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. So, with a crankshaft speed of 5000 min⁻¹ and a piston stroke of, say, 75 mm, the total distance traveled by the piston per minute is 375 m. Over an hour of engine operation, this distance will be 2 km 250 m, and over a month of operation of 8 hours a day, excluding weekends (which is, of course, unlikely for the average car), the piston will travel a distance of 460 km. With intensive use of the car in 5 years (this is the duration of vehicle operation before a major engine overhaul that statistics confirm) the piston will cover a distance of 24,000 km!
So, wear of the piston and its mating parts (engine cylinder) inevitable. However, the wear values of the piston group (pistons-piston rings) before major overhaul for engines of different companies differ greatly from each other. So, the maximum wear of pistons and piston rings of engines Mercedes-Benz, Volkswagen, BMW, for most American and Japanese companies, this occurs after a mileage of about 300,000 km.
At the same time, engines of other, say, less advanced models, require replacement of pistons and piston rings after just 50,000 km of mileage (almost 10 times less).
Attention! What's the reason for this? And how does the longevity of these parts depend on operating conditions? To answer these questions, let's consider two typical piston group designs for gasoline and diesel engines. Let us first 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 gasoline engine—either carbureted or with direct fuel injection—it's 40–55 kg/cm², while in a diesel engine it's 70–80 kg/cm². Therefore, the pistons of gasoline and diesel engines differ from each other, although their fundamental 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 the piston head from seizing in the cylinder when heated to operating temperature. The piston's 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 conical in height: the diameter at the top is smaller than at the bottom. In addition, there are two steel temperature-regulating inserts 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. With 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 piston pin hole in modern engines is usually shifted from the piston's 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, which should face the front of the engine. This offset is made 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 supplying a jet of oil to the loaded side of the cylinder mirror (in some engines these holes are missing). The bearing shells and the big end cap are also provided with corresponding marks for correct 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 engine manufacturers today use a system according to which pistons are typically divided into five or six classes by outside diameter, in 0.01 mm increments. They are also divided into three or four categories, in 0.004 mm increments, based on the piston pin bore diameter. Engine cylinders also have a similar division into five classes. This system allows for a more precise selection of a piston for any cylinder, even a worn one, and a piston pin of the required category for the bore in the piston bosses and the connecting rod. For major engine repairs, which usually involve boring (increasing diameters) cylinders, spare parts manufacturers produce so-called oversized repair pistons.
The piston of a modern diesel engine is designed to withstand 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 combustion of the fuel-air mixture occurs when the piston is near top dead center, the hot gases heat the piston head more strongly, and the walls of the upper part of the cylinder heat up somewhat less than in gasoline engines. To ensure 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 three upper grooves. Two oil scraper rings are located in the lower grooves. Many companies produce rectangular compression rings that are practically no different from the rings used in gasoline engines. However, a more progressive, although more expensive, design is one with a conical upper working surface of the ring. The cone angle of these rings is typically set at 10°. The use of conical rings provides some increase in their durability, since during the power stroke, the component of the gas pressure force on the conical surface of the ring additionally presses it against the cylinder bore. A special feature of servicing and repairing pistons with conical compression rings is the need for precise clearance 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 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 significantly improves the piston's fit to the cylinder and increases its service life before major repairs. A similar treatment of the rubbing surfaces of pistons is used today in gasoline engines.
In addition to wear on the skirt surfaces, the piston compression ring grooves also wear out. In addition, the oil scraper ring groove also wears out, although such wear is usually much less. As the grooves wear out, the piston 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 effect manifests itself in the engine's ever-increasing consumption of motor oil. When the oil enters 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 will not improve the situation much. There is an objective need to replace the entire piston group, and it is highly desirable to bore the cylinders to the repair size. All the described types of wear and tear are a natural and, unfortunately, inevitable process.
However, this natural wear and tear can be spread out over time, thus extending the life of the engine. There is no need to discover America here. You just need to strictly follow the manufacturer's requirements for vehicle operation, use high-quality motor oil and oil filters, and properly adjust the fuel system. Good results are achieved by using high-quality oil and fuel modifiers and preparations that change the microstructure of the surface layers of engine friction surfaces.
Along with this, engine wear, as well as that of the entire vehicle, largely depends on the driver, his qualifications and technical literacy. It's no wonder that cars of the same brand serve some drivers for a long time and without failure, while others need repairs almost every week. An experienced driver almost never allows the engine to operate under overload, and especially with detonation. He constantly listens to his car's engine running and immediately reacts to any overload, usually accompanied by a low-pitched booming sound at low crankshaft speed. The car acceleration mode 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 is 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 steep driving style becomes a habit, engine repair will be provided twice as soon as stipulated by the technical conditions.
Often, another type of wear and tear is observed that is not provided for by any instructions. This is an emergency failure of the connecting rod and piston group elements, and primarily 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 the fuel-air mixture in the cylinder, accompanied by a sudden increase in pressure in the combustion chamber. This is equivalent to a sharp blow with a sledgehammer on a stationary piston and rings. The parts, naturally, 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 engine running on gasoline with an octane number lower than that specified by the technical specifications, as well as overheating and running on an over-enriched fuel mixture. An experienced driver must be able to 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 rotation. They can be barely audible against the background of other engine sounds, especially with slightly advanced ignition timing, and disappear with a very slight reduction in fuel (gas) delivery. Such barely noticeable detonation indicates correctly adjusted ignition timing, but sometimes detonation knocks appear immediately upon pressing the accelerator pedal, which is, of course, unacceptable. Continuing to drive in this mode is tantamount to smashing the insides of the engine with a hammer.
Diesel engines are not as sensitive to changes in fuel composition, although they can still experience problems that lead to increased wear of the crank mechanism parts. This is primarily engine overheating and the associated decrease in oil viscosity, especially if the oil quality is low. Increased wear can also be a consequence of improper adjustment of the high-pressure fuel pump and deterioration of fuel atomization 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, the following conclusions can be drawn. The longevity of your car's engine, 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 maintenance, for which the driver is ultimately responsible. This should be kept in mind both when purchasing a car and when preparing and training drivers.