The Sequential Turbocharging System of the diesel engine can effectively solve the matching problem between the diesel engine and the turbocharger at low speeds and low loads, and effectively improve the power performance, economy and emission characteristics of the diesel engine under low operating conditions. It is widely used in ship propulsion diesel engines.
This article introduces the structure and related characteristics of the Sequential Turbocharging System of a certain type of marine diesel engine. Based on the common faults accumulated through long-term maintenance tests of this type of engine, it summarizes, generalizes and studies the solutions, and proposes relevant measures.
This article has significant reference value for the maintenance and repair process of this type of Sequential Turbocharging System, as well as for eliminating similar faults.
I. Structure, working process and characteristics of the Sequential turbocharging system
Sequential turbocharging of diesel engines refers to the configuration where a single diesel engine is equipped with two or more turbochargers that are connected in parallel, together with the exhaust and intake systems of the ship's diesel engine to form a turbocharging system. All the turbochargers operate according to a certain logic either sequentially or simultaneously. It is also known as sequential turbocharging.
Due to the excellent matching performance, power, and economy of sequential turbochargers, they are widely used in high-power and diesel engines for ships with variable loads.
1. Structure and Successive Boosting Process
Taking the sequential turbocharging system of a certain type of marine diesel engine as an example, we will introduce the structure of the sequential turbocharging system.
This type of diesel engine is equipped with two identical turbochargers, which are produced by a domestic company. Under the condition that the structure of the diesel engine and the turbocharging system remains basically unchanged, this sequential turbocharging system is designed to meet various operating conditions of the ship. It uses one of the turbochargers to operate at partial loads, adapting to partial load conditions, so that the diesel engine has a high combustion excess air coefficient and the maximum possible torque.
When entering a high-load condition, the controlled turbocharger (the second one) is put into operation.
Figure 1 Structure Diagram of Sequential Turbocharging System for Diesel Engines
Its structure is shown in Figure 1. After the diesel engine starts successfully, the booster TCA in column A immediately operates, while the booster TCB in column B does not work. Thus, all the exhaust gases from columns A and B are centrally supplied to the booster TCA in column A, driving the turbine of the booster TCA to rotate. The booster TCA then compresses the air and pumps it into the intercooler. The compressed air is cooled by the intercooler and evenly enters the cylinders of columns A and B.
The booster TCB in column B is a controlled booster, mainly controlled by the air valve and the gas valve to achieve the start-stop control of the controlled booster. The gas valve controls the connection and disconnection between the exhaust pipe of column B and the turbine end of TCB, while the air valve controls the connection and disconnection between the compressor of TCB and the intake pipe of column B. When the speed, load, and boost pressure increase, the STC (Sequential Turbocharging System) control system will automatically open the gas valve and the air valve respectively, and the controlled booster TCB will be put into operation.
From the above process, it can be seen that the successive boosting of this type of diesel engine can be regarded as a non-continuous variable-area turbocharging system.
At high loads, both boosters operate simultaneously;
At low loads, the booster TCB stops operating, which is equivalent to reducing the flow area of the turbine nozzle, so that there is still a relatively high pressure before and after the turbine, generating a higher boost pressure, meeting the requirements for air intake volume when the diesel engine operates at low speed and large torque.
2. The characteristics of this type of sequential supercharging
(1) The diesel engine and the sequential turbocharging system have a good matching characteristic.
The good matching is specifically manifested as follows: under any load condition, by adjusting the air pressure and flow rate entering and exiting the supercharger and the diesel engine, the sequential turbocharging system can always work efficiently. The transient response is fast, low-speed supercharging is sufficient, high-speed does not overheat or overspeed, acceleration is fast and no black smoke is emitted. The fuel consumption rate, exhaust temperature, emissions, etc. of the diesel engine all reach the optimal state.
(2) The sequential turbocharging system enables the marine diesel engine to have excellent fuel economy and efficiency under low-load conditions, as well as high power performance and stability under high-load conditions.
The operation of a marine diesel engine during navigation requires it to work under various load conditions, and frequent switching of the engine load is necessary. A well-matched sequential turbocharging system enables the marine diesel engine to meet these requirements.
