Particularly subject to wear are the main parts of the engine - pistons with piston rings, connecting rods and cylinders. The performance 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. So, with a crankshaft speed of 5000 rpm and a piston stroke of 75 mm, the total piston travel per minute is 375 m. For an hour of operation, this distance will already be 2 km 250 m, and for a month of operation, 8 hours a day, excluding weekends, the piston will move 460 km. With intensive operation of the car for 5 years (namely, such a duration of operation of the car before overhaul is confirmed by statistics) the piston will cover a distance of 24,000 km!
So, wear of the piston and associated parts is inevitable. However, the wear values of the piston groups (piston-piston rings) before the overhaul of engines of various companies are very different from each other. So, the wear limit 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 advanced models need to be replaced by pistons and piston rings after 50,000 km of run (almost 10 times less!).
What is the reason here? And how does the durability of these parts depend on the operating conditions? To answer these interrogations, consider two typical designs of piston groups of a gasoline engine and a diesel engine. Recall, first of all, that the pressure of gases inside the cylinders of these engines at the beginning of the working stroke differs by about two times. In a carbureted engine or in an engine with gasoline direct injection, it is 40-55 kg/cm2, in a diesel engine it is 70-80 kg/cm2. Therefore, the pistons of gasoline 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 workability to the cylinder mirror: Its upper part - the head - has a diameter smaller by 0.1 mm than the inner diameter of the cylinder. This is done to prevent the head from jamming in the cylinder when heated. Two compression rings and one oil scraper ring are placed in the annular grooves. The lower part of the piston - the skirt - is oval in cross section, and has a conical shape in height: in the upper part there is a smaller diameter than in the lower one. In addition, two steel thermostatic inserts are fused inside the piston bosses with holes for the piston pin. All this is done to prevent an increase in friction between the skirt and the cylinder mirror when the piston is heated. Having a lower thermal expansion coefficient 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. For the correct assembly of the piston with the connecting rod and their installation in the engine cylinder, there is a mark near the hole of the boss, which must be turned towards the front of the engine. Such a displacement is done to reduce the lateral component of the gas pressure force pressing the piston to one of the sides of the cylinder on the stroke "working stroke".
The connecting rod must also be correctly oriented in the engine. On its front side there are holes for a directed oil jet on the loaded side of the cylinder mirror (some engines do not have this hole). The bearings and the connecting rod cap are also marked for correct assembly. From the accuracy of the manufacture of the piston and its correct selection to the cylinder bore, its further performance and durability significantly depend. Leading automotive manufacturers today use a system according to which pistons are usually divided into five or six classes by 0.01 mm in terms of outer diameter. In addition, they are divided into 3-4 categories by 0.004 mm according to the diameter of the piston pin hole. Engine cylinders also have a similar division into five classes. Such a system allows you to most accurately select the piston of the appropriate class to any, even worn-out cylinder, and the pin of the desired category to the hole in the bosses and to the connecting rod. For overhaul of engines, which usually consists in boring cylinders, firms 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 considered earlier. The main difference is the placement of the combustion chamber directly in the piston head. Because combustion occurs when the piston is near top dead center, hot gases heat up the piston head more, and the walls of the top of the cylinder heat up relatively less than in gasoline engines. For 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 top three grooves. In the lower grooves there are two oil scraper rings. Many companies manufacture rectangular compression rings, which are practically no different from gasoline engine rings. However, more progressive, although more expensive, is the design with a conical top working surface. The angle of inclination of the generatrix of the cone for such rings is usually 10°. The use of cone rings provides some increase in their durability, since "working stroke" the component of the gas pressure force on the conical surface of the ring additionally presses it against the cylinder mirror. A feature of the maintenance and repair of pistons with tapered compression rings is the precise control of clearances. The gaps between the groove and the oil scraper rings are controlled in the same way as in gasoline engines.
