**1.3. Cylinder**

The cylinder of a reciprocating engine is the part through which the piston travels. The cyl‐ inder may be sleeved or sleeveless depending on the metal used for the engine block. For example, a cast iron engine block generally does not require cylinder sleeve because the iron is hard enough to resist wear between the piston ring and the cylinder wall. However, for aluminum alloy engine blocks that can be found in almost all daily drive cars, cylinder sleeves are required since the aluminum alloy is not hard enough to resist wear between the piston ring and the cylinder wall interface.

Cylinder liners, or cylinder sleeves, are manufactured nowadays using the centrifugal casting process. The centrifugal casting process refers to the technique for casting, which has a perma‐ nent mold spinning continuously along its center line at a constant speed. At the same time, molten metal is poured to the mold and thrown toward the inside wall of the mold. Then, the molten metal is solidified after cooling. The spinning orientation of the casting machine can be either horizontal or vertical, depending on the parts it is producing. Horizontal spin is preferred for long and thin cylinder, while vertical spin is preferred for short and wide cylinders. Aluminum engines without sleeves can also be found. The aluminum cylinders are 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. This leads to reduction of viscous friction as claimed.

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‐ sumption increases.

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

$$
\delta R = \sum\_{i \neq 0} \left( A\_i \cos(i\theta \,) + B\_i \sin(i\theta \,) \right) \tag{1}
$$

where *δR* is the deviation from roundness, *<sup>A</sup> <sup>i</sup>* and *B*<sup>i</sup> are Fourier coefficients and *i* is the order of the series.


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

**Table 2.** Cylinder bore distortion.

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

The cylinder of a reciprocating engine is the part through which the piston travels. The cyl‐ inder may be sleeved or sleeveless depending on the metal used for the engine block. For example, a cast iron engine block generally does not require cylinder sleeve because the iron is hard enough to resist wear between the piston ring and the cylinder wall. However, for aluminum alloy engine blocks that can be found in almost all daily drive cars, cylinder sleeves are required since the aluminum alloy is not hard enough to resist wear between the piston

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

Cylinder liners, or cylinder sleeves, are manufactured nowadays using the centrifugal casting process. The centrifugal casting process refers to the technique for casting, which has a perma‐ nent mold spinning continuously along its center line at a constant speed. At the same time, molten metal is poured to the mold and thrown toward the inside wall of the mold. Then, the molten metal is solidified after cooling. The spinning orientation of the casting machine can be either horizontal or vertical, depending on the parts it is producing. Horizontal spin is preferred for long and thin cylinder, while vertical spin is preferred for short and wide cylinders. Aluminum engines without sleeves can also be found. The aluminum cylinders are

into the groove and eventually back to the crankcase.

168 Improvement Trends for Internal Combustion Engines

**1.3. Cylinder**

ring and the cylinder wall interface.
