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Engine Component Damage (UK TPI)(2029773/4)

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with diagnosis.
In the first instance, the shape of the fracture can determine the direction of force. The following
diagrams show the direction in which the ring land moved as a result of excessive pressure from
either above or below. In the first image, as with the example above, it’s clear from the shape that the
piece moved in the downward direction – whereas the second image shows overstressing in the
upward direction (usually during installation).

There are three possible reasons for piston land fractures
1.

Knocking combustion

2.

Hydraulic locks

3.

Installation errors

1.1 Knocking Combustion
This means that the octane rating of the fuel is not capable of covering the entire operational needs of
the engine. In which case, either the compression ratio has increased or the octane rating of the fuel
has decreased. Although the former is more common in the development phase, for service
workshops in the aftersales environment, the likely cause is that the octane rating of the fuel is not
correct (diesel mixed in with the fuel). This is usually confirmed by the fracture being evident on the
pressure side of the piston.

When considering a diesel engine, the only reason for this type of damage is ignition delay.
1.2 Reasons for knocking combustion on petrol engines:


The anti-knock properties of the fuel are not suitable to cover the engine’s octane requirements
across the entire operating range. Therefore, the fuel does not correspond to EN228 standard



Diesel in with the petrol which lowers the octane rating – even a small amount will lower the
octane rating dramatically



Engine oil in the cylinder due to excessive oil consumption



Excessively high compression ratio caused by incorrect head gasket, build-up of carbon on the
piston crown/head or a machined cylinder head in the service workshop



Ignition timing too advanced



Mixture too lean which leads to high combustion temperatures and pre-ignition



Excessively high intake air temperatures caused by poor ventilation or high exhaust backpressure

1.3 Knocking combustion on diesel engines

file:///D:/ElsaWin/docs/hs2/A/en-GB/src/2b/714283/master.htm

12/05/2015