ALEXANDER KHRULEV, "ABS" By themselves, defects 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 is not easy to understand them, especially when the components of the piston group are damaged. The piston group is a traditional source of trouble for the driver operating the car and the mechanic repairing it. Engine overheating, negligence in repairs, - and please - increased oil consumption, blue smoke, knocking. When "opening" such a motor, scuffs on pistons, rings and cylinders are inevitably found. The conclusion is disappointing - expensive repairs are required. And the question arises: what was the fault of the engine, that it was brought to such a state? It's not the engine's fault, of course. It is simply necessary to foresee what these or those interventions in its work lead to. After all, the piston group of a modern engine is "thin matter" in every sense. The combination of the minimum dimensions of parts with micron tolerances and the enormous forces of gas pressure and inertia acting on them contributes to the appearance and development of defects, ultimately leading to engine failure. In many cases, simply replacing damaged parts is not the best engine repair technique. 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, like a grandmaster, needs to think several moves ahead, calculating 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, operating 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 is a detail that at first glance is simple, but extremely responsible and at the same time complex. Its design embodies the experience of many generations of developers. And to some extent, the piston forms the appearance of the entire engine. In one of our previous publications, we even expressed such an idea, paraphrasing a well-known aphorism: "Show me a piston, and I'll tell you what kind of engine you have." So, with the help of a piston in the engine, several problems are solved. The first and main thing is to perceive the gas pressure in the cylinder and transfer the resulting pressure force through the piston pin to the connecting rod. This force is then converted by the crankshaft into engine torque. It is impossible to solve the problem of converting gas pressure into torque without reliable sealing of the moving piston in the cylinder. Otherwise, a breakthrough of gases into the engine crankcase and oil from the crankcase into the combustion chamber are inevitable. To do this, a sealing belt with grooves is organized 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 bottom of the piston (fire zone), 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 - heat is transferred through them from the bottom to the cylinder wall and then to the coolant. However, in some of the most loaded designs, additional oil cooling of the pistons is done, supplying oil from below to the bottom using special nozzles. Sometimes internal cooling is also used - the nozzle supplies oil to the internal annular cavity of the piston. For reliable sealing of cavities from the penetration of gases and oils, 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 "shifts" that cause the "hanging" of the piston in the cylinder, adversely affect the sealing and heat transfer properties of the rings, increase the noise of the engine. The piston skirt is designed to hold the piston in this position. The requirements for the skirt are very contradictory, namely: it is necessary to provide a minimum, but guaranteed, clearance between the piston and the cylinder both in a cold and in a fully warmed up engine. The task of designing a skirt is complicated by the fact that the temperature coefficients of expansion of the materials of the cylinder and piston are different. Not only are they made of different metals, their heating temperatures vary many times over. To prevent the heated piston from jamming, modern engines take measures to compensate for its thermal expansion. First, in the cross section, the piston skirt is shaped like an ellipse, the major axis of which is perpendicular to the axis of the pin, and in the longitudinal section, a cone tapering towards the piston bottom. This shape allows the skirt of the heated piston to conform 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 light aluminum alloys for the manufacture of pistons is not a whim of designers. At high speeds, typical for modern engines, it is very important to ensure a low mass of moving parts. Under such conditions, a heavy piston will require a powerful connecting rod, a "mighty" crankshaft and an overly heavy block with thick walls. Therefore, there is no alternative to aluminum yet, and you have to go to all sorts of tricks with the shape of the piston. There may be other "tricks" in the piston design. One of them is a reverse cone at the bottom of the skirt, designed to reduce noise due to the "relocation" of the piston in dead spots. A special micro-profile on the working surface - micro-grooves with a step of 0.2-0.5 mm helps to improve the lubrication of the skirt, and a special anti-friction coating helps to reduce friction. The profile of the sealing and fire belts is also defined - here is the highest temperature, and the gap between the piston and the cylinder in this place should not be large (there is an increased likelihood of gas breakthrough, the risk of overheating and breakage of the rings) or small (there is a high risk of jamming). Often, the resistance of the fire belt is increased by anodizing. Everything that we have said is far from a complete list of requirements for a piston. The reliability of its operation also depends on the parts associated with it: piston rings (dimensions, shape, material, elasticity, coating), piston pin (clearance in the piston bore, method of fixation), cylinder surface condition (deviations from cylindricity, microprofile). But it is already becoming clear that any, even not too significant, deviation in the operating conditions of the piston group quickly leads to defects, breakdowns and engine failure. In order to repair the engine in the future with high quality, it is necessary not only to know how the piston is arranged and works, but also to be able to determine by the nature of the damage to the parts why, for example, a scuff has occurred 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 and force effects from gases in the combustion