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Maintenance of piston of Marine diesel engine.

Date:2019-05-24 Author: Click:

1. Simple live plug

Piston is one of the main moving parts of diesel engine, and is an integral part of combustion chamber.
The piston of diesel engine can be divided into two categories: crosshead diesel engine piston and cylinder piston diesel engine.
The former consists of piston head, piston skirt, piston ring, piston rod and piston cooling mechanism, which consists of piston body, piston ring and piston pin.

1.1 piston action.

The piston is not only the component part of the combustion chamber, but also the connecting rod, crankshaft and other parts.
The main function of the piston is to complete the compression and expansion process under the condition of ensuring the sealing, and transfer the pneumatic strength through the connecting rod to the crankshaft.
In a cylindrical piston diesel engine, the piston takes the side thrust and plays the role of the slider.
In the two-stroke diesel engine, the air port is also opened to control the ventilation.

1.2 working conditions of piston.

In the work of diesel engine, the piston is affected by high temperature, high pressure, ablation and corrosion.
Its thermal load and mechanical load are very high, and the mechanical properties of the piston material are reduced at high temperature, so the piston is prone to crack and deformation in the work.
Between piston and cylinder, friction and impact occur in relative motion.
In the cylinder, the piston is not able to establish liquid dynamic lubrication between the cylinder and the cylinder because of the high temperature, gas scour and reciprocating motion. Therefore, the friction loss is large and the wear is serious.
In the middle and high speed diesel engine, the piston has a large reciprocating inertia force, which makes the vibration of diesel engine increase.

Check the basic method of failure.

2.1 causes of piston damage.

Piston is one of the main moving parts of diesel engine, and is an integral part of combustion chamber.
Piston works with a lot of mechanical stress and thermal stress while also bearing friction.
Therefore, the piston is easy to be damaged in use, especially for high - pressure diesel engines.
The main damage forms of piston are: outer surface and ring groove wear, crack and crack, top ablation, etc.

2.2 causes of wear and tear on the outer surface of the piston.

Generally, the outer surface of cylinder piston skirt of small and medium sized diesel engine is prone to wear.
This is due to the operation of the piston skirt leading role and the bearing side thrust results.
Large crosshead movement of the piston movement parts of a diesel engine is established by guide, slide block up the guiding role, and inherit the side thrust, and clearance between the piston and cylinder liner is bigger, so the normal operation of live round surface is not worn beyond the Great Wall, only if the piston motion device is with cylinder and other abnormal situations can occur.
After the outer surface of the piston skirt is worn, the diameter of the skirt decreases the gap between the piston and the cylinder. The piston cross section produces roundness error and the longitudinal interface produces roundness error.
These directly affect the piston's performance and diesel power.


2.3 causes of ring groove wear.

Piston ring groove wear is a common form of piston damage, especially for aluminum piston.
The wear of the ring groove is mainly due to the relative motion of the ring in the ring groove, including the reciprocating motion of the ring in the ring groove, the radial expansion motion of the ring and the rotation and distortion of the ring in the ring groove.
        Second is due to the new rigid particles of dust, carbon particles in the gas, especially when burning heavy oil of larger, more hard granular and these formed between hard particles in the ring and ring groove end face grinding grain, accelerate the end of the ring groove wear.
In addition, if the high temperature of the combustion chamber causes the piston head and ring groove to deform, the material performance is reduced, and the oil film between the ring and the end of the ring groove is damaged, the ring groove wear is more serious.

Wear ring groove face flush with the increase of ring fit clearance, this will bring down the piston ring sealing, produce gas leak, the compression pressure and explosion pressure is reduced, at the same time into the rings on the back of the gas increases, high pressure gas to press the ring to the cylinder wall ring is easy to break.
Ring groove multi-face wear makes the shape of the ring groove section change from rectangle to trapezoid or grinding platform, and the first and second ring grooves wear out as fast.
The wear rate of the end face of the piston ring groove is less than 0.01mm/kh for normal wear.

2.4 reasons for piston crack and fracture.

The crack in the piston head contact surface is the radial or circumferential crack, the crack of the lifting hole and the crack of the first ring groove at the top of the piston.

