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As the core component of the diesel engine fuel injection system, the performance of the diesel injector directly determines the fuel atomization quality, combustion efficiency and pollutant emission level. This paper systematically explains the working principle, key characteristic indicators, influencing factors and optimization technology of the diesel injector. Combining test data and engineering application cases, it reveals its core role in improving engine power, economy and environmental protection, and looks forward to future technology development trends.

Introduction
Diesel engines are widely used in heavy vehicles, ships, engineering machinery and other fields due to their advantages such as high torque, low fuel consumption and strong reliability. With the increasingly stringent global environmental regulations (such as China's National VII and EU Stage VI standards), higher requirements are placed on the fuel economy and emission control of diesel engines. As the execution unit for achieving precise fuel injection, the injector has become a key breakthrough in balancing engine power and environmental protection by controlling the injection pressure, spray shape and injection timing of the fuel. In-depth research on the characteristics of the injector is of great significance to promoting the upgrading of diesel engine technology.

Working principle and structural characteristics of diesel fuel injector
2.1 Working principle
Diesel fuel injector realizes fuel injection based on electromagnetic control principle:
Signal reception: The engine control unit (ECU) sends pulse voltage signal to the injector electromagnetic coil according to the sensor signals such as speed and load.
Electromagnetic drive: After the electromagnetic coil is energized, a magnetic field is generated, which attracts the armature to drive the needle valve to move upward to overcome the spring force and open the spray hole.
High-pressure injection: Under the action of high pressure (100-200MPa) in the common rail pipe, the fuel is sprayed out at high speed through the spray hole to form mist particles.
Precise control: By adjusting the width (injection pulse width, 0.5-20ms) and frequency of the pulse signal, the dynamic adjustment of the injection amount is realized.

2.2 Core structure
Spray hole assembly: The spray hole diameter is 0.1-0.3mm, laser drilling or electrochemical processing is used, and the surface roughness Ra≤0.2μm ensures smooth fuel flow.
Needle valve pair: The clearance between the needle valve and the valve seat is only 1-3μm, and it needs to be precisely ground to ensure sealing performance and movement flexibility.
Pressure chamber: Stores high-pressure fuel, and its volume affects the injection rate and fuel atomization effect.
Key characteristic indicators of diesel injectors

3.1 Atomization characteristics

Droplet size: measured by Sauter mean diameter (SMD), the ideal range is 20-50μm. The smaller the SMD, the more fully the fuel and air are mixed, and the higher the combustion efficiency. Studies have shown that when the SMD drops from 60μm to 30μm, the combustion rate increases by 50% and the fuel consumption decreases by 8%.
Atomization uniformity: The degree of discreteness of the distribution of each droplet size, usually expressed by the particle size distribution index (N). The larger the N value, the better the uniformity.

3.2 Spray form
Penetration distance: The axial movement distance of the spray front end per unit time, which needs to match the depth of the combustion chamber. For example, heavy-duty diesel engines require a penetration distance of 150-200mm to cover a large combustion chamber.
Spray cone angle: The spray diffusion angle affects the spatial distribution of fuel in the combustion chamber. A cone angle that is too large can easily cause the fuel to hit the wall and produce carbon deposits, while a cone angle that is too small may cause uneven mixing.

3.3 Injection law
Injection rate: The amount of fuel injected per unit time directly affects the combustion heat release rate. The ideal injection rate curve is "slow first and then fast" to reduce combustion noise and NOx emissions.
Injection timing: The correspondence between the start time of injection and the crankshaft angle. Too early or too late will lead to deterioration of combustion and affect power and emissions.

Main factors affecting injector characteristics
4.1 Injection pressure
Injection pressure is the decisive factor affecting the atomization effect. When the pressure increases from 100MPa to 200MPa, the droplet SMD decreases from 55μm to 32μm, and the penetration distance increases by 25% (as shown in Figure 2). However, excessive pressure will increase the wear of the nozzle hole, and a balance needs to be struck between atomization requirements and reliability.

4.2 Nozzle hole structural parameters
Aperture: Reducing the aperture can increase the fuel injection speed, but it will increase the flow resistance and the risk of clogging. Tests show that when the aperture is reduced from 0.3mm to 0.2mm, the injection speed increases by 20%, but the fuel impurity sensitivity coefficient increases by 30%.
Number of holes and angle between holes: Multi-hole design (6-10 holes) can improve spray uniformity, and the angle between holes needs to be optimized according to the shape of the combustion chamber. For example, the ω-type combustion chamber is suitable for a 120° uniformly distributed hole shape.