chamber, and mechanical problems. At the same time, 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 scuffing on the guide belt leads to a violation of the sealing and heat transfer properties of the piston rings. Ultimately, this can cause burnout of the fire belt. We also note that with almost all malfunctions of the piston group, increased oil consumption occurs. With severe damage, thick, bluish exhaust smoke, a drop in power and difficult starting 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 defect in the piston group can be determined even without disassembling the engine according to the above external signs. But most often such "indiscriminate" diagnosis is inaccurate, since different causes often give almost the same result. Therefore, the possible causes of defects require a detailed analysis. Violation of piston cooling is perhaps the most common cause of defects. This usually occurs when the engine cooling system malfunctions (chain: "radiator-fan-fan switch-on sensor-water pump") or due to damage to the cylinder head gasket. In any case, as soon as the cylinder wall ceases to be washed from the outside by liquid, its temperature, and with it the temperature of the piston, begin to rise. The piston expands faster than the cylinder, moreover, unevenly, and eventually the clearance in certain places of the skirt (usually near the pin hole) becomes equal to zero. Scuffing begins - the seizure and mutual transfer of materials of the piston and cylinder mirror, and with further engine operation, the piston jams. After cooling, the shape of the piston 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 piston burr extends into the sealing belt, rolling the rings into the piston grooves. Then the cylinder, as a rule, turns off from work (compression is too low), and it is generally difficult to talk about oil consumption, since it will simply fly out of the exhaust pipe. Insufficient piston lubrication is most often characteristic of starting conditions, especially at low temperatures. Under such conditions, the fuel entering the cylinder washes away the oil from the cylinder walls, and scoring occurs, which are usually located in the middle part of the skirt, on its loaded side. Double-sided scuffing of the skirt usually occurs during prolonged operation in the oil starvation mode associated with malfunctions of the engine lubrication system, when the amount of oil falling on the cylinder walls decreases sharply. The lack of lubrication of the piston pin is the reason for its jamming in the holes of the piston bosses. This phenomenon is typical only for designs with a pin pressed into the upper head of the connecting rod. This is facilitated by a small gap in the connection between the pin and the piston, so "sticking" of the fingers is more often observed in relatively new engines. Excessively high thermal force effect on the piston from hot gases in the combustion chamber is a common cause of defects and breakdowns. So, detonation leads to the destruction of the jumpers between the rings, and glow ignition - to burnouts. In diesel engines, an excessively large fuel injection advance angle causes a very rapid increase in pressure in the cylinders ("rigidity" of work), which can also cause breakage of the jumpers. The same result is possible when using various fluids that make it easier to start a diesel engine. The bottom and fire belt can be damaged if the temperature in the diesel combustion chamber is too high, caused by a malfunction of the injector nozzles. A similar picture also occurs when the piston cooling is disturbed - for example, when the nozzles supplying oil to the piston, which has an annular internal cooling cavity, become coked. Seizure that occurs on the top of the piston can also spread 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 both "from above", due to dust entering through a torn air filter, and "from below", when abrasive particles circulate in the oil. In the first case, the cylinders in their upper part and the compression piston rings are the most worn, and in the second case, the oil scraper rings and the piston skirt. By the way, abrasive particles in the oil can appear not so much from untimely maintenance of the engine, but as a result of the rapid wear of any parts (for example, camshafts, pushers, etc.). Rarely, piston erosion occurs at the “floating” pin hole when the retaining ring pops out. The most likely causes of this phenomenon are the non-parallelism of the lower and upper heads of the connecting rod, which leads to significant axial loads on the pin and the “knocking out” of the retaining ring from the groove, as well as the use of old (lost elasticity) retaining rings when repairing the engine. The cylinder in such cases turns out to be damaged by a finger so much that it can no longer be repaired by traditional methods (boring and honing). Sometimes foreign objects can get into the cylinder. This most often occurs with careless work during engine maintenance or repair. A nut or bolt, caught between the piston and the head of the block, is capable of many things, including simply “failing” the piston bottom. The story about defects and breakdowns of 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 tell... 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 power supply, lubrication and cooling systems of the engine in good condition, to service them in time, not to overload a 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 bring it "to the handle", when the repair will no longer cost "little blood". When repairing, it is necessary to add and strictly follow a few more rules. The main thing, in our opinion, is that one should not strive to ensure minimum piston clearances in cylinders and in ring locks. The "small gap disease" epidemic that once afflicted many mechanics is still not over. Moreover, practice has shown that attempts to "tighter" install the piston in the cylinder in the hope of reducing engine noise and increasing its resource almost always end in the opposite: piston scuffing, knocking, oil consumption and repeated repairs. The rule "better clearance is 0.03 mm more than 0.01 mm less" always works for any engine. The rest of the rules are traditional: high-quality spare parts, proper processing of worn parts, thorough washing and careful assembly with mandatory control at all stages.
