ALEXANDER KHRULEV, "ABS"
Defects per se in the mechanical part of the engine, as you know, do not appear. Practice shows: there are always reasons for damage and failure of certain parts. It’s not easy to understand them, especially when the components of the piston group are damaged.
Piston group. the traditional source of trouble waiting for the driver operating the car, and the mechanic repairing it. Engine overheating, repair negligence. and please. increased oil consumption, gray smoke, knock.
At "autopsy" such a motor will inevitably show scuffing on pistons, rings and cylinders. The conclusion is disappointing. expensive repairs required. And the question arises: what caused the engine to blame, what brought it to such a state?
The engine, of course, is not to blame. It is simply necessary to foresee what these or other interventions in its work lead to. After all, the piston group of a modern engine. "thin matter" in all senses. The combination of the minimum dimensions of parts with micron tolerances and the enormous gas pressure forces and inertia acting on them contributes to the appearance and development of defects, which ultimately lead to engine failure.
In many cases, simple replacement of damaged parts. not the best engine repair technology. The reason for the appearance of the defect remained, and if so, then its repetition is inevitable.
To prevent this from happening, a competent minder, as a grandmaster, needs to think a few moves ahead, considering the possible consequences of his actions. But this is not enough. it is necessary to find out why the defect occurred. And here, without knowledge of the design, working conditions of parts and processes occurring in the engine, as they say, there is nothing to do. Therefore, before analyzing the causes of specific defects and breakdowns, it would be nice to know.
How does a piston work?
The piston of a modern engine. the detail at first glance is simple, but extremely responsible and at the same time complex. The experience of many generations of developers is embodied in its design.
And to some extent, the piston forms the appearance of the entire engine. In one of the previous publications, we even expressed this idea, paraphrasing a well-known aphorism: "Show me the piston and I’ll tell you what engine you have".
So, using a piston in the engine, several tasks are solved. First and foremost. to perceive the gas pressure in the cylinder and transfer the arising pressure force through the piston pin to the connecting rod. Further, this force will be converted by the crankshaft into engine torque.
It is impossible to solve the problem of converting gas pressure to torque without a reliable seal of the moving piston in the cylinder. Otherwise, a gas breakthrough in the engine crankcase and oil from the crankcase into the combustion chamber are inevitable.
For this purpose, a sealing belt with grooves is arranged on the piston, in which compression and oil scraper rings of a special profile are installed. In addition, special holes are made in the piston to discharge oil.
But this is not enough. During operation, the piston bottom (fire belt), in direct contact with hot gases, heats up, and this heat must be removed. In most engines, the cooling problem is solved using the same piston rings. through them heat is transferred from the bottom to the cylinder wall and beyond. coolant. However, in some of the most loaded designs, additional oil cooling of the pistons is done by supplying oil from below to the bottom using special nozzles. Sometimes internal cooling is also used. the nozzle supplies oil to the inner annular cavity of the piston.
To reliably seal the cavities against the penetration of gases and oil, the piston must be held in the cylinder so that its vertical axis coincides with the axis of the cylinder. All sorts of distortions and "shifting"causing "hanging out" piston in the cylinder, negatively affect the sealing and heat transfer properties of the rings, increase the noise level of the engine.
A guide belt is designed to hold the piston in this position. piston skirt. The requirements for the skirt are very contradictory, namely: it is necessary to ensure a minimum, but guaranteed, clearance between the piston and the cylinder in both a cold and a fully warmed engine.
The task of constructing a skirt is complicated by the fact that the temperature expansion coefficients of the materials of the cylinder and piston are different. Not only are they made of various metals, their heating temperatures vary many times over.
To prevent the heated piston from sticking, modern engines take measures to compensate for its thermal expansion.
Firstly, in the cross section the piston skirt is shaped like an ellipse, the major axis of which is perpendicular to the axis of the finger, and in the longitudinal. cone tapering to the bottom of the piston. This form allows you to ensure that the skirt of the heated piston corresponds to the cylinder wall, preventing jamming.
