When the wear of a cylinder liner (the increase in liner bore diameter at and above the top ring reversal point) is plotted against running hours, the points are scattered because liner wear is not constant. Reasons for the scatter:
- Wear is highly dependent on load and speed; the engine rarely runs at constant power. Running hours give no account of the actual loading, so at equal running hours different ships produce different wear.
- The measurement is done at intervals and at slightly different circumferential positions; the point of maximum wear may not coincide with the fixed measurement line each time, and gauge/reading errors occur.
- Environmental and operating factors vary: fuel quality, fuel sulphur/BN match, cylinder oil feed rate, water temperature (cooling conditions), ambient air temperature, and specific operating events (slow steaming, manoeuvring, scavenge fires, contamination).
- Thermal and mechanical factors: at low load the liner runs cooler, increasing cold corrosion wear; high load increases abrasive/mechanical wear. The proportion changes.
- Cold corrosion and scuffing can cause sudden local wear steps which are not linear.
- Wear is not uniformly distributed around the bore; the most wear is near TDC and on the thrust/anti-thrust sides, so a standard measurement may bounce between maxima and minima.
To show the wear rate in a more acceptable form: wear should be plotted as a function of a load/time factor rather than raw hours, e.g. as wear against equivalent hours at a reference load (e.g. wear per 1000 hours at 85% MCR), or normally the mean wear rate in mm per 1000 hours is computed by fitting a best straight line (least-squares) through the points rather than connecting them point-to-point; the slope of the best-fit line gives a uniform average wear rate. Outliers are examined and the wear rate expressed as an average plus its scatter band. Very often the wear is also compared with the maker's allowable rate.
To forecast the length of life of a liner:
- Determine the maximum allowable liner wear (the maximum wear limit) allowed by the manufacturer/society, often quoted as a percentage of the bore (e.g. 0.6 to 1.0% of bore) at which the liner should be renewed/withdrawn.
- Using the well-established average wear rate (from the best-fit line), the life is calculated as: Life (hours) = (allowable wear, mm)/(average wear rate, mm per hour).
- Because wear rate is not constant, use the average worn rate at the reference load and compare with margin; also take into account that wear accelerates once the top ring groove or bore becomes worn, so the forecast is a first estimate, and the liner should be re-measured at intervals and the forecast revised accordingly, with the decision subject to the actual point of maximum wear and to the condition of the running surface, and to the remaining thickness for cylinder wall strength.