The energy source of a diesel engine is the ignition combustion of fuel. As one of the most important systems of a diesel engine, the fuel injection system not only achieves the thorough mixture of gas and oxygen in the cylinder, but also provides the essential fuel for combustion.
The quality of the fuel injection system directly determines the stability, economy, power output, and exhaust emissions of the diesel engine, among other important performance indicators.
Throughout the entire development stage of the diesel engine, the fuel injection and combustion state have always been the focus of research. Through continuous experiments and research, scientists have made significant progress.
In recent years, the continuously developing testing technologies and more comprehensive monitoring methods have enabled the research on diesel engines to enter a golden period of rapid development. People can understand each process of combustion more conveniently and intuitively, and carry out targeted optimization of combustion, thereby accelerating the development of diesel engines and the improvement of manufacturing processes.
The fuel injection process is a series of complex changes at the physical level.
Due to the compression characteristics of fluids, the elasticity of pressure-resistant pipelines, the inertial force of fuel flow, and the throttling effect of the system, the physical changes of fuel in the high-pressure system, that is, the process of fuel being transported from the fuel injection pump outlet valve to the fuel injector needle valve under high pressure, are affected.
The influence of these factors can lead to time lag in the injection stage and unstable fuel pressure, further deteriorating the injection quality.
Currently, the traditional diesel engine injection systems mostly adopt the plunger pump direct injection system, with the core component being the fuel injection pump. A complete injection unit usually includes the fuel injection pump, fuel injector, and high-pressure hose.
I. Selection of Fuel
With the increasing tension of international energy resources, fuel prices have risen significantly, and the proportion of fuel costs in the operating costs of ships has become increasingly heavy.
Due to the low price of low-quality fuel, it is widely used, and it also has a significant effect in the rational utilization of petroleum.
In recent years, the international community has continuously promoted the protection of marine environment and climate environment, and various environmental protection regulations have been revised, especially the emission requirements for each restricted area have become increasingly strict.
In fuel management, not only the applicability of diesel engines needs to be considered, but also the requirements for emission standards stipulated by laws and regulations need to be taken into account. To achieve the best balance between environmental protection regulations and economic benefits.
1. Viscosity
To avoid abnormal combustion and carbon deposits in the main engine, the viscosity of the fuel supplied to the main engine needs to be controlled first.
The oil separator works best in separating fuel with a low viscosity. Before the fuel is injected into the combustion chamber, suspended particles in the fuel will settle, causing the mechanical components to get stuck.
Therefore, it is very important to carry out sufficient separation before the fuel enters the main engine.
2. Catalytic Particulate Matter
After a large amount of analysis and statistics, it has been found that with the advancement of refining technology, the content of catalytic particles (hard particles, commonly known as Catafines, mainly composed of Si+Al) in the new low-sulfur oil is higher than that of the previous high-sulfur oil and shows an upward trend.
The reason for this problem is that fuel residues catalytic particles during the desulfurization process, and the diameters of the catalytic particles are all less than 10μm, becoming increasingly miniaturized. If the content is too high, it will cause excessive wear and even seizing of components such as the combustion chamber, fuel injection equipment, fuel injection pump, and fuel injectors.
ISO8217/2012 and GB/T17411-2012 stipulate that the aluminum+silicon content in RME180 oil should not exceed 50mg/kg, and most engine manufacturers recommend an inlet content of <15ppm.
Therefore, higher requirements have been put forward for the separation effect, and the fine filter before the main engine is also very crucial.
Experimental results show that using an effective and correct separation method can greatly reduce the content of catalytic particles and other inorganic precipitates.
First, it is necessary to ensure that the particle content of the fuel before entering the main engine is low to 15pm (preferably in the single digit) to ensure the safety of the main engine.
The separation effect of the oil separator is particularly important. Within a reasonable range, a higher oil separation temperature should be selected.
It has been found that there are still many areas for improvement in the separation and purification treatment on board, including how to allocate and use the oil tanks, and how to operate the oil separator.
3. Stability and Compatibility
Stability must first ensure that the total sediment meets the requirements of the marine fuel oil standard. During actual fuel usage, the principle of use the fuel that has been added first should be followed to avoid prolonged storage.
In daily management, if stratification is suspected, the first step can be to use the transfer pump to fully circulate the fuel in the oil tank to achieve a uniform mixture of the fuel.
At the same time, it is also necessary to pay attention to the differences in density and viscosity. Once there is a tendency of stratification, it will lead to changes in sulfur content, resulting in uneven distribution of sulfur in the fuel, thereby causing excessive sulfur content.
Stability and compatibility have differences. The former emphasizes the inherent state of the fuel, such as blended oil; the latter focuses on the stability of asphaltene after the fuel is mixed.
Asphaltene is not flammable and will prolong the afterburning period, causing black smoke and greatly increasing the formation of system paint film and carbon deposits, thereby leading to excessive wear.
Once stability and compatibility are compromised, the fuel system and combustion of the diesel engine will suffer a disastrous state: and if stratification occurs in the fuel, it will cause the sulfur content in the fuel to exceed the standard during use, putting the ship at risk of being detained.
II. Structure and Working Characteristics of the Injection Pump
Secondly, the phenomenon of the high-pressure oil pump getting stuck can also be attributed to the working environment (i.e., the characteristics of the flight route).
Firstly, from the analysis of the structure of the piston pair in the high-pressure oil pump, the gap between the piston and the sleeve must be very small (the manufacturer requires it to be 0.011-0.013mm) to ensure the sealing effect.
