**2. Ring pack dynamics**

The piston ring dynamics is closely related to their functions, especially for gas control and oil control. Although the top ring is the most important part for gas sealing, while the oil control ring has the highest effect in control oil flow and consumption, the second ring also has significant influence on both gas and oil control. This section discusses the ring dynamics of the second compression ring. The theories can also be applied on the top compression ring and oil control ring as well and the details of the ring dynamics models can be found from Refs. [15–20].

As discussed in Section 1, there are two types of ring dynamics: ring fluttering and ring radial collapse. Piston ring fluttering is the axial movement under the consequence of external force unbalance, especially between gas pressure force and inertial force. The other loads acting on the ring, including friction force, oil film squeezing force, and so on, are relatively small in comparison [6]. It is noted that while the second ring friction is relatively low, the friction forces of the oil ring and the top ring during high cylinder pressures can be large. Also, only the second ring flutter and collapse that occurs around top dead center (TDC) firing condi‐ tions is described here. This region is also considered the most important region for ring flut‐ tering and collapse because of its significance on blowby and oil consumption.

Another phenomenon, radial collapse, can occur if the ring is lifted and seated against the top of the ring groove. When the ring is on the top side of the ring groove, the pressure force not only pushes the ring downward but also acts on the front face of the ring pushing it inward. The ring seals the gas pressure at the top, meaning the pressure behind the ring can be much lower. When the pressure force on the ring face exceeds the ring tension and the pressure force behind the ring, the ring collapse will occur. Once it collapses, the gases will escape past the ring face and equalize all around the rings. Once again, there will be no net gas pressure on the ring and the elastic tension of the ring will force the ring back out to the cylinder wall. As can be expected, there is no sealing between the ring face and the cylinder wall. As a result, the gas flow can pass the ring face resulting in high blowby. The ring collapse is one of the unstable behaviors of the ring.

It will depend on the ring and piston design as well as the operating conditions if ring flutter or collapse can occur. It is also possible that both ring flutter and collapse occur at the same time. In either case, the second ring loses its sealing capability allowing gases flowing either around the ring (in the fluttering case) or past the ring face (in the ring collapse case).

to move down, it will be less likely to collapse. Conversely, the positive‐twisted ring will be more difficult to push down; therefore, the ring will be more likely to collapse radially inward as the pressure above the ring can become higher. In summary, the configuration with posi‐ tive static twist tends to increase the pressure force holding the second ring down and pro‐ moting second ring stability. This positive twist ring is also more susceptible to collapse. On the contrary, the configuration with a negative static twist will tend to promote ring flutter.

Power Cylinder System for Internal Combustion Engines http://dx.doi.org/10.5772/intechopen.69762 171

When the second ring flutters or collapses, the blowby will generally be higher. This is because the ring does not seal the gases and the gases flow past the ring. While this can cause high blowby, the pressure in the second land will be very low. This will prevent reverse blowby, which is beneficial for oil consumption. More discussions about ring pack dynamics can be

Nowadays, researchers from the industry and the academic area are developing the ring pack dynamics model in three‐dimensional (3D) in order to capture the variation along the ring

However, ring radial collapse is less susceptible to occur.

**Figure 12.** Ring‐seating stability: (a) bottom‐seating stability and (b) top‐seating stability.

found from Refs [17–20].

The ring design itself also has significant influence on its stability, for example, ring static twist. A negative‐twisted second ring forms an outer edge seal between the ring and groove bottom sides when the ring is at the flank bottom. This allows gases to flow underneath the ring resulting in a very low net downward gas pressure force. In this case, the ring can be eas‐ ily lifted by the inertial force acting on the ring (**Figure 12a**). On the other hand, for a positive static twisted second ring, the seal between the ring and the groove bottom occurs at the inner bottom corner. This prevents higher‐pressure gas moving between the ring bottom and the groove bottom, resulting in a higher downward pressure force. The ring is not easy to be lifted by inertial force. **Figure 12a** depicts a simplistic illustration showing the gas pressure forces acting on the sides of the rings.

