Friday, 1 June 2012

Cylinder covers

Cylinder covers
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 Cylinder heads are exposed to maximum gas pressures and temperatures. They must therefore have adequate strength and cooling. This results in complex structures of strengthening ribs and cooling water passages. The design of heads is further complicated by the need to house various valves, fuel, air start, relief etc. Where exhaust valves are situated in the head the structure design has to take into account the relatively high local temperatures around the valve which can cause thermal stressing. The combustion chamber may be formed by either shaping the cylinder cover or the piston crown. A flat piston crown is usually used with a shaped cover further complicating design and construction.
As the head runs at a fairly high temperature the cooling water must also be at a reasonably high temperature. This further thermal stressing. It is therefore usual to have the cooling water for the head in series with the jacket. The covers are attached to the cylinder block by means of large diameter bolts. The gas loads acting on the head are thus transferred to the cylinder block from which the tie bolts transfer it to the bedplate and then to the hull of the ship.
The original Sulzer engines employed single piece cylinder covers, but thermal stress cracks developed in relatively uncooled section were the conical part of the combustion chamber changed to the flat top.
In order to avoid this problem some allowance was required for thermal expansion, and this was provided by having a two part cover with an inner and outer section.
The inner section was of cast iron due to the complicated shape and the outer section cast steel for strength, a soft iron ring provided the joint between inner and outer sections. When the two parts are bolted together the head may be treated as a single unit.
For recent engines the single piece bore cooled steel cylinder cover has been developed and presents no particular problems.

Sulzer RD cylinder cover

Sulzer RD cover

B & W cylinder safety valve

This type of valve is suitable for use with the special long studs used in modern engines. The safety valve is small an indicates the onset of over load. To release excessive pressure from the cylinder the cover is able to stretch the studs release the pressure and reseat. Hopefully cleanly.< class ="noindent">

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