor at a desired value, during the split retard combustion mode in the catalyst deactivated state. 17. A method of executing an exhaust-emission reduction control function for an exhaust purifying device including at least a NOx trap catalyst that traps nitrogen oxides contained in exhaust gases when an exhaust air-fuel mixture ratio is lean and disposed in an exhaust passage of an internal combustion engine capable of recovering an operating condition of the exhaust purifying device, the method comprising: disposing a catalyst temperature sensor downstream of the NOx trap catalyst for detecting a catalyst temperature of the NOx trap catalyst; disposing a concentration sensor downstream of the NOx trap catalyst for detecting at least one of a concentration of hydrocarbons (HC) emissions at a catalyst outlet of the NOx trap catalyst and a concentration of carbon monoxide (CO) emissions at the catalyst outlet; determining that the NOx trap catalyst is in a catalyst deactivated state, when the catalyst temperature detected by the catalyst temperature sensor is less than or equal to a predetermined temperature threshold value; determining that the NOx trap catalyst is in the catalyst deactivated state, when either one of the concentration of HC emissions and the concentration of CO emissions, detected by the concentration sensor exceeds a predetermined concentration threshold value; estimating the operating condition of the exhaust purifying device; determining, based on the operating condition of the exhaust purifying device, whether a predetermined condition, including at least one of a request for a rise in an exhaust temperature and a request for an engine operating mode at an excess air factor corresponding to a rich air-fuel mixture ratio, is satisfied; executing, by way of fuel injection control when the predetermined condition is satisfied, a split retard combustion mode in which a main combustion needed to produce a main engine torque and at least one preliminary combustion occurring prior to the main combustion are both achieved, wherein the preliminary combustion takes place near top dead center on a compression stroke, and wherein the main combustion initiates after the preliminary combustion has been completed; and simultaneously executing an exhaust-emission reduction control that reduces the HC and CO emissions exhausted from a combustion chamber of the engine, while keeping the excess air factor at a desired value, during the split retard combustion mode in the catalyst deactivated state. 18. The method as claimed in claim 17, wherein: the exhaust-emission reduction control, in which the HC and CO emissions are reduced, while keeping the excess air factor at the desired value, during the split retard combustion mode in the catalyst deactivated state, comprises exhaust gas recirculated (EGR) decrease control by which an EGR rate is reduced. 19. The method as claimed in claim 18, further comprising: initiating EGR increase control by which the EGR rate is increased, responsively to a transition from the catalyst deactivated state to the catalyst activated state. 20. The method as claimed in claim 18, further comprising: advancing an injection timing of main fuel injection for the main combustion simultaneously with the EGR decrease control. 21. The method as claimed in claim 19, further comprising: retarding the injection timing of main fuel injection for the main combustion simultaneously with the EGR increase control. 22. The combustion control system as claimed in claim 1, wherein the system is adapted to execute a preliminary fuel injection on the compression stroke prior to executing the at least one preliminary combustion occurring prior to the main combustion. 23. The method of claim 16, further comprising executing a preliminary fuel injection on the compression stroke prior to executing the at least one preliminary combustion occurring prior to the main combustion.