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 min-1 and a piston stroke of, say, 75 mm, the total path traveled by the piston per minute is 375 m. For an hour of engine operation, this distance will be 2 km 250 m, and for a month of operation 8 hours a day, excluding weekends (which, of course, is unlikely for an average car), the piston will cover a distance of 460 km. With intensive use of the car for 5 years (namely, such a duration of operation of the car before the overhaul of the engine is confirmed by statistics) the piston will cover a distance of 24,000 km!
So, the wear of the piston and its mating parts (engine cylinder) inevitable. However, the wear values of the piston group (piston-piston rings) before overhaul for engines of different companies are very different from each other. So, the limiting wear of pistons and piston rings of engines Mercedes-Benz, Volkswagen, BMW, most American and Japanese firms comes 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).
Attention! What is the reason? And how does the durability of these parts depend on the operating conditions? To answer these questions, consider two typical designs of piston groups of a gasoline engine and a diesel engine. First of all, let us recall that the pressure of the gases inside the cylinders of these engines at the beginning of the working stroke differs by about a factor of two. In a gasoline engine - carbureted or with direct fuel injection, it is 40–55 kg / cm2, in a diesel engine - 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 gasoline engine piston is cast from an aluminum alloy and coated on the outside with a layer of tin to improve running-in to the cylinder bore. The diameter of its upper part - the head - is 0.1 mm smaller than the inner diameter of the cylinder. This is done to prevent jamming of the piston head in the cylinder when heated to operating temperature. In the annular grooves of the piston there are two compression rings and one oil scraper. The lower part of the piston - the skirt - is oval in cross section, and conical in height: the diameter in the upper part is smaller than in the lower one. In addition, inside the piston bosses with holes for the piston pin there are two steel thermostatic inserts. All this is done to prevent an increase in friction between the skirt and the cylinder mirror when the piston is heated. With a coefficient of thermal expansion lower than that of 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 of the piston 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 against one of the sides of the cylinder during the power stroke.
The connecting rod must also be properly oriented in the engine. On its front side, holes are made for supplying a jet of oil to the loaded side of the cylinder mirror (Some engines don't have these holes). The bearings and the connecting rod cap are also marked with appropriate marks for proper 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 engine manufacturers today use a system according to which pistons are usually divided into five or six classes in terms of outer diameter in increments of 0.01 mm. In addition, they are divided into three or four categories in increments of 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 more accurately match the piston to any, even worn-out cylinder, and the piston pin of the desired category - to the hole in the bosses and connecting rod. For overhaul of engines, usually consisting in boring (diameter increase) cylinders, manufacturers of spare parts produce so-called oversized repair pistons.
The piston of a modern diesel engine is designed to perceive higher pressures, so the thickness of its bottom and bosses is greater. In addition, the design of the diesel piston is somewhat different from that discussed above. The main difference is the placement of the combustion chamber directly in the piston head. Since the combustion of the air-fuel mixture occurs when the piston is near top dead center, hot gases heat up the piston head more strongly, and the walls of the upper part of the cylinder heat up somewhat less than in gasoline engines. For reliable sealing of the piston in the cylinder, five grooves are made on its outer surface for installing piston rings. 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 upper working surface of the ring. The angle of inclination of the generatrix of the cone for such rings is usually made equal to 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 surface. A feature of the maintenance and repair of pistons with tapered compression rings is the need for 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. It greatly improves the running-in of the piston to the cylinder and increases its life before overhaul. A similar treatment of the rubbing surfaces of pistons is used today on gasoline engines.
In addition to wear on the skirt surfaces, the grooves of the piston compression rings also wear out. In addition, the oil ring groove wears out, although this wear is usually much less. As the grooves wear, the piston rings begin to move more and more rapidly up and down the groove height and the so-called pumping action of the rings becomes more and more noticeable. This action is manifested in an ever-increasing consumption of engine oil by the engine. Once in the combustion chamber, the oil burns there, forming a bluish smoke that comes 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, while it is highly desirable to bore the cylinders to the repair size. All described types of wear are a natural and, unfortunately, inevitable process.
However, this natural wear can be extended over time, thus extending the life of the engine. America does not need to be discovered here. You just need to strictly follow the manufacturer's requirements for the operation of the car, use high-quality engine oil and oil filters, and properly 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 layers of friction surfaces 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 with detonation. He constantly listens to how the engine of his car is working, and immediately reacts to any overload, usually accompanied by a low-pitched booming sound at a reduced crankshaft speed. Vehicle acceleration 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, engine repairs will be provided twice as soon as specified 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 elements of the connecting rod and piston group and, above all, the rings and bridges of the annular grooves of the piston. In gasoline engines, this is primarily due to detonation. Recall that detonation is an explosive combustion of an air-fuel mixture in a cylinder, accompanied by an abrupt increase in pressure in the combustion chamber. This is equivalent to a sharp blow with a sledgehammer on a fixed piston and rings. Parts, of course, are not designed for such a load and can break, then damaging the cylinder mirror with their fragments. There are several reasons for detonation. However, the main one is 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 re-enriched 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 rotation. They can be barely audible over other running engine sounds, especially when the ignition is slightly early, and disappear with a very slight decrease in fuel supply (gas). Such a barely noticeable detonation indicates a correctly adjusted ignition timing, but it also happens that detonation knocks appear immediately when you press the accelerator pedal, which, of course, is unacceptable. To continue moving in this mode is tantamount to breaking the insides of the engine with a hammer.
Diesel engines are not so sensitive to changes in the composition of the fuel, although troubles occur in them, leading to increased wear of the parts of the crank mechanism. This is primarily overheating of the engine and the associated decrease in oil viscosity, especially if the quality of the oil is low. Increased wear can also be the result of improper adjustment of the high pressure fuel 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 all that has been said, we can draw the following conclusions. The longevity of your car's engine, as well as that of the vehicle as a whole, depends on two factors: the quality of workmanship, which is the responsibility of the manufacturer, and the level of maintenance, which is ultimately the responsibility of the driver. This must be remembered both when buying a car and when preparing and educating drivers.