4.1 surge of Marine diesel engine supercharger.
 

When work supercharger compressor discharge will reduce to a certain extent, the flow when the compressor working point in its surge area, the compressor discharge pressure will be high and low, thus cause the strong vibration of the machine, at the same time have a heavy breathing.
When the pressure of the supercharger is reduced, the air intake of the diesel engine will be reduced, and the power will decrease accordingly. Therefore, the diesel fuel consumption will increase correspondingly and emit a lot of black smoke.
After a long period of operation of the supercharger, the gap of the internal sealing liner will gradually increase, allowing the exhaust to enter the bearing box.
As the pressure of the bearing box becomes larger, the fixed bolts on it will become loose, and some of the oil will flow out.
Reducing the amount of lubricating oil will not only increase the wear rate of the bearing, but also cause its heat dissipation, which will increase the temperature of the supercharger, which is unfavorable to the supercharger.
 

1. The performance of supercharger surge is usually caused by violent fluctuation of air flow in the compressor terminal, accompanied by a roar and vibration.
 

2. Reasons for surge of supercharger.

The general reasons for the surge of supercharger can be divided into the following:

The load of the ship diesel engine suddenly has a large range of negative fluctuation.
Because the rotor speed of the supercharger cannot quickly follow the decline of the diesel engine speed, it will cause a surge of the diesel engine in the transition process.
Supercharger and need mutual cooperation between ship diesel engine works, when the speed of the ship diesel engine to reduce, load, unload, the amount of air cylinders will need also sharply reduce, turbocharger rotor speed, however, cannot be synchronized rapidly decline.
The pressure of the compressor is more than that of the diesel engine, causing the excess air to be blocked in the compressor and causing the surge.
 

The low speed needle valve (with stepless speed regulation) or the drive to reduce the needle valve can easily cause a short surge.
It is also due to the large size of the needle valve, so that the ship diesel engine decelerate the process too fast.
The speed of the rotor speed of the supercharger can not keep up.
 

The combustion state of a ship's diesel engine will deteriorate when the load fluctuates or overloads.
As a result, the exhaust temperature is increased, the exhaust gas energy of the exhaust manifold is greatly increased, which leads to the increase of the rotor speed of the supercharger, which causes the air compressor to have more air intake than the air volume.
(4) the nozzle ring of the supercharger is coals, or the area of the nozzle ring is too small.
Gas flows through the narrowed the nozzle ring circulation section when the gas flow rate increases, prompting the rotor speed rises (each reduce lcm2, rotor speed will rise about 250 r/min), the compressor inlet flow rate also increase, which makes part of the air blocking in the compressor of the population to generate flow disturbance, lead to surge.
 

4.2 damage to the ship's diesel engine bearing.

The ratio of Marine diesel engine bearing damage to diesel engine failure is about 30%.
Bearing damage is generally used in assembly and make caused by improper operation, the impurities into the ship diesel engine is embedded into the bearing damage caused by accidents accounted for the most, is about 45%.
Bearing shell is the vulnerable parts on the ship diesel engine, so the analysis of a common cause of bearing damage, targeted to take effective measures of prevention and maintenance to prolong the service life of bearing will be of great significance.
  

The damage of the bearing shell is mainly due to the damage of the wear-resistant alloy layer on the bearing bush.
The main damage forms of bearing bush are as follows:

4.2.1 early wear of the bearing bush.

(1) damage characteristics: the circular surface of the inner circle of the bearing is marked with a ribbon and linear shape;
Or flaky, blocky scratches;
Or in the oil groove, the leaf shaped erosion marks and so on.
  

Causes: (2) the surface of bearing inner circle, can appear in the oil film breaks and shaft neck or other hard the interparticle friction surface, due to the effect of friction temperature rise and friction, it metal molecules generated displacement and even from the metal layer.
The combination of hard particles, inner surface and outer surface roughness of the axial surface and the outer surface roughness of the oil film is greater than that of the oil film thickness, which is the two basic factors that cause the oil film discontinuity which leads to the abrasion of the bearing.
  

(iii) prevention methods:

In the case of assembly, the debris, burr, sand and other impurities should be cleaned up in time.
 