Secondly, in some cases, steel plates are poured into the piston skirt. When heated, they expand more slowly and limit the expansion of the entire skirt.
The use of lightweight aluminum alloys for the manufacture of pistons. not a whim of designers. At high speeds characteristic of modern engines, it is very important to ensure a low mass of moving parts. In such conditions, a heavy piston will need a powerful connecting rod, "mighty" crankshaft and too heavy block with thick walls. Therefore, there is no alternative to aluminum, and you have to go to all sorts of tricks with the shape of a piston.
There may be others in the design of the piston. "tricks". One of them. reverse cone at the bottom of the skirt, designed to reduce noise due to "shifting" piston in dead spots. A special microprofile on the work surface helps improve the lubrication of the skirt. micro grooves in increments of 0.2-0.5 mm, and reduce friction. special anti-friction coating. The profile of the sealing and fire zones is also defined. here the temperature is the highest, and the gap between the piston and cylinder in this place should not be large (the probability of gas breakthrough, the risk of overheating and ring breakage increases), nor small (there is a high risk of jamming). Often the resistance of the fire zone is increased by anodizing.
Video: Why a Trimmer Piston Burnt Out
All that we have told. far from a complete list of requirements for the piston. The reliability of its operation also depends on the parts mating with it: piston rings (dimensions, shape, material, elasticity, coating), piston pin (clearance in the piston bore, fixing method), cylinder surface condition (deviation from cylindricity, microprofile). But it is already becoming clear that any, even not too significant, deviation under the conditions of the piston group operation quickly leads to defects, breakdowns and engine failure. In order to subsequently repair the engine qualitatively, it is necessary not only to know how the piston is arranged and working, but also to be able to determine by the nature of the damage to the parts why, for example, there was scoring or.
Why did the piston burn out?
An analysis of various piston damages shows that all causes of defects and breakdowns are divided into four groups: cooling failure, lack of lubrication, excessively high thermal force from the gases in the combustion chamber, and mechanical problems.
However, many causes of piston defects are interrelated, as are the functions performed by its various elements. For example, defects in the sealing belt cause overheating of the piston, damage to the fire and guide belts, and scoring on the guide belt leads to a violation of the sealing and heat transfer properties of the piston rings.
Ultimately, this can provoke burnout of the fire belt.
We also note that in almost all malfunctions of the piston group, an increased oil consumption occurs. In case of serious damage, thick, bluish exhaust smoke, a drop in power and a difficult start due to low compression are observed. In some cases, the sound of a damaged piston is heard, especially on a cold engine.
Sometimes the nature of the piston group defect can be determined without disassembling the engine according to the above external signs. But more often than not "in-place" diagnosis is inaccurate, because different reasons often give almost the same result. Therefore, the possible causes of defects require a detailed analysis.
Disturbance of piston cooling. perhaps the most common cause of defects. This usually happens when the engine cooling system is malfunctioning (chain: "radiator-fan-sensor for turning on the fan-water pump") or due to damage to the cylinder head gasket. In any case, as soon as the cylinder wall ceases to be washed by the liquid from the outside, its temperature, and with it the temperature of the piston, begin to increase. The piston expands faster than the cylinder, moreover, unevenly, and ultimately the gap in some places of the skirt (usually near the hole for the finger) becomes equal to zero. Scuffing begins. setting and mutual transfer of materials of the piston and the mirror of the cylinder, and with further operation of the engine, the piston seizes.
After cooling, the piston shape rarely returns to normal: the skirt is deformed, i.e. compressed along the major axis of the ellipse. Further operation of such a piston is accompanied by knocking and increased oil consumption.
In some cases, the seizure on the piston extends to the sealing belt, rolling the rings into the piston grooves. Then the cylinder, as a rule, turns off from work (compression is too small), and it is generally difficult to talk about oil consumption, since it will simply fly out of the exhaust pipe.
