Injector Types and Related Requirements
Function and Requirements
The primary task of an injector is to finely atomize fuel into minute droplets according to the characteristics of diesel engine mixture formation, and accurately inject them into the designated location within the combustion chamber. The injector must meet the specific spray characteristics required by various combustion chamber types. It must possess sufficient penetration capability to ensure fuel droplets reach deep into the combustion chamber. Additionally, it requires an appropriate spray cone angle to cover a sufficient combustion chamber area. Furthermore, excellent atomization quality is essential to ensure thorough fuel-air mixing. Finally, the absence of dripping after fuel injection concludes is also a critical performance indicator for the injector.
Types of Injectors
Injectors are primarily categorized into orifice-type and needle-type injectors. Orifice-type injectors feature a simple structure, ease of manufacturing, and widespread application in diesel engines. Needle-type injectors are specifically designed for partitioned combustion chambers, with a configuration that minimizes carbon deposit accumulation.
Orifice Injector Analysis
Structural Characteristics and Applications
Orifice injectors are renowned for their unique structural features. They utilize multiple orifices to finely atomize fuel into minute droplets, which are then injected at high velocity into the combustion chamber. This injector type is simple in structure, easy to manufacture, and widely used in diesel engines. Orifice injectors achieve fuel atomization through multiple nozzles and come in two structural designs: long and short, suitable for unified combustion chambers. The long injector design extends the nozzle body, positioning the needle valve guide away from the combustion chamber. This reduces the risk of thermal deformation and prevents the needle valve from seizing within its housing, making it ideal for diesel engines with high thermal loads. The needle valve incorporates two conical surfaces: a pressure-bearing surface that withstands high-pressure oil chamber forces, generating upward axial thrust to overcome the preload of the pressure-regulating spring and friction between the needle valve and body, enabling fuel injection; and a sealing surface that mates with the sealing surface within the needle valve body to ensure internal chamber sealing. Additionally, the nozzle hole design is a critical component of orifice-type injectors. Based on the number of nozzle holes, they can be classified as single-hole, dual-hole, or multi-hole injectors.
Typically, the nozzle hole diameter ranges from 0.2 to 0.5 mm. It is important to note that the nozzle hole diameter should not be excessively small to ensure manageable manufacturing complexity and prevent carbon buildup and blockage during operation. Pressure Regulation Mechanism: The preload force of the pressure-regulating spring is transmitted to the needle valve via the push rod, causing it to tightly seal against the sealing cone surface within the needle valve body and thereby close the fuel nozzle. The magnitude of this preload can be finely adjusted using the pressure-regulating screw.
Working Principle
When high-pressure diesel fuel supplied by the injection pump enters the injector via the high-pressure fuel line, it passes through the inlet fitting, injector filter element, and precision fuel passages within the injector body and needle valve body before reaching the pressure chamber inside the nozzle. Here, the fuel pressure acts upon the pressure-bearing cone surface of the needle valve, generating an upward axial thrust. Once this thrust overcomes the preload force of the pressure-regulating spring, the needle valve rises, simultaneously opening the orifice. This allows high-pressure diesel to be smoothly injected into the combustion chamber. When the injection pump ceases fuel delivery, the pressure in the nozzle's pressure chamber rapidly drops. Under the force of the pressure-regulating spring, the needle valve quickly resets, sealing the orifice and halting the injection process.
Analysis of Axial-Needle Injectors
Structure and Principle
The axial needle-type injector resembles the orifice-type injector but is better suited for compartmentalized combustion chambers. Its design minimizes carbon deposit buildup. The axial needle moves vertically during injection, and its unique nozzle design prevents carbon accumulation while enabling self-cleaning. Additionally, the diverter-type nozzle features a diverter hole on the needle valve body, further optimizing fuel injection characteristics.
Function of the Diverting Nozzle
During diesel engine startup, low rotational speed and reduced fuel pressure cause most diesel fuel to be injected through the diverting holes against the airflow into the vortex chamber center. This counter-current injection promotes thorough fuel atomization, while the higher temperature at the vortex chamber center facilitates diesel ignition and combustion. This ensures reliable engine starting in cold conditions. Once the engine starts and enters normal operation, increased needle valve lift directs most fuel through the main orifice in the direction of the airflow into the swirl chamber.
Impact of Needle Pin Shape
The orifice of the needle-pin type injector forms a circular ring due to the design of the needle pin below the sealing cone surface of the needle valve. The needle pin passes through the orifice in the needle valve body and protrudes slightly beyond it. This design produces a hollow cylindrical spray pattern, directly influencing the spray cone angle. Specifically, cylindrical shaft needles generate a narrower spray cone angle, while truncated cone-shaped shaft needles produce a wider spray cone angle. The unique shaft needle design enables the creation of diverse spray patterns, allowing shaft needle injectors to adapt flexibly to the requirements of various combustion chamber geometries.