The head crack of piston is mainly caused by thermal stress and mechanical stress.
Diesel engine is running, the piston at the top of the uneven distribution of temperature, the top edge of the central or the highest temperature, cast steel piston can reach 450 ℃;
Aluminium piston can reach 300 ~ 300 ℃, top surface the temperature of the cooling side and a piston ring groove is in commonly 200 ℃ or so.
Under normal working conditions, there are mechanical stress of temperature difference stress and high pressure gas in the head of piston, and these stresses are periodic.
When the oil injection timing is not positive, fuel atomization or flame contact piston top surface will cause local overheating, causing excessive thermal stress;
Thermal stress can also occur when a diesel engine is overcharged or when the top of the piston is cooled.
Excessive thermal stress can cause cracks.
Frequent starting and stopping of diesel engine can cause thermal fatigue crack.
The piston top surface crack is also caused by the insufficient cooling of the piston top surface.
For example, when the cooling side of the water-cooled piston is serious or the piston top area is serious, it will overheat the top surface of the piston and cause the crack.
In general, when the scale or carbon layer is over 0.5mm, the likelihood of cracking due to overheating increases sharply.
Therefore, in order to prevent the crack generation, the diesel engine should be carried out regularly, strengthening the regular treatment of cooling water and other maintenance work.
The lifting hole edge of the piston top and the root of the first ring groove will cause cracks in the stress concentration.

2.5 the cause of piston ablation.

Due to the piston at the top of the first direct contact with the fuel gas and fire, the temperature is very high, especially when the fuel injection timing is not right or fuel injector installation bad or cooling side fouling caused local overheating, the top temperature is higher;
Second, as a result of the diesel engine burning heavy oil containing vanadium, too much sodium, will be on the top of the piston temperature as high as 550 ℃ or more parts of high temperature corrosion.
At the same time, when the piston material overheats, it oxidizes and decarbonates and changes its chemical composition.
Under the effect of comprehensive above factors, the piston at the top of the metal
layer upon layer peeling gradually thinning, the top thickness of vanadium corrosion pitting or pits, size, distribution of two-tone on top of the piston, the phenomenon as the piston at the top of the ablation.
Burn through the top when serious.
At the top of the piston, the roof thickness is reduced, the strength is reduced, and even the cylinder compression ratio is affected, and the performance of the diesel engine is reduced.

Repair of piston damage.

The appearance of the above four phenomena is the main damage form of piston in work.
In general, there is no condition to complete the repair of heavy damage on the ship, and it usually depends on whether the damage is renewed or continued.
Here is a brief description of the four damage fixes.

3.1 repair of surface abrasion of the piston surface.

When the outer surface of the piston skirt is not worn seriously, the outer circle of the light car group is used to eliminate the geometric error.
After the light vehicle meets the piston and cylinder clearance can continue to use.
Otherwise, different countermeasures are adopted according to the piston material: the aluminum piston is replaced by a new one, and the cast iron piston can be recovered by thermal spraying and iron plating.
The cast steel piston adopts the recovery size of plated iron and surfacing.
The wear ring should be replaced by excessive wear, severe strain or loose.

3.2 measurement and repair of ring groove wear.

The ring groove wear is determined by measuring the height of the ring groove with the sample and the gauge.
The template is made with the ring groove height of the new piston, and a new piston ring can be used as the sample.
Measurement, the level of the model will be inserted into the ring groove and clingy ring groove end face, with a feeler ring and ring groove on the distance between the end face, namely fitting clearance of ring and ring groove, called plane gap or clearance of heaven and earth.
The measured value is compared with the specification or standard. When the measured value exceeds the limit value, it indicates that the ring groove is severely worn and should be repaired.
The following method can be used to repair the wear condition of the ring groove.

(1) repair size method.
Light vehicle or grinding ring groove end face, with the processing of the repair size of the corresponding increase size of piston ring, ensure that the plane clearance meets the original requirements.
For example, the piston ring of MAN type diesel engine makes the ring groove height more than 0.6mm above the original design slot, which can be fitted with a larger piston ring (up to 1.0mm).
This method will reduce the thickness of the groove (the axial height between the grooves) and reduce the intensity.
In order to avoid excessive thinning of the ridge, the thickness of the grooved ridge should not exceed 20%~25% of the design thickness of the original groove.
It is required that no two ring grooves on the same piston should be repaired by this method.
Because of the different repair sizes of the grooves, the different sizes of each piston ring are different when the different grooves in the same piston are repaired.
A piston with a variety of piston rings will cause trouble for spare parts supply and management.
(2) restoration dimension method.
After the end face of the optical car ring groove, the original size of the ring groove is restored by spray welding, surfacing and chrome plating.
For example, when the maximum plane gap between the piston ring and ring groove of MAN B&W L60MC/MCE diesel engine exceeds 0.7mm, the recovery size method can be used to repair the end face of the ring groove, so that the plane clearance value can be restored to 0.4~0.45mm.

