Method for estimating NOx creation in a combustion process of an engine including a variable volume combustion chamber includes monitoring engine sensor inputs including a cylinder pressure within the combustion chamber. A mass fraction burn value for combustion can be modeled within the combustion chamber based upon said sensor inputs, wherein said mass fraction burn value indexes a crank angle at which a selected percentage of injected fuel is burned in a combustion cycle. The state of combustion within the combustion chamber can be estimated based upon the mass fraction burn value, the state of combustion including a combustion phasing and a combustion strength. NOx creation within the combustion chamber can be estimated with a non-linear function based upon said state of combustion.