**1. Introduction**

The internal combustion engine converts thermal energy of the combustible fuel into mechan‐ ical energy that moves the piston and eventually the crankshaft. This energy conversion pro‐ cess occurs within the engine power cylinder system. The power cylinder system comprises the following components: piston, piston rings, cylinder liner, wrist pin, and connecting rod.

and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, The piston is the main component that delivers the mechanical energy through reciprocating motion. And this reciprocating motion is transmitted into rotational motion of the crankshaft to output power through the connecting rod. The connecting rod's small end is connected to the piston through the wrist pin, and the rod's big end is connected to the crankshaft. Combustion occurs above the piston in the combustion chamber, which is sealed by the ring pack, especially the top compression of the ring pack. **Figure 1** shows these major components of the power cylinder system.

power. About 4–15% of that energy is wasted as mechanical friction loss. And the rest of the energy, which is almost over half of the chemical energy, is dissipated as other forms, for exam‐ ple, heat transfer, blowby loss, and so on, as shown in **Figure 3** from the study by Richardson [1].

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And about half of the mechanical friction loss is attributed to the friction in the power cylinder system, including the piston, ring pack, and the connecting rod as shown in **Figure 4** [1]. The other part is due to the friction of other components, for example, the valve train system, the

The friction loss distribution among piston, piston ring pack, and the connecting rod for the power cylinder system can be found in **Figure 5** [1]. As can be found, the piston and ring pack

crankshaft bearings, and so on.

**Figure 4.** Mechanical friction power distribution.

**Figure 3.** Power distribution for diesel engines.

account for higher friction loss than the connecting rod.

**Figure 1.** Power cylinder system.

A complete engine cycle consists of four different strokes for a four‐stroke engine along with the piston‐reciprocating motion. These four strokes are intake stroke, compression stroke, expansion stroke, and exhaust stroke as shown in **Figure 2**.

As for a modern diesel engine, which is known for its better efficiency over its gasoline coun‐ terpart, only about 40% of the energy produced by the engine is converted to the engine output

**Figure 2.** Four strokes for a complete engine cycle.

power. About 4–15% of that energy is wasted as mechanical friction loss. And the rest of the energy, which is almost over half of the chemical energy, is dissipated as other forms, for exam‐ ple, heat transfer, blowby loss, and so on, as shown in **Figure 3** from the study by Richardson [1].

**Figure 3.** Power distribution for diesel engines.

The piston is the main component that delivers the mechanical energy through reciprocating motion. And this reciprocating motion is transmitted into rotational motion of the crankshaft to output power through the connecting rod. The connecting rod's small end is connected to the piston through the wrist pin, and the rod's big end is connected to the crankshaft. Combustion occurs above the piston in the combustion chamber, which is sealed by the ring pack, especially the top compression of the ring pack. **Figure 1** shows these major components

A complete engine cycle consists of four different strokes for a four‐stroke engine along with the piston‐reciprocating motion. These four strokes are intake stroke, compression stroke,

As for a modern diesel engine, which is known for its better efficiency over its gasoline coun‐ terpart, only about 40% of the energy produced by the engine is converted to the engine output

expansion stroke, and exhaust stroke as shown in **Figure 2**.

of the power cylinder system.

162 Improvement Trends for Internal Combustion Engines

**Figure 1.** Power cylinder system.

**Figure 2.** Four strokes for a complete engine cycle.

And about half of the mechanical friction loss is attributed to the friction in the power cylinder system, including the piston, ring pack, and the connecting rod as shown in **Figure 4** [1]. The other part is due to the friction of other components, for example, the valve train system, the crankshaft bearings, and so on.

The friction loss distribution among piston, piston ring pack, and the connecting rod for the power cylinder system can be found in **Figure 5** [1]. As can be found, the piston and ring pack account for higher friction loss than the connecting rod.

**Figure 4.** Mechanical friction power distribution.

**Figure 5.** Power cylinder system friction power distribution.

#### **1.1. Piston**

The piston of an internal combustion engine is the main component to transmit the thermal energy into mechanical energy. High‐pressure gas from the combustion of the fuel‐air mix‐ ture pushes the piston downward to deliver mechanical energy. Thus, the working condition for the piston is severe. Pistons in small engines are made of aluminum while for large lower speed applications, the pistons are made of cast iron [2]. As the load continues to increase for engines, especially in the heavy‐duty industry, steel pistons become widely used nowadays. **Figure 6** shows a typical piston for diesel engine with the definitions of the key geometries shown in **Table 1**.