(3) set repair.
Ring grooved end face insert ring to restore the original size.
Diesel engine at low speed steel piston ring groove end face serious wear and tear, can adopt the method of padded ring fixed: first light car ring groove end face, remove the geometric error, and then under the ring groove end face with wear-resisting backing ring to restore original ring groove size and clearance, this is the ring groove padded tour DE France.
Welding process is used to weld the gasket ring to the lower end of the ring groove to form a permanent connection. This ring is called a dead ring, and this method is known as the dead ring method.
This method is firmly connected, and the ring will not fall off, but it will be difficult to repair after wearing.
        The ring is inserted into the lower end of the ring groove with the interference fit.
Because the gasket ring is not fixed in the lower end of the ring groove, it is easy to replace the gasket ring after re-wear, but the cushion ring is easy to loosen and fall off into the cylinder to cause an accident.

3.3 repair of piston crack.

Piston top surface cooling side, the piston pin seat of trunk piston cracks are very common, is mainly caused by excessive mechanical stress, as well as poor and blank production due to poor design, material defects in these areas such as the existence of stress concentration and accelerate the production of cracks.
The piston crack can be examined by observation or by staining.
The top crack of steel and aluminum piston is relatively light, which can be repaired by welding repair process, and local replacement is adopted when the crack of steel piston top is serious.
Piston annulus groove root crack, piston penetration crack and unrepaired cooling side crack should replace the piston.

3.4 piston ablation measurement and repair.

The piston top ablation can be measured with a piston top sample and a feeler.
When measuring, place the sample on the top of the piston and measure the maximum clearance between the sample and the top of the piston.
Measurement, but also should make the model around the axis of the piston movement, every turn 45 ° Angle measurement, taking the maximum.
New piston should be replaced when over 15mm.
The following measures may be adopted in the absence of spare parts or emergency situations.

(1) change the position of the piston.
When the ablation is not too serious and the piston structure is allowed, change the piston mounting Angle.
For example, B&W diesel engine piston top ablation site corresponds to the fuel injector direction, and the fuel is burned in this part and the oil is cooled with poor cooling effect, resulting in ablation of this part.
Due to the piston structure allows, when produce less serious ablation install piston area around 90 ° Angle, so avoid the ablation site continue to use the injection direction.

(2) welding repair.
When the ablation is serious (the maximum thickness is close to the normal value), the surfacing welding process can be used to repair and weld after the mechanical processing to restore the top shape of the piston.

(3) new.
When the maximum ablation thickness at the top of the piston exceeds the specification value or the top thickness of the piston is reduced to half of the design thickness, the piston should be scrapped and replaced.

Check the piston after repair.

Marine engineering is a system engineering, the host is the power source of the ship, the piston is the power of inherit and pass directly, like all sorts of people move to the heart to provide energy to support, any movement of the ship to complete implementation of piston to the host.
The host piston has a problem, like a heart attack, and can have dire consequences at any moment.
Therefore, in the case of the above mentioned problems in the piston, the repair work of the piston should be carried out in a timely manner, and the inspection must be carried out before the installation, ensuring the quality and the normal operation after the installation.
Engineer in addition to the piston size, shape, and surface roughness do general examination, also deal with the position of the piston platform inspection accuracy, size, shape, position precision of the piston and the surface roughness shall conform to the requirements of design drawings.

The technical requirements and testing methods of piston position accuracy are as follows:

(1) the center line of the piston pin hole is perpendicular to the center line of the piston, and the vertical dimension is not greater than 0.025mm/100mm.
When the piston is placed on the platform, the center line of the piston is perpendicular to the platform.
When the center line of the piston pin hole is parallel to the platform, it is shown that the center line of the piston pin hole is perpendicular to the center line of the piston, and the reading difference of the percentage meter in the measurement length is the verticality error.

(2) the center line of the piston pin hole should meet the center line of the piston, and its position requires that the piston diameter shall be no greater than 0.10mm when the diameter is less than 200mm, and the piston diameter shall be no more than 0.20mm when the diameter of the piston is >200mm.
When the test is done, the piston vertical platform of the pin hole is placed and the distance of the vertical right Angle plate to the left and right side of the piston is measured with a micrometer.
Half of the difference between the two sides is the position error.

(3) the piston ring groove should be perpendicular to the center line of the piston, and the perpendicularity should not be greater than 0.02mm.
When the test is done, the surface of the ring groove is calculated as the vertical error of the piston plane and the piston center line.

(4) inspection of piston top shape.
Test piston top shape with sample and feeler, and meet the requirement of drawing.



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