Gas mixture temperature estimation apparatus for internal combustion engine

A gas mixture temperature estimation apparatus for an internal combustion engine assumes that a pilot gas mixture (after combustion) produced by pilot injection is stagnating in an annular configuration in the vicinity of the side wall of a combustion chamber (a side surface of a cavity) when fuel injected by means of main injection (i.e., main gas mixture) travels within the combustion chamber. During a period during which a forefront portion of the main gas mixture has not yet reached the stagnating pilot gas mixture, the temperature of the main gas mixture is estimated by making use of the temperature and specific heat of the cylinder interior gas which serves as a peripheral gas. After the main gas mixture forefront portion has reached the stagnating pilot gas mixture, the temperature of the main gas mixture is estimated by making use of the temperature and specific heat of the stagnating pilot gas mixture which serves as the peripheral gas.