When you think of piston rings, have you ever considered that they are the smallest component of the internal combustion engine, yet have the largest responsibility? When you assemble an engine, you never really grasp what the piston ring is going to do during its lifespan, making the performance of this diminutive component even larger in reality.
The piston rings have three major tasks to ensure the engine makes consistent power efficiently.
Piston rings commonly used on small engines include the compression ring, wiper ring, and oil ring.
Compression Ring
A compression ring is the piston ring located in the ring groove closest to the piston head. The compression ring seals the combustion chamber from any leakage during the combustion process. When the air-fuel mixture is ignited, pressure from combustion gases is applied to the piston head, forcing the piston toward the crankshaft. The pressurized gases travel through the gap between the cylinder wall and the piston and into the piston ring groove. Combustion gas pressure forces the piston ring against the cylinder wall to form a seal. Pressure applied to the piston ring is approximately proportional to the combustion gas pressure.
Wiper Ring
A wiper ring is the piston ring with a tapered face located in the ring groove between the compression ring and the oil ring. The wiper ring is used to further seal the combustion chamber and to wipe the cylinder wall clean of excess oil. Combustion gases that pass by the compression ring are stopped by the wiper ring.
Oil Ring
An oil ring is the piston ring located in the ring groove closest to the crankcase. The oil ring is used to wipe excess oil from the cylinder wall during piston movement. Excess oil is returned through ring openings to the oil reservoir in the engine block.
Material Selection
Cast Iron
A popular material used for piston rings is cast iron, often referred to as grey iron. The biggest advantage in using cast iron to manufacture pistons rings is that it will not gall or scuff the cylinder bore. And as long as the cast iron ring is sufficient in size, it will provide adequate seal. If the operating loads are increased or the size is decreased for the application, then ring seal can become an issue. When cast iron is used for the top ring, it is usually coated with molybdenum or chrome to prevent bore wear. If cast iron is used for the second ring, no coating is applied. Cast iron material is very brittle; under a microscope, the grain structure of cast iron is rectangular and sharp. This is why if you were to try to twist a cast iron ring it will break because the grain structure is easily fractured. Cast iron is popular because it is somewhat cost-effective to manufacture. The drawback to its use is that several manufacturing steps are required for completion–and it’s not ideal for high-performance engines.
Ductile Iron
Ductile iron is another material used in the manufacture of piston rings; it has been around for quite a few years and is still common today. The forming process for ductile iron piston rings is extremely similar to that used to manufacture a cast iron ring. The composition of the material is taken from cast iron by extracting the carbon flakes–which is mostly graphite–and forming that material into a cylindrical mold to set the outside dimension. Then the inside dimension can be cut out. The rings can then be sliced from the “gun barrel” and heat-treated. Under a microscope, ductile iron has round nodular shaped grains that are very strong, unlike the grain structure for cast iron. If you were to take the ductile iron ring and try to break it you would find that it will only bend and twist into a pretzel shape. Ductile iron is twice as strong as cast iron and is used in high-output applications. Since most diesel engines are turbocharged, ductile iron rings were commonly used for their resistance to failure in high compression situations with high operating cylinder pressures.
The up and down motion of the piston keeps the keystone ring loaded in the ring groove of the piston and as a byproduct, also keeps the ring groove clean from the soot of the diesel fuel. The uniquely-shaped ductile iron keystone ring is not commonly used today, however. Because the use of Exhaust Gas Recirculation has become standard on nearly all internal combustion engines, when this shape is used, carbon packing tends to stick the ring in the piston groove, causing failure.
If you are not sure of what material your rings are made of, don’t try to bend them. An easy way to test them is to drop them on a table in your shop. If the ring makes a ringing sound it is ductile iron, and if it simply thuds onto the table, it is manufactured from cast iron.
Steel
Today–especially in high-performance and severe-duty applications–steel is used to construct piston rings. The advantages of steel rings are many: they are easier to manufacture, stronger and harder than ductile iron, and resist breakage especially in those demanding power-adder applications. The disadvantage? The materials are more expensive.
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