FAQ Orals MEP (Scavenging & Scavenge Fire)

Question Answer On The Topic Scavenging And Scavenge Fire

What Is Scavenging?
The Process Of Exchanging Gases Present In The Combustion Spaces After Combustion With The Charged Fresh Air Is Called Scavenging.
 
What Is Uniflow Scavenging?
With Uniflow Scavenging, The Incoming Air Enters At The Lower End Of The Cylinder And Leaves At The Top. The Advantage Of Uniflow Is The Simple Design Of The Liner, Plus It Is The Most Efficient.

What Is Cross Flow Scavenging?
In Cross Flow Scavenging, The Incoming Air Is Directed Upwards, Pushing The Exhaust Gases Before It.  The Exhaust Gases Then Travel Down And Out Of The Exhaust Port.  Advantage Is No Exhaust Valve.
 
What Is Loop Scavenging In A 2-Stroke Engine?
Loop Scavenging Is Where The Incoming Air Passes Over The Piston Crown, Then Rises Towards The Cylinder Head Forcing Exhaust Gases Down And Out The Exhaust Ports, Just Above The Inlet Port.  The Advantage Of Loop Is That No Exhaust Valve Is Required.
 
What Is Compression And Explain Why Compression Is Required?
Compression Is Where The Air That Is Drawn Into The Cylinder Is Compressed On The Upward Stroke Of The Piston, Both Inlet And Exhaust Valves Being Closed On This Stroke.  As The Air Is Compressed By The Rise Of The Piston, The Air Temperature Is Consequently Raised.  As The Piston Reaches The TDC, Fuel Is Injected, Causing Ignition And Very Rapid Rise In Pressure Of The Gases Forcing The Piston Down.
 
 
What Could Start A Scavenge Fire?
The Main Cause Of A Scavenge Fire Is The Fouling Of Scavenge Spaces By The Blow By Of Products Of Incomplete Combustion Caused By Incorrect Cylinder Lubrication, Faulty Fuel Injection Equipments, Accumulation Of Such Mixtures In The Scavenge Space Can Be Set Alight By Sparks Or Flame Blow By.
 
What You Can Do To Prevent Scavenge Fires?
Conscientious Maintenance Of The Engine And Regular Inspection And Cleaning Of Scavenge Air Spaces Will Help Prevent Scavenge Fires.
 
What Are The Symptoms Of A Scavenge Fire?
When A Scavenge Fire Has Broken Out, The Scavenge Air Temperature And The Exhaust Temperature Of The Unit In Question Will Increase Considerably And In Some Cases The Turbocharger May Start Surging.  Even Hot Spots On The Scavenge Trunking Can Be Found.
 
What Would You Do In The Event Of A Scavenge Fire?
If A Scavenge Fire Were To Start, The Two Main Objectives Are To Confine The Scavenge Fire To The Scavenge Space And To Minimise Damage To The Engine.
In The Event Of The Fire Breaking Out, Inform Bridge That The Engine Is To Be Brought To Dead Slow Ahead And Also Inform The Chief Engineer.
The Fuel Should Be Cut Off To That Particular Cylinder.  The Cylinder Lub Oil Should Be Increased To Prevent Seizure And Wear.
If Fixed Fire Fighting Equipment Is Attached To The Scavenge Trunking, This Can Be Brought Into Operation, Depending On Severity Of Situation.  But In Most Cases The Fire Will Generally Subside Within 5-15 Minutes.
Once The Fire Is Out And Navigational Circumstances Allow It, The Engine Must Be Stopped.
Do Not Open Scavenge Space Doors Or Crankcase Doors Before Site Of Fire Has Cooled Down.  When Opening Up, Care Must Be Taken To Keep Clear Of Any Flame.
After Opening Up, All Scavenge Spaces Must Be Thoroughly Cleaned And All Debris Removed.  The Piston Rods And Cylinder Liner Should Be Examined For Surface Blemishes, Straightness, Etc., And The Diaphragm Glands (Stuffing Box) Examined To Ensure That They Are Operational And Not Damaged.
Also Piston Rings Should Be Checked, As Blow By May Have Been The Ignition Source Of The Fire.  If Possible The Piston Head In Question Should Be Renewed At The Earliest Possible Moment And The Damaged Unit Overhauled.
On Engines Fitted With Tie Bolts, It May Be Necessary To Re-tighten The Bolts Adjacent To The Fire.
When Starting The Engine Again, Care Must Be Taken After Switching On The Fuel To The Cylinder In Question, And That Also The Cylinder Lub Oil Quantities Are Reduced To Normal.
What Is Scavenging?
The Process Of Exchanging Gases Present In The Combustion Spaces After Combustion With The Charged Fresh Air Is Called Scavenging.
 
What Is Compression And Explain Why Compression Is Required?
Compression Is Where The Air That Is Drawn Into The Cylinder Is Compressed On The Upward Stroke Of The Piston, Both Inlet And Exhaust Valves Being Closed On This Stroke.  As The Air Is Compressed By The Rise Of The Piston, The Air Temperature Is Consequently Raised.  As The Piston Reaches The TDC, Fuel Is Injected, Causing Ignition And Very Rapid Rise In Pressure Of The Gases Forcing The Piston Down.