In order to maintain and improve the performance of the filter, all the particles in the lubricating oil greater than 15 mu m are filtered out.
More than 95% of the particles above the m are filtered out;
More than 90% of the particles above the m are filtered out.
 

The roughness of the surface and the inner circle should be Rz less than or equal to 1.
It should rotate the crankshaft in the opposite direction when repairing the neck, and the rotation direction of the shaft should be the same as that of his work.
 

In the process of manufacture and assembly, the tolerance and size of the control position are strictly controlled.
 

To improve the embedding of the inner circle surface of the bearing.
 

Change the oil in time and ensure the quality of the oil.
 

Select the appropriate work space.
 

In the early stage of the wear, the new bearing should be replaced in time.
  

4.2.2 the fatigue spalling of the bearing bush.

(1) the characteristics of spalling: the surface of the round alloy layer in the bearing is in a parallel, reticulated fissure, scaly or alloy in point, flake and block.
 

(2) cause: the bearing load of bearing alloy exceeds the maximum carrying capacity allowed by the alloy layer material, which will cause fatigue damage of the bearing.
The root cause of fatigue is usually the maximum periodic load repetition.
The causes of early fatigue are: in the design or manufacturing process, the reasonable selection of the diesel engine is not carried out according to the different USES of the diesel engine, or the safety factor is too small.
The bonding strength of steel back and alloy layer is less than the tensile strength of alloy.
The fitting degree between the bearing and the tile is too small; surface of exhaust valve valve line is also to soot pollution, and even lead to host starting difficulty, this becomes the next host drive soon after oil and exhaust valve fault hidden trouble, so these bad habits of manipulation and maintenance Marine diesel engine is a Marine diesel engine valve fault factors.

4.3.2 exhaust valve burning loss of exhaust valve is the most common failure of exhaust valve.
The main reason is that the exhaust valve is not tightly sealed, which causes the high temperature gas leakage, which causes the place to overheat, and even melts the metal material.
The following figure shows the maximum allowable ablation of several major models.

4.3.3 valve seal cone wear too fast in the combustion chamber pressure under the action of the seat and valve disc elastic deformation occurs, valve seat impact also causes the elastic deformation of valve seat and valve disc, which can make the valve plate cone repeatedly wedge is fashionable, seal cone of relative movement, cause the seal cone wear.
Valve clearance is too large, the valve plate and valve seat stiffness is insufficient, air valve and valve seat material performance can not meet the requirements or do not match, the heavy oil contains many harmful elements such as vanadium, sodium, sulfur, high load operation or combustion deterioration, cooling, the valve stem and duct clearance is too large, the valve seat valve body vibration velocity is too large, can make the wear rate increases.
 Improper selection of structural parameters;

Poor assembly;
Poor lubrication;
Poor maintenance;
When the oil film is discontinuous or is cut off, it causes a high pressure gradient, which causes the shear stress and temperature fluctuation inside the alloy layer.
As well as careless operation, and so on.
 

(3) prevention measures: (1) the practice proved that the design and manufacture should be according to the different USES of diesel engine bearing shell required units under maximum load and fatigue strength values of various bearing alloy to select suitable material and safety factor, this is one of the important measures to overcome fatigue;
The bonding strength of the steel back and the alloy layer should not be less than the tensile strength of the alloy.
There should be no less than 85 percent of the fitting between the shaft and the hole, and two or more points should be scattered in the unbonded area, otherwise the concentration will be affected by the transmission of load and heat.
In structural parameters, thin wall, narrow shape, small clearance and large diameter should be recommended.
  

4.3 common failure of exhaust valve.