(3) The Sequential Turbocharging System is relatively complex.
This turbocharging system requires the addition of a set of control boosters (TCB) for gas and air control valves as well as a control system. This increases the number of failure points and failure rates, and raises the maintenance costs.
In addition, during the design process, precise matching calculations for the boosters and the intake and exhaust flow and pressure of the diesel engine are needed, and the working timing of the controlled boosters TCB must be determined.
II. Common Faults and Cause Analysis
1. Surge of the turbocharger
Surge of the turbocharger refers to the phenomenon where the rotational speed of the turbocharger remains constant, but if the air flow entering the compressor of the turbocharger decreases to a certain level, the direction of the gas entering the compressor impeller and diffuser deviates from the optimal value, causing significant fluctuations in the gas pressure within the compressor. The turbocharger enters an unstable working state, and emits roaring or wheezing sounds at the turbocharger end.
This fault frequently occurs in marine diesel engines with sequential turbocharging systems. Surge can accelerate the fatigue damage of internal parts such as blades or accelerate the expansion of existing cracks. In severe cases, it can lead to the destruction of the turbocharger.
Surge of marine diesel engines occurs frequently in sequential turbocharging systems. This system consists of a complex diesel engine intake and exhaust system and turbochargers and intake and exhaust systems. There are several main reasons for this.
(1) Insufficient or unsmooth intake air volume of the turbocharger
Due to certain reasons, the intake air volume at the compressor end of the diesel engine turbocharger is insufficient or not smooth. This will cause air vortices or unstable pressure at the inlet and outlet of the compressor, leading to surge in the turbocharger.
In addition, the turbine nozzles of the turbocharger are prone to being clogged with dirt or foreign objects during operation. This will cause the air in the turbine blades to change from laminar flow to turbulent flow, altering the optimal angle of impact on the turbine blades, and also causing surge in the turbocharger.
(2) Unstable diesel engine speed or uneven power output of each cylinder
If the diesel engine speed fluctuates, the energy of the gas pushing the turbocharger turbine becomes discontinuous, which leads to unstable operation of the turbocharger and causes poor air flow at the compressor end. As a result, surge in the turbocharger occurs.
In addition, when this type of ship's diesel engine operates at low speed, there are often situations where some cylinders do not ignite. This reduces the demand for air, which may even be lower than the supply capacity of the compressor. Excess air flows back to the compressor outlet, causing blockage of the air flow at the compressor outlet, poor air flow, increased back pressure of the compressor, and other conditions that lead to surge in the turbocharger.
(3) The sequential turbocharging system operates inconsistently with the diesel engine's operation.
The sequential turbocharging system's operation does not match the diesel engine's operation. The control system of the sequential turbocharging system malfunctions and fails to meet the requirements of the diesel engine's operating conditions, causing it to either stop or have the controlled supercharger operate under control.
In simple terms, when operating under low conditions, the controlled supercharger TCB should only be used when TCA is employed, meaning both TCA and TCB are used simultaneously;
However, when using both TCA and TCB during high conditions, the controlled supercharger TCB does not operate, and only TCA is used.
This phenomenon occurs very frequently in a certain type of marine diesel engine and is mainly due to control system failures.
The conditions for the controlled supercharger TCB to be put into operation in this type of marine diesel engine are:
The main engine speed ≥ * * * r/min, the supercharger speed ≥ * * *00 r/min, and the supercharging pressure ≥ 0. * * MPa.
During the operation of the diesel engine, the entire process is automatically controlled by the STC control instrument, and the pneumatic actuator with a control air pressure of 0. * MPa completes the opening and closing of the gas valve and air valve and realizes automatic switching. Additionally, the control air also presses the valve piston to prevent the valve from opening or closing due to vibration or other reasons.
Figure 2 STC Control System
Therefore, if any component in the STC control system (as shown in Figure 2) or its actuators fails during the operation of the diesel engine, it will lead to a matching problem between the turbocharger and the diesel engine, causing a surge fault in the turbocharger.
