Page 31 - Introduction to Marine Engineering

Diesel engines 19

In cross scavenging the incoming air is directed upwards, pushing the
exhaust gases before it. The exhaust gases then travel down and out of
the exhaust ports. Figure 2.10(a) illustrates the process.
In loop scavenging the incoming air passes over the piston crown then
rises towards the cylinder head. The exhaust gases are forced before the
air passing down and out of exhaust ports located just above the inlet
ports. The process is shown in Figure 2.10(b).
With uniflow scavenging the incoming air enters at the lower end of
the cylinder and leaves at the top. The outlet at the top of the cylinder
may be ports or a large valve. The process is shown in Figure 2.10(c).
Each of the systems has various advantages and disadvantages. Cross
scavenging requires the fitting of a piston skirt to prevent air or exhaust
gas escape when the piston is at the top of the stroke. Loop scavenge

Scavenge
air in
Tinir

Opposed piston Exhaust valve

Figure 2.10 Scavenging methods, (a) Cross-flow scavenging, (b) loop scavenging,
(c) uniflow scavenging