Exhaust gas recirculation (hereinafter also referred to as EGR) is known as a technique of reducing nitrogen oxides (NOx) in exhaust gas discharged from a diesel engine. An engine system employing the EGR includes an exhaust gas recirculation apparatus (EGR apparatus) that merges a part of the exhaust gas discharged from an engine (EGR gas) with fresh air supplied to the engine. The fresh air and the EGR gas merged by the EGR apparatus are supplied to the engine.
Generally, pressure difference between the EGR gas and the fresh air is utilized to introduce the EGR gas into the EGR apparatus. Thus, it is easy to introduce the EGR gas into the EGR apparatus while the engine is in a low load range involving a large pressure difference between the EGR gas and the fresh air, but it is difficult to introduce the EGR gas into the EGR apparatus while the engine is in a high load range involving a small pressure difference between the EGR gas and the fresh air.
In view of this, in an EGR apparatus disclosed in PTL 1, a double pipe structure is formed by an EGR path and an air supply path. In the EGR apparatus, the EGR gas is sucked by a mechanism of an ejector, so that the introduction of the EGR gas is facilitated.
When the EGR gas is supplied to a multi-cylinder engine, mixing between the EGR gas and the fresh air is an important factor. Thus, in an exhaust gas recirculation apparatus disclosed in PTL 2, an EGR gas introduction chamber having an annular shape is formed around an EGR gas mixing pipe forming an air intake path. Furthermore, an EGR gas supply port connecting the air intake path and the EGR gas introduction chamber is formed in the EGR gas mixing pipe. A plate is disposed in the EGR gas mixing pipe, and the EGR gas and the fresh air each collide with the plate to be mixed while swirling.