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Case study of two failed injectors

Failure investigation of two injectors from a V12 marine diesel engine using μ-CT.

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The problem

Two injectors from a V12 marine diesel engine were investigated using μ-CT.  Injector A2 showed a significant amount of contamination on its tip and would open only when a pressure of 280 bar was applied while injector B2 would not open even applying a pressure of 400 bar.

The A2 injector

The tip area of the contaminated A2 injector was scanned producing a 3D volume which was ‘virtually’ sliced to determine the extent of the contamination.  The scan clearly showed that the density of the contaminant (blue) was significantly lower than that of the metallic nozzle tip (yellow) and needle (pink).  A single slice unmistakably displayed that four nozzles out of eight were significantly blocked by a deposit which had solidified across its surface. This deposit would hinder the release of fuel and it was thought to explain the malfunction of this injector. µ-CT confidently resolved this problem; if the tip was cut to be embedded, part of the deposit could have broken down partially destroying crucial evidence compromising the investigation.

B2 Injector

The B2 injector

The B2 injector was inoperable, a fast scan revealed a spring-loaded needle valve that closed or opened the nozzle holes. It was hypothesized that a spring failure of some form could prevent the correct operation of the injector and a shadow in the scan of the B2 injector was noted adjacent to one of the active coils of the spring.  A higher resolution scan was performed revealing the suspected foreign object (green) being flake shaped with its shortest side (through thickness < 0.5 mm) in the radial direction of the spring and the longest (> 2 mm) following the circumference of the coil preventing the operation of the needle valve.

The cause

A destructive strip and examination of the injector sub-assemblies was directed from the CT data.  Upon opening of the injector, a flake of debris of size 3 mm x 1.8 mm x 0.4 mm was found.

Image of debris found in the tip

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