Why did the piston burn out?

Analysis of various piston damage shows that all causes of defects and breakdowns are divided into four groups: impaired cooling, lack of lubrication, excessively high thermal force from gases in the combustion chamber and mechanical problems.

At the same time, many of the 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 disruption of the sealing and heat transfer properties piston rings.

Ultimately, this can cause the fire belt to burn out.

We also note that for almost all malfunctions piston group increased oil consumption occurs. At serious damage thick, blue smoke exhaust, loss of power and difficult starting due to low compression. In some cases, a knocking sound from a damaged piston can be heard, especially when the engine is not warmed up.

Sometimes the nature of the defect in the piston group can be determined without disassembling the engine using the above external signs. But more often than not, such “in-place” diagnostics are inaccurate, because different reasons often give almost the same result. That's why possible reasons defects require detailed analysis.

Impaired piston cooling is perhaps the most common cause of defects. This usually occurs when there is a malfunction of the engine cooling system (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 the liquid, its temperature, and with it the temperature of the piston, begin to rise. The piston expands faster than the cylinder, and also unevenly, and ultimately the gap in selected places skirt (usually near the finger hole) becomes zero. Scuffing begins - the seizing and mutual transfer of materials of the piston and cylinder mirror, and with further operation of the engine the piston jams.

After cooling, the shape of the piston rarely returns to normal: the skirt turns out to be deformed, i.e. compressed along the major axis of the ellipse. Further operation of such a piston is accompanied by knocking and increased consumption oils

In some cases, piston scuffing extends to the sealing belt, forcing the rings into the piston grooves. Then the cylinder, as a rule, stops working (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 modes, especially when low temperatures. Under such conditions, the fuel entering the cylinder washes away the oil from the cylinder walls, and scuff marks appear, which are usually located in the middle part of the skirt, on its loaded side.

Double-sided skirt lifting usually occurs when long work in mode oil starvation associated with malfunctions of the engine lubrication system when quantity of oil falling on the cylinder walls decreases sharply.

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 of the pin with the piston, so “sticking” of the pins is more often observed in relatively new engines.

Excessively high thermal force impact on the piston from hot gases in the combustion chamber - common reason defects and breakdowns. Thus, detonation leads to the destruction of the bridges between the rings, and glow ignition leads to burnouts.

In diesel engines, an excessively large fuel injection advance angle causes a very rapid increase in pressure in the cylinders (“hardness” of operation), which can also cause breakage of the jumpers. The same result is possible when using various liquids, making it easier to start a diesel engine.

The bottom and fire belt can be damaged if too high temperature in the diesel combustion chamber caused by a malfunction of the injector nozzles. A similar picture arises when the cooling of the piston is disrupted - for example, when the nozzles that supply oil to the piston having an annular cavity become coked internal cooling. Seizure that occurs on the top of the piston can spread to the skirt, trapping the piston rings.

Mechanical problems, perhaps, give the most big variety 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”, with the circulation of abrasive particles 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 - oil scraper rings and piston skirt. By the way, abrasive particles in oil can appear not so much from untimely engine maintenance, but as a result rapid wear any parts (for example, camshaft, pushers, etc.).

Rarely, erosion of the piston at the hole of the “floating” pin occurs when the retaining ring pops out. Most probable reasons This phenomenon is the non-parallelism of the lower and upper heads of the connecting rod, leading to significant axial loads on the finger and “knocking” the retaining ring out of the groove, as well as the use of old (lost elasticity) retaining rings when repairing the engine. In such cases, the cylinder becomes so damaged by the finger that it can no longer be repaired using traditional methods (boring and honing).

Sometimes foreign objects can get into the cylinder. This most often occurs due to careless work during engine maintenance or repair. A nut or bolt, once between the piston and the block head, can do a lot, including simply “falling through” 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 determine...

How to avoid burnout?

The rules are very simple and stem from the characteristics 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 maintain the power supply, lubrication and cooling systems of the engine in good working order, to service them on time, and not to overload them cold engine, avoid use low quality fuel, oil and inappropriate filters and spark plugs. And if something is wrong with the engine, do not let it get to the point where repairs are no longer costly.

When repairing, it is necessary to add and strictly follow a few more rules. The main thing, in our opinion, is that you cannot strive to ensure minimal piston clearances in the cylinders and in the ring locks. The epidemic of “small clearance disease” that once afflicted many mechanics has not yet passed. Moreover, practice has shown that attempts to install the piston “more tightly” in the cylinder in the hope of reducing engine noise and increasing its service life almost always end in the opposite: piston scuffing, knocking, oil consumption and repeated repairs. The rule “a gap of 0.03 mm more is better than a gap of 0.01 mm less” always works for any engine.