The friction forces between the surfaces of the piston skirt and the cylinder bore are higher in diesel engines than in gasoline engines. To increase durability, a layer of a special colloidal graphite coating is applied to the surface of the piston skirt in modern firms. It greatly improves the running-in of the piston to the cylinder and extends its life before overhaul. A similar treatment of the rubbing surfaces of pistons is used today on gasoline engines.
In addition to the wear of the skirt surfaces, the grooves for the piston compression rings also wear out. In addition, the oil scraper ring groove wears out, although this wear is usually much less. As the grooves wear, the rings begin to move more and more rapidly down and up the height of the groove, and the so-called pumping action of the rings becomes more and more noticeable. This is manifested in the ever-increasing consumption of engine crankcase oil. Once in the combustion chamber, it burns there, forming blue smoke coming out of the car's exhaust pipe. With significant wear of the grooves, replacing the rings with new ones does not improve the situation much. There comes an objective need to replace the entire piston group with a highly desirable cylinder bore for a repair size. All described types of wear are a natural and, unfortunately, inevitable process.
Reference
The natural wear of the engine can be successfully dealt with, extending its performance. America should not be opened here. You just need to scrupulously fulfill the requirements for operating the car, use high-quality oil and oil filters, and correctly adjust the fuel equipment. Good results are obtained by the use of high-quality oil and fuel modifiers, preparations that change the microstructure of the surface friction layers of engines.
Along with this, the wear of the engine, as well as the entire car as a whole, largely depends on the driver, on his qualifications and technical literacy. After all, it is not in vain that cars of the same brand serve for some drivers for a long time and without fail, for others they are repaired almost every week. An experienced driver almost never allows the engine to work with overload, and even more so - detonation. It constantly listens to how its engine is running and reacts immediately to overload, accompanied by a booming, low-pitched sound at a reduced speed of the crankshaft. The acceleration mode of the car is also accompanied by increased engine wear. Here the analogy with a horse and a rider suggests itself: a caring owner will not whip his four-legged friend unnecessarily, forcing him to run off the bat, especially when the horse has not yet warmed up. Of course, in critical situations, the driver can afford to famously, extremely sharply disperse the car. But if such a steep driving style becomes a habit, the repair of the engine of such a reckless driver is probably provided twice as soon as it is stipulated by the technical conditions.
Often there is another type of wear that is not provided for by any instructions. This is an emergency breakdown of the connecting rod and piston group and, above all, the rings and bridges of the piston annular grooves. In gasoline engines, this is primarily due to detonation. Recall that detonation is an explosive combustion of a combustible mixture in a cylinder, accompanied by an abrupt increase in pressure in the combustion chamber. This is tantamount to a sharp blow with a heavy sledgehammer on a fixed piston and rings. Parts, of course, are not designed for the load and can break, then damaging the cylinder mirror with sharp fragments. There are several reasons for detonation. However, the main ones are the operation of the engine on gasoline with an octane number lower than specified by the technical conditions, as well as overheating and operation on a rich combustible 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 knocking sound is a high-pitched metallic click that matches the frequency of the crankshaft revolutions. They can be barely audible against the background of other sounds of a running engine, especially with slightly early ignition, and disappear with a very slight decrease in fuel supply (gas). Such barely noticeable detonation indicates a correctly adjusted ignition timing, but it also happens that detonation knocks appear immediately when you press the gas pedal, which, of course, is unacceptable, and continuing to move in this mode is tantamount to breaking the engine insides with a hammer.
Diesel engines are not so sensitive to changes in the composition of diesel fuel, although troubles occur in them, leading to increased wear of parts of the crank group. This is, first of all, overheating of the engine and the associated decrease in oil viscosity, especially if it is of low quality. Increased wear can be the result of both incorrect adjustment of the high pressure pump and poor atomization of fuel in the combustion chambers due to malfunctioning injectors. And, of course, a lot depends on the driver himself.
So, from the foregoing, we can draw such generalized conclusions. The longevity of your car, as well as that of the entire vehicle, depends on two factors: the quality of workmanship, for which the manufacturer is responsible, and the level of maintenance, for which the driver is ultimately responsible. This must be remembered both when buying a car and when preparing and educating drivers.