Basic knowledge of marine diesel engines

1. What is exhaust gas turbocharger surge? What are the various causes of exhaust gas turbocharger surge? How can it be eliminated and prevented?

Answer: Exhaust gas turbocharger surge refers to the "surge" of the centrifugal compressor. Surge is characterized by unstable operation of the centrifugal compressor, with periodic fluctuations in discharge pressure and flow, resulting in a whine (surge sound), vibration of the rotor shaft, and mechanical jolt of the entire turbocharger.This phenomenon is called turbocharger surge.

1. Causes of turbocharger surge in a separate turbocharger system:

In this type of turbocharger system, when the air inlet or air cooler is clogged, the turbocharger's back pressure increases, reducing flow, which can cause turbocharger surge under low load.

 Furthermore, when a ship sails in hot waters, the high ambient temperature can also reduce air flow, potentially causing surge.

To prevent this type of surge, regularly maintain cleanliness of the air inlet and air cooler to keep the flow passages unobstructed.

2. Causes of turbocharger surge in a series-supercharging system:

In this supercharging system, since all the exhaust air from the turbocharger enters the reciprocating pump for secondary supercharging, the occurrence of surge is largely determined by the coordination between the turbocharger and the reciprocating pump during engine load fluctuations.

Generally, diesel engines do not experience surge at low loads. This is because the exhaust energy of the diesel engine is very low at low loads, causing the turbocharger's performance to decrease rapidly, resulting in low displacement or even no air delivery. 

However, the diesel engine still maintains a certain speed, and the reciprocating pump it drives continues to operate at a certain speed. 

At this point, the turbocharger's displacement cannot meet the reciprocating pump's intake and exhaust needs, forcing the reciprocating pump to draw air from the outside through the turbocharger. 

The reciprocating pump's suction action correspondingly increases the air flow through the turbocharger. 

Therefore, at these speeds, the turbocharger will not experience surge.

However, series-supercharging systems are prone to surge at high loads. 

As the diesel engine speed increases, although the reciprocating pump's pumping capacity also increases, it's not as rapid as the turbocharger's output. 

This results in a significant increase in the turbocharger's displacement, while the reciprocating pump cannot absorb the entire turbocharger's displacement. This conflict causes a sharp rise in pressure in the turbocharger's discharge line.

The increased back pressure in the turbocharger reduces the turbocharger's flow rate at the corresponding speed. This phenomenon becomes more severe at higher loads. Clearly, series turbocharging systems are prone to surge under high loads. 

Blockage of scavenge ports, exhaust ports, and air coolers, as well as excessively high ambient temperatures, are also major causes of turbocharger surge in this type of system.

If this type of turbocharging system surges under high load or the aforementioned conditions, the load can be reduced until the surge is eliminated.

3. Causes of turbocharger surge in parallel turbocharging systems:

In this type of system, the air volume required by the diesel engine is the sum of the displacement of the turbocharger and the space below the piston. 

Diesel engines don't surge under normal loads, but they are prone to surge at low loads.

 This is because the turbocharger's displacement decreases rapidly at low loads, while the reciprocating pump's displacement also decreases, but to a lesser extent. 

Since the turbocharger and reciprocating pump's discharge ports are connected in parallel, the reciprocating pump's higher displacement creates backpressure on the turbocharger, reducing turbocharger flow and making it more susceptible to surge at low loads.

Such turbocharging systems can also experience surge if severe carbon buildup on the scavenge and exhaust ports significantly reduces the passage area, or if the air cooler becomes contaminated or clogged. High ambient temperatures, when airflow is significantly reduced, can also cause surge.

When surge is caused by these factors, the scavenge air box can be appropriately bled (usually with a dedicated bleed valve) to increase the turbocharger's displacement and eliminate surge. 

In addition to using the bleed air method to eliminate surge, some models feature a conversion device between the turbocharger and the piston chamber, which switches the turbocharger and piston chamber from parallel operation to series operation. This allows surge to be eliminated when low loads cause it.

4. Causes of Surge When Two Turbochargers Operate in Parallel:

When the exhaust air from two turbochargers is fed into the same scavenge air box, severe load imbalances between cylinders or a cylinder failing to fire can cause surge in the turbocharger connected to that cylinder, leading to alternating surge in both turbochargers.

When this occurs, the cause should be investigated and, if possible, the engine can be shut down to correct the fault. If shutdown is not possible, one of the cylinders connected to the normally operating turbocharger can be deactivated to align the speed, load, and discharge pressure and flow of the two turbochargers. 

This can avoid surge. Once the fault is corrected, normal operation can be resumed.

2. What causes the vibration and noise of the exhaust gas turbocharger?

 Answer: The exhaust gas turbocharger is a high-speed rotating machine, but if vibration and noise are found, it must be taken seriously and the fault must be eliminated. 

The causes of the fault are: （1）Vibration of the exhaust gas turbocharger: (1) The turbine blades are broken or bent.Replace the blades or perform the same repair on the blades in the symmetrical part of the damaged blades.

 (2) Bearing damage or excessive bearing clearance will also cause rotor vibration and must be repaired.

 (3) The rotor is bent. If there is no reliable way to straighten it, it should be replaced. The repaired rotor should be re-calibrated for dynamic balance. 

2. Noise during exhaust gas turbine operation: (1) The air seal is not installed properly, causing friction.

 (2) The turbine blades or compressor impeller rub against the fixed part. 

(3) Insufficient or cut-off lubricating oil causes the bearings to burn and sink, and the rotor and the casing to be damaged. 

(4) Foreign matter such as piston ring fragments is blown into the exhaust gas turbine intake. The noise and vibration during operation are often a precursor to a major accident. Therefore, in addition to paying attention to the lubrication and cooling of the exhaust gas turbocharger, the engine crew must also regularly listen to the noise and vibration of the casing. Once an abnormality is found, measures must be taken immediately to eliminate it to prevent major engine damage. 

3. What are the reasons for the scavenging pressure of the supercharged diesel engine being too low or too high? What impact does it have on the diesel engine?

 Answer: 1. The reasons for the low scavenging pressure are: (1) The turbocharger intake muffler is too dirty, the intake resistance increases, and the intake vacuum is too high.

 (2) The air flow path of the air cooler is dirty, which increases the flow resistance.

 (3) Due to the failure of the exhaust turbine itself, the turbocharger speed is reduced and the displacement is reduced. 

(4) The centrifugal compressor gas seal gap is too large, and the air leakage is serious. 

(5) The scavenging pump piston ring is worn or the valve plate of the valve group is broken, which reduces the pumping effect of the scavenging pump. 

(6) The centrifugal compressor flow passage (impeller, diffuser) is heavily contaminated by dust and oil, which increases the flow resistance and reduces the efficiency of the centrifugal compressor. 

2. The reasons why the scavenge air pressure is higher than the normal value are: (1) Due to the fault of the diesel engine itself, the turbocharger is overspeeding.

 For example, the valve mechanism is improperly adjusted, the exhaust valve cannot be closed tightly, or the exhaust valve is seriously leaking due to other reasons; combustion deteriorates, afterburning is aggravated, and overload operation causes the exhaust gas temperature to increase, the turbine to overspeed, and the centrifugal compressor displacement to increase. 

(2) The air port is blocked. At this time, the air flow into the scavenge air port (or exhaust port) is blocked, the air consumption is reduced, and air accumulates in the scavenge air box. Both low scavenge air pressure and high scavenge air pressure caused by air port blockage are not conducive to scavenge air, worsening combustion, causing overheating of the engine parts and reducing the power of the diesel engine; too high scavenge air pressure may cause the turbocharger to surge.