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Horror Stories

A Catalogue of Errors Causes Crankcase Explosion

 

 

It was lucky that no one was injured in this crankcase explosion which  lifted and buckled the engine room floor plating,  blew the spare gear store door (which was 2 decks above the crankcase) through the frame , blew the oil mist detector off the engine, and caused extensive damage to lighting, vent trunking, paintwork and of course the crankcase relief valves and doors. The workshop door was buckled and de-laminated The control room door wire reinforced glass was blown through into the control room where shards of glass became embedded in adjacent woodwork.

 

The explosion happened at about 1800 hrs, not long after the staff had left engine room for the day.

 

On investigation the cause of the explosion was found to have been a hole in number 1 cylinder piston crown. Hot combustion gases had then been able to pass down the piston oil cooling return to the crankcase, forming and then igniting the oil mist.

 

The photograph shows the oil puddle on top of the piston, with a pen lying next to the crack for scaling.

 

Burn away was excessive on the piston crowns, with numbers 1 and 6 out of tolerance. (number 6 piston crown was also cracked)

 

 

 

The hole in number one piston crown had been caused by erosion due to poor fuel injection. All pistons were examined, and erosion damage and cracking found on all of them. The "burning" of the piston crowns giving them the telltale "elephant skin" surface. The underside of the piston crowns had 3mm build up of carbon deposits which would have compromised the cooling of the piston.

 

The fuel valves were examined: Blockages were found, the nozzles were not genuine spare parts and the push rod profiles were incorrect. The injectors were lifting at 30bar below the recommended pressure, and the nozzle holes had worn so that a jet of fuel was being injected rather than an atomised mist. Bearing damage had resulted from overloading due to poor combustion.

The crankcase relief valves were found to have been incorrectly assembled with defective flame arrestors. This caused them to jam open once they had lifted, and this may have contributed to a violent secondary explosion.

The oil mist detector had alarmed in the five second period between the first abnormality indication - ME governor minor fault and the ME cyl no.1 outlet high temp. Due to the short time-span between first indication and explosion, it would be doubtful if the monitor could have slowed the engine quickly enough to prevent an explosion.

Information of this event was taken from the Isle of Man Accident Investigation website website. The full report can be downloaded here in pdf format

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