One Riga resident borrowed a diesel car from a friend and drove to Lithuania. The car was brought from Lithuania in tow - something with a motor. The engine did not start, measured the compression - it was not there. Dismantled - and saw melted pistons (see photo). Why did this happen - the fault of the driver, or is it something else?
Situation
According to the driver, it was on the highway. I felt a loss of traction, stopped. What is strange - the speed is not reset to idle. Moreover, they suddenly began to grow spontaneously, and rose to almost 4000 rpm. Turning off the ignition did not help - the engine continued to run, and it turned out to be turned off only in a barbaric way - holding the brake, sticking the fifth gear in place and dropping the clutch. The owner of the car did not believe at first - in his opinion, the driver simply "fired" on the Lithuanian expressways, and ruined the diesel engine. But the mechanics, seeing the symptoms, said that the driver had nothing to do with it. On the contrary, he did the right thing, that he was not at a loss, and turned off the engine. Otherwise, he would have completely peddled, and the consequences would have been much worse.
Causes
The first thing they thought was the turbine. It happens that due to the destruction of its bushings, oil begins to flow into the combustion chambers, and the engine spreads very quickly from this. But the turbine was in order, the oil level too. It was the injectors. One of them - in the first cylinder - hung in the open position. That is, the fuel was pouring constantly. By the way, the piston in this cylinder was damaged the most. Why does this happen? As a rule - from wear and tear or, more often, clogging of nozzles due to low-quality fuel. It is probably impossible to be 100% safe from such a phenomenon. However, you can still reduce the likelihood of it happening. Often, when one or more injectors start to malfunction, the sound of the motor becomes harsher. True, hearing this on the go is often problematic even for an experienced person. That is, prevention remains.
Actions
Ideally, some manufacturers recommend changing injectors after 120-150 thousand kilometers. It is clear that in our conditions such extravagance is not particularly acceptable - for example, for a diesel VW Passat B5, one nozzle costs about 200 lats. But at least flushing the nozzles at such a mileage probably won't hurt. The least expensive option is to add a detergent additive to the fuel, and the most effective one is to check the condition “alive” and flush on the stand (on average - 25-50 lats per nozzle). By the way, if you had to refuel with dubious fuel, then flushing may be required after 20-30 thousand km. By the way, if the diesel engine did not start well in the morning in cold weather, the compression, as well as all its components and assemblies, seemed to be in order, one of the reasons was partially clogged nozzles. And if, for example, the engine does not start well “hot”, then one of the options is the shut-off valve hanging until the nozzle cools down. That is, with such symptoms, in order to avoid the above-mentioned consequences, it will not be superfluous to be puzzled by the state of the injectors in advance.
Semyon ZAKHAROV
R?ga automotive
The study of various piston damage shows that all causes of defects and breakdowns are divided into 4 groups:
- cooling interruptions
- lubricant imperfection
- unreasonably large thermal and force influence from the gases in the combustion chamber
- mechanical problems.
At the same time, many causes of piston defects are interrelated, as are the functions performed by its various elements. In particular, defects in the sealing belt cause piston overheating, damage to the fire and guide belts, and scuffing on the guide belt leads to a violation of the sealing and heat transfer properties of the piston rings.
Ultimately, this will most likely provoke burnout of the fire belt.
