Rajan Wilcox
Diesel fuel injectors are an essential component of any diesel engine, and their design has become increasingly complex over the last two decades. The injector type is determined by the type of engine and what will fit in the cylinder head. The injected quantity of fuel must match the engine and air system, and the nozzle spray angle and hole count must match the piston bowl, cylinder head swirl, and overall combustion system to perform at its best. The start of injection (SOI) is the time at which injection of fuel into the combustion chamber begins. The fuel is sprayed with the help of a nozzle that is opened and closed with a solenoid-operated needle valve. The fuel injector is effectively a spray nozzle that performs the final stage in the delivery of fuel into the engine.
| Characteristics | Values |
|---|---|
| Purpose | To deliver fuel into the engine cylinders |
| Type | Mechanical, common rail, unit injector systems (HEUI), pump-line-nozzle, air-blast injection |
| Spray Angle | 10 to 15 degrees |
| Spray Pattern | Directed by the director plate |
| Fuel Atomization | Comparable to the disc type injector |
| Fuel Pressure | 4,000 psi, up to 44,000 psi in third-generation common-rail diesels |
| Injection Timing | Controlled by the Electronic Control Unit (ECU) |
| Injection Rate | Controlled by piezo injectors |
| Injection Duration | The period when fuel enters the combustion chamber from the injector |
| Injected Fuel Quantity | Amount of fuel delivered to an engine cylinder per power stroke |
| Injection Delay | The difference between the start of delivery and the start of injection |
| End of Injection | The time in the cycle when fuel injection stops |
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What You'll Learn
Injector design and function
The basic design of a diesel injector is fairly simple. Diesel fuel enters the mechanical injector body from the injection pump and starts to build pressure. Once the pressure reaches about 4,000 psi, a check valve in the injector is lifted, and the fuel sprays through. The excess fuel remaining after the valve opens is returned to the injection pump. However, over the years, injectors have become increasingly complex, with advancements in injector technology driving notable improvements in diesel engine performance.
The injector has several components, each with a unique role in ensuring precise fuel injection. At the heart of the injector is the nozzle, which atomizes the fuel into a fine mist before it enters the combustion chamber. This atomization is critical for rapid and efficient combustion. The nozzle is precision-engineered to ensure a consistent fuel spray pattern, contributing to even and complete combustion. The nozzle holder or injector body refers to the part that the nozzle is mounted on. In conventional injection systems, this part primarily serves the function of nozzle mounting and nozzle needle spring preloading. In contrast, in common rail systems, it houses the main functional components: the servo-hydraulic circuit and the hydraulic actuator (electromagnetic or piezoelectric).
The needle valve, also known as the control valve, works in tandem with the nozzle to regulate fuel flow. It opens and closes the injector's nozzle, allowing for precise timing and metering of fuel delivery. The solenoid, an electromagnetic coil within the injector, acts as a valve by responding to electrical signals from the Electronic Control Unit (ECU). It either opens or closes the injector, enabling precise fuel delivery at the right time and aiding efficient combustion.
The injector pressure can reach extremely high levels, up to 36,000 PSIG, while the temperature inside the chamber can reach 1000 degrees Fahrenheit. These extreme conditions highlight the importance of injector technology advancements, which can account for a significant portion of the engine's total cost. The design of the injector, including the number and size of holes, is driven by horsepower requirements and combustion recipes. Larger injectors, for instance, can flow more fuel into the engine, resulting in increased power output.
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Injector failure
One of the main issues is that diesel fuel can carry impurities such as dirt, water, and debris, which can clog or damage injectors over time. Fuel filters are designed to prevent these contaminants from reaching the injectors, but if they become clogged or fail, the injectors are at risk of damage. Another issue is that diesel fuel that does not meet industry standards can lead to the buildup of deposits within the injectors, affecting their ability to deliver fuel properly and resulting in incomplete combustion and increased wear on the injector components.
The internal components of diesel injectors can also wear out over time due to the constant stress and high pressure they endure. This can lead to a gradual decline in performance and eventual failure. Modern common-rail diesel injectors are particularly susceptible to this type of failure, as they fire two or three times per engine cycle, which doubles the wear on the injector compared to older diesel injection systems.
Some common symptoms of injector failure include a noticeable decrease in power, acceleration, and overall engine responsiveness. The engine may struggle to reach higher speeds or maintain consistent performance under load. Faulty injectors can also cause an imbalance in the fuel-air mixture, leading to incomplete combustion, higher fuel usage, and poor combustion efficiency. Other signs of injector failure include rough idling, misfires, and the smell of diesel fuel, which can indicate a leak.
To prevent injector failure, it is important to maintain the fuel injection system and ensure that fuel filters are replaced regularly. It is also crucial to use diesel fuel that meets industry standards to avoid the buildup of deposits within the injectors.
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Injector selection
Engine Type and Cylinder Head Fit
The type of engine is a primary driver for injector selection. Different engines have unique cylinder head designs, which dictate the injector type that will fit. Bosch, for instance, supplies fuel systems for various engines, and while their injectors may differ in length, the overall design and operation can be similar. Ensuring the correct fit between the injector and the cylinder head is essential.
