The ship is installed with YANMAR 6EY18ALW and Mitsubishi MET18SRC, rated power of 615Kw.Shortly after the departure of a certain voyage, NO.1's turbine vibrated continuously at generator high load (about 430Kw). Switched to No.3 generator, the turbine vibrated continuously too after a few hours later. The vibration started soon, even switched to both no.1 & no.3 generators ran together, the load of No.3 generator is only 300kw. In addition, the scavenging pressure of the two auxiliary generators was significantly higher before the vibration, but the exhaust temperature of the two was not improved, and the phenomenon of rising instead of falling.

It is only approx. 500 hours from the last turbine overhaul, after all not long, turbine is disassembled and found that the turbine is very clean, casing is not dirty, just accumulated on the nozzle ring leaves a layer of different thickness, where the airway narrower convex up high, not very hard, like soil of grey scale, see the attached pictures. After removing the fouling on the nozzle ring blade, install and retest, the vibration phenomenon is eliminated.

The inspection records showed that the turbine overhaul of the auxiliary engine was frequent: the turbocharger of NO.1 generator was disassembled and inspected 4 times in one and half years (about 5300hours),the running time is only 500 hours since last inspection; the turbocharger of NO.2 generator was disassembled and inspected 4 times in one years (about 3500hours) the turbocharger of NO.3 generator was disassembled and inspected 6 times in one and half years (about 5600hours), and the last two times have only been separated by about 500 hours.

In addition, after the latest overhaul for the turbocharger NO. 2 generator, scavenging pressure relative to the low before the overhaul some low (by about 0.02 MPa), but the overall temperature is abnormal much high, engineer had to  rarely use it during the voyage, just used as at berth.

It is easy to understand the vibration happened to working under high load if dirty plugging. But why does the vibration occur at relatively low loads? More surprisingly, the scavenging pressure is obviously higher before the surge, but the exhaust temperature rises instead of falling. Why? In addition, what is the reason that the turbine vibrated in a short period of time, has to frequent inspection? However,How to explain  the exhaust temperature of no.2 increased abnormally after turbocharger disassembled and inspection.

The reasons for the above questions are as follows:

Compared with other generators, the YANMAR 6EY18ALW accessory can be made so "small and exquisite", no doubt increase the power of the single cylinder by turbocharger. Therefore, no matter the load is high or low, its operation is very dependent on the state of turbocharger, plus scavenging pressure has a relatively large influence on the exhaust temperature.

However, the scavenging pressure is not as high as possible in the use of the rated matched turbocharger and engine.

Because, after clean and in good condition of the turbocharger, in the course of usage, based on the same load, the higher the scavenging pressure indicates that the turbine speed is faster, usually also means that the narrower exhaust gas port  (that is, the turbine will be dirtier). So, the pressure of front exhaust manifold (i.e., the exhaust back pressure)will be also higher. Meanwhile, because the engine has a larger angle of inlet and exhaust overlap (80 °), therefore, exhaust back pressure is equal to the scavenging back pressure.

On the one hand, as the turbine is more and more dirty, scavenging back pressure is more and more high, It pushes the turbocharger works at high speed high back pressure even if on lower load, once the load changes slightly, it is extremely easy to vibrate. This is also the reason why "under high load, the turbine has a continuous vibration", gradually develops to "relatively low load, there will also be a vibration ".

On the other hand, more and more dirty, despite the scavenging air pressure is higher and higher. (note: scavenging air pressure is relative increased to exhaust port of the narrowing, but after reaching a peak, will begin to decline, more and more low), at the same time, exhaust back pressure is becoming more and more high. The quality of the air conditioning depends on the pressure difference between scavenging pressure and exhaust back pressure, not scavenging pressure. Therefore, in the case of both increasing at the same time, the air exchange effect may not be better or even worse. This is also the reason why the scavenging pressure of the auxiliary engine is obviously higher before the turbine vibrates, but its exhaust temperature is not improved, even the phenomenon of rising instead of falling.

Obviously, for this engine, if the scavenging pressure is significantly increased during the usage of turbocharger, but the exhaust temperature is not decreased correspondingly, or even to converse, then it means the turbocharger should be disassembled and cleaned. Otherwise, the turbocharger may starts to vibrate soon.

The vibration can be solved by removing and cleaning the turbocharger. Of course, the cleaning methods and procedures and installation precision must meet the related requirements of the specification. Otherwise, if the cleaning is improper, it may be counterproductive, or make the vibration happen again very soon, or make the exhaust temperature rise abnormally.

As mentioned above, the drag of the internal exhaust passage directly affects the scavenging back pressure. Therefore, the demolition inspection and cleaning of turbocharger should be treated differently according to different situations:

1. There is no doubt that the scale accumulation on the turbocharger must be completely removed.

2. The deposition on the nozzle ring must also be completely removed to ensure smooth and smooth gas passage. In this way, even if the efficiency slightly reduced relatively, scavenging pressure decreases, but the scavenging back pressure is relatively lower, which had little impact on air exchange quality, so, for the effect of exhaust temperature is little; On the contrary, if the cleaning is insufficient, although it has little influence on the quality of air exchange and temperature discharge, it will certainly shorten the overhaul period of turbine.

3. For the scale accumulation on the turbocharger casing, a light grinding and smoothing should be adopted to ensure the appropriate clearance. Otherwise, the gap is too big, although relatively lower exhaust back pressure, but air leakage from clearance will increase substantially,reducing efficiency of turbine, the scavenging pressure drop too much, the air exchange quality was also affected by the larger, most likely would result in higher exhaust temperature anomalies.

To sum up, the turbocharger has to be overhauled frequently, the main reason is that poor quality fuel oil, diesel engine long-term low load operation, and poor combustion cause parts such as turbocharger nozzle ring carbon deposition, and maintenance of nozzle ring "clean enough" thoroughly; The scavenging pressure of no.2 generator has been relatively low since the last overhaul, but the exhaust temperature has been abnormally high, most likely due to the "too clean" of the turbocharger casing.

In a word, the operation of YANMAR 6EY18ALW auxiliary generator is very dependent on the operation state of turbocharger. Therefore, the usage and maintenance of turbine cannot be taken lightly.

Of course, it must be mentioned that the overall exhaust temperature of the auxiliary engine is high or the turbocharger vibates, which is also closely related to the technical condition of the air cooler and the fouling of the inlet pipe of the cylinder head. Generally, the more severe the air cooler's air side or inlet pipe is blocked, the higher the exhaust temperature will be, and the higher the back pressure of the turbine will be and the more likely it will be to cause vibration.

At present, many ships use a central cooling system. Air coolers use fresh water to cool them. However, it is common for bulk carriers, which often carry loose coal or ore, more chance to get  fouling on the gas side of air cooler. The air side of the auxiliary air cooler is seldom equipped with instruments to monitor the pressure drop. Therefore, it is not easy to find out whether there is dirty plug. Therefore, don't be misled by some superficial phenomena. For example, the inspection shows that the surface of the air cooler core is clean, or the air temperature after the air cooler is normal, etc. These phenomena do not really indicate that the air side is clean. If the overall exhaust temperature of the auxiliary machine is high, or if the turbine is not very dirty, the vibration will occur again. Before the problem is solved, do not give up the inspection and cleaning of the air side of the air cooler. Even if the problem cannot be completely solved, it will at least help to reduce the exhaust temperature and will never be in vain. No doubt, this applies to all types of main or auxiliary engine.

Note: the technical status of auxiliary engine cylinder, valves, nozzles or fuel oil pump, as well as the influence of oil injection, timing of inlet and outlet or temperature scavenging on the exhaust temperature are not discussed in this paper.