In addition to the barrel/parabolic profile in the axial direction, piston skirt usually has oval‐ ity in the circumference direction as well. The ovality is defined as the difference between the diameter in the thrust axis and the diameter in the pin axis. The ovality is introduced to reduce wear and the risk of scuffing. Development work related to piston dynamics, friction,

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The ring pack is typically composed with three rings: two compression rings and one oil con‐

**1.** To seal the combustion chamber in conjunction with the piston lands and the cylinder wall, in order to prevent the high‐pressure gas from leaking into the crankcase that is wasted in

**2.** To control the lubrication oil from getting into the combustion chamber from below the

**3.** To transfer heat from the piston to the cylinder wall and eventually to the cooling system. Since the piston crown is exposed to the combustion chamber, it is critical to reduce the

piston as well as to distribute the lubrication oil evenly on the cylinder wall.

piston temperature in order to guarantee the piston's working condition.

**Figure 8** shows typical ring packs for modern gasoline and diesel engines.

scuffing, and so on can be found from the references by different researchers [3–11].

trol ring. The main functions of the ring pack are listed as follows:

**No. Definitions** 1 Piston crown 2 Piston skirt 3 Top land

5 Top groove

**Table 1.** Definitions of key piston geometries.

4 Second and third land

6 Second and third groove

**1.2. Ring pack**

producing power.

**Figure 7.** Piston skirt profile.

The piston skirt generally has a barrel/parabolic profile that promotes hydrodynamic lubri‐ cation due to the edge effect (**Figure 7**). This skirt profile needs to be optimized in order to minimize piston friction. Piston skirt also grows outward in the radial direction at a high temperature during engine operation.

**Figure 6.** Key piston geometries.


**Table 1.** Definitions of key piston geometries.

In addition to the barrel/parabolic profile in the axial direction, piston skirt usually has oval‐ ity in the circumference direction as well. The ovality is defined as the difference between the diameter in the thrust axis and the diameter in the pin axis. The ovality is introduced to reduce wear and the risk of scuffing. Development work related to piston dynamics, friction, scuffing, and so on can be found from the references by different researchers [3–11].

#### **1.2. Ring pack**

**1.1. Piston**

shown in **Table 1**.

temperature during engine operation.

**Figure 6.** Key piston geometries.

**Figure 5.** Power cylinder system friction power distribution.

164 Improvement Trends for Internal Combustion Engines

The piston of an internal combustion engine is the main component to transmit the thermal energy into mechanical energy. High‐pressure gas from the combustion of the fuel‐air mix‐ ture pushes the piston downward to deliver mechanical energy. Thus, the working condition for the piston is severe. Pistons in small engines are made of aluminum while for large lower speed applications, the pistons are made of cast iron [2]. As the load continues to increase for engines, especially in the heavy‐duty industry, steel pistons become widely used nowadays. **Figure 6** shows a typical piston for diesel engine with the definitions of the key geometries

The piston skirt generally has a barrel/parabolic profile that promotes hydrodynamic lubri‐ cation due to the edge effect (**Figure 7**). This skirt profile needs to be optimized in order to minimize piston friction. Piston skirt also grows outward in the radial direction at a high The ring pack is typically composed with three rings: two compression rings and one oil con‐ trol ring. The main functions of the ring pack are listed as follows:


**Figure 8** shows typical ring packs for modern gasoline and diesel engines.


**Figure 7.** Piston skirt profile.

*1.2.2. Second compression ring*

result, blowby gas may increase.

this chapter.

*1.2.3. Oil control ring*

**Figure 10.** Second compression ring cross section.

possible that the two conditions occur simultaneously.

The second ring is a scraper ring which is recognized as 80% for scraping the lubrication oil down and 20% for sealing the combustion chamber. Because of the wedge effect, the scraper ring promotes hydrodynamic lubrication during the up‐strokes (compression and exhaust strokes) and scrapes oil down during the down‐strokes (intake and expansion strokes). **Figure 10** shows two types of second rings: one is scraper ring and the other is Napier ring. For the second ring, static twist is usually introduced by cutting off the ring material at one of the back corners. If the lower inside corner is cut off, the ring is a negative static twisted ring, while if the upper inside corner is cut off, the ring has a positive static twist configuration.