The remaining rules are traditional: high-quality spare parts, proper processing of worn parts, thorough washing and careful assembly with mandatory control at all stages.

Why did the piston burn out?

Why did the piston burn out?

ALEXANDER KHRULEV, Candidate of Technical Sciences

As is known, defects in the mechanical part of the engine do not appear by themselves. Practice shows: there are always reasons for damage and failure of certain parts. Understanding them is not easy, especially when the components of the piston group are damaged.

The piston group is a traditional source of troubles that await the driver operating the car and the mechanic repairing it. Engine overheating, negligence in repairs - and please - increased oil consumption, blue smoke, knocking.

When you “open” such an engine, scuffs on the pistons, rings and cylinders are inevitably discovered. The conclusion is disappointing - it is required expensive repairs. And the question arises: what was wrong with the engine that it was brought to such a state?

The engine, of course, is not to blame. It is simply necessary to foresee the consequences of certain interventions in its work. After all, the piston group of a modern engine is “delicate matter” in every sense. Combination minimum sizes parts with micron tolerances and enormous gas pressure forces and inertia acting on them contribute to the appearance and development of defects, ultimately leading to engine failure.

In many cases easy replacement damaged parts - no best technology engine repair. The reason for the appearance of the defect remains, 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 possible consequences of your actions. But this is not enough - you need 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 simple part at first glance, but extremely important and at the same time complex. Its design embodies the experience of many generations of developers.

And to some extent, the piston shapes the appearance of the entire engine. In one of our previous publications, we even expressed this idea, paraphrasing the well-known aphorism: “Show me the piston, and I’ll tell you what kind of engine you have.”

So, using a piston in an engine solves several problems. The first and main thing is to perceive the gas pressure in the cylinder and transmit the resulting pressure force through the piston pin to the connecting rod. This force will then be converted by the crankshaft into engine torque.

It is impossible to solve the problem of converting gas pressure into torque without a reliable seal of the moving piston in the cylinder. Otherwise, gases will inevitably break through into the engine crankcase and oil will enter the combustion chamber from the crankcase.

For this purpose, the piston has a sealing belt with grooves in which compression and oil scraper rings of a special profile are installed. In addition, special holes are made in the piston to drain oil.

But this is not enough. During operation, the piston bottom (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 - through them, heat is transferred from the bottom to the cylinder wall and then to the coolant. However, in some of the most loaded structures, additional oil cooling pistons, 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.

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. Various kinds of distortions and “shifts” that cause the piston to “hang” in the cylinder negatively affect the sealing and heat transfer properties of the rings and increase the noise of the engine.

The purpose of holding the piston in this position is the guide belt - the piston skirt. The requirements for the skirt are very contradictory, namely: it is necessary to ensure a minimum, but guaranteed, gap 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.

To prevent a heated piston from jamming, modern engines take measures to compensate for its temperature expansion.

Firstly, in the cross section the piston skirt is given the shape of an ellipse, the major axis of which is perpendicular to the axis of the pin, and in the longitudinal section it is shaped like 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. On high frequencies rotations characteristic of modern engines, it is very important to keep the mass of moving parts low. In 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 we have to resort 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 in the lower part of the skirt, designed to reduce noise due to the “shifting” of the piston in dead spots. A special microprofile on the working surface—microgrooves with a pitch of 0.0.5 mm—helps improve the lubrication of the skirt, and a special antifriction coating helps reduce friction. The profile of the sealing and fire belts is also certain - here the temperature is highest, and the gap between the piston and the cylinder in this place should not be large (the likelihood of gas breakthrough increases, the danger of overheating and breakage of the rings) nor small (the danger of jamming is high). Often the resistance of the fire belt is increased by anodizing.

Everything we have said is far from full list piston requirements. 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), condition of the cylinder surface (deviations from cylindricity, microprofile). But it is already becoming clear that any, even not very significant, deviation in the operating conditions of the piston group quickly leads to the appearance of defects, breakdowns and engine failure. In order to properly repair the engine in the future, it is necessary not only to know how the piston is designed and works, but also to be able to determine by the nature of the damage to the parts why, for example, scuffing or...

Why did the piston burn out?

Analysis of various piston damage shows that all causes of defects and breakdowns are divided into four groups: impaired cooling, lack of lubrication, excessively high thermal force from gases in the combustion chamber and mechanical problems.

At the same time, many of the 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 disruption of the sealing and heat transfer properties of the piston rings.

Ultimately, this can cause the fire belt to burn out.

