General

In cooperation with a major automotive manufacturer a new measurement instrument has been developed. The instrument measures the internal diameter of fuel injectors and pipework along their length (ISO/CD 123296). The electro-pneumatic technique employed allows measurement of both straight and curved pipework over lengths of upto 1m and for a range of internal diameters from 1 - 3 mm. The internal diameter can be measured continuously and the following parameters determined:

• Change of effective diameter along the bore expressed as an equivalent diameter with an accuracy of + 3 µm.
• Changes in cross sectional area due to bends, kinks, blockages and deformations caused by faulty  assembly.
• Local surface features such as roughness, waviness, tool marks and cavitation.
• Volume and local volume changes from the averaged cross section and the corrected total length for straight and curved pipes.
• The high reproducibility of measurement allows determination of possible cavitation damage inside injectors or pipework that have been in use.

A range of connectors are available to allow pipework with different connection flanges to be fitted directly to the instrument.

Measurement principle:

By means of a compressed air jet a reference sphere is forced through the pipe to be measured. The reference sphere is restrained by means of thread, as the sphere travels through the pipe the thread is fed in a controlled manner from a reel. The length of thread that has been fed out is measured by means of an inductive position sensor. The gap between sphere and internal wall of the pipe or bore is determined by measuring the pressure drop detected by a precision pressure sensor. The pressure value is converted to a cross sectional area measurement based on calibration measurements of bores of known diameters made previously. The pressure drop caused by the pipe length is corrected for by the system software after measurement of pressure from both pipe ends.

A PC combines the measurement and control signals and produces a screen display of diameter as a function of position along the pipe:

Knowledge

Cavitation in fuel pipes

The so-called cavitation arises when in a flowing liquid the vapor pressure is locally falling short and then vapor bubbles are formed. This flow cavitation can occur inside fuel pipes at bending points in case the cross section of the pipe changes. Therefore one must make sure that the cross section doesn’t get smaller around bends of the fuel pipe.

The emerging vapor bubbles are unstable and implode when the reach areas of a higher vapour pressure. The vapor bubble doesn’t decay in a uniform manner but by forming a needle-like liquid beam which is oriented towards the pipe face and hits the surface with a velocity of up to 1 km/s. The material is initially deformed plastically then it passes a strengthening phase and finally it fatigues. Cracks are formed and a gradual destruction of the metal structure. This erosion cannot be avoided by using different materials. The proper remedy is to make sure that there are steady flow conditions inside the pipe.   Lit:  PhiuZ, 1/2011, page 44

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