In addition, the controlled turbocharger TCB is mainly activated by the opening and closing control of the gas valve and air valve. After the diesel engine operates for a long time, ash and carbon deposits accumulate in the pipeline, which can prevent the gas valve or air valve from closing completely. This will result in leakage of the gas discharged by the diesel engine or the air entering it, and in severe cases, it can also cause the start-stop fault of the controlled turbocharger TCB, thereby causing a surge in the turbocharger.
2. Decrease in boost pressure
The sequential turbocharging system' s boost pressure dropping is also a relatively common fault. The main reasons include a decrease in the boost pump's rotational speed, poor air intake in the boost pump's intake channel, and damage to the compressor blades. The specific analysis is as follows.
(1) Decrease in boost pump rotational speed
The decrease in the boost pump's rotational speed is usually caused by insufficient gas entering the boost pump. First, the gas passage's smoothness and sealing should be checked. Long-term operation of the diesel engine may cause soot to accumulate on the inner wall of the exhaust pipe, resulting in a smaller inner diameter of the exhaust pipe's flow path and poor gas flow. Additionally, the nozzle of the boost pump should also be checked for smoothness.
This type of engine is mainly inspected in three aspects. First, the sealing of the exhaust pipe should be checked. This type of diesel engine's exhaust main pipe is divided into 8 parts, each part is integrated with 8 cylinder exhaust manifolds and installed on the cylinder head. The 8 parts of each cylinder's exhaust main pipe are fixed together by pipe clamps. Long-term operation of the diesel engine may cause the sealing rings at the connection points of each main pipe to age due to high temperatures, and there may also be leakage of gas at the connection points between each exhaust main pipe and the cylinder exhaust manifold and the cylinder head.
Second, check if there is any looseness at the connection point between the exhaust main pipe and the turbocharger's turbine end due to vibration or other reasons. There may be gas leakage and poor sealing.
Third, if the low rotational speed of the boost pump fault occurs during low-load operation of the diesel engine, that is, when the controlled boost pump TCB is not running, first check the sealing of the gas valve controlling the controlled boost pump TCB. If the gas valve is not closed tightly, some gas will leak through the poorly sealed gas valve into the controlled boost pump, resulting in a decrease in the gas volume entering the A column boost pump TCA during low-load operation of the diesel engine, causing the boost pump rotational speed to be low during low-load operation. The basis for this fault judgment can be to check if the controlled boost pump TCB is running at a low speed. If TCB is running at a low speed, it indicates that the gas valve is not closed tightly, a small amount of gas leaks into TCB, causing rotation, resulting in a decrease in the boost pressure during low-load operation of the diesel engine. In severe cases, it may cause surge of the boost pump.
(2) The smoothness of the intake passage and the deterioration of the sealing performance of the diesel engine's intake pipeline are some of the issues for this type of diesel engine when used as a marine diesel engine. The intake method varies among different types of ships. There are mainly two methods: engine room intake and external intake.
For diesel engines using the engine room intake method, if all the engine room fans are exhaust fans, it will cause a negative pressure in the engine room, resulting in a decrease in the intake pressure and intake volume of the turbocharger.
For diesel engines using the external intake method, after long-term operation, during the installation of the intake passage and intake filter, external impure air will cause dirt and blockage in the intake filter, intake pipeline, muffler, and intercooler, which will lead to reduced intake resistance, resulting in a decrease in intake pressure and intake volume, and ultimately causing a decrease in boost pressure.
In addition, due to the vibration of the ship and the diesel engine, problems of loosening at the connection points of the intake pipeline occur from time to time, which will lead to leakage of the intake pipeline and a decrease in intake pressure.
During low-load operation, if the air valve of the controlled turbocharger is not closed tightly, part of the air from the controlled turbocharger will leak through it, resulting in a decrease in the intake pressure of the diesel engine.
Finally, it is necessary to check the integrity of the compressor blades of the turbocharger. If this turbocharger has ever experienced surge, the compressor blades, especially the blade tips, may deform or have other damage to the compressor rotor, which will cause a decrease in compressor efficiency and a reduction in compressor volume, resulting in a decrease in boost pressure.
3. Abnormal noise or vibration
During the high-speed operation of the supercharger, there may also be other types of noise faults besides the surge noise, such as the piercing sound of metal friction or impact, which may cause vibration of the supercharger.
