What is exhaust gas dilution?

Exhaust gas dilution is something I did not find an elaborate explanation for on the internet so here goes.

                In a naturally aspirated I.C. Engine, there are basically three ranges of operation:

1.       Idling Range

2.       Cruising Range

3.       Power Range

Exhaust gas dilution mainly occurs during idling and is insignificant during the other two ranges. The pressure distribution in the Inlet manifold, Exhaust manifold and Cylinder head while idling is given in the diagram below. We see that the pressure at the inlet manifold is very low when compared to the other two regions. This occurs due to the closed throttle.

Case 1: When fresh charge is admitted into the cylinder while idling, the quantity of fresh charge is quite less when compared to the charge admitted while throttling. The fresh charge and the exhaust gas in the cylinder mix up to reduce the air fuel ratio significantly. This results in poor combustion and as a result, a loss of power.

Case 2: With Nearly closed throttle, the pressre in the intake manifold is significantly less when compared to the other regions. The pressure differential causes a backward flow of exhaust gasses into the intake manifold as soon as the inlet valve opens. When the piston moves down, sucking the charge, the charge is already diluted with the exhaust gasses leading to poor combustion.

Case 1 and 2 both act together to dilute the intake charge to an extent where the engine stalls and shuts down.

This can be avoided by sending a richer mixture into the cylinder such that the air-fuel ratio stays within the combustible range even after dilution.
Image is from the book Internal Combustion Engines(2nd Edition)-V. Ganesan