What is Piston Rings | Types of Piston Rings.

 

Piston Rings- 

Piston rings are fitted into the grooves of the piston to maintain a good seal between the piston and the cylinder wall. There are three functions of the piston rings as follows : 

1. To provide a pressure seal to prevent blow-by of burnt gases. blow-by is the name that describes the escape of burned gases from the combustion chamber, past the piston, and into the crankcase. 
2. To form the main path for the conduction of heat from the piston crown to the cylinder walls. 
3. To control the flow of oil to the skirt and rings themselves in adequate quantity while preventing an excessive amount from reaching the combustion chamber with consequent waste and carbonization. 

Material- 

Piston rings are usually made of fined-grained alloy cast iron. This material possesses excellent heat and wear-resisting quantities inherent in its graphitic structure. The elasticity of this material is also sufficient to impart radial expansion and compression which is necessary for the assembly and removal of the ring: and particularly to enable it to exercise flexible pressure on the cylinder walls. 

Types of Piston Rings- 

There are two types of piston rings: 

1. Compression rings
2. Oil control rings.

Compression rings seal the air-fuel mixture as it is compressed, and the combustion pressure as the mixture burns. 

Oil control rings scrap off excess oil from the cylinder wall and returns it to the oil pan.

The piston rings are split so that they can be expanded and slipped over the piston head and into the recessed grooves cut into the piston. The rings usually have butt joints, but in some heavy-duty engines, the joint may be angled, lapped, or of the sealed type.

The outer diameter of the ring is somewhat larger than the cylinder bore and the split joint is open. When it is installed, it is compressed thus giving it an initial tension; the joint is nearly closed. When in position, it presses tightly against the cylinder wall.

Compression Rings : 

In modern engines, there are two or three compression rings fitted into the top grooves. The number of compression rings tends to increase the compression ratio. An oil control ring is fitted into the lower groove of the piston. Generally, the second and third compression rings are taper-faced and supplied to improve oil sealing. Taper-sided compression rings are used to overcome ring-sticking problems in high-output engines. It must not be fitted in grooves other than ones of the same section. A special ridge-dodger ring having a small step is specially designed for use in worn engines so that the wear ridge left by the original compression ring is cleared.

In many cases, oil control rings have a series of slots that transfer excess oil through holes in the piston groove to the inside of the piston and so to the sumps but leave sufficient oil to lubricate cylinder walls. Oil control rings give slightly more radial pressure than compression rings.

Counter board or scrapper rings- 

In many engines, counterbored and scrapper rings are used for the top and second compression rings. During the suction stroke, the rings twist due to the internal forces produced by cutting away a corner of the rings. Thus, as the rings move down they scrape all the oil that might have been left over the cylinder wall by the oil control rings. During the combustion stroke when the rings moved upward, they tend to skate over the oil film on the cylinder wall. Thus, less oil is carried up into the combustion chamber. During the power stroke, the combustion pressure causes them to untwist the rings, thus they have full-face contact with the cylinder walls for effective sealing. 

Headland rings- 

The Headland ring is a special type of compression ring, having an L-shaped cross-section. It covers or shields the headland area of the piston. This is the area of the piston between the top ring groove and the head of the piston. This area holds a certain amount of air-fuel mixture that does not burn because the cylinder wall and piston coll this air-fuel mixture below the combustion point. This unburned air-fuel mixture passes out of the engine and can produce smog. 

The headland ring also has the advantage of good sealing during the power stroke. As combustion starts, the pressure acts quickly on the upper lip of the ring. Forcing out it, thus having good sealing with the cylinder wall. 

Why two compression rings? 

Usually, two compression rings are installed on the piston. During the power stroke, the pressure increases as high as 70kg/Cm^2, and would be difficult for a single compression ring to hold this much pressure. If there are two rings, this pressure will be divided between the two rings. The loss of pressure past the upper ring is reduced. The load on the upper ring is also reduced so that it does not press quite so hard on the cylinder wall. Wearing of cylinders and rings is also reduced. 

Oil control ring : 

Some connecting rods have an oil split hole that splits oil from the oil pan onto the cylinder wall during each revolution of the crankpin. Far more oil reaches the cylinder wall than is needed. It must be scraped off and returned to the oil pin. Otherwise, it will go into the combustion chamber and burn. This would increase oil consumption so that the engine would require the addition of oil at frequent intervals. Also, the burned oil would foul the spark plug, increase the possibility of knocking and hamper the action of compression rings.

Instead of having a cooling, sealing, clearing, and lubricating effect on the cylinder walls, the oil must be removed from the cylinder wall every time so that it may not enter the combustion chamber. This is done by the oil control ring.

Oil control rings are of three general types : 

1. One piece slotted cast iron type.
2. One-piece pressed steel type.
3. Three-piece steel rail type with expander. 

One piece slotted cast iron type- 

This type of oil control ring has slots between the upper and lower faces that bear on the cylinder wall. The oil scrapped off the cylinder wall passes through those slots in the back of the oil ring grooves in the piston and from there, it returns to the oil pan. Some rings of this type are installed with expander rings. The expander spring increases the pressure of the ring on the cylinder wall which improves the oil-scraping effect.

One piece pressed steel type- 

This type of oil control ring is mostly used in engines with worn cylinder walls. It is made of pressed steel instead of cast iron. It can seal against only one side of the ring groove in the piston at a time, thus leaving an open path through which oil can pass upward toward the combustion chamber.

Three-piece steel rail type with expander- 

In this type of oil control ring, the expander spring forces the rails not only upward into contact with cylinder walls but also upward and downward against the upper and lower sides of the ring grooves in the piston. This provides a more effective seal at these three vital points to provide effective oil control.

Why only one oil control ring? 

Usually, four-piston rings were installed on the long skirt pistons of earlier passenger car engines. The lower two rings were oil control rings. But the use of lower hood lines reduces the number of rings to three. Because two compression rings are necessary to withstand the high combustion pressure, hence there remains only one oil control ring. It is quite possible to use one oil control ring because of engineering and manufacturing improvements and the more effective action of the modern oil control ring.