Insufficient lubrication of the piston is most often characteristic of starting conditions, especially at low temperatures. Under such conditions, the fuel entering the cylinder flushes the oil off the cylinder walls, and scoring occurs, which are usually located in the middle part of the skirt, on its loaded side.
Two-sided skirt scuffing is usually found during prolonged operation in oil starvation mode, associated with malfunctions of the engine lubrication system, when the amount of oil falling on the cylinder walls decreases sharply.
Lack of piston pin lubrication. the reason for its jamming in the holes of the piston bosses. This phenomenon is characteristic only for designs with a finger pressed into the upper head of the connecting rod. This is facilitated by a small gap in the connection of the finger with the piston, therefore "hooks" fingers are more commonly seen in relatively new engines.
Excessively high thermal stress on the piston from the hot gases in the combustion chamber. A common cause of defects and breakdowns. So, detonation leads to the destruction of jumpers between the rings, and ignition ignition. to burnouts.
In diesel engines, an excessively large lead angle of fuel injection causes a very rapid increase in pressure in the cylinders ("rigidity" work), which can also cause jumper breakage. The same result is possible when using various fluids that facilitate starting the diesel engine.
The bottom and the fire belt can be damaged if the temperature in the combustion chamber of the diesel engine is too high due to a malfunction of the nozzle nozzles. A similar picture occurs when the cooling of the piston is disturbed. for example, during coking of nozzles supplying oil to a piston having an annular cavity of internal cooling. Scoring that occurs on the top of the piston can also extend to the skirt, trapping the piston rings.
Mechanical problems, perhaps, give the largest variety of piston group defects and their causes. For example, abrasive wear of parts is possible as "from above"due to the ingress of dust through a torn air filter, and "from below"when circulating abrasive particles in oil. In the first case, the most worn out cylinders are in their upper part and compression piston rings, and in the second. oil scraper rings and piston skirt. By the way, abrasive particles in the oil may appear not so much from the untimely maintenance of the engine, but as a result of the rapid wear of any parts (for example, camshaft, pushers, etc.).
Rarely, erosion of the piston at the hole occurs "floating" finger when the snap ring pops up. The most likely causes of this phenomenon. the parallelism of the lower and upper connecting rod heads, leading to significant axial loads on the finger and "knocking out" retaining rings from the grooves, as well as the use of old (lost elasticity) retaining rings during engine repair. The cylinder in such cases is damaged by a finger so much that it can no longer be repaired by traditional methods (boring and honing).
Foreign objects can sometimes get into the cylinder. This most often occurs with careless work during engine maintenance or repair. A nut or bolt, being between the piston and the head of the block, can do a lot, including just "fail" piston bottom.
The story of defects and breakdowns of the pistons can be continued for a very long time. But what has already been said is enough to draw some conclusions. At least you can already determine.
How to avoid burnout?
The rules are very simple and follow from the features of the piston group and the causes of defects. However, many drivers and mechanics forget about them, as they say, with all the ensuing consequences.
Although this is obvious, it is still necessary during operation: to keep the engine power, lubrication and cooling systems in good working order, to service them on time, not to overload the cold engine, to avoid the use of low-quality fuel, oil and inappropriate filters and spark plugs. And if something is wrong with the engine, do not finish it "to the handle"when repair is no longer cost "low blood".
During the repair, it is necessary to add and strictly follow a few more rules. The main thing, in our opinion. one cannot strive to ensure minimum piston clearances in cylinders and in ring locks. Epidemic "small gap diseases", which once struck many mechanics, still failed. over, practice has shown that attempts "denser" to install a piston in a cylinder in the hope of reducing engine noise and increasing its life almost always end up with the opposite: piston scuffing, knocking, oil consumption and repeated repairs. The rule "better clearance is 0.03 mm more than 0.01 mm less" It always works for any engine.
The rest of the rules are traditional: high-quality spare parts, proper processing of worn parts, thorough washing and accurate assembly with mandatory control at all stages.
Scuffing on the skirt may result from insufficient clearance or overheating. In the latter case, they are located closer to the hole of the finger.