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How to avoid scavenge fires - Internal combustion engine procedure

A basic part of the cycle of an internal combustion engine is the supply of fresh air and removal of exhaust gases. This is the gas exchange process. Scavenging is the removal of exhaust gases by blowing in fresh air. Charging is the filling of the engine cylinder with a supply or charge of fresh air ready for compression. With supercharging a large mass of air is supplied to the cylinder by blowing it in under pressure.

Efficient scavenging is essential to ensure a sufficient supply of fresh air for combustion. Cylinder oil can collect in the scavenge space of an engine. Unburned fuel and carbon may also be blown into the scavenge space as a result of defective piston rings, faulty timing, a defective injector, etc.

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A build-up of this flammable mixture presents a danger as a blow past of hot gases from the cylinder may ignite the mixture, and cause a scavenge fire. A loss of engine power will result, with high exhaust temperatures at the affected cylinders. The affected turbo-chargers may surge and sparks will be seen at the scavenge drains.

Once a fire is detected the engine should be slowed down, fuel shut off from the affected cylinders and cylinder lubrication increased. All the scavenge drains should be closed. A small fire will quickly burn out, but where the fire persists the engine must be stopped. A fire extinguishing medium should then be injected through the fittings provided in the scavenge trunking. On no account should the trunking be opened up.

To avoid scavenge fires occurring the engine timing and equipment maintenance should be correctly carried out. The scavenge trunking should be regularly inspected and cleaned if necessary. Where carbon or oil build up is found in the scavenge, its source should be detected and the fault remedied. Scavenge drains should be regularly blown and any oil discharges investigated at the first opportunity..

Uniflow scavenging
Fig: Uniflow scavenging

Dealing Scavenge Fires (Slow & Medium Speed 2-Strokes)

Fires in the scavenge spaces of diesel engines can be caused by wear of mechanical components, faulty fuel injection, inefficient exhaust system and incorrect cylinder lubrication.

It is also important that oil leaks into the scavenge spaces are prevented. The scavenge trunking must be kept in good clean condition so that the risk of scavenge fires is kept to a minimum. When inspecting scavenge trunks, the condition of piston rings should also be checked, remember to carefully ventilate scavenge spaces before inspecting.

Scavenge fires are extremely dangerous and it is important that quick and effective action is taken to rectify the situation.
  1. Engine revolutions must immediately be reduced to a minimum.
  2. If possible the fuel is to be cut off from the cylinder unit concerned.
  3. If more than one cylinder is involved then it will be more effective if the engine is stopped and the turning gear engaged and continuous turning commenced.
  4. The cylinder lubricators are then to be advanced to the maximum setting on the effected cylinders.
  5. Turbocharger air intakes are to be blanked off.
  6. Scavenge space relief valves where possible can be secured, and isolating flaps shut.
  7. All external scavenge space surfaces must be carefully scrutinised for outbreaks of fire and strategic cooling applied as necessary.
  8. There are various types of scavenge space fire extinguishing systems in use i.e. steam, dry powder, and CO2.

The system applicable to your engine is to be operated strictly according to the manufacturer’s instructions. Particular attention must be paid to any precautions which have to be taken, e.g. the engine must only be operated within the rpm ranges specified by the manufacturer.

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