Piston Ring Geometry of Running Service

When discussing piston ring geometry, one must consider medium speed 4-stroke engines and large bore slow speed 2-stroke engines as separate topics.

4-stroke engines

The ring geometry (profile of running face) on a medium speed 4 stoke engine has to be accurately produced to its final profile during manufacture. This is because the piston ring in a modern 4-stroke diesel engine is usually chrome plated and does not have the possibility to run in as quickly as an uncoated ring. Therefore the piston ring profile will vary little during its effective working life from it profile when new. The principal relevant force when considering ring profile is the twisting of the compression rings as they receive the force of the combustion and as they change direction during upward and downward strokes of the piston. This twisting (torsion) varies with the pressure and occurs to a much greater extent when the pressure is high. As the ring twists it “rolls” against the cylinder wall and develops a natural asymmetric barrel shape. This natural shape is copied during piston ring manufacture and the finished chrome plated ring has this shape profile ground onto the working face.

It is the top compression rings which are the most subject to twisting or rolling, and so it is these which are often fitted with asymmetric barrel faced rings. To reduce this twisting effect these rings are frequently manufactured with a large torsional chamfer on the inside top corner of the ring.

In the lover grooves, taper faced chrome or concentric barrel faced chrome plated rings are frequently used.

At the bottom of the piston or on the piston skirt there is usually one sometimes two spring loaded chrome plated oil scrapers to scrape away excessive lube-oil from the cylinder wall and return it to the crankcase.

2-stroke engines

In large bore 2 stroke engines, the design of the piston ring geometry has much more to do with the running in properties of the ring. One can say that during operation the piston ring acquires the profile that best suits it! The reason for this is that the majority of piston rings for 2-stroke engines are uncoated and the only possibility open to the manufacturer is to give the new ring a certain shape so as to best assist smooth running in. When it is impossible to accurately pre-define the final running shape, we at Precision Products believe that the best solution is to supply a new piston ring with a slightly tapered working face. The taper will allow the ring to touch the cylinder all at the same point no matter how much it twists. This single contact point gives higher contact load against the cylinder wall and this helps prevent blowby in the early hours of the rings life. It is these first hours after a new piston ring is fitted that are so crucial to its good running and long life. If significant blowby occurs at this time it will damage the oil film between ring and liner which may lead to direct contact metal to metal, overheating and possible micro seizure/scuffing. Even discounting these possible catastrophic problems, the tapered ring gives a better seal allowing a cooler liner surface. This permits a more stable viscosity of cylinder oil leading to better lubrication and therefore lower ring and cylinder wear throughout the running period.

Piston Ring joints

There are various joints that may be specified when ordering piston rings. They each have different performance characteristics and are fitted by engine builders according to the varying demands of different engine models.

Vertical and angle cut (scarf joints)

These are the most common and are used for several applications where there are no special operating conditions. The advantage of these rings is that they are less expensive to manufacture and rather strong against breakage. These two joints are similar and fulfil the same demands. Vertical cut rings are today used on the great majority of 4-stroke diesel engines and scarf joints are specified on most modern 2-stroke diesel engines. Sulzer specify their rings to have a scarf joint at 60º with all rings left hand cut whereas MAN-B&W specify scarf joints cut at 45º alternatively right and left cut.

Lap joint

An overlapping or lap joint is used when one wishes to prevent the gas passing straight through it. However it allows the gas to pass around the joint and down to the lower rings in the pack. For this reason it is not a fully gastight joint.

Axial Lap Joint

An axial lap joint is used in place of a normal vertical of scarf joint. The design of this type of joint makes this ring similar in characteristics to the normal vertical or scarf joint so far as the pressure drop is concerned, but it has the advantage that it seals the gas from behind the ring. This means that the gas behind the ring is prevented from escaping and will not be replaced by hot new gas on every stroke as usually happens. This will keep the ring cooler and cleaner reducing the tendency of coke to build up behind the ring.

Boxlap joint

This joint is used where there is a demand for a totally gastight seal. Piston rings equipped with this design of joint are normally used for two different purposes.

1. To avoid blow-by. This can occur if all normal vertical or scarf joints move into a row. To prevent it, a ring with a boxlap joint is fitted into groove no. 3or 4. When using a boxlap ring for this purpose, one must be aware that there is a danger of de-stabalising the ring pack and causing flutter by so doing.

2. The second purpose of using a boxlap ring is in the top groove. Where a top ring is not moving in its groove or where that movement is very limited, the hot gas passing through the gap of a normal piston ring joint may cause the cylinder liner at that point to rise in temperature around 100ºC higher than elsewhere on the liner. This may cause severe problems with lubricating oil and even cracked cylinder liners. By inserting a ring with a boxlap joint in the top groove you may prevent this hot gas passing through the joint and therefore keep the liner temperature down. Note that a boxlap joint is directional and is only gastight when pressure comes from above. Care must be taken when fitting the ring. These rings are always marked TOP.