At the time of construction, the manufacturer also took into account the issue of leakage, and allowed a certain amount of leakage because, from the structure of the piston pair, there are 3 ring grooves inside the sleeve from top to bottom. The first groove is connected to the top of the piston and serves as a lubrication function. The second and third grooves are respectively connected to the axial openings on the sleeve body. Among them, the second groove is connected to the external diesel engine discharge pipe of the injection pump through the axial opening, and the third groove is required by the manual to be connected to the lubricating oil pipeline when using light diesel for the diesel engine to prevent the light diesel from leaking into the camshaft chamber. Usually, it is sealed with a pipe plug.
So, the slightly leaked fuel flows down along the gap between the piston and the sleeve from the top of the piston, first gathering in the first ring groove, then continuing to flow down to the second ring groove, passing through the axial opening and flowing upward to the main engine fuel discharge pipe, as shown in Figure 1.
Figure 1 Fuel Injector
If the diesel engine keeps running, the leaked fuel will gradually flow into the discharge pipeline.
This ship is a ferry liner. Due to its short sailing time and long docking time, and because the high-pressure oil pump body and the discharge pipeline are not wrapped with insulation materials, and in addition, during daily operations, in order to check the pipeline leakage situation and facilitate the quick disassembly and adjustment of the fuel injection pump when the plunger gets stuck, the protective cover outside the fuel injection pump is always not installed.
This results in the gradually cooling of the leaked fuel during the period of non-sailing, and then it condenses in the pipeline. When sailing, the leaked fuel is difficult to break through the discharge pipeline, and eventually, the fuel discharge pipeline gets blocked, and the leaked fuel fills the two upper grooves and continues to flow downward, accumulating more and more at the bottom of the plunger, forming abrasive particles, which further aggravates the wear of the plunger.
From the layout of the fuel injection pump on the diesel engine and the leakage path of the fuel, its leakage path is complex and tortuous. It first flows downward, then passes through several twists and turns before walking upward at an angle, and finally flows into the discharge pipeline. Moreover, its discharge pipeline and the high-pressure oil pipe are connected in a straight pipe leakage discharge pipeline.
This also accelerates the accumulation speed of the leaked fuel in the pipeline, as shown in Figure 2.
Figure 2 Layout of the diesel engine discharge pipeline
III. Management Suggestions and Technological Improvements
Based on the above analysis, the following management suggestions and technical improvements are proposed:
1. For the catalyst powder remaining in the fuel oil during the refining process, two methods are generally adopted. One is to use additives to reduce the residual catalyst powder in the fuel oil; the other is to adopt the following methods:
(1) Regularly discharge the residue.
Catafines are hydrophilic and can easily settle in low-viscosity and low-sulfur oil.
However, if the fuel oil contains a high water content, it will cause the catalytic particles to be dispersed and suspended in the fuel oil, making it difficult to settle.
Taking advantage of its higher specific gravity, the residue can be discharged from the bottom of the oil tank, sediment tank, daily use tank, and oil collection well to reduce its content.
(2) Make full use of the characteristics of the two sets of sediment tanks and daily use tanks, and use them alternately to ensure sufficient sedimentation.
(3) Use two fuel oil separators to separate the fuel oil, adopting a series connection method of separating water first and then separating impurities.
(4) Under the premise of ensuring safety, increase the separation temperature to ensure the efficiency of the separator;
Avoid excessive flow rate to ensure the purification effect.
(5) Regularly perform effective internal cleaning of the fuel oil daily use tank and sediment tank.
(6) The fuel storage tank can be cleaned according to the ship's dry dock maintenance plan.
2. Insulate the fuel injection pump and discharge pipe of the diesel engine.
3. Connect the fuel discharge pipeline of the diesel engine fuel injection pump separately and install a vacuum device. This will form negative pressure in the pipeline system, and the fuel leaking from the piston pair will be immediately sucked away, not accumulating at the piston pair.
4. Install a homogenizing pump at the fuel inlet of the diesel engine.
5. During warm-up, manually move each fuel injection pump to lubricate it. This method is to test the flexibility of the piston pair and prevent the piston pair from getting stuck. At the same time, other moving parts should also be inspected.
In addition, pay attention to checking and adjusting the adjustment mechanism of the regulating device to ensure the accuracy and reliability of the fuel injection pump's fuel injection volume.
6. During equipment operation, if an unexpected situation occurs and a single cylinder needs to be shut off for oil supply, it must be carried out strictly in accordance with the manufacturer's guidance and recommendations. Use the special oil supply tool provided with the machine to lock the roller mechanism of the fuel injection pump, and do not blindly close the inlet and outlet oil paths of the fuel injection pump, which may cause the piston pair to get stuck due to lack of effective lubrication.
7. In daily management, monitor the combustion state of each cylinder. Through smoke color, indicator diagram, and exhaust temperature changes, determine the operating status of the fuel injection equipment. Make timely adjustments according to the actual situation.
Ⅳ.Conclusion
Be familiar with the meanings of various fuel parameters and the corresponding handling measures.
Before refueling, understand the type, characteristics and quantity of the fuel.
When there are significant differences in the viscosity and density of the fuel, avoid the risk of mixing fuels during actual operation.
Effectively prevent the phenomenon of fuel pump sticking due to fuel quality issues, which may affect the normal operation of the diesel engine.
At the same time, refer to the manufacturer's guidance suggestions and regularly carry out equipment maintenance and upkeep. Make reasonable adjustments based on the operating parameters of the equipment to ensure good combustion state of the diesel engine.