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Engine Operation

Piston Rings For Large Marine Diesel Engines

 

 

 

 

Piston rings are manufactured using the pot casting method, producing a short cylinder of oval cross section from which the rings are machined. This produces  a balanced and homogenous casting around the entire circumference of the ring.

Materials

Generally speaking the piston ring must be harder than the liner in which it runs. Rings are cast and machined from grey cast iron. This may include some additions such as chromium, molybdenum, vanadium, titanium, nickel and copper. In some engines nodular or spheroidal graphite iron is used, which has greater hardness and tensile strength, as has vermicular or compact graphite cast iron, the use of which is becoming more widespread. Alloying elements such as titanium and vanadium form hard insoluble carbides within the matrix.

Manufacture

The primary sealing effect of the piston ring is achieved by the inherent tension of the ring which makes it contact the cylinder wall around its whole circumference. Rings in early engines were hammered around their circumference to induce a tension in a circular ring. Two methods are used today; the thermally tensioned ring and the oval pot cam turning method. The cheaper method employed mainly for small engines is the Thermally Tensioned Ring. In this method the ring is machined from a circular pot to the cylinder diameter. A gap is cut in the ring into which a distance piece is inserted, thus expanding the ring. The ring and distance piece is then placed in an oven to relieve the induced stresses. This method suffers from the disadvantage the rings lose their tension in the heat of the engine.

The Oval Pot Cam Turning Method is more expensive but produces a ring which retains its tension when working in the engine. The rings are machined from the cast oval pots in a cam turning lathe. The lathe is equipped with an interchangeable copying cam , which controls the cutting tools. By changing the shape or ovality of the cam, the circumferential pressure distribution around the ring can be altered.

The advantage of this method is that when the ring is in storage, it is under no artificially induced tensions. Also when the ring is expanded for fitting over the piston the stresses induced in the back of the ring are not in addition to any previous built in stresses. A ring cast by this method always tends to return to its natural stress free state by pressing against the cylinder walls.

When the pot has been machined and the ring parted off, the gap is cut in the ring (7 - 13% dia) and the ring ground on its flanks to ensure good sealing in the piston ring grooves.

Plating of Rings

Piston rings need to combine strength with wear resistance. To enhance the wear resistant properties the rings can be plated with a layer of different material.

Chrome plating is the most common form of surface treatment It is used on the running surface of compression rings and on the flanks (landing surfaces). It is also used on oil scraper rings. Chromium plating combines good wear resistance with low friction and corrosion resistance. The plating must be of high quality to operate under the arduous conditions of load and temperature without peeling, breaking away or softening.

Plasma coating provides a running surface with better properties than chrome plating. A gas mixture is directed through an electric arc generated between a tungsten electrode and a water cooled copper tube . Very high temperatures are generated, the gas molecules atoms dissociate and into this plasma state metal, carbides, or ceramics are sprayed as a fine powder, these melt, and coat the ring surface.

 

Chromium plated, and plasma sprayed coatings are limited in thickness and the strength of the bonding to the parent metal. A method of laser treatment which hardens the running surface has been developed, which produces a wear resistant outside layer several times thicker than conventional coatings. The treated zone has a complex material matrix with high hardness and good sliding properties.

 

Soft Coatings for Running In

 

Copper Plating is plated directly on to the base material of the ring or on to the chromium plated layer. The plating is very thin and has a life long enough to allow bedding in of the ring whilst running in.

Plasma rings can be graphite coated to aid with running in.

Plain rings can be ferrox or phosphate coated. This creates a thin protective layer on the piston ring which absorbs oil and aids protection of the ring and liner during the first hours of use.

 

Ring Pack Designs

 

2 Stroke Ring Packs

4 Stroke Ring packs

Two Manufacturers of Marine Diesel Engine Piston Rings are Daros

 and Standard Piston Ring Co.

 

 

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