Mercedes Mullins
Common rail direct fuel injection systems are used in diesel engines to improve performance, reliability, and efficiency, while reducing noise and emissions. The system is composed of an injection pump, the rail, injectors, and sensors that detect engine conditions for a centralized electronic unit (ECU) to manage all components. The high-pressure pump is the heart of the system, pressurizing the fuel and transferring it to the rail, a reservoir and pressure accumulator. The pressure is regulated by a valve so that the correct pressure is maintained, and fuel is sprayed into the cylinders at the desired pressure. Injectors can inject fuel up to 7 times during one combustion event, improving combustion, fuel economy, and power.
| Characteristics | Values |
|---|---|
| Purpose | To increase engine power, reduce emissions and make diesel engines run smoother |
| Type of injection | Direct fuel injection |
| Fuel pressure | Over 2,000 bar or 200 MPa or 29,000 psi |
| Injectors | Injectors are controlled and operated by the ECM/ECU |
| Injection quantity | Injectors can inject up to 7 times during one combustion event |
| Injection pressure | Injection pressure is independent of engine speed and load conditions |
| Fuel spray | Fuel is sprayed into the cylinders at the desired pressure |
| Fuel consumption | Greater injection pressure produces a finer spray of fuel that burns more efficiently, leading to better fuel consumption |
| