Yesterday, a friend asked me, saying that there are many auto repair masters on the Internet who are saying that engine carbon deposits will trap the piston, which will lead to engine burning oil. He wondered, how did this carbon deposit trap the piston, and did the carbon deposit really cause the engine to burn the oil? How can this be avoided? In addition, are the methods of removing carbon deposits mentioned on the Internet useful, can they really release the piston ring and restore elasticity?
This is really a good question, after I gave him an answer, I thought that there should be many people who have such doubts, so I wrote out the process of answering for your reference.
First, let's talk about the piston rings of the engine. Today's car engines, called reciprocating piston engines, the piston moves up and down in the cylinder, converting the heat energy after fuel combustion into mechanical energy for external output. Here, the degree of sealing between the piston and the cylinder greatly affects the efficiency of the engine. To this end, the engineers designed the piston ring, which fits snugly on the inner wall of the cylinder liner to strengthen the seal between the piston and the cylinder, and there is a gap of 0.05 to 0.1mm between the piston and the cylinder liner.
A piston ring is a metal ring, which is divided into two types: a gas ring and an oil ring. The main role of the air ring is to seal, generally there are two; the main role of the oil ring is to scrape the oil cloth oil, improve the lubrication of the cylinder wall, and generally have one. The piston ring is mounted on the piston, and the installation site is called the piston ring groove. When the piston ring is loaded into the cylinder, three gaps are formed between the piston ring groove, the piston ring and the cylinder wall: end gap, side gap and back gap. These three gaps are very small, generally 0.3 to 0.5 mm, side gaps of 0.05 to 0.1 mm, and back gaps of 0.1 to 0.3 mm.
These three gaps form two sealing surfaces and a labyrinth-type air leakage channel between the piston ring and the piston and cylinder wall, of which the first air ring seals about 80 to 90% of the gas, the second air ring seals about 10 to 20% of the gas, and the oil ring seals about 5% of the gas. Coupled with the secondary sealing effect of the high-pressure gas, the gas that eventually leaks from the combustion chamber to the crankcase is only 0.2% to 1.0% of the intake air volume, and the sealing is relatively successful, which is also one of the important reasons why the reciprocating piston engine still has strong vitality.
In addition, since the piston ring has a "three-gap", it moves up and down in the cylinder. When the piston moves downward in the cylinder, the oil on the cylinder wall enters the backlash through the side gap of the piston ring; when the piston moves upward in the cylinder, the oil in the backlash will enter the side gap and then be sent to the combustion chamber to participate in combustion; this phenomenon is repeated during the operation of the engine, and the oil is gradually consumed. This phenomenon is called "piston ring oil pumping action", which is one of the inherent characteristics of reciprocating piston engines, and any brand and model of engine will consume oil, but to varying degrees. The larger the three-gap of the piston ring, the more intense the pump oil action and the more oil is consumed. During the use of the engine, due to wear and tear will cause the three gaps to become larger and larger, so the oil consumption will be more and more.
It is precisely because the piston ring cannot be completely sealed, coupled with the presence of the piston ring pump oil action, so the piston ring part will inevitably generate carbon deposits. But the formation mechanism of gas ring and oil ring carbon deposit is different, let's talk about it separately.
First, let's talk about the gas ring. The gas ring is in direct contact with the combustible gas at high temperature and high pressure, and the temperature is extremely high when working. High temperature and high pressure combustible gas from the gap between the cylinder wall and the piston into the piston ring groove, encounter the piston ring pump oil up the oil, will lead to oil coking, curing; in addition, when the engine shutdown, the last injection into the combustion chamber of the fuel is not burned, in the piston residual high temperature action, will also coking curing, and eventually will form carbon deposited in the piston ring groove. This is the formation principle of gas ring carbon deposition. Only in this part of the gas ring, the engine oil brought up by the piston ring pump oil is very small, mostly gasoline is not completely burned deposited carbon deposits. That is to say, most of the carbon deposited in the gas ring is caused by gasoline, and the quality and combustion state of gasoline directly affect the amount and nature of carbon deposits.
The piston ring in the piston ring groove is constantly moving up and down, while the cylinder is not absolutely round, the piston in the cylinder up and down reciprocating movement, the piston ring is also constantly compressing and stretching, in the piston ring end gap and side gap part of the carbon deposition, constantly squeezed, is not retained, only the carbon deposit in the back gap can remain. That is to say, the carbon deposit is only left in one side of the air ring, it is impossible to hold the gas ring to death, nor can it cause the engine to burn the oil. At the same time, carbon deposition is deposited in the backlash, which objectively plays a role in reducing the backlash, but can strengthen the sealing and reduce the role of the pump oil, so it is said that the slight carbon deposition is good for the engine.
