During the upward strokes, the bottom rail/segment scrapes oil into the groove through the expander. 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. Generally, holes at the back of the oil control ring groove can be found along the circumference in order to allow the oil draining to the crankcase. The top rail/segment scrapes oil back into the groove through the oil control ring expander. During the downward strokes, the bottom rail/segment scrapes oil directly back into the crankcase. The oil control ring is a twoвЂђdirection scraper ring that scrapes oil in both upward strokes and downward strokes. 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 threeвЂђpiece oil control ring consists of two segments and an expander in between the two segments. The twoвЂђpiece oil control ring consists of a ring body with two rails and a helical spring on the back providing the ring tension force. There are generally two types of oil control rings: twoвЂђpiece oil control ring and threeвЂђpiece oil control ring ( FigureВ 11). The oil control ring is used to meter and distribute lubrication oil onto the cylinder wall. This will be discussed in the вЂњRing dynamicsвЂќ section later in this chapter. 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.

The second ring with a negative static twist is more likely to flutter than a positive static twist second ring. It was found that the static twist has significant influence on the second ring fluttering and radial collapse.

It is also possible that the two conditions occur simultaneously. It depends on the ring and piston design which of the two conditions occur, ring fluttering or ring collapse. Severe engine blowby can occur at this ring collapse condition. 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. This radial movement is known as ring radial collapse. It is also possible for the second ring to move inward in the radial direction. When the ring fluttering occurs, another gas flow path between the ring and groove sides opens.

This unstable axial inвЂђgroove motion is recognized as ring fluttering. This process may repeat depending on the pressure buildup above the second ring when it is top seated. The inertial force may lift the second ring up at late compression stroke such that the second ring stays against the top flank of the groove. Due to this lowerвЂђpressure gradient across the second ring, the ring inertial force becomes competitive to gas pressure force. 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.

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