Combustion control system of internal combustion engine

A required nominal-torque heat-release-rate characteristic value that satisfies the required nominal torque, required engine speed and required ignition timing is specified as the maximum value of a heat-release-rate waveform. It is determined whether a limiting-value heat-release-rate characteristic value based on a restriction, such as combustion noise, is within a permissible range of the restriction, with respect to one of the required nominal-torque heat-release-rate characteristic values which meets the above requirements, and the required nominal-torque heat-release-rate characteristic value is changed to another required nominal-torque heat-release-rate characteristic value that meets the above requirements, until the limiting-value heat-release-rate characteristic value falls within the permissible range of the restriction. Where there a plurality of required nominal-torque heat-release-rate characteristic values that meet the respective requirements, one of the characteristic values which is closest to the compression top dead center is used, and an actuator, such as an injector, is driven so as to provide the nominal-torque heat-release-rate characteristic value.