Why did the piston burn out
Poor combustible mixture
For 1 liter of gasoline, it contains more than 16 kg of oxygen. It doesn't burn very fast. the motor is overheating, energy drops, as a result, the entire engine overheats. The piston in this list is key, because it is aluminum (except for tuned pistons) and is located directly in the fuel combustion zone. As is well known, aluminum melts at about 660 ° C, and when to think about what maximum allowable engine temperature only 150 degrees, and then 200 ° C no longer lubricates any oil, then it does not take long to calculate that a poor mixture is still capable of heating up parts in the middle of the motor more than 4 times more.
Bad gasoline
Body gasoline burns through the piston for the same reason - as a result of overheating. Because the gasoline that we pour into our gas tank often cannot be called gasoline. Proper gasoline burns at a sufficiently low temperature, while expanding very strongly, because the essence of any gas driven to top dead center ( TDC), lies in the fact that it expands to the maximum relative to its original volume, thereby extremely convincingly pushing the piston down, and combustion and the generated temperature are all side effects, without which the engine would be fine. In bad gasoline, as in normal, there are components such as benzene, gasoline and other bad substances. The fact is that they are contained in low-quality “gasolines” in absolutely different proportions, but in fact in larger proportions than standards allow.
Burnt piston signs and symptoms
These two wonderful components burn, releasing more heat and at the same time have a small expansion factor during combustion, and at the same time, being part of gasoline, they slow down the rate of combustion, reducing power. Therefore, when driving on such fuel, in order to achieve proper traction, it is required to twist the throttle stick more than when driving on normal gasoline, but here's the misfortune: we get the necessary traction complete with a much higher combustion temperature and excessive fuel consumption, and the result of this is a hole in piston.
Many factors affect the operation of the piston and it is impossible to give an unequivocal answer whether a particular piston will burn out or some other defect will occur. You can estimate the probability of an event occurring. And in order to prevent the onset of such an unpleasant event as piston burnout you must follow the rules written in the RE. After all, piston burnout is a purely operational defect.
The zone of the bottom and the top zone is completely destroyed. The hot zone burned out to the reinforcing insert. The molten piston material has moved along the piston skirt and caused damage and scuffing there as well. The reinforcing insert of the first compression ring is partially preserved only on the left side of the piston.
The rest of the reinforcing insert detached from the piston during operation and caused other damage in the combustion chamber. Parts of the piston flew off with such force that they fell through the intake valve into the intake manifold and thus also into the adjacent cylinder and caused damage there (impact marks).
to fig. 2: in the direction of injection by one or more jets of nozzles, erosive burnouts appeared on the piston bottom and on the edge of the heat zone. The piston skirt and piston ring area are free of burrs.
Damage assessment
Damage of this kind occurs especially in direct injection diesel engines. This applies to pre-chamber diesel engines only if one of the pre-chambers is damaged and, as a result, the pre-chamber engine turns into a direct injection engine.
If the injector of the corresponding cylinder does not maintain injection pressure after the end of the injection process and the pressure drops, vibrations in the high pressure fuel line can once again raise the injector needle, so that after the end of the injection process, fuel is injected again into the combustion chamber (mechanical injectors).
If the oxygen in the combustion chamber is exhausted, then individual drops of fuel flow through the entire combustion chamber and fall on the bottom of the piston moving down closer to the edge. They quickly burn out there with a lack of oxygen, and quite a lot of heat is generated. At the same time, the material in these places softens. The dynamic forces and erosion of the fast-flowing combustion gases pull out individual particles from the surface or remove the head completely, resulting in damage.
Possible causes of damage
- Leaky nozzles or hard moving or stuck nozzle needles.
- broken or weakened injector springs.
- defective pressure reducing valves in the high pressure fuel pump injection quantity and injection timing not adjusted according to the engine manufacturer's instructions.
- in prechamber engines: a defect in the prechamber, but only in combination with one of the above reasons.
- ignition delay due to insufficient compression as a result of too much clearance, incorrect valve timing or leaking valves
- too long delay due to non-flammable diesel fuel (too low cetane number)
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 and force effects from gases in the combustion chamber, and mechanical problems.
At the same time, 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 scuffing on the guide belt leads to a violation of the sealing and heat transfer properties of the piston rings.
Ultimately, this can cause burnout of the fire belt.
We also note that with almost all malfunctions of the piston group, increased oil consumption occurs. With severe damage, thick, bluish exhaust smoke, a drop in power and difficult starting 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 defect in the piston group can be determined even without disassembling the engine according to the above external signs. But more often than not, such an “indiscriminate” diagnosis is inaccurate, since different causes often give almost the same result. Therefore, the possible causes of defects require a detailed analysis.