We also note that with almost all malfunctions of the piston group, increased oil consumption occurs. In case of serious damage, thick, bluish exhaust smoke, a drop in power and difficult starting due to low compression are observed. In some cases, a knocking sound from a damaged piston can be heard, especially on a cold engine (for more details on piston knocking, see No. 8.9/2000).

Sometimes the nature of the defect in the piston group can be determined without disassembling the engine using the above external signs. But more often than not, such “in-place” diagnostics are inaccurate, since different causes often give almost the same result. Therefore, possible causes of defects require detailed analysis.

Impaired piston cooling is perhaps the most common cause of defects. This usually occurs when there is a malfunction of the engine cooling system (chain: “radiator - fan - fan switch 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 the liquid, its temperature, and with it the temperature of the piston, begin to rise. The piston expands faster than the cylinder, and unevenly, and ultimately the gap in certain places of the skirt (usually near the pin hole) becomes zero. Scuffing begins - the seizing and mutual transfer of materials of the piston and cylinder mirror, and with further operation of the engine the piston jams.

After cooling, the shape of the piston rarely returns to normal: the skirt turns out to be 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, piston scuffing extends to the sealing belt, forcing the rings into the piston grooves. Then the cylinder, as a rule, stops working (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 modes, especially at low temperatures. Under such conditions, the fuel entering the cylinder washes away the oil from the cylinder walls, and scuff marks appear, which are usually located in the middle part of the skirt, on its loaded side.

Double-sided skirt scuffing usually occurs during prolonged operation in oil starvation mode associated with malfunctions of the engine lubrication system, when the amount of oil reaching the cylinder walls sharply decreases.

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 of the pin with the piston, so “sticking” of the pins is more often observed in relatively new engines.

Excessively high thermal force on the piston from hot gases in the combustion chamber is a common cause of defects and breakdowns. Thus, detonation leads to the destruction of the bridges between the rings, and glow ignition leads to burnouts (for more details, see Nos. 4, 5/2000).

In diesel engines, an excessively large fuel injection advance angle causes a very rapid increase in pressure in the cylinders (“hardness” of operation), which can also cause breakage of the jumpers. The same result is possible when using various liquids 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 arises when the cooling of the piston is disrupted - for example, when the nozzles that supply oil to the piston, which has an annular cavity of internal cooling, become coked. Seizure that occurs on the top of the piston can spread to the skirt, trapping the piston rings.

Mechanical problems perhaps provide the widest variety of piston 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 most worn are the cylinders in their upper part and the compression piston rings, and in the second - the oil scraper rings and the piston skirt. By the way, abrasive particles in the oil can appear not so much from untimely engine maintenance, but as a result of rapid wear of some parts (for example, camshaft, pushers, etc.).

Rarely, erosion of the piston at the hole of the “floating” pin occurs 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 “knocking out” of the retaining ring from the groove, as well as the use of old (lost elasticity) retaining rings during repairs. In such cases, the cylinder becomes so damaged by the finger that it can no longer be repaired using traditional methods (boring and honing).

Sometimes foreign objects can get into the cylinder. This most often occurs due to careless work during engine maintenance or repair. A nut or bolt, once between the piston and the block head, can do a lot, including simply “falling through” 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 determine...

How to avoid burnout?

The rules are very simple and stem from the characteristics 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, during operation it is still necessary to: maintain the power supply, lubrication and cooling systems of the engine in good condition, service them on time, do not unnecessarily load a cold engine, avoid the use of low-quality fuel, oil and inappropriate filters and spark plugs. And if something is wrong with the engine, do not let it get to the point where repairs are no longer costly.

When repairing, it is necessary to add and strictly follow a few more rules. The main thing, in our opinion, is that you cannot strive to ensure minimal piston clearances in the cylinders and in the ring locks. The epidemic of “small clearance disease” that once afflicted many mechanics has not yet passed. Moreover, practice has shown that attempts to install the piston “more tightly” in the cylinder in the hope of reducing engine noise and increasing its service life almost always end in the opposite: piston scuffing, knocking, oil consumption and repeated repairs. The rule “a gap of 0.03 mm more is better than a gap of 0.01 mm less” always works for any engine.

The remaining rules are traditional: high-quality spare parts, proper processing of worn parts, thorough washing and careful assembly with mandatory control at all stages.

