Exhaust gas purification system for an internal combustion engine

An exhaust gas purification system for an internal combustion engine comprising:

• a low pressure EGR mechanism that is equipped with a low pressure EGR passage which serves to introduce a part of an exhaust gas flowing through a portion of an exhaust passage downstream of a turbine of a centrifugal supercharger to a portion of an intake passage upstream of a compressor as a low pressure EGR gas, and a low pressure EGR valve which serves to change a channel cross section of said low pressure EGR passage;
• a selective reduction type catalyst that is arranged in a portion of the exhaust passage upstream of a connection portion of said low pressure EGR passage;
• a supply device which serves to supply an ammonia derived compound to said selective reduction type catalyst, and is arranged in such a manner that the ammonia derived compound is able to be supplied into the exhaust passage at a location upstream of the connection portion of said low pressure EGR passage;
• and
• a control unit that:
• causes said supply device to supply the ammonia derived compound therefrom when said low pressure EGR valve is in a valve open state,
• calculates a saturation amount of ammonia by using a bed temperature of the selective reduction type catalyst as a parameter, and
• controls said supply device so that an amount of ammonia more than the saturation amount is supplied to the selective reduction type catalyst when said low pressure EGR valve is in the valve open state.