Let's talk about oil rings. The main function of the oil ring is to scrape off the excess lubricating oil on the cylinder wall, and then flow back to the oil pan through the return oil hole on the piston, while placing a thin oil film on the cylinder wall. There are two types of oil rings: integral and combined. The integral structure is relatively simple, similar to the air ring, opening some small holes or narrow grooves for oil return on the circumference of the ring, and there is an elastic lining spring on the back, its advantage is that the structure is simple, the cost is low, and the disadvantage is that the oil scraping effect is general; the combined oil ring structure is more complex, composed of scraping steel sheets and elastic lining springs, there is no side gap in the installation state, the lining spring elasticity is large, the oil return ability is strong, the adaptability to the cylinder wall is strong, and the scraping effect is better than the integral type. A German engine known for burning oil in the early days was to use an integral oil ring, and the oil consumption was huge; later changed to a combined oil ring, and the oil consumption dropped significantly.
So how is the carbon deposit in this area formed? Here, the high temperature and high pressure gas in the combustion chamber is blocked and intercepted by the two gas rings above, and the gas that enters the oil ring is very small, and the impact on the formation of carbon deposits is almost negligible. Carbon deposits in this area are mainly formed by motor oil. The main function of the oil ring is to scrape the oil cloth oil, so it is always covered with oil. Under the action of the piston at high temperatures, these oils will slowly coke, solidify, and eventually form carbon deposits. This is the formation mechanism of carbon deposits in the oil ring area.
These carbon deposits are deposited on the oil ring or piston back oil hole, which will shrink or even block the oil return hole, reduce the oil scraping effect of the oil ring, and the oil scraped off cannot be smoothly returned to the oil pan; if the carbon deposit is deposited on the elastic lining spring, it will reduce the elasticity of the lining spring, resulting in a decrease in the overall elasticity of the oil ring, the adaptability to the cylinder wall is weakened, and the scraping effect is reduced; if there is too much carbon deposition, the entire oil ring is deposited with carbon deposits, then the oil ring is really "locked", without any elasticity, and the scraping effect is greatly reduced. The consumption of engine oil has risen sharply, which is the so-called "burning oil".
To sum up, the gas ring cannot be caught by carbon deposition, and the carbon deposit in the gas ring part has no effect on burning the oil; and the oil ring is likely to be caught by carbon deposition, and it will indeed cause the engine to burn the oil after it is held. Those big names on the Internet say that carbon accumulation hugs the piston ring, mainly refers to the oil ring being held, but some people deliberately say it is not so clear.
Knowing the formation mechanism of carbon deposits, we can prevent the speed and quantity of carbon deposits in a targeted manner. Carbon deposition in the combustion chamber and gas ring part is mainly affected by the quality of gasoline and the combustion state of gasoline, so the use of high-quality gasoline and the complete combustion of gasoline as much as possible are the main methods to prevent carbon deposition, such as avoiding long-term low-load operation of the engine, avoiding long-term idle speed, regular running high speed, etc.; and the carbon deposition of the oil ring part is mainly affected by the quality of the oil, so the use of high-quality, pure oil is the main method to reduce the carbon deposition of the oil ring.
Finally, let's talk about carbon deposit cleaning. Now cleaning engine carbon deposits is already a huge turnover business, low cost, high profits. Almost all 4s shops and repair shops have a variety of carbon deposit cleaning methods, such as walnut sand, dry ice, hanging bottles, cleaning agents and so on. However, these methods, without exception, can only clean the carbon deposit on the surface, such as the piston head, the combustion chamber wall, etc., but the carbon deposition in these parts has almost no impact on the performance of the engine; the oil ring carbon deposit that really has the greatest impact on the engine performance cannot be washed at all, and it is impossible to release the oil ring and restore the elasticity, even if the oil is added to the cleaning agent, it is impossible to remove the carbon deposit on the oil ring in a short time. Therefore, Lao Hou summed up these carbon deposits in a sentence: the place where it should be washed cannot be washed, and what is washed off is flashy.
Therefore, if you want to use the method of cleaning carbon deposits to control the engine burning oil, it is tantamount to seeking fish from wood and digging for the sky, which is simply impossible. When the engine has the symptoms of burning oil, it is generally the air ring wear is larger, the oil ring has been locked by carbon deposits, and the oil that enters the combustion chamber to participate in the combustion increases, and the carbon deposit inside the combustion chamber and the gas ring also increases, resulting in further increased wear, more air channeling, more carbon deposits, forming a vicious circle, and even carbon deposits falling between the piston and the cylinder wall, resulting in a pull cylinder. In this case, the engine can only be overhauled. Cleaning the carbon deposit at this time, like giving painkillers to patients with advanced cancer, can only relieve pain and will not cure the disease.