Marine Engineering Study Materials

Reasons for Cylinder Cover Cracking

Cylinder covers are exposed to high levels of working stress resulting from the thermal and pressure stresses of combustion, which are imposed on the  stress  resulting from initial head tensioning. These stresses could be increased by:

1. Excess cylinder combustion pressures. This would increase the mechanical or pressure stress on the head.
2. Excess cylinder thermal stress produced by excess cylinder temperatures, incorrect cylinder cooling or insufficient cooling water treatment. These would lead to larger temperature differences across the cylinder head material leading to larger thermal stresses.
3. Incorrect i.e. over-tensioning of the cylinder cover on assembly. The increase in initial tension would increase the total stress on the cylinder cover, possibly enough to produce cracks in the highly stressed areas of the cylinder cover.

How to Prevent Possibility of Cylinder Cover Cracking

The possibility of cracking would be reduced if all of the above causes were eliminated by correct operation and maintenance of the engine.

1. The cylinder combustion pressure would be monitored by indicator cards or peak pressure readings. Engine load could also be measured on-line by exhaust temperatures and turbocharger revolutions, and relating these to the engine model curves or test bed readings. The engine room staff would be told to monitor these readings and ensure the maximum levels given were not exceeded.
2. The temperature of the cylinder cooling would be closely monitored to ensure that the temperatures were not excessive (leading to overheating of the liner), or too cold (leading to higher temperature gradients across the liner wall, and hence higher thermal stress). Temperature monitoring would be via control room gauges, and alarm settings.
3. Initial tensioning of the cylinder head should be carried out to the recommended hydraulic tension or torque settings. Thus the gauges that are used to determine these levels must be calibrated regularly.
4. The reserve of cooling water nitrates must be maintained to reduce scaling or fouling of the cylinder cover heat transfer surfaces, and to maintain the fatigue life of the covers.