Piston Ring Function - Heavy Truck Parts Store
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Air Dryer
2020-09-27
Automatic adjustment arm
Automatic adjustment arm
2020-10-19

Piston Ring Function

Top Ring

Looking at a typical ring set for today’s automotive engine application, we’ll first examine the Top Ring.

Shown is a top ring for an 86mm diameter cylinder bore. The ring’s axial width (height) is 1.2mm and its radial thickness is 3.1mm. The ring in the photo is shown in the so-called “free state”, that is, what the ring looks like before installation into the engine. The ‘free gap’ (open portion of the ring) is about 9mm.

When the ring is installed (on the piston) in the engine, it is ‘squeezed’ closed to its effective diameter of 86mm. Since the ring wants to expand back to its free state, this built-in ‘springiness’ makes the ring fit tightly against the cylinder wall. The force needed to close the ring to 86mm can be measured and is usually called ‘ring tangential tension’. Tangential tension is an important ring characteristic and is always considered carefully during the design phase.

Looking at the outside shape of the ring, we see the peripheral working face (The working or running face refers to that portion of the ring that is sliding in contact with other surfaces.) of the ring. This ‘running face’ stays in contact with the cylinder wall, usually cushioned on a thin layer of oil. This surface ‘slides’ on the cylinder wall and some resistance, called sliding friction, is generated.

Piston Ring
Piston Ring

Here we show a profile of the ring’s ‘running face’ magnified 1000 times. Under magnification, the surface appears to be composed of minute convex spherical surfaces. This surface shape plays an important role in forming an oil film during high-speed operation. A hard film of CrN (nitrided chromium) is formed on the running face by an ion plating (known as “PVD”) process. Other surface treatments for the ring’s running face include nitirding and hard chrome plating.

Second Ring

Next comes the second ring, also for an 86mm cylinder bore, with an axial width (height) of 1.2mm and a radial thickness of 2.9mm.

Looking at the appearance of the second ring’s running face, we can see that it serves to complement the top ring in its gas sealing function, but it also provides oil control. The photo shows the free gap and running face of the second ring.

Piston Ring
Piston Ring

When squeezed closed to its 86mm working diameter, the ring’s tension will hold it tightly against the cylinder wall, providing a sealing function. At the same time, the ring’s ‘undercut’ bottom side produces a running face profile that acts to scrape oil downward.

A magnified measurement trace shows the ring’s running face in profile. This face is not straight-vertical, but is ‘tapered’. This ring shows a taper face angle of approximately . This taper-face profile provides good gas sealing action, efficient oil scraping action, and allows for a fast running-in period (break-in phase).

Oil Ring

Engine lubricating oil performs an essential protective function in the engine by allowing parts to slide and by preventing them from making direct metal-to-metal contact (reducing friction that occurs when the outer periphery of the piston ring rubs the inner periphery of the cylinder), which causes wear, and in the worst case, total engine failure. Engine oil reduces operating friction, making the engine more efficient; while also serving to cool critical engine components and trapping harmful dirt particles.

Appearance of the Oil Ring
Piston Ring
Piston Ring

Can you see how it is assembled? This 3-piece oil ring is installed into the piston groove and functions as an assembly to generate the required performance. The oil ring assembly above has a diameter of 86mm, an axial width of 2.0mm and a radial thickness of 2.5mm.

Cross-Sectional Shape of the Oil Ring Side Rails

The thin, upper and lower plate-shaped rings are referred to as side rails. Each side rail is about 0.4mm wide. Each oil ring assembly requires two side rails: an upper side rail and a lower rail. The periphery of each side rail makes contact with the cylinder walls and slides. A hard film of CrN (nitrided chromium) has been applied to the running face by a PVD process. Other surface treatments for the side rails’ running face include nitriding and hard chrome plating.

Appearance of the Oil Ring Spacer expander

The wave design seen in the picture below allows the spacer expander to act as a spring. The spring action of the spacer expander presses the side rails against the cylinder wall until the correct tension and unit wall pressure are achieved.

Piston Ring
Piston Ring

As mentioned above, the two side rails and the spacer expander form an oil ring set or assembly. This is why it is referred to as a three-piece ring.

2-Piece Oil Ring

The oil ring illustrated above is a two-piece assembly that is used most often for diesel engines and some of gasoline engines applications. We refer to this ring as a oil ring assembly. This oil ring assembly consists of a M-shaped steel rail and a cylindrical coil spring. The DVM is characterized by its lightweight, high conformability and its ability to accurately control oil and reduce oil consumption.

Piston Ring
Piston Ring
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