Basic knowledge of marine diesel engines

1. What are the characteristics of the independent supercharging system used in large, low-speed, two-stroke marine diesel engines?

Answer: Independent supercharging systems used in large, low-speed, two-stroke diesel engines utilize only an exhaust gas turbine, without any additional auxiliary machinery. This type of supercharging system is easily implemented only on diesel engines that use pulse supercharging and have high scavenging quality.

The exhaust gas turbocharger consists of an exhaust gas turbine and a coaxial centrifugal compressor. The turbine is driven by the exhaust gas from the cylinders, which in turn drives the coaxial centrifugal compressor at high speed. The turbocharger draws air from the engine compartment, compresses it to a certain pressure, and then discharges it through an air pipe to an intercooler for cooling. After cooling, it enters the diesel engine scavenge box for use by the diesel engine.

During startup, this independent supercharging system uses compressed starting air discharged from the diesel engine's exhaust port (valve) to blow the turbine, ensuring that the turbocharger's speed reaches a level sufficient to provide sufficient air volume, resulting in excellent starting performance. To ensure continued engine operation in the event of turbocharger failure, this independent supercharging system is equipped with an electric emergency blower. It supplies a sufficient amount of air to maintain the engine's low-speed cruising capability.

This independent supercharging system ensures normal operation under all loads for low- and medium-pressure diesel engines. However, in high-pressure engines with a boost pressure of 0.20 to 0.25 MPa, the increased boost pressure also increases exhaust backpressure (pressure before the turbine), reducing the utilization rate of exhaust gas pulse energy and resulting in poor engine performance during starting and low loads. Therefore, an electric blower is required to assist with air supply during starting.

2. What are the characteristics of the series supercharging system used in large, low-speed, two-stroke marine diesel engines?

Answer: The series supercharging system utilizes an exhaust gas turbocharger and a reciprocating pump driven by the main engine in a two-stage supercharging system. This system was previously used on older engines. The turbocharger in this supercharging system acts as the first stage, compressing the air to approximately 70% to 95% of the boost pressure. The compressed air then passes through an intercooler for cooling before being sent to a reciprocating boost pump connected in series with the supercharger for a second compression, reaching the rated boost pressure. After the second stage of compression, the air is cooled again by the intercooler before being sent to the scavenge air box for use as scavenging air for the diesel engine.

Diesel engines using this supercharging system are relatively reliable and have good starting performance. If the turbocharger fails, the reciprocating boost pump can still enable the diesel engine to reach 70% to 80% of its rated speed.

A typical series supercharging system uses a constant-pressure turbocharger connected in series with a reciprocating pump that drives the turbocharger in stages.

3. What are the characteristics of the series bypass supercharging system used in large, low-speed, two-stroke marine diesel engines? How does it operate? A: The series bypass system is a special case of the series supercharging system. Its characteristic is partial series supercharging, meaning that the scavenging process is series-driven in the early stages, but the series-driven process is disabled in the later stages, with the turbocharger supplying air independently.

In this supercharging system, the impulse turbocharger serves as the first stage of supercharging, while the working space below the diesel engine piston acts as an auxiliary pump, serving as the second stage of series supercharging. The scavenge air box is divided into two sections: the outer scavenge air box is shared by all cylinders, while the inner section is separated for each cylinder and forms a scavenge air chamber with the working space below the piston. A check valve is installed between the outer and inner scavenge air chambers. The turbocharger's compressed air is first cooled by the intercooler before entering the scavenge air box and then passing through a check valve into the scavenge air chamber.

The operating principle is as follows: As the piston moves upward, the volume below the piston increases, causing the pressure in the inner scavenge air chamber to drop. At this time, a large amount of air from the supercharger is forced through the check valve from the outer scavenge air box into the inner scavenge air chamber. As the piston descends, the check valve automatically closes when the pressure in the scavenge chamber exceeds that in the outer scavenge box. As the piston continues to descend, the volume of the scavenge chamber rapidly decreases, increasing the pressure of the air within. When the scavenge port opens, the air in the inner scavenge chamber is already at a higher pressure and can enter the cylinder to begin scavenging. Because the pressure differential between the scavenge chamber, the cylinder, and the exhaust pipe is large at the beginning of scavenging, this pressure differential causes air to rush into the cylinder quickly, creating a pressure surge and intensifying scavenging. As the large amount of air rushes into the cylinder to scavenge, the pressure in the scavenge chamber begins to drop. When the piston reaches bottom dead center, its compression effect ceases, and the pressure in the scavenge chamber continues to decrease. When the pressure in the scavenge chamber drops below that in the outer scavenge box, air from the scavenge box enters the scavenge chamber through the check valve and continues scavenging the cylinder until the scavenge port is blocked by the piston. As the piston continues to move upward, the volume of the scavenge chamber continues to increase, reducing the pressure. Compressed air continues to flow from the scavenge box into the scavenge chamber for storage, to be compressed and scavenged again the next time the piston descends.

The main advantage of this series bypass supercharging system is that, because high pressure builds up in the scavenge chamber as the piston descends, the resulting pressure peak occurs precisely when the scavenge port opens. Since the scavenge chamber pressure is high at this time, backwashing is prevented.

In the event of supercharger failure, the pumping action of the space below the piston allows the diesel engine to continue operating at 70% of the rated speed, eliminating the need for a separate emergency blower.

This series bypass supercharging system ensures good operation at full load at low and medium boost pressures. However, at high boost pressures, the benefit of utilizing the exhaust gas pulse energy is minimal, so pulse supercharging is not used. Instead, constant pressure supercharging is employed in series with the space below the piston. This supercharging system can still provide air even at low loads. However, to improve low-load performance, an additional electric blower is often added for use during starting and low-load conditions. This significantly improves maneuverability during starting.

4. How did the parallel supercharging system used in large, low-speed, two-stroke marine diesel engines work?

Answer: A parallel supercharging system achieves supercharging by combining the turbocharger and the piston chamber in parallel.

In this supercharging system, the diesel engine's boost air is supplied in parallel by a pulse exhaust turbocharger and the piston chamber, which operates in parallel. Each of these systems draws air from the engine compartment, compresses it to the required boost pressure, and then flows through the intercooler into the scavenge air box to provide scavenging air for the diesel engine. The turbocharger typically supplies 75% to 80% of the total air volume, with the remainder supplied by the piston chamber. This reduces the amount of piston chamber used. This parallel turbocharging system significantly reduces the turbocharger's air supply at low loads, while the air supply to the space below the piston is also far from sufficient. Consequently, the diesel engine's performance is poor at low loads, necessitating the use of an electric blower for low-load operation.

In addition to the pulse turbocharger mentioned above, parallel turbocharging systems also employ a constant-pressure exhaust gas turbocharger operating in parallel with a piston pump.