	Skirting can occur as a result of insufficient clearance or overheating. In the latter case, they are located closer to the finger hole.
	Insufficient lubrication caused one-sided lifting of the skirt (a). With further operation in this mode, the scuffing spreads to both sides of the skirt (b).
	The pin caught in the hole of the piston bosses occurred immediately after starting the engine. The reason is a small gap in the connection and insufficient lubrication.
	Occurrence of rings in grooves and scuffing as a result of too high temperature in the combustion chamber (a). At insufficient cooling bottom scuffing extends to the entire upper part of the piston (b)
	Poor oil filtration caused abrasive wear on the skirt, cylinders and piston rings.
	A deformed connecting rod usually results in an asymmetrical skirt-to-cylinder contact patch due to piston misalignment.

ALEXANDER KHRULEV, "ABS"

As is known, defects in the mechanical part of the engine do not appear by themselves. Practice shows: there are always reasons for damage and failure of certain parts. Understanding them is not easy, especially when the components of the piston group are damaged.

The piston group is a traditional source of troubles that await the driver operating the car and the mechanic repairing it. Engine overheating, negligence in repairs - and please - increased oil consumption, blue smoke, knocking.

When you “open” such an engine, scuffs on the pistons, rings and cylinders are inevitably discovered. The conclusion is disappointing - expensive repairs are required. And the question arises: what was wrong with the engine that it was brought to such a state?

The engine, of course, is not to blame. It is simply necessary to foresee the consequences of certain interventions in its work. After all, the piston group of a modern engine is “delicate matter” in every sense. The combination of minimal dimensions of parts with micron tolerances and enormous gas pressure forces 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 remains, and if so, then its repetition is inevitable.

To prevent this from happening, a competent mechanic, 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 simple part at first glance, but extremely important and at the same time complex. Its design embodies the experience of many generations of developers.

And to some extent, the piston shapes the appearance of the entire engine. In one of our previous publications, we even expressed this idea, paraphrasing the well-known aphorism: “Show me the piston, and I’ll tell you what kind of engine you have.”

So, using a piston in an engine solves several problems. The first and main thing is to perceive the gas pressure in the cylinder and transmit the resulting pressure force through the piston pin to the connecting rod. This force will then be converted by the crankshaft into engine torque.

It is impossible to solve the problem of converting gas pressure into torque without a reliable seal of the moving piston in the cylinder. Otherwise, gases will inevitably break through into the engine crankcase and oil will enter the combustion chamber from the crankcase.

For this purpose, the piston has a sealing belt with grooves in which compression and oil scraper rings of a special profile are installed. In addition, special holes are made in the piston to drain oil.

But this is not enough. During operation, the piston bottom (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 - through them, heat is transferred from the bottom to the cylinder wall and then to the coolant. However, in some of the most heavily 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.

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. Various kinds of distortions and “shifts” that cause the piston to “hang” in the cylinder negatively affect the sealing and heat transfer properties of the rings and increase the noise of the engine.

The purpose of holding the piston in this position is the guide belt - the piston skirt. The requirements for the skirt are very contradictory, namely: it is necessary to ensure a minimum, but guaranteed, gap 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.

To prevent a heated piston from jamming, modern engines take measures to compensate for its temperature expansion.

Firstly, in the cross section the piston skirt is given the shape of an ellipse, the major axis of which is perpendicular to the axis of the pin, and in the longitudinal section it is shaped like 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. On high rotation speeds characteristic of modern engines, it is very important to ensure a low mass of moving parts. In 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 we have to resort 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 in the lower part of the skirt, designed to reduce noise due to the “shifting” of the piston at dead points. A special microprofile on the working surface - microgrooves with a pitch of 0.2-0.5 mm - helps to improve the lubrication of the skirt, and a special anti-friction coating helps reduce friction.

The profile of the sealing and fire belts is also certain - here the temperature is highest, and the gap between the piston and the cylinder in this place should not be large (the likelihood of gas breakthrough increases, the danger of overheating and breakage of the rings) nor small (the danger of jamming is high).

Why did the piston burn out?

Analysis of various piston damage shows that all causes of defects and breakdowns are divided into four groups: impaired cooling, lack of lubrication, excessively high thermal force from gases in the combustion chamber and mechanical problems.

At the same time, many of the 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 disruption of the sealing and heat transfer properties of the piston rings.

Ultimately, this can cause the fire belt to burn out.

We also note that with almost all malfunctions of the piston group, increased oil consumption occurs. In case of serious damage, thick, bluish exhaust smoke, a drop in power and difficult starting due to low compression are observed. In some cases, a knocking sound from a damaged piston can be heard, especially on a cold engine.

Sometimes the nature of the defect in the piston group can be determined without disassembling the engine using the above external signs.

But more often than not, such “in-place” diagnostics are inaccurate, since different causes often give almost the same result. Therefore, possible causes of defects require detailed analysis.

