U.S. Pat. No. 5,590,488, assigned to the assignee of the prevent invention, which is incorporated herein by reference ('488 patent), describes an intake manifold having an Exhaust Gas Recirculation (EGR) passage formed therein extending generally parallel to a cooling passage formed in the manifold. The EGR flow in the '488 patent is considered ported because EGR flows through secondary EGR passages extending from the EGR supply passage to individual runners in the intake manifold.
In an EGR system according to the '488 patent, the EGR ports are provided in close proximity to the cylinder head. The EGR ports are provided within the individual intake runners, which results in communication between the ports. Such communication short circuits a tuned induction system, resulting in a degradation of engine performance and can provide for unequal distribution of EGR. This unequal distribution can also affect engine performance and emissions because the EGR is not properly controlled between cylinders. This makes it difficult to control the EGR to ensure smooth running of the engine.
U.S. Pat. Nos. 5,535,717 and 5,492,093 provide a system for introducing EGR into the an air induction system. These patents provide introduction of EGR within a balance tube, or into one of the runners, or a primary runner. This system does not promote equal distribution or mixing, as air is typically stagnant in the primary runner unless balance tube valve and the intake valves for the cylinders are open, and therefore distribution of the EGR is uneven between the cylinder banks. As described above, this unequal distribution may produce undesirable operation of the vehicle. A valve for such a balance tube is open normally in mid-range operation, such as between approximately 3000 and 4500 RPM. Thus, outside this range (at low speed or high speed operation), the balance valve is closed and the EGR gas is not properly mixed, and therefore an Inefficient, unequal cylinder bank distribution exists.
It would therefore be desirable to provide an EGR system which is balanced between the cylinders of a multi-cylinder engine and which provides proper mixing of the recirculated exhaust gas with the intake air at separate or coincidental reservoirs.