Similarly, the ring top‐seating stability (**Figure 12b**) can be explained similarly as for the bot‐ tom‐seating condition. However, it should be noted that since a negative‐twisted ring is easier

**Figure 12.** Ring‐seating stability: (a) bottom‐seating stability and (b) top‐seating stability.

control ring has the highest effect in control oil flow and consumption, the second ring also has significant influence on both gas and oil control. This section discusses the ring dynamics of the second compression ring. The theories can also be applied on the top compression ring and oil control ring as well and the details of the ring dynamics models can be found from

As discussed in Section 1, there are two types of ring dynamics: ring fluttering and ring radial collapse. Piston ring fluttering is the axial movement under the consequence of external force unbalance, especially between gas pressure force and inertial force. The other loads acting on the ring, including friction force, oil film squeezing force, and so on, are relatively small in comparison [6]. It is noted that while the second ring friction is relatively low, the friction forces of the oil ring and the top ring during high cylinder pressures can be large. Also, only the second ring flutter and collapse that occurs around top dead center (TDC) firing condi‐ tions is described here. This region is also considered the most important region for ring flut‐

Another phenomenon, radial collapse, can occur if the ring is lifted and seated against the top of the ring groove. When the ring is on the top side of the ring groove, the pressure force not only pushes the ring downward but also acts on the front face of the ring pushing it inward. The ring seals the gas pressure at the top, meaning the pressure behind the ring can be much lower. When the pressure force on the ring face exceeds the ring tension and the pressure force behind the ring, the ring collapse will occur. Once it collapses, the gases will escape past the ring face and equalize all around the rings. Once again, there will be no net gas pressure on the ring and the elastic tension of the ring will force the ring back out to the cylinder wall. As can be expected, there is no sealing between the ring face and the cylinder wall. As a result, the gas flow can pass the ring face resulting in high blowby. The ring collapse is one of the

It will depend on the ring and piston design as well as the operating conditions if ring flutter or collapse can occur. It is also possible that both ring flutter and collapse occur at the same time. In either case, the second ring loses its sealing capability allowing gases flowing either

The ring design itself also has significant influence on its stability, for example, ring static twist. A negative‐twisted second ring forms an outer edge seal between the ring and groove bottom sides when the ring is at the flank bottom. This allows gases to flow underneath the ring resulting in a very low net downward gas pressure force. In this case, the ring can be eas‐ ily lifted by the inertial force acting on the ring (**Figure 12a**). On the other hand, for a positive static twisted second ring, the seal between the ring and the groove bottom occurs at the inner bottom corner. This prevents higher‐pressure gas moving between the ring bottom and the groove bottom, resulting in a higher downward pressure force. The ring is not easy to be lifted by inertial force. **Figure 12a** depicts a simplistic illustration showing the gas pressure forces

Similarly, the ring top‐seating stability (**Figure 12b**) can be explained similarly as for the bot‐ tom‐seating condition. However, it should be noted that since a negative‐twisted ring is easier

around the ring (in the fluttering case) or past the ring face (in the ring collapse case).

tering and collapse because of its significance on blowby and oil consumption.

Refs. [15–20].

170 Improvement Trends for Internal Combustion Engines

unstable behaviors of the ring.

acting on the sides of the rings.

to move down, it will be less likely to collapse. Conversely, the positive‐twisted ring will be more difficult to push down; therefore, the ring will be more likely to collapse radially inward as the pressure above the ring can become higher. In summary, the configuration with posi‐ tive static twist tends to increase the pressure force holding the second ring down and pro‐ moting second ring stability. This positive twist ring is also more susceptible to collapse. On the contrary, the configuration with a negative static twist will tend to promote ring flutter. However, ring radial collapse is less susceptible to occur.

When the second ring flutters or collapses, the blowby will generally be higher. This is because the ring does not seal the gases and the gases flow past the ring. While this can cause high blowby, the pressure in the second land will be very low. This will prevent reverse blowby, which is beneficial for oil consumption. More discussions about ring pack dynamics can be found from Refs [17–20].

Nowadays, researchers from the industry and the academic area are developing the ring pack dynamics model in three‐dimensional (3D) in order to capture the variation along the ring circumference with the consideration of cylinder liner ID deformation. In addition, the influ‐ ence from piston secondary motion can also be implemented to further understand the ring pack behavior. This will capture the gas flow in the circumference direction, which the current commercial two‐dimensional (2D) models are not capable of. As a result, the ring dynamics, oil consumption, friction, and wear for the ring pack can be better modeled and understood to guide design. The next section is an introduction to the 3D modeling work for the ring pack.

The 2D ring pack dynamics model is still widely used in the automotive and heavy‐duty industries during product development, given the experience and fidelity built on this approach. Some improving activities include implementing wear model at the ring face and side based on different wear mechanism, oil consumption model due to oil evaporation, oil throw‐off, oil scraped back to the combustion, and so on. In addition, 3D ring pack dynamics models are being developed using different approaches, including full FEA with hexahedron element, discretizing the ring using space beam elements, and so on, with different orders of success. The 3D model approach will be discussed in the next section with more detail.