4.3 Fuel physical properties
Fuel viscosity and surface tension directly affect the atomization efficiency. For every 1mm²/s increase in diesel viscosity, the droplet SMD increases by 10%-15%. Reducing the viscosity by preheating the fuel (such as heating to 50°C) can increase the atomization efficiency by 12%-18%.

4.4 Cavitation phenomenon
Cavitation refers to the formation of bubbles due to a sudden drop in pressure when the fuel flows through the nozzle. When the bubbles burst, energy is released to promote atomization, but excessive cavitation will cause nozzle erosion. Studies have found that when the radius of the nozzle entrance corner increases from 0.05mm to 0.2mm, the cavitation intensity decreases by 45% and the nozzle wear rate decreases by 60%.

Injector characteristic optimization technology and engineering application
5.1 Structural optimization design
Variable cross-section nozzle: adopts a "contraction-expansion" type nozzle (Venturi structure) to enhance atomization and reduce wear by using the cavitation effect. Tests show that this structure can reduce the droplet SMD by 18% and extend the nozzle life by 50%.
Multi-stage injection strategy: Through the combination of pre-injection (reducing combustion noise), main injection (providing power), and post-injection (promoting soot oxidation), NOx and PM emissions can be reduced by 15%-20% at the same time.

5.2 Material and process innovation
Wear-resistant materials: The spray holes are made of silicon nitride ceramic or diamond coating (hardness HV1800-2200), which improves corrosion resistance by 3-5 times and is suitable for fuel environments with high sulfur content.
Precision manufacturing process: Magnetorheological polishing technology can control the roughness of the inner wall of the spray hole to Ra≤0.1μm, reduce fuel turbulence, and make the flow deviation rate of each hole less than 3%.

5.3 Intelligent control technology
Pressure closed-loop control: The injector with integrated micro pressure sensor can feedback the injection pressure data in real time, and the ECU dynamically adjusts the drive signal according to the deviation, with a control accuracy of ±0.5MPa.
AI algorithm optimization: Through machine learning algorithms to analyze massive working condition data, the injection pulse width and timing are automatically optimized, so that the fuel consumption of the engine under complex working conditions is reduced by 5%-7%.

5.4 Engineering Application Cases
A heavy truck diesel engine (displacement 12L) is equipped with an optimized injector (injection pressure 200MPa, 8-hole spray hole), and the measured data are as follows:
Power: Maximum torque increased by 10%, from 2200N・m to 2420N・m;
Economy: Fuel consumption per 100 kilometers decreased from 38L to 34L, and fuel saving rate was 10.5%;
Emission: PM emissions decreased from 0.04g/kWh to 0.02g/kWh, and NOx emissions decreased from 4.0g/kWh to 3.2g/kWh, meeting the National VII emission standards.

Challenges and Future Trends
6.1 Technical Challenges
Reliability under ultra-high pressure: As the injection pressure breaks through 300MPa, the fatigue resistance of the spray hole material and the sealing technology face higher requirements.
Multi-fuel adaptability: The viscosity and lubricity of new fuels such as biodiesel and ammonia fuel vary significantly, requiring the injector to have wide-range adaptability.

6.2 Development Trends
Intelligence and integration: "Intelligent injectors" that integrate sensors and microprocessors can achieve self-diagnosis and self-calibration, and deeply cooperate with the engine thermal management system.
Green manufacturing technology: Additive manufacturing (3D printing) is used to achieve complex internal flow channel design, reduce material waste, and improve atomization performance.
Hydrogen fuel injection expansion: In view of the low density and easy leakage of hydrogen fuel, high-pressure hydrogen injectors are developed to pave the way for the commercialization of hydrogen fuel internal combustion engines.

Conclusion
The study of the characteristics of diesel injectors is the core link to improve the performance of diesel engines, and its technological progress depends on the cross-innovation of fluid mechanics, materials science and control engineering. By optimizing the spray hole structure, improving manufacturing accuracy, and introducing intelligent control, the injector is evolving from a "single actuator" to a core unit of "intelligent perception-precise control". In the future, with breakthroughs in ultra-high pressure injection, new materials and alternative fuel technologies, injectors will play a more critical role in achieving the "dual carbon" goals.