The scavenging process is an incompressible process, which infers that it is a volumetric process. As such, its success depends on the volume of scavenging air. For a given port configuration, the higher the scavenging air volume the higher, the trapped fresh air volume in the cylinder. But this method leads to a less trapped fresh air mass in cylinder because of its lower density.
The aim of two-stage scavenging is to increase the amount of the trapped fresh air mass. For this purpose the scavenging air is divided into two portions, namely warmed and unwarmed. During the first stage the cylinder is scavenged with the warmed portion resulting in a higher volumetric purity factor due to the higher scavenging air volume. The unwarmed air portion having a higher density is in the second stage to fill the already well-scavenged cylinder. The maximum fresh air mass in the cylinder is obtained if the two portions are divided almost equally.
Two-stage scavenging offers the following advantages: increased exhaust energy, reduced dimensions of the scavenging air cooling system, better combustion and more reliable piston running conditions at part loads.
