Piston & Piston Ring Assembly - Piston & Piston Ring Assembly
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Piston & Piston Ring Assembly

Recently received many inquiries from customers for Four matchings (piston, piston rings, piston pin, liner). Pistons and piston rings as a key explanation:
We’ve removed a piston & ring assembly from the engine so we can look at it in detail.

Top Ring

This is referred to as the ‘upper compression ring'(The designation ‘upper’ is standard nomenclature; this is the ring that is closest to the piston’ stop, or crown.). The upper compression ring is the piston ring that operates under the harshest conditions with respect to thermal and mechanical loading. Its job is to form a gas-tight barrier between the piston and cylinder wall in order to ‘seal the combustion chamber.

Second Ring

This is referred to as the ‘lower compression ring’. One of its jobs is to work together with the top ring in order to ‘seal’ the combustion chamber.

Oil Ring

As its name ‘oil control ring’ implies, this ring scrapes excess lubricating oil off the cylinder wall, maintaining proper lubrication while keeping oil consumption within proper limits.

Assembling the rings onto the piston

One of each of the above ring types, ‘top’, ‘second’, and ‘oil ring’ is usually installed on each piston in a typical engine. Such an arrangement makes for what we call a ‘three ring set’ or a ‘three-ring pack’. Using this arrangement, how many rings in total would be installed in a four-cylinder engine? Twelve is the correct answer, i.e., 4 cylinders x 3 rings/cylinder = 12 rings. This can be considered ‘the usual’ number of rings found in a standard 4-cylinder truck engine. A 6-cylinder engine would then have 18 rings, an 8-cylinder engine 24 rings, and so on. Each of the three rings in a set is a ‘specialist’ and will use a distinct combination of shapes, materials, heat treatment and/or surface coatings in order to perform its assigned function in an optimal way

Looking more closely at the top and second rings, an observant person notices immediately that there are open joints in the periphery of these rings. That is, they are not closed circles. Such a person would also notice something like a spring in the middle of the oil ring. In fact, this expander section does behave like a spring, generating a uniform tension in the oil ring so as to keep it forced tightly against the cylinder wall.

Basic Ring Function

Gas Seal Function

This means that the combustion chamber must be made as gas-tight as possible, so that the pressure generated by the quickly burning combustion gases will move the piston in the cylinder causing the crankshaft to turn, making power available. Not only important for the combustion/expansion stroke, gas-tightness is also very important for the intake, compression and exhaust strokes as well. This general function can be simply called “gas sealing”.

Heat transfer Function

The piston rings act to carry heat away from the hot piston into the cooled cylinder wall/block of the engine. Heat energy flows from the piston groove into the piston ring and then into the cylinder wall, where it eventually will be transferred into the engine coolant. This heat transfer function is very important to maintaining acceptable temperatures and stability in the piston and piston rings, so that sealing ability is not impaired.

Oil Control Function

The piston rings require some oil for lubrication, however, it is desirable to keep this amount to a minimum. The rings act in a scraping manner, keeping excess oil out of the combustion chamber. In this way, oil consumption is held at an acceptable level and harmful emissions are reduced.

The three boxes above show the primary functions performed by piston rings. In almost all cases, each individual ring in a three-ring set will be designed so as to optimize or help the functions of the other two rings. In this way, it’s easy to understand that while each ring in the three-ring pack is unique, the ring pack as a whole is really designed as a ‘system’, where each ring is ‘tuned’ to make the complete 3-piece set work most effectively in the engine.

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 (Running 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.

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.

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

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.

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 gasoline engines applications. We refer to this ring as an oil ring assembly. This oil ring assembly consists of an 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.

With the increasingly stringent requirements for engine power, economy, environmental protection and reliability of the entire automobile, tThe piston has developed into a high-tech product that integrates many new technologies such as light-weight and high-strength new materials, special-shaped outer circular composite profiles, and special-shaped pin holes, to ensure the heat resistance, wear resistance, stable guidance and good sealing function of the piston, reduce the frictional power loss of the engine, and reduce fuel consumption, noise and emissions.  In order to meet the above functional requirements, the outer circle of the piston is usually designed as a special-shaped outer circle (convex to ellipse), that is, the cross section perpendicular to the axis of the piston is an ellipse or a modified ellipse, and the ellipticity changes according to a certain rule along the axis direction, and the ellipticity accuracy is 0.005mm; the outer contour of the piston longitudinal section is the fitting curve of the higher order function, and the contour accuracy 0.005~0.01mm; In order to improve the bearing capacity of the piston and increase the power of the engine, the pin hole of the high-load piston is usually designed into a micro-inner cone or normal stress curved surface (shaped pin hole), and the pin hole size accuracy is up to IT4 level, contour accuracy is 0.003mm.

As a typical key part of automobile, piston has strong technological characteristics in cutting and processing. Piston manufacturing industry usually consists of general-purpose machine tools and special equipment that combines the characteristics of piston technology to form a machining production line. Therefore, special equipment has become the key equipment for piston cutting processing, and its function and accuracy will directly affect the quality indicators of the key characteristics of the final product.

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