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Although the gas pressure gradient across the second compression ring is much lower than that of the top ring, the second ring also has a noticeable effect on gas flow and gas dynamics. Due to this lower‐pressure gradient across the second ring, the ring inertial force becomes competitive to gas pressure force. The inertial force may lift the second ring up at late com‐ pression stroke such that the second ring stays against the top flank of the groove. This pro‐ cess may repeat depending on the pressure buildup above the second ring when it is top seated. This unstable axial in‐groove motion is recognized as ring fluttering [12]. When the ring fluttering occurs, another gas flow path between the ring and groove sides opens. As a

It is also possible for the second ring to move inward in the radial direction. This radial move‐ ment is known as ring radial collapse [12]. When the ring radial collapse occurs, the gas above the ring can flow past the ring directly between the ring face and the cylinder wall to the lower land. Severe engine blowby can occur at this ring collapse condition. It depends on the ring and piston design which of the two conditions occur, ring fluttering or ring collapse. It is also

It was found that the static twist has significant influence on the second ring fluttering and radial collapse. The second ring with a negative static twist is more likely to flutter than a positive static twist second ring. However, if the second ring is lifted against the top flank of the groove, the positive static twist configuration will be more likely to collapse than the negative twist configuration. This will be discussed in the "Ring dynamics" section later in

The oil control ring is used to meter and distribute lubrication oil onto the cylinder wall. There are generally two types of oil control rings: two‐piece oil control ring and three‐piece oil

**Figure 8.** IC engine ring pack: (a) gasoline engine and (b) diesel engine.

#### *1.2.1. Top compression ring*

The top compression ring is the first ring and the main component sealing the combustion chamber for engine blowby control. The top ring is also under the most severe working condi‐ tion since it is directly exposed to the combustion gas and usually under high pressure and high temperature.

The top compression rings for gasoline engine usually have a rectangular cross section. However, for diesel engine operation, the top compression rings usually are keystone rings (**Figure 9**) which promotes the breakup of the deposits between the ring and piston groove, thus reduc‐ ing the possibility of micro‐welding between the piston ring and the piston groove. The top compression ring usually has a parabolic or a barrel profile at its front face in order to enhance the hydrodynamic lubrication between the ring face and the cylinder wall interface (**Figure 9**).

The sealing capability of the top compression ring has significant influence of engine blowby because of the high gas pressure gradient across the top ring. Engine blowby is recognized as the high‐pressure gas leaking into the crankcase through the ring pack. Thus, the top com‐ pression ring is desired to conform to the cylinder wall evenly along the ring circumference. Also, due to the high gas pressure gradient across the top ring, the top ring stays against the bottom side of the piston groove most of the time during the engine cycle.

**Figure 9.** Top compression ring cross section.

#### *1.2.2. Second compression ring*

The second ring is a scraper ring which is recognized as 80% for scraping the lubrication oil down and 20% for sealing the combustion chamber. Because of the wedge effect, the scraper ring promotes hydrodynamic lubrication during the up‐strokes (compression and exhaust strokes) and scrapes oil down during the down‐strokes (intake and expansion strokes). **Figure 10** shows two types of second rings: one is scraper ring and the other is Napier ring. For the second ring, static twist is usually introduced by cutting off the ring material at one of the back corners. If the lower inside corner is cut off, the ring is a negative static twisted ring, while if the upper inside corner is cut off, the ring has a positive static twist configuration.

Although the gas pressure gradient across the second compression ring is much lower than that of the top ring, the second ring also has a noticeable effect on gas flow and gas dynamics. Due to this lower‐pressure gradient across the second ring, the ring inertial force becomes competitive to gas pressure force. The inertial force may lift the second ring up at late com‐ pression stroke such that the second ring stays against the top flank of the groove. This pro‐ cess may repeat depending on the pressure buildup above the second ring when it is top seated. This unstable axial in‐groove motion is recognized as ring fluttering [12]. When the ring fluttering occurs, another gas flow path between the ring and groove sides opens. As a result, blowby gas may increase.

It is also possible for the second ring to move inward in the radial direction. This radial move‐ ment is known as ring radial collapse [12]. When the ring radial collapse occurs, the gas above the ring can flow past the ring directly between the ring face and the cylinder wall to the lower land. Severe engine blowby can occur at this ring collapse condition. It depends on the ring and piston design which of the two conditions occur, ring fluttering or ring collapse. It is also possible that the two conditions occur simultaneously.

It was found that the static twist has significant influence on the second ring fluttering and radial collapse. The second ring with a negative static twist is more likely to flutter than a positive static twist second ring. However, if the second ring is lifted against the top flank of the groove, the positive static twist configuration will be more likely to collapse than the negative twist configuration. This will be discussed in the "Ring dynamics" section later in this chapter.

#### *1.2.3. Oil control ring*

*1.2.1. Top compression ring*

166 Improvement Trends for Internal Combustion Engines

**Figure 8.** IC engine ring pack: (a) gasoline engine and (b) diesel engine.

**Figure 9.** Top compression ring cross section.

high temperature.