Impaired piston cooling is perhaps the most common cause of defects. This usually occurs when there is a malfunction of the engine cooling system (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, and unevenly, and ultimately the gap in certain places of the skirt (usually near the pin hole) becomes zero. Scuffing begins - the seizing and mutual transfer of materials of the piston and cylinder mirror, and with further operation of the engine the piston jams.

In some cases, piston scuffing extends to the sealing belt, forcing the rings into the piston grooves. Then the cylinder, as a rule, stops working (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 modes, especially at low temperatures. Under such conditions, the fuel entering the cylinder washes away the oil from the cylinder walls, and scuff marks appear, which are usually located in the middle part of the skirt, on its loaded side.

Double-sided skirt scuffing usually occurs during prolonged operation in oil starvation mode associated with malfunctions of the engine lubrication system, when the amount of oil reaching the cylinder walls sharply decreases.

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 of the pin with the piston, so “sticking” of the pins is more often observed in relatively new engines.

Excessively high thermal force on the piston from hot gases in the combustion chamber is a common cause of defects and breakdowns. Thus, detonation leads to the destruction of the bridges between the rings, and glow ignition leads to burnouts.

In diesel engines, an excessively large fuel injection advance angle causes a very rapid increase in pressure in the cylinders (“hardness” of operation), which can also cause breakage of the jumpers. The same result is possible when using various liquids 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 arises when the cooling of the piston is disrupted - for example, when the nozzles that supply oil to the piston, which has an annular cavity of internal cooling, become coked. Seizure that occurs on the top of the piston can spread to the skirt, trapping the piston rings.

Mechanical problems perhaps provide the widest variety of piston 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 are the most worn. By the way, abrasive particles in the oil can appear not so much from untimely engine maintenance, but as a result of rapid wear of some parts (for example, camshaft, pushers, etc.).

Rarely, erosion of the piston at the hole of the “floating” pin occurs when the retaining ring pops out. The most likely reasons for 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 “knocking out” of the retaining ring from the groove, as well as the use of old (lost elasticity) retaining rings when repairing the engine. In such cases, the cylinder becomes so damaged by the finger that it can no longer be repaired using traditional methods (boring and honing).

Sometimes foreign objects can get into the cylinder.

How to avoid burnout?

This most often happens due to careless work during engine maintenance or repair. A nut or bolt, caught between the piston and the block head, can do a lot, including simply “falling through” 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 determine...

When repairing, it is necessary to add and strictly follow a few more rules. The main thing, in our opinion, is that you cannot strive to ensure minimal piston clearances in the cylinders and in the ring locks. The epidemic of "small clearance disease" that once afflicted many mechanics has not yet passed. Moreover, practice has shown that attempts to install the piston “more tightly” in the cylinder in the hope of reducing engine noise and increasing its service life almost always end in the opposite: piston scuffing, knocking, oil consumption and repeated repairs. The rule “a gap of 0.03 mm more is better than a gap of 0.01 mm less” always works for any engine.

The remaining rules are traditional: high-quality spare parts, proper processing of worn parts, thorough washing and careful assembly with mandatory control at all stages.

The bottom and heat zone area is completely destroyed. The heat belt burned through to the reinforcing insert. The melted piston material moved up the piston skirt and caused damage and scuffing there as well. The reinforcing insert of the first compression ring was partially preserved only on the left side of the piston.

The remainder of the reinforcing insert became 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 hit through inlet valve in intake manifold and thus also into the adjacent cylinder and damage was also caused there (traces of impacts).

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 hot zone. The piston skirt and piston ring area are free from scoring.

Damage assessment

This type of damage occurs especially in diesel engines. direct injection. Prechamber diesel engines This only applies if one of the prechambers is damaged and as a result the prechamber engine turns into a direct injection engine.

If the injector of the respective cylinder does not maintain the 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 again injected into the combustion chamber (mechanical injectors).

If the oxygen in the combustion chamber is depleted, then individual drops of fuel flow through the entire combustion chamber and land on the bottom of the downward moving piston 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 fast-flowing combustion gases tear individual particles from the surface or remove the head completely, resulting in damage.

Possible causes of damage

1. leaking injectors or difficult to move or jammed injector needles.
2. Broken or weakened injector springs.
3. defective pressure reduction valves in fuel pump high pressure, the amount of fuel injected and the injection timing are not adjusted according to the engine manufacturer's instructions.
4. in pre-chamber engines: a defect in the pre-chamber, but only in combination with one of the above reasons.
5. Ignition delay due to insufficient compression resulting from too much large gap, incorrect valve timing or leaking valves
6. too long delay due to non-flammability diesel fuel(cetane number too low)