Explosions and fires in the exhaust manifold of large two stroke engines can occur if cylinder oil or unburnt fuel passes to the manifold an subsequently ignites.

LOW LOAD RUNNING

POOR MAINTENANCE

Choked scavenge ports will result in poor scavenging of the cylinder, with partially burnt or unburnt fuel passing into the exhaust manifold. Poor atomisation from injectors may have the same result. A scavenge fire could cause depletion of oxygen in the inlet air to the affected cylinder, and in turn cause incomplete combustion of the fuel injected into the affected cylinder. This unburnt fuel would then pass into the exhaust and mix with oxygen from the scavenging air from other cylinders. Combustion of this fuel would then be possible. If a fire, or sufficient after burning occurs within the exhaust manifold, the manifold becomes a high volume combustion chamber with the fire rapidly heating and expanding the hot gases exhausted from the cylinders before they enter the turbocharger. The energy from even a small amount of unburnt fuel when added to the exhaust gas flow entering the turbocharger would be sufficient to cause a significant energy imbalance, leading to uncontrolled acceleration of the turbocharger rotor. This can lead to disintegration of the compressor wheel, loss of blading due to high centrifugal forces on the turbine wheel and subsequent bursting of the casing.

Examples of Fires in exhaust manifolds leading to turbocharger failure:

MV Marine Express, 24 August 2000

• Combustible gas between the cylinder cover and piston crown flowed out through deep scratches in the surface of the No. 8 cylinder liner. [There were two such scratches vertically from the lubricating holes to the scavenging ports. One piston ring was broken and the others worn.]
• There was a fire in the scavenging space.
• The fire consumed the air in the scavenging space.
• Lack of air made fuel combustion worse.
• The main engine governor activated to increase revolutions (to maintain a certain range).
• More fuel was supplied to the combustion chamber.
• Non-combusted/rich fuel gas discharged to exhaust gas manifold.
• There was an explosion in exhaust gas manifold due to blow-by.
• The turbochargers over-sped.
• The turbo charger impeller touched the air casing and the turbine blades were torn off.
• The bearing around the rotor shaft was stuck.

MV Goliath, 22 September 2002 and 12 February 2003

• The similarity of the compressor disc failures supports the conclusion that both turbochargers underwent an uncontrolled transient overspeed event similar to the second failure.

• While it is not possible to state with certainty, the two possible mechanisms which led to the turbocharger failure were a slight slip of the compressor disc or a scavenge fire but was more likely to have been a scavenge fire based on the evidence.