The top compression ring is the first ring and the main component sealing the combustion chamber for engine blowby control. The top ring is also under the most severe working condi‐ tion since it is directly exposed to the combustion gas and usually under high pressure and

The top compression rings for gasoline engine usually have a rectangular cross section. However, for diesel engine operation, the top compression rings usually are keystone rings (**Figure 9**) which promotes the breakup of the deposits between the ring and piston groove, thus reduc‐ ing the possibility of micro‐welding between the piston ring and the piston groove. The top compression ring usually has a parabolic or a barrel profile at its front face in order to enhance the hydrodynamic lubrication between the ring face and the cylinder wall interface (**Figure 9**). The sealing capability of the top compression ring has significant influence of engine blowby because of the high gas pressure gradient across the top ring. Engine blowby is recognized as the high‐pressure gas leaking into the crankcase through the ring pack. Thus, the top com‐ pression ring is desired to conform to the cylinder wall evenly along the ring circumference. Also, due to the high gas pressure gradient across the top ring, the top ring stays against the

bottom side of the piston groove most of the time during the engine cycle.

The oil control ring is used to meter and distribute lubrication oil onto the cylinder wall. There are generally two types of oil control rings: two‐piece oil control ring and three‐piece oil

**Figure 10.** Second compression ring cross section.

control ring (**Figure 11**). The two‐piece oil control ring consists of a ring body with two rails and a helical spring on the back providing the ring tension force. The three‐piece oil control ring consists of two segments and an expander in between the two segments. The expander provides the radial force to conform the ring to the cylinder wall and also the axial force to push the ring against the top and bottom sides of the groove. The oil control ring is a two‐ direction scraper ring that scrapes oil in both upward strokes and downward strokes. During the downward strokes, the bottom rail/segment scrapes oil directly back into the crankcase. The top rail/segment scrapes oil back into the groove through the oil control ring expander. Generally, holes at the back of the oil control ring groove can be found along the circumfer‐ ence in order to allow the oil draining to the crankcase. In some piston design, instead of using these holes at the back of the groove, cast slots are introduced at the bottom edge of the groove for oil drain as an easier solution. During the upward strokes, the bottom rail/segment scrapes oil into the groove through the expander. The recovery of oil scraped by the top rail/segment during these upward strokes depends on the external force on the top rail/segment. At times, the external axial force on the oil control ring overcomes the expander force. As a result, an oil flow crevice is formed between the oil control ring and the groove sides allowing the oil drain into the groove and eventually back to the crankcase.

treated with nickel silicone alloy coating or other plasma coating that help reduce cylinder wear. Other techniques have also been explored by the researchers in order to reduce engine friction. One method is to introduce dimples at the mid‐stroke to the cylinder walls [13]. This helps reduce friction because at the mid‐stroke, the piston rings are generally under hydrody‐ namic friction when the piston speed is high. By introducing the dimples to the cylinder wall, the effective area of contact between the ring faces and the cylinder wall has been reduced.

Typical surface roughness for cylinder liner is 0.4–0.5. This roughness has been reduced sig‐ nificantly, which could help reduce engine oil consumption. Rougher cylinder walls can help retain lubrication oil on the liner surface between micro‐valleys, which is similar to the dimple liner [13]. As a result, friction between the ring/cylinder wall and the piston skirt/cylinder wall interfaces can be reduced due to the lubrication oil in the micro‐valleys. However, this micro‐valley‐retained oil is not scraped from the liner during engine down‐strokes and can stay exposed to high‐temperature gases. As a result, more oil is evaporated and the oil con‐

Cylinder liners are no longer circular when the engine is in operation. The deformation results from mechanical distortion from bolting the cylinder block to the cylinder head, thermal dis‐ tortion when the thermal load on the liner is not uniform, mechanical load when piston is slapping into the liner, the pressure load from the combustion event, and so on. Cylinder bore distortion is measured from an experiment by researchers [14]. For modeling concern, the

and *B*<sup>i</sup>

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

(*Ai* cos(*iθ* ) + *Bi* sin(*iθ* )) (1)

are Fourier coefficients and *i* is the order

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This leads to reduction of viscous friction as claimed.

cylinder bore distortion is usually defined by a Fourier series [4, 5]:

**Zero order Change in bore diameter**

First order Bore eccentricity Second order Oval deformation Third order Three‐lobe deformation Fourth order Four‐lobe deformation

*i*=4

*δR* = ∑*<sup>i</sup>*=0

**2. Ring pack dynamics**

**Table 2.** Cylinder bore distortion.

where *δR* is the deviation from roundness, *<sup>A</sup> <sup>i</sup>*

The orders of the distortion are recognized in **Table 2**.

sumption increases.

of the series.

**Figure 11.** Oil control ring: two‐piece oil control ring (left), three‐piece oil control ring (right).
