The main engine parts are particularly susceptible to wear - pistons with piston rings, connecting rods and cylinders. The work of the engine pistons is most impressive. After all, moving back and forth between the top and bottom dead centers, they cover a huge distance. Thus, with a crankshaft rotation speed of 5000 rpm and a piston stroke of 75 mm, the total piston travel per minute is 375 m. In an hour of operation, this distance will already be 2 km 250 m, and in a month of operation for 8 hours a day, excluding weekends, the piston will move 460 km. With intensive operation of the car in 5 years (and this is the duration of vehicle operation before major repairs that statistics confirm) the piston will cover a distance of 24,000 km!
So, wear of the piston and its associated parts is inevitable. However, the wear values of piston groups (pistons-piston rings) before major overhauls, engines from different companies differ greatly from each other. Thus, the maximum wear of pistons and piston rings of Mercedes-Benz, Volkswagen, BMW engines, most American and Japanese companies occurs after a run of about 300,000 km. At the same time, engines of other, say, less perfect models, need to have pistons and piston rings replaced after 50,000 km of run (almost 10 times less!).
What is the reason for this? And how does the durability of these parts depend on operating conditions? To answer these questions, let's consider two typical designs of piston groups of a gasoline engine and a diesel engine. Let us first of all remember that the gas pressure inside the cylinders of these engines at the beginning of the working stroke differs by approximately two times. In a carburetor engine or in an engine with direct petrol injection it is 40-55 kg/cm², in a diesel engine it is 70-80 kg/cm². Therefore, the pistons of petrol and diesel engines differ from each other, although their main design solutions are the same.
A typical carburetor engine piston is cast from an aluminum alloy and coated on the outside with a layer of tin to improve the running-in ability to the cylinder surface: Its upper part - the head - has a diameter 0.1 mm smaller than the inner diameter of the cylinder. This is done to prevent the head from seizing in the cylinder when heated. The annular grooves contain two compression rings and one oil scraper ring. The lower part of the piston - the skirt - is oval in cross section and has a conical shape in height: the upper part has a smaller diameter than the lower part. In addition, two steel temperature-regulating inserts are fused inside the piston bosses with holes for the piston pin. All this is done to prevent increased friction between the skirt and the cylinder wall when the piston heats up. Having a lower coefficient of thermal expansion than aluminum, these inserts tighten the skirt in a direction perpendicular to the axis of the piston pin.
The hole for the piston pin in modern engines is usually shifted from the axis of symmetry to the right side of the engine. To correctly assemble the piston with the connecting rod and install them in the engine cylinder, there is a mark near the boss hole that should face the front of the engine. This displacement is done to reduce the lateral component of the gas pressure force that presses the piston to one side of the cylinder during the "working stroke".
The connecting rod must also be oriented correctly in the engine. On its front side there are holes for directing a jet of oil onto the loaded side of the cylinder mirror (in some engines this hole is missing). The bearing shells and the big end cap are also marked accordingly for proper assembly. The piston's future performance and durability depend significantly on the precision of its manufacture and its correct selection for the cylinder bore. Leading automobile manufacturers today use a system in which pistons are usually divided into five or six classes by outside diameter, each in 0.01 mm. In addition, they are divided into three or four categories, each in 0.004 mm, according to the diameter of the piston pin hole. Engine cylinders also have a similar division into five classes. This system allows you to most accurately select a piston of the appropriate class for any, even worn-out cylinder, and a pin of the required category for the hole in the bosses and for the connecting rod. For major engine repairs, which usually involve boring the cylinders, companies produce oversized repair pistons.
The piston of a modern diesel engine is designed to withstand higher pressures, so it has a thicker bottom and bosses. In addition, the design of the diesel piston is somewhat different from that discussed earlier. The main difference is the placement of the combustion chamber directly in the piston head. Since combustion occurs with the piston near top dead center, the hot gases heat the piston head more strongly, while the walls of the upper part of the cylinder heat up relatively less than in gasoline engines. To ensure reliable sealing of the piston in the cylinder, five grooves for piston rings are made on its outer surface. Compression rings are installed in the three upper grooves. Two oil scraper rings are located in the lower grooves. Many companies manufacture rectangular compression rings that are practically no different from the rings of gasoline engines. However, a more progressive, although more expensive, design is one with a conical upper working surface. The angle of inclination of the cone generatrix for such rings is usually made 10°. The use of conical rings provides some increase in their durability, since during the "working stroke" the component of the gas pressure force on the conical surface of the ring additionally presses it against the cylinder mirror. A special feature of servicing and repairing pistons with conical compression rings is precise gap control. The gaps between the groove and the oil scraper rings are controlled in the same way as in gasoline engines.