The reasons for this problem are numerous. For example, poor lubrication of the supercharger leads to severe wear of the supporting bearings;
Long-term operation of the diesel engine causes the connecting bolts between the diesel engine and the supercharger to loosen;
Uneven torque of the connecting bolts during the installation of the supercharger;
Foreign metal objects damaging the rotor may cause the supercharger to deviate or have friction with the outer edge of the housing;
The timely operation of the controlled supercharger TCB is not timely, resulting in the excessive rotational speed of the A column supercharger TCA, and so on.
III. Common solutions or preventive measures for successive pressure increase system failures
Through long-term participation in the maintenance of this type of marine diesel engine, we have accumulated and summarized the common troubleshooting measures and preventive methods for the common faults of this type of engine's sequential turbocharging system.
Because the sequential turbocharging system is a relatively complex system composed of the diesel engine intake and exhaust piping system, the supercharging unit intake and exhaust piping system, and the controlled supercharging unit control system, and each component is interdependent and mutually influential, the causes of the faults are not isolated. The solutions for each fault are not unique. Based on relevant practices, the following common troubleshooting measures and preventive methods for the successive supercharging unit system are summarized.
(1) When the diesel engine turbocharger experiences surge or has low boost pressure, relevant parameters should be comprehensively analyzed to determine the cause. If necessary, the engine should be shut down for inspection.
If abnormal sounds or vibrations occur and the cause cannot be determined, the engine should be shut down for inspection first to avoid further expansion of the fault scope.
(2) According to the actual usage conditions, in accordance with the maintenance requirements, timely maintenance and upkeep of the diesel engine intake and exhaust pipelines and the turbocharger's intake and exhaust pipelines should be carried out. This mainly includes regular cleaning of the inner walls of the pipelines to prevent blockage and ensure smoothness; strengthening the inspection of pipeline sealing, especially paying attention to sealing issues caused by pipe wall corrosion or connection problems, to prevent leakage of exhaust and smoke.
(3) Strengthen the inspection of exhaust back pressure to prevent poor exhaust flow and reduce surge in the turbocharger. First, check whether the exhaust pressure at the compressor end of the turbocharger is too high, and check the dirt and blockage of the air cooler. Regularly clean and maintain the air cooler to keep it unobstructed. Secondly, check the exhaust flow of the gas and exhaust from the turbine end of the turbocharger. Some ships install smoke treatment devices such as noise reduction, and they should be regularly cleaned and maintained.
(4) To prevent the noise and vibration of the turbocharger, in combination with repair or maintenance, regularly inspect and replace the rotor structure and blade shape. Keep the shape intact and without deformation. For newly installed or factory-repaired turbochargers, perform dynamic balance tests on the rotor. If the dynamic balance is unqualified, grinding adjustment should be carried out.
(5) Regularly inspect and maintain the STC control system of the successive turbochargers to ensure that the data acquisition, transmission, and processing modules of the control system work normally. According to the set logic, accurately grasp the status of the controlled turbochargers and send start or stop work signals as required.
(6) Ensure the normal operation of the gas valves, air valves, gas source devices, etc. of the controlled turbochargers. Prevent valve sticking, causing exhaust or air leakage. Prevent the inability to start or stop the gas valve and air valve normally due to faults in other executive mechanisms, thereby affecting the start and stop of the controlled turbocharger.
(7) In the STC successive turbocharging system of a certain type of diesel engine studied in this paper, columns A and B use turbochargers of the same specification. To improve the service life of the turbocharging system and reduce the failure rate, the controlled turbocharger in column B has a shorter operating time. It is recommended to alternate the rotation of the two columns of turbochargers according to the working time, so that the service cycles of the two turbochargers can be approximately the same and the utilization rate of the turbochargers can be improved.
IV. Conclusion
This article briefly introduces the structure and related characteristics of the successive Turbocharging system of a certain type of marine diesel engine. Based on the long-term practical experience of the maintenance tests for this type of engine, the author summarizes and concludes common faults, and proposes related solutions such as the regular interchange of A and B sets of superchargers. This article has good quality and economic benefits for the maintenance and repair of the sequential turbocharging system of this type of engine.