Horsepower Requirements and Combustion
Injector Size
When selecting an injector, it is generally advisable to choose the smallest injector that can support the desired power target. This approach reduces the amount of tuning required to optimise drivability. Larger injectors can flow more fuel and produce more power, but they may also impact idle quality and drivability. It is important to consider the trade-off between injector size and the overall performance of the vehicle.
Injector Data and Compatibility
Accurate injector data is crucial for proper control of the fuel injector by the on-board computer. When selecting an injector, ensure that the manufacturer provides detailed and reliable injector data. This data assists tuners in characterising the injector, ensuring optimal performance in idle and light throttle driving situations.
Fuel Type and System
The type of fuel used influences injector selection. For example, E85 fuel requires a higher fuel flow capacity compared to gasoline to achieve the same air-fuel ratio (AFR) level. As a result, the fuel system must be sized accordingly. Additionally, the selection of a fuel injector for alcohol-based applications (e85) may require the injector to remain open longer to deliver the necessary fuel quantity.
Maintenance and Reliability
Consider the maintenance requirements and reliability of the injectors. Some injector designs are more prone to issues like ball seat erosion, which can lead to improper injection. Regular maintenance, including filtration, is essential to prevent injector problems. Contaminated fuel is a common issue that can impact the performance and longevity of diesel injectors.
In conclusion, injector selection is a complex process that requires careful consideration of various factors. By understanding the engine's specific requirements, performance goals, and fuel system characteristics, one can make an informed decision about injector type, size, and compatibility.
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Injection timing
The injection timing is influenced by several factors, including the type of engine, horsepower requirements, and combustion recipes. The injected quantity of fuel needs to be carefully calibrated relative to the amount of oxygen available for burning. Modern common rail fuel systems offer infinite adjustability in terms of fuel quantity, injection timing, and injection pressure, allowing for precise control over the fuel-air mixture.
The start of injection (SOI) is the specific moment when the injection of fuel into the combustion chamber begins. It is usually expressed in crank angle degrees (CAD) relative to the top dead centre (TDC) of the compression stroke. The SOI can be indicated by an easily measured parameter, such as the activation of an electronic trigger or a signal from a needle lift sensor. However, due to the mechanical response of the injector, there may be a delay between the indicated SOI and the actual SOI when the fuel exits the injector nozzle. This delay is known as injector lag and can be influenced by factors such as the length of the line between the high-pressure pump and the injector.
The end of injection (EOI) marks the time in the cycle when fuel injection stops. The injected fuel quantity is the volume of fuel delivered to an engine cylinder per power stroke, typically measured in mm3/stroke or mg/stroke. The injection duration is the period during which fuel enters the combustion chamber, calculated as the difference between the EOI and SOI. It is important to note that the rate of injection can vary during the injection duration, and the injection pattern may not be consistent.
Advancements in diesel injector technology have led to increasingly complex designs, allowing for more precise fuel delivery at higher pressures. This results in better optimisation of the fuel-air mixture, leading to improved combustion efficiency and overall engine performance.
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Injection systems
There are several types of injection systems, including pump-line-nozzle, unit injector, and common rail. Modern injection systems utilise very high injection pressures and advanced electronic control methods. The unit injector, for instance, controls the metering and is called an injection valve, while those that perform all three functions (injection, spark, and metering) are called unit injectors.
The common rail system is a popular type of injection system that has been used in OE diesel fuel systems for many years. In this system, fuel from the fuel tank is supplied to a common header (accumulator) and then sent through tubing to the injectors, which inject it into the combustion chambers. The accumulator maintains pressure using a high-pressure relief valve and returns excess fuel to the fuel tank. The fuel is sprayed with a nozzle that opens and closes with a solenoid-operated needle valve. Third-generation common-rail diesels use piezoelectric injectors for increased precision, achieving fuel pressures up to 300 MPa or 44,000 psi.
Another type of injection system is the pump-line-nozzle system, which was commonly used in early diesel engines. In this system, a 'jerk pump' is used to dispense fuel oil at high pressure to an injector. The manifold injection system, designed by Johannes Spiel in 1884, is another example of an early injection system.
The size and number of holes in an injector impact the flow of fuel into the engine and, consequently, the power produced. Custom injectors are specified by the number of holes multiplied by the hole size, for example, 5x13 injectors have five holes at thirteen-thousandths of an inch. Larger injectors flow more fuel, resulting in increased power output.
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Frequently asked questions
Diesel fuel injectors deliver fuel into the engine cylinders, controlling the injection timing, fuel atomization, and other parameters.
Diesel fuel enters the mechanical injector body and starts to build pressure. Once the pressure is high enough (about 4,000 psi), a check valve in the injector is lifted, and fuel sprays through.
The difference between start of delivery and SOI is affected by the length of time it takes for a pressure wave to travel between the pump and injector. It is influenced by the length of the line between the pump and injector and the speed of sound in the fuel.
Signs of white smoke coming from the engine often point to a bad fuel injector. White smoke occurs when there is a lack of fuel entering the injector's body.
Direct injection atomizes the fuel through a small nozzle under high pressure, while carburetion relies on suction created by intake air.
