The 'skeletal joints' of the engine - piston rings

Similar to human bones and joints,Generator set'Piston rings' are divided into gas rings and oil rings according to their functions. They have the following functions.Piston rings are the parts that move in three directions among all engine parts.(i.e. axial movement, radial movement and rotational movement in the circumferential direction), and it is also a part with harsh usage conditions.At the moment of explosion in the engine combustion chamber, the gas temperature can reach 2000℃-2500℃, and its explosion pressure reaches an average of 50kg/cm2. The temperature of the piston head is generally not lower than 200℃.The piston makes reciprocating motion, and its speed and load are very large.Therefore, piston rings work under high temperature and high pressure conditions.In particular, the first gas ring withstands high temperatures and poor lubrication conditions. In order to ensure that it has the same or higher durability as the other rings, the working surface of the first gas ring is often treated with porous chrome plating. .The porous chromium plating layer has high hardness and can store a small amount of lubrication to improve the lubrication conditions and increase the life of the ring by 2-3 times.In recent years, most motorcycle engines use pistons whose length is shorter than the cylinder diameter. The head of this piston will swing when the upper stroke turns to the lower stroke, causing the upper and lower edges of the outer circle of the piston ring to be in tight contact with the cylinder wall. This causes the edge of the piston ring to be loaded and cause scratches.In order to avoid this abnormal phenomenon, the outer circle of the first gas ring is generally made into an arc shape, so that the edge angles of its upper and lower end surfaces do not touch the cylinder wall, and it is easy for the initial running-in of the engine. This kind of gas ring is called a barrel. Face rings are used in current high-power and high-speed internal combustion engines.
1. Maintain air tightness
Although today's manufacturing technology is very sophisticated and parts differences are kept to a minimum, due to the material, heat treatment and mechanical deformation after assembly, there are always very few leakage points in the air seal in the cylinder, which requires the engine to be inspected in the early stages of use. Good running-in and proper preheating after starting will gradually eliminate the uneven spots of the friction pair.If the cylinder seal is poor due to various reasons, it will cause the compression pressure to drop and the combustion gas to leak. The high-pressure and high-temperature gas will pass through the tiny gap between the cylinder wall and the piston ring, and the resulting failure will be damage. The necessary oil film between the piston ring and the cylinder wall forms a dry friction state of direct contact between metals, which leads to burns and strain on the piston, piston rings and cylinder due to dry friction, causing abnormal wear of the engine.The leaked high-temperature gas flows into the crankcase, causing the engine oil to deteriorate and produce hard sludge, causing piston rings to stick and other problems.From this point of view, it is important to ensure the airtightness of the piston ring in the cylinder to prevent any leakage.
2. Heat transfer effect
The piston of an internal combustion engine works under the action of high-temperature and high-pressure gas in each burst.Therefore, if the heat from the top of the piston is not dissipated and cooled in time, the upper part of the piston will be seriously overheated.The result is scratches due to abnormal expansion of the piston, early wear of the piston due to reduced hardness, sticky rings due to oil deterioration, damage due to reduced strength at the piston top and piston pin seat, etc.These all bring fatal danger to the normal operation of the engine.It can be seen that the function of the piston ring includes transmitting the high temperature of the piston caused by the combustion gas to the cylinder, that is, cooling the piston.According to relevant information, 70%-80% of the heat received by the atmosphere at the top of the piston is transmitted to the cylinder wall through the piston and dissipated.
3. Supporting function
The piston reciprocates due to gas pressure. This reciprocating motion is converted into rotational motion through the crankshaft, so the piston ring bears the side thrust component.Therefore, piston rings fill the gap between the piston and the cylinder, and are often in contact with the cylinder wall for sliding movement.It not only prevents blow-by and controls engine oil, but also prevents strong contact between the piston and the cylinder wall.The high-pressure gas reaches the back gap of the ring, and its pressure presses the outer circle of the piston ring against the inner wall of the cylinder, keeping the piston in a floating state.It can be considered that at this time, there must be appropriate side clearance and backlash between the piston ring and the ring groove.Under normal circumstances, the backlash has two functions: the first is to prevent the ring from sticking in the groove due to the expansion of the piston ring and the piston; the second is to increase the contact pressure of the sliding surface of the piston ring.It prevents strong contact between the piston and the cylinder wall.
4. Control engine oil
The piston rings slide back and forth along the cylinder wall under high load and in a high temperature atmosphere.In order to better perform its function, it is necessary to have a small amount of engine oil to lubricate the cylinder and piston, and the excess engine oil attached to the cylinder wall must be properly scraped off to prevent it from jumping up to keep the oil consumption moderate.
As we all know, during the intake stroke of a four-stroke engine, the pressure in the combustion chamber is lower than the pressure in the crankcase. Since this pressure difference acts as a pump for oil, the oil passes through the tiny gaps between the piston rings, piston and cylinder. And is sucked into the combustion chamber, resulting in a large increase in oil consumption due to oil channeling.Especially when the engine is idling, the throttle valve is basically closed, and when the negative pressure in the cylinder is large, this phenomenon becomes more serious.In order to control the upward channeling of engine oil, the outer circle of the second air ring on the piston is generally made into a tapered surface.The tapered ring can not only spread an oil film on the sliding surface of the piston when it moves upward, but also scrape off excess oil at the lower end of the cylinder wall when the piston ring moves downward. It can really kill two birds with one stone.In order to scrape away the oil splashed to the lower part of the cylinder wall, a steel plate combined oil scraper ring is added to the lower part of the second gas ring of the piston.This kind of ring is only characterized by its high contact pressure, and because the upper and lower scrapers can act separately, it has good adaptability even for cylinders with poor roundness.What's more important is that each scraper not only maintains a seal with the sliding part between the cylinder, but also maintains an air-tight effect on the engine oil between the upper and lower ends of the annular groove, so the oil sealing effect is good.

Two-stroke engines generally use oil mist lubrication.Part of the oil mist after the engine oil is mixed with gasoline and air lubricates the crankshaft connecting rod bearing and the lower part of the cylinder piston, and the other part enters the combustion chamber during the scavenging process. Under the high temperature and high pressure, some carbon molecules will always remain on the top of the piston and the first piston. In the ring groove of the gas ring.For this reason, wedge-shaped rings came into being.Its effectiveness lies in the fact that when the wedge-shaped ring moves radially, the gap becomes larger, and conversely, when the wedge-shaped ring moves inward, the gap becomes smaller.Therefore, the sludge remaining in the ring groove is ground and discharged together with the oil and exhaust gas, thus automatically removing carbon deposits.The wedge-shaped ring is generally installed on the first gas ring. In some engines, due to the large oil flow, in order to enhance the function of removing carbon deposits, wedge-shaped rings are installed on both gas rings.