Basic knowledge of marine diesel engines

1. What is fuel atomization? What factors influence fuel atomization?

Answer: High-pressure fuel enters the injector and is ejected through the nozzle at high velocity. Due to the resistance of the compressed air in the cylinder and the turbulence within the fuel column as it flows through the tiny nozzle hole, the ejected fuel breaks down into fine particles, forming a mist (atomized body). This process is called fuel atomization.

The main factors influencing fuel atomization are the fuel injection pressure, the diameter of the injector nozzle hole, and the fuel viscosity. Increasing the injection pressure increases the fineness of the atomized body, as well as the length and taper of the atomized body, improving atomization quality.

Fuel viscosity that does not meet the injector's requirements, or changes to the nozzle hole, can affect fuel atomization. Enlarging the nozzle hole reduces the fineness of the oil mist, increases the length of the atomized body, and decreases its taper. Conversely, reducing the nozzle hole increases the fineness of the oil mist, increases the taper of the atomized body, and decreases its length. For a given injector, the shape and distribution of the atomized body are precisely matched to the combustion chamber. Therefore, when the fuel injection holes become larger due to wear or smaller due to partial blockage, fuel atomization and combustion quality will be affected.

Good fuel atomization is essential for even mixing with air and is a crucial factor for complete combustion. Therefore, regular inspection of the fuel injectors and high-pressure fuel pump to ensure they are in good condition and good fuel atomization is crucial for ensuring proper diesel engine operation.

2. What is a combustible mixture? How is it formed?

Answer: Oil mist evaporates into oil and gas under heat in the cylinder, which then mixes with air to form a combustible mixture. To achieve a good combustible mixture in a very short time, two methods are generally used: First, inject the fuel into the cylinder at high injection pressure to achieve good atomization. Second, during fuel injection, the air is stirred as much as possible during combustion to facilitate rapid mixing of the fuel mist and air.

Large, low-speed diesel engines generally use higher injection pressures to disperse the fuel mist and mix it with the air. In the past, some small, high-speed diesel engines often used concave surfaces under the piston crown or separated combustion chambers to create strong air turbulence and promote uniform mixing of fuel, gas, and air.

3. How does the fuel combustion process occur in the cylinder?

A: Fuel is injected into the combustion chamber, atomized, and mixed to form a combustible mixture. During the formation of the combustible mixture, a preliminary oxidation reaction occurs. This reaction gradually intensifies, ultimately causing the fuel to ignite and burn. To fully utilize the heat released by the fuel and ensure reliable diesel engine operation, the entire combustion process must occur near top dead center and be completed in a very short time.

4. What is afterburning? What are the main causes of afterburning?

A: Afterburning is the continuation of the combustion process during the expansion stroke. During the main combustion period, combustion products in the cylinder continuously increase, while oxygen gradually decreases. The high-temperature flame causes the injected fuel to begin burning before it can fully mix with the air. Furthermore, the combustion products hinder mixing between the fuel and air, resulting in incomplete combustion. This phenomenon becomes more severe as the combustion progresses. Fuel that lacks oxygen will be delayed until the expansion stroke, where it burns, resulting in afterburn.

Afterburn is a detrimental phenomenon. Not only does it prevent diesel from fully realizing its thermal efficiency, but it also increases exhaust temperatures, leading to overheating and failure of components such as the exhaust valves, ports, pistons, cylinder heads, and exhaust turbines. In severe cases, it can cause black smoke to billow from the chimney or even catch fire.

The main causes of afterburn are:

1. Too little injection advance, even starting after top dead center (TDC), delaying the entire combustion period in the cylinder.

2. Ending injection too late. Even if the injection advance angle is correct, the injector needle valve may not close tightly, causing delayed injection or dripping, which inevitably contributes to afterburn.

3. Poor fuel atomization results in an uneven combustible mixture, which impairs the fuel's combustion preparation (heating, evaporation, diffusion, and ignition) in the cylinder, also delaying combustion.

4. Engine load and speed also affect afterburn. As the load increases, the fuel injection volume also increases, resulting in a relatively smaller amount of air reaching the fuel compared to a light load. As the diesel engine speed increases, the combustion time inevitably shortens. This can also lead to insufficient combustion preparation and afterburning, which is particularly noticeable in high-speed diesel engines. Therefore, afterburning is more severe in high-speed diesel engines than in low-speed engines, resulting in higher exhaust temperatures. However, as an engine operator, by regularly inspecting and adjusting the working components of the fuel injection system to ensure that the injection advance angle, atomization quality, and fuel cut-off timing are maintained at appropriate levels, afterburning can be controlled to a certain extent.

5. How does the length of the ignition delay period affect diesel engine operation? What factors influence the length of the ignition delay period?

Answer: If the ignition delay period is too long, excessive fuel will accumulate. Once combustion occurs, the pressure will suddenly increase, causing the diesel engine to operate roughly. In more serious cases, it may cause the diesel engine to "knock" and the safety valve to trip—a blowout. This increases the impact load on moving parts, shortening the engine's life and affecting operational reliability.

If the ignition delay period is too short, the entire combustion process will be delayed, not only affecting the engine's performance but also exacerbating afterburning and causing overheating.

The main factors affecting the length of the ignition delay period include fuel quality, injection timing, cylinder thermal conditions, and the quality of the fuel atomization.

1. Fuel quality is primarily influenced by the cetane number. A higher cetane number shortens the ignition delay period; a lower cetane number shortens it.

2. If injection is too early, the ignition delay period will be extended due to lower cylinder pressure and temperature. Conversely, if injection is too late, the ignition delay period will be too short, delaying the entire combustion process and causing poor combustion. Therefore, every diesel engine has its own inherent injection advance angle, which cannot be arbitrarily changed and must be maintained accurately. However, modern engines are equipped with a device that automatically adjusts the start angle according to load changes to maintain a higher combustion pressure at low loads and a constant maximum combustion pressure at high loads.

3. The thermal state of the cylinder is primarily influenced by the temperature and pressure at the end of compression. Higher temperatures and pressures within the cylinder accelerate fuel combustion preparation and shorten the ignition delay period. These temperatures and pressures depend on the compression ratio, the temperature and pressure at the end of intake, the cylinder's temperature, and any leaks.

4. Good fuel atomization results in a uniform mixture of combustibles, greatly facilitating fuel combustion preparation and ignition. Consequently, the ignition delay period is shortened; otherwise, it is prolonged.