4.3.1 ship diesel engine exhaust valve out of the cause of the problem in Marine diesel engine valve working conditions is very bad, the air valve base, direct contact with high temperature combustion products during the valve opening is under high temperature (900 ~ 1000 ℃) and corrosive gas high speed (up to 600 m/s) flushing, valve core temperature as high as 700 ~ 800 ℃, in the middle of valve plate and valve stem transition arc temperature is 600 ~ 700 ℃, exhaust valve working temperature distribution as shown in figure 4-3.
Excessive temperature can reduce the mechanical properties of metal materials and heat deformation of materials.
When the valve seal is not tight, it will cause the high temperature gas to burn the valve surface.
Valve seat, valve and valve seat under the effect of inertia force and spring force, also bear the considerable impact of alternating load, beating in the valve or valve clearance increases, the load will be significantly increased.
The impact of valve and seat is easy to form a sealing surface deformation and serious wear.
For Marine ship more than most of the diesel engine is turbocharged Marine diesel engine, due to the stress of fresh air in the intake port to prevent oil from valve catheter may, therefore, prone to dry friction between metal.
However, there is a layer of lubricating oil, such as lubricating oil or smoke oil, between the air valve of the general Marine diesel engine and the exhaust valve seat alloy surface of the supercharged ship diesel engine.
In addition, there will be wear and tear between the stem and the pipe, and the top of the stem will be impacted and worn by the rocker arm.
Because of rising fuel prices and fierce competition in the shipping market, shipowners use cheap, low-quality fuel to reduce costs to improve their competitiveness and gain more profits.
The fuel has a high viscosity, long period of combustion, and high content of vanadium, sodium and sulfur.
The burning fuel on diesel engine of the ship, the residue in the emissions (fuel ash) are only part of it with the exhaust gas to leave machine, and the rest remain in some high temperature (497-797 ℃) in the engine parts.
For example, the exhaust width and the piston top, formed deposition, resulting in the so-called "high temperature corrosion".
So far, there has been no economically rational process to remove corrosion elements from residual oil, and even high-grade alloy steels and surfacing steels are corroded by fuel.
In ship diesel engine running in violation of the provisions of car maintenance, low temperature start ship diesel engine, the low temperature force load, rapid changes in ship diesel engine cylinder combustion temperature, in the condition of ship diesel engine load, handle a sharp transformation, worsen ship diesel engine cylinder combustion condition, a lot of bad atomizing bulky heavy oil particles sprayed into the cylinder, causing severe after burning and not over a burning, serious carbon deposit to the

4.3.4 fracture in the valve plate and valve plate and valve stem stem the stem of set up checked posts block grooves and transition fillet in, due to the stress of these areas is easy to focus, when the stress concentration to a certain degree of fatigue fracture damage occurs.
The causes of fracture are: the gap between the stem and the pipe is too large;
Deformation of disc and seat causes local force to be too large;
The air valve clearance is too large, and the percussion is serious fatigue damage;
Vibration of the valve mechanism.
The groove of the valve stem is the weakest part of the air valve, and fatigue fracture can occur if the groove processing technology is bad or the impact of the closed valve is heavier.
 

4.3.5 gas valve card dead valve card die mainly because of the small clearance between the valve stem and the pipe, and when the thermal expansion, the gap between the two gaps occurs.
In addition, the stem card dies in the pipe when the stem is bent.

4.3.6 the spring of the valve spring is not reasonable in its structure, and there are defects in it, and the distortion or fatigue limit in the process of improper processing or use can lead to fracture in the work.
The valve spring break directly affects the normal work of the Marine diesel engine. When serious, the valve may fall into the cylinder and damage the cylinder.
 

4.4 wear of cylinder of Marine diesel engine.

The wear of cylinder liner of Marine diesel engine will directly affect the working status and service life of the diesel engine.
According to the wear mechanism classification, the wear of cylinder liner mainly includes the following three conditions: adhesion wear, abrasive wear and corrosion wear.
  

(I) reasons

(1) normal wear is mainly caused by friction under the boundary lubrication condition of frictional due to the direct contact of metals and metal inevitable cause, and the quality of the lubricating oil, the viscosity is too low or can wear.
 

The abrasive wear is mainly caused by external hard material, such as dust entering the cylinder when inhaling.
Generally speaking, there are parallel vertical lines on the circular surface of the cylinder liner with abrasive wear, which can be observed by the naked eye and can be touched by hand.
 

This wear and tear is caused by the presence of sulphur in the fuel.
It is proved that the wear amount of the first piston ring at the upper part of cylinder jacket is the largest.
 

The wear is mainly caused by the lubrication of the piston skirt, or because of the excessive clearance between the piston skirt and the worn cylinder sleeve.
 

(2) reduce the wear of cylinder liner.

To strengthen the management of fuel and combustion;
 

Ensure good lubrication condition of cylinder;
 

To control the temperature of cooling water;
 

The normal clearance between piston and cylinder liner is maintained, so that the piston motion device has a good neutral.