Maintenance and Repairs

Gauging a Cylinder Liner

Measuring the wear on a marine diesel engine cylinder liner to establish the rate of wear
 

Gauging a liner is carried out for two reasons: To establish the wear rate of the liner, and to predict if and when the liner will require changing.

Although on a 2 stroke engine the condition of the liner can be established by inspection through the scavenge ports (evidence of blowby, scuffing etc.), the liner is gauged during the routine unit overhaul (15000 hrs), or if the unit has to be opened up for any reason

Because of the action of the piston rings, the varying gas pressure and temperature in the cylinder, the wear will not be even down the length of the liner. Consider the piston just beginning the power stroke. The gas pressure pushing the piston rings against the liner wall is at its highest; The liner surface temperature up at this part of the liner is about 200C, so the viscosity of the lubricating oil is low. The relative speed of the piston is low, and so the lubrication is only boundary. Because of these factors wear at the top of a liner increases to a maximum a few centimetres below the position of the top ring at TDC, and then decreases as the ring pressure and liner wall temperature decreases and the piston speed increases building up a hydrodynamic film between liner and ring surfaces. Then as the piston slows down and the rings pass over the port bars, the wear will increase due to boundary lubrication, a reduction in surface area, and oil being blown out into the scavenge space.

 

A liner is gauged by measuring the diameter of the liner at fixed points down its length. It is measured from port to stbd (athwartships) and fwd to aft. An internal micrometer is used because of its accuracy (within 0.01mm). To ensure that the liner is always measured in the same place, so that accurate comparisons may be made, a flat bar is hung down the side of the liner with holes drilled through where the measurements are to be taken.

Gauging a liner on a large bore RTA engine.

(Thanks to Emyr Davies)

Measurements are taken at more frequent intervals at the top of the liner where wear rate is expected to be highest.

To ensure accuracy, the micrometer gauge is checked against a standard, and the liner and micrometer should be at ambient temperature. If the temperature is higher then a correction factor can be applied. To ensure micrometer and liner are at the same temperature, lay the micrometer on the entablature for a few minutes before starting.

The readings can be recorded in tabular form, and from the data obtained the wear rate/1000 hours can be calculated. Wear rate varies, but on a large 2 stroke crosshead engine ideally should be about 0.05mm/1000 hours. On a medium speed trunk piston engine where the procedure for gauging is similar, the wear rate is around 0.015mm/1000 hours.

Cylinder Number: 1

Nominal Dia: 840mm

Total  Running hours:

60000

Running hours since last calibration:

15000

Gauging

 point

P - S F - A

Wear rate

(average)

P - S

Wear rate

(average)

F - A

last

calib.

P - S

wear

rate

P - S

last

calib.

F - A

wear

rate

F - A

1 841.2 841.26 0.02 0.021 840.95 0.017 841 0.017
2 841.38 841.44 0.023 0.024 841.1 0.019 841.17 0.018
Etc                

Figures are for illustration only.

Manufacturers quote max wear for a cylinder liner at about 0.8% of original diameter. If the wear rate is kept to a minimum, then the liner may last the life of the engine.

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