The frictional forces between the surfaces of the piston skirt and the cylinder wall in diesel engines are higher than in gasoline engines. To increase durability, modern companies apply a layer of special colloidal graphite coating to the surface of the piston skirt. It significantly improves the piston's adaptation to the cylinder and extends its service life before major repairs. Similar treatment of piston rubbing surfaces is used today in gasoline engines.
In addition to wear on the skirt surfaces, the grooves for the piston compression rings also wear out. In addition, the groove for the oil scraper ring wears out, although such wear is usually much less. As the grooves wear out, the rings begin to move more and more intensively up and down the height of the groove, and the so-called pumping action of the rings becomes more and more noticeable. This is reflected in the ever-increasing consumption of engine crankcase oil. Once in the combustion chamber, it burns there, forming blue smoke that comes out of the car's exhaust pipe. If the grooves are significantly worn, replacing the rings with new ones does little to improve the situation. There is an objective need to replace the entire piston group with a highly desirable reboring of the cylinders to a repair size. All the described types of wear are a natural and, unfortunately, inevitable process.
Reference
Natural engine wear and tear can be successfully combated, extending its service life. There is no need to discover America here. You just need to scrupulously follow the vehicle operating requirements, use high-quality oil and oil filters, and properly adjust the fuel equipment. Good results are achieved by using high-quality oil and fuel modifiers, preparations that change the microstructure of the surface friction layers of engines.
Along with this, engine wear, as well as the entire car as a whole, largely depends on the driver, his qualifications and technical literacy. It is not for nothing that cars of the same brand serve some drivers for a long time and without failure, while for others they are repaired almost every week. An experienced driver almost never allows the engine to operate under overload, and even less so - to detonate. He constantly listens to his engine running and immediately reacts to overload, accompanied by a booming, low-pitched sound at low crankshaft speed. The acceleration mode of the car is also accompanied by increased engine wear. An analogy with a horse and rider comes to mind: a caring owner will not whip his four-legged friend unnecessarily, forcing him to run at full speed, especially when the horse has not yet warmed up. Of course, in critical situations the driver can allow himself to accelerate the car recklessly and extremely sharply. But if such a cool driving style becomes a habit, then the engine of such a daredevil will probably need to be repaired twice as soon as is stipulated by the technical conditions.
Often, another type of wear is observed that is not provided for by any instructions. This is an emergency failure of the connecting rod and piston group and, above all, the rings and bridges of the piston ring grooves. In gasoline engines, this is primarily due to detonation. Let us remember that detonation is an explosive combustion of a combustible mixture in a cylinder, accompanied by a sudden increase in pressure in the combustion chamber. This is equivalent to a sharp blow with a heavy sledgehammer on a stationary piston and rings. The parts, naturally, are not designed to withstand the load and can break, damaging the cylinder mirror with sharp fragments. There are several reasons for detonation. However, the main ones are the engine running on gasoline with a lower octane number than specified by the technical conditions, as well as overheating and running on a rich fuel mixture. An experienced driver must hear detonation knocks when the engine is running and immediately reduce the fuel supply during acceleration, and then eliminate the causes of detonation. The sound of detonation is a high-pitched metallic clicking sound that matches the frequency of the crankshaft revolutions. They can be barely audible against the background of other sounds of the running engine, especially - with slightly early ignition, and disappear with a very slight decrease in fuel (gas) supply. Such barely noticeable detonation indicates a correctly adjusted ignition advance angle, but it also happens that detonation knocks appear immediately when pressing the gas pedal, which, of course, is unacceptable, and continuing to drive in this mode is equivalent to smashing the insides of the engine with a hammer.
Diesel engines are not so sensitive to changes in the composition of diesel fuel, although they also experience problems that lead to increased wear of the crankshaft and connecting rod components. This is, first of all, engine overheating and the associated decrease in oil viscosity, especially if it is of low quality. Increased wear can be a consequence of both incorrect adjustment of the high-pressure pump and deterioration of fuel atomization in the combustion chambers due to malfunctioning of the injectors. And, of course, a lot depends on the driver himself.
So, from all of the above, we can draw the following general conclusions. The longevity of your car, as well as the entire vehicle, depends on two factors: the quality of workmanship, for which the manufacturer is responsible, and the level of technical operation, for which the driver is ultimately responsible. This should be kept in mind both when purchasing a car and when preparing and training drivers.
[The text was taken from the website: HONDABOOK.ru]