Reason for surge
Usually, when selecting a turbocharger, the compressor is selected at an appropriate position on the right side of the surge line B according to the working characteristics of different boosting systems. At this point, it can ensure that the diesel engine meets the predetermined boost target and the turbocharger operates in the high-efficiency zone, while also ensuring that the turbocharger does not surge within the entire operating range of the diesel engine.
Therefore, under normal circumstances, surge generally does not occur. However, when working conditions change, such as blockage of the boost system channel, high or low load, uneven diesel engine load, and sudden load changes, the working line will partially or completely enter the surge zone, causing surge. Below are some possible causes of turbocharger surge. One of the direct consequences of blockage in the turbocharger flow channel is an increase in the resistance of the airflow in the system. When a diesel engine is running, the gas flow path of the turbocharging system is: compressor inlet filter and muffler → compressor impeller → compressor diffuser → air cooler → scavenging box → diesel engine inlet (valve) → exhaust port (valve) → exhaust pipe → exhaust gas turbine nozzle ring → exhaust gas turbine impeller → chimney. The circulation area of each component is fixed. If any of the above flow paths are blocked, such as dirt, carbon deposition, deformation, etc., the increase in flow resistance will cause the back pressure of the compressor to rise, the flow rate to decrease, and surge to occur. The components that are prone to dirt are the compressor inlet filter, compressor impeller and diffuser, air cooler, diesel engine inlet (scavenging) port and exhaust port (valve), exhaust turbine nozzle ring, and exhaust turbine impeller. Usually, blockage of the turbocharger airflow channel is the main cause of its surge. Regular inspection and cleaning of the above-mentioned components should be carried out in management to prevent or eliminate any surges caused by them.
Blocking factors
When the operating conditions (load, speed) of a diesel engine change, the diesel engine operates at low speed and high load. When the diesel engine malfunctions or the external load increases due to full load, headwind, or fouling of the ship, the speed of the diesel engine decreases. At this time, the governor automatically increases the fuel supply, allowing the diesel engine to operate at low speed and high load. Due to the increase in fuel supply and exhaust energy, it is inevitable that the turbocharger speed will increase, and the compressor displacement and discharge pressure will increase. At this time, the low speed and low gas consumption of the diesel engine disrupt the supply and demand balance between the turbocharger gas supply and the diesel engine gas consumption. The back pressure of the compressor increases, causing a decrease in flow rate.
When entering a low-temperature sea area, the air density increases due to the decrease in ambient temperature, resulting in an increase in the intake volume of the compressor and an increase in the energy obtained by the turbine. The increase in the speed of the turbocharger causes the operating line to move lower, and the surge margin increases; However, when entering high-temperature waters, the opposite is true, where a decrease in surge margin is more likely to result in surge. For some ships, when their turbocharged diesel engines without air coolers are matched in low-temperature navigation areas and sailing in high-temperature waters, or when their matched turbocharged diesel engines with air coolers are matched in high-temperature navigation areas and sailing in low-temperature waters, the matching relationship between the two changes, and the operating point is prone to approach the wheezing zone, causing surge.
Oil injection
When the fuel injection system malfunctions and low-quality heavy oil is used, the afterburner becomes heavier and the exhaust temperature is higher. Whether it is full load or partial load, and no matter how severe the afterburning is, its cooperative operating point is on the normal cooperative operating line. As the combustion endpoint is delayed, the exhaust temperature increases, the turbocharger speed increases, the compressor flow rate increases, the pressure ratio increases, and the operating point moves towards a higher point on the curve, resulting in less surge margin.
Cooling deterioration
When the cooling capacity of the air cooler decreases, the exhaust temperature of the diesel engine increases, the compressor speed increases, and the operating point moves to a higher position, reducing the surge margin.
In short, non channel blockage factors will not affect the flow characteristics and will not change the position of the turbocharger and diesel engine operating line. Only changing the position of the turbocharger and diesel engine operating point on this line is the secondary cause of diesel engine turbocharger surge.
