By Maklub Al Mostofa
Q. What is scavenging? Name the types of scavenge used for large two stroke engine. Describe the advantage of Uniflow scavenging.
It is the removal of residual exhaust gas and its replenishment with fresh air in an internal combustion of the engine. The fresh air intake and exhaust gas expel operation are not simultaneous fully but some degrees of overlap period are provided for better efficiency.
There are 3 types of scavenging process.
1. Loop scavenging
2. Cross scavenging
3. Uniflow scavenging
In this type of scavenging air passes over the piston crown and rises to form a loop. Ports are cut in the cylinder liner wall for this operation. The scavenge ports and exhaust ports are in the same side of the liner. Read more
For any fire to begin, the fire tringle needs to be completed. To complete a fire tringle there must be present a combustible material, oxygen or air to support combustion and a source of heat at a temperature high enough to start combustion.
In the case of scavenge fires:
the combustible material is oil. The oil can be cylinder oil which has drained down from the cylinder spaces, or crankcase oil carried upwards on the piston rod because of a faulty stuffing box. In some cases the cylinder oil residues may also contain fuel oil. The fuel may come from defective injectors, injectors with incorrect pressure setting, fuel particles striking the cylinders and other similar causes.
The oxygen necessary for combustion comes from the scavenge air which is in plentiful supply for the operation of the engines.
The source of heat for ignition comes from piston blow-by, slow ignition and afterburning, or excessive exhaust back pressure, which causes a blowback through the scavenge ports.