Basic knowledge of marine diesel engines(Ⅶ）

1.Why is waste turbocharging widely used in modern marine diesel engines? How does a waste turbocharger work?

Answer: Modern ships are getting bigger and bigger, and the speed requirements are getting higher and higher, and there is an urgent need for large marine diesel engines to increase power. Of course, there are many ways to increase the horsepower of the machine, for example, increase the number of cylinders in the main engine; increase the diameter of the cylinder; increase the piston stroke; Increasing the speed of the machine is a feasible way. However, if there are too many cylinders, the reliability of the crankshaft will be reduced; Increasing the diameter of the cylinder will bring many difficulties to the heat dissipation and processing technology of the cylinder. In addition, measures such as increasing the piston stroke are somewhat limited on already large machines. Under the premise of keeping the original and its size basically unchanged, make full use of the energy of the exhaust of the machine, promote the exhaust gas turbine, further drive the compressor to compress the air in advance, and realize the intake pressurization, which can improve the cylinder inflation density and increase the fuel injection volume of each cylinder, which is an effective way to improve the power and economy of the diesel engine.

Of course, using the lower space of the main engine cylinder to make a piston bottom pump is also a pressurization method. But this method obviously consumes part of the power emitted by the host, so it is not very economical and reasonable. Therefore, exhaust gas turbocharging is used in new diesel engines.

If the energy of the exhaust gas is used to impulse the blades on the turbine rotor, so that the rotor rotates at high speed, and a compressor is installed on the same shaft of the turbine, which is specially used to suck fresh air, and compress the air to a certain extent for the main engine to take in. In this way, the exhaust gas turbine and compressor form a unit, which is actually equivalent to a blower driven by exhaust gas. In the exhaust gas turbine, the exhaust gas is first introduced into a snail shell channel, which takes advantage of the gradual reduction of the cross-section of the vortex shell channel to make the exhaust gas flow rate higher and higher, and finally blows the blades on the turbine rotor through the nozzle to make the rotor rotate at high speed. This is a process that converts the pressure energy and flow energy of exhaust gas into mechanical energy for the rotation of a turbine. In the compressor, it is the opposite, it uses the high-speed rotation of the rotor impeller of the compressor to suck fresh air, and push the air to make a circular motion, and finally drive the air into a gradually expanded expansion tube or vortex shell with a channel section at a considerable speed, so that the air flow rate gradually decreases, but its pressure is getting higher and higher, which completes the process of pressurizing the air.

2. What are the ways of exhaust gas turbocharging? What are their characteristics?

Answer: The exhaust gas emitted by diesel engine has two energies: speed energy and pressure energy, or pulse energy and constant pressure energy. Some superchargers focus on using the pressure energy of exhaust gas to push the rotor to work, called constant pressure turbochargers; Some superchargers focus on using the pulse kinetic energy of exhaust gas to drive the rotor to work, so it is called a pulse turbocharger.

1. Constant voltage turbocharging:

Constant pressure turbocharging is to make all the exhaust gases in the cylinder enter a common large-capacity exhaust head, and only one exhaust gas turbocharger is installed for six cylinders and below, as shown in Figure 13a).

Due to the large volume of the exhaust main pipe, when the exhaust gas flows into the exhaust main pipe from the cylinder, it will throttle at the exhaust port (valve), expand and diffuse in the main pipe, and quickly stabilize, so the kinetic energy of the exhaust gas cannot be utilized. This pressurization method uses only the constant pressure energy in the exhaust gas.

In constant pressure turbocharging, because the pulse energy in the exhaust gas is not used, the power emitted by the turbine cannot meet the power required by the centrifugal compressor at low load, resulting in a decrease in the speed of the compressor and insufficient air supply. Therefore, this pressurization method requires an auxiliary sweeping pump or emergency blower to meet the needs of the main engine for air scavenging at low load. Modern marine engines are powered by high-efficiency constant pressure turbocharging.

2. Pulse turbocharging:

Pulse turbocharging is used, and there is no large-capacity exhaust main pipe, but each cylinder has a small cross-sectional area and volume exhaust branch, as shown in Fig. 13b). In the pulse supercharger, the exhaust branch pipes of each cylinder should be grouped, and each group can be equipped with an exhaust gas turbocharger, so that several turbochargers can be set up at the same time, so that the exhaust gas turbine can use most of its pulse energy in addition to the exhaust gas constant pressure energy.

The key to the pulse boosting method is how to effectively utilize the pulse energy of the exhaust gas. The utilization of pulse energy is related to factors such as exhaust pipe section and volume. Generally, it is required to minimize the throttling loss when the exhaust gas is discharged from the air outlet (or air valve), that is, it is required to be as fast as possible when the exhaust port or exhaust valve is opened, so that the pulse can be fully utilized, so the exhaust pipe should be curved and thin and have few elbows.

Because the pulse pressurization method can effectively utilize the pulse energy in the exhaust gas, the exhaust gas energy is used more. Although the turbine works under the impulse of unstable airflow, which makes the turbine itself work slightly less efficiently, it can still maintain a large amount of power. In addition, because the pressure in the exhaust pipe is in the low pressure stage in the scavenging stage (forced exhaust period), there is a large pressure difference between the scavenger box and the exhaust pipe, which is conducive to the scavenging (even when the diesel engine is at low load), so when equipped with an exhaust gas turbocharger that makes good use of pulse energy, the diesel engine does not need to have an auxiliary scavenging pump. The booster air produced by the exhaust gas turbine at starting and low load can still meet the needs of diesel engine scavenging. However, due to its low efficiency, modern diesel engines are no longer used.