Basic knowledge of marine diesel engines

1.What is the injection advance angle? Why not inject fuel when the piston reaches the top dead center? What effect does injecting fuel too early or too late have on the diesel engine?

Answer: Before the piston reaches the top dead center, the crank angle between the crank position and the top dead center when the high-pressure oil pump starts to supply fuel to the cylinder is what we often call the injection advance angle. In theory, the injection, ignition and combustion of the diesel engine should start when the piston reaches the top dead center, but in fact, there is an "ignition delay" (ignition delay period) between the start of injection and the complete combustion of the fuel. In order to allow the fuel injected into the cylinder to ignite when the piston moves to the top dead center, thereby generating an ideal explosion pressure to push the piston downward to perform work, in fact, the high-pressure oil pump starts to supply fuel to the injector before the piston reaches the top dead center, and injects fuel into the cylinder. Injecting fuel too early or too late will have the following adverse effects on the operation of the diesel engine:

(1). Too early injection time: - Too early injection will inevitably increase the amount of oil injected into the cylinder during the ignition delay period, that is, the time after ignition. Once the fuel in the cylinder ignites, it will inevitably generate excessive combustion pressure, resulting in knocking of the diesel engine due to explosion. This will increase the impact load on the parts, shorten the life of the parts, and affect its working reliability.

(2) Too late injection time: Too late injection time will delay the entire combustion process, increase the afterburning, and increase the heat load. Due to poor heat utilization, the working capacity is reduced. At the same time, due to this situation in individual cylinders, the operation of the diesel engine is often unbalanced. At this time, the average temperature and exhaust temperature in the working stroke are also significantly higher than those in normal working conditions. In severe cases, black smoke will be emitted from the exhaust. Due to the increase in afterburning, the increase in the average temperature in the working stroke will affect the efficiency of the cylinder lubricating oil and accelerate the wear of the cylinder liner and piston ring. Therefore, full attention should be paid to the injection timing, and regular inspections and adjustments should be carried out, especially for old machines.

2. What impact will a faulty fuel injection system have when a diesel engine is running at low speed?

Answer: In a multi-cylinder diesel engine, although the high-pressure fuel pump and fuel injector structures are identical in each cylinder, the clearances between their corresponding components, such as the plunger and sleeve, the needle valve and valve seat, and the valve body, are not always identical. This can lead to variations in leakage during operation. The technical condition of the cylinders and the injector needle valve components can also vary due to processing and wear. While these differences still exist when the diesel engine is running at normal speed, the impact of these differences is not significant due to the high fuel injection volume during normal operation. However, when the diesel engine is running at low speed, the impact of these differences on the fuel injection volume of each cylinder becomes more pronounced, resulting in increased unevenness in injection volume and, in severe cases, even misfiring in some cylinders. This can cause large speed fluctuations, unstable operation, and even automatic shutdown of the diesel engine at low speeds.

3. What are the main components and operating principles of the fuel injection system?

Answer: The main function of the fuel injection device is to inject a certain amount of fuel into the cylinder of the diesel engine at a very high pressure at a certain time, and form a good atomization to facilitate complete combustion. The amount of fuel and the injection pressure are both generated by the high-pressure fuel pump, and the atomization of the fuel is mainly formed by the fuel injector during the injection process. Therefore, the components of the fuel injection device are mainly the high-pressure fuel pump and the fuel injector.

4.What basic requirements should the high-pressure fuel pump meet?

Answer: The high-pressure fuel pump is the heart of the fuel injection system. In addition to providing the high pressure required for the fuel to form a good atomization, it also plays a quantitative and timing control role in each injection of fuel. Therefore, its performance plays an extremely important role in the normal operation of the diesel engine. To this end, it should meet the following basic requirements:

(1)Have a certain injection pressure: The oil pressure should be quickly established at the beginning of the injection; and at the end of the injection, the fuel should be cut off immediately. The oil pressure should not fluctuate during the injection process to ensure that the combustion process can proceed well.

(2)The injection amount can be adjusted: The injection amount should be adjusted according to the change of diesel engine load (to achieve total adjustment); at the same time, in order to achieve uniform injection amount of each cylinder of a multi-cylinder diesel engine, the injection amount of the high-pressure oil pump can be adjusted individually (monotonically). Otherwise, at low load, some cylinders will not fire due to too little oil intake; at normal load, some cylinders will be overloaded due to too much oil intake. Therefore, it is required that the high-pressure oil pump can not only be adjusted in total, but also be able to adjust the oil amount individually.

(3) Suitable and adjustable injection timing: The high-pressure oil pumps of different types of diesel engines have their own optimal injection timing, and the oil supply time in the high-pressure oil pump can be adjusted accordingly according to the requirements of different loads. However, due to the differences in the structure and application of high-pressure oil pumps, the adjustment methods of their oil supply time are also different. Some adopt the method of changing the starting point and end point of oil supply at the same time; some keep the starting point (or end point) of oil supply unchanged, and only change the end point (or starting point) of oil supply to adjust the total oil supply time. Regardless of the method used, the injection timing of a diesel engine will always change due to wear and tear after long-term operation, which will affect the normal combustion process of the diesel engine. In this case, the high-pressure fuel pump has the ability to adjust and restore the original optimal injection timing, which ensures that the diesel engine can always operate at the optimal injection timing.