Violation of piston cooling is perhaps the most common cause of defects. This usually occurs when the engine cooling system malfunctions (chain: “radiator-fan-fan switch-on sensor-water pump”) or due to damage to the cylinder head gasket. In any case, as soon as the cylinder wall ceases to be washed from the outside by liquid, its temperature, and with it the temperature of the piston, begin to rise. The piston expands faster than the cylinder, moreover, unevenly, and eventually the clearance in certain places of the skirt (usually near the pin hole) becomes equal to zero. Seizure begins - the seizure and mutual transfer of materials of the piston and cylinder mirror, and with further engine operation, the piston jams.
After cooling, the shape of the piston 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 piston burr extends into the sealing belt, rolling the rings into the piston grooves. Then the cylinder, as a rule, turns off from work (compression is too low), and it is generally difficult to talk about oil consumption, since it will simply fly out of the exhaust pipe.
Insufficient piston lubrication is most often characteristic of starting conditions, especially at low temperatures. Under such conditions, the fuel entering the cylinder washes away the oil from the cylinder walls, and scoring occurs, which are usually located in the middle part of the skirt, on its loaded side.
Double-sided scuffing of the skirt usually occurs during prolonged operation in the oil starvation mode associated with malfunctions of the engine lubrication system, when the amount of oil falling on the cylinder walls decreases sharply.
The lack of lubrication of the piston pin is the reason for its jamming in the holes of the piston bosses. This phenomenon is typical only for designs with a pin pressed into the upper head of the connecting rod. This is facilitated by a small gap in the connection between the pin and the piston, so the "sticking" of the fingers is more often observed in relatively new engines.
Excessively high thermal force effect on the piston from hot gases in the combustion chamber is a common cause of defects and breakdowns. So, detonation leads to the destruction of the jumpers between the rings, and glow ignition - to burnouts.
In diesel engines, an excessively large fuel injection advance angle causes a very rapid increase in pressure in the cylinders (“rigidity” of work), which can also cause breakage of the jumpers. The same result is possible when using various fluids that make it easier to start a diesel engine.
The bottom and fire belt can be damaged if the temperature in the diesel combustion chamber is too high, caused by a malfunction of the injector nozzles. A similar picture also occurs when the piston cooling is disturbed - for example, when the nozzles supplying oil to the piston, which has an annular internal cooling cavity, coke. Seizure that occurs on the top of the piston can also spread 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 both “from above”, due to dust entering through a torn air filter, and “from below”, when abrasive particles circulate in the oil. In the first case, the cylinders in their upper part and the compression piston rings are the most worn, and in the second case, the oil scraper rings and the piston skirt. By the way, abrasive particles in the oil can appear not so much from untimely maintenance of the engine, but as a result of the rapid wear of any parts (for example, camshafts, pushers, etc.).
Rarely, piston erosion occurs at the “floating” pin hole when the retaining ring pops out. The most likely causes of this phenomenon are the non-parallelism of the lower and upper heads of the connecting rod, which leads to significant axial loads on the pin and the “knocking out” of the retaining ring from the groove, as well as the use of old (lost elasticity) retaining rings when repairing the engine. The cylinder in such cases turns out to be damaged by a finger so much that it can no longer be repaired by traditional methods (boring and honing).
Sometimes foreign objects can get into the cylinder. This most often occurs with careless work during engine maintenance or repair. A nut or bolt, caught between the piston and the head of the block, is capable of many things, including simply “failing” the piston bottom.
The story about defects and breakdowns of 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 tell...
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 power supply, lubrication and cooling systems of the engine in good condition, to service them in time, not to overload a 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 bring it "to the handle", when the repair will no longer cost "little blood".
When repairing, it is necessary to add and strictly follow a few more rules. The main thing, in our opinion, is that one should not strive to ensure minimum piston clearances in the cylinders and in the locks of the rings. The epidemic of "small gap disease", which once struck many mechanics, is still not over. Moreover, practice has shown that attempts to "tighter" install the piston in the cylinder in the hope of reducing engine noise and increasing its resource almost always end in the opposite: piston scuffing, knocking, oil consumption and repeated repairs. The rule “better clearance is 0.03 mm more than 0.01 mm less” always works for any engine.
The rest of the rules are traditional: high-quality spare parts, proper processing of worn parts, thorough washing and careful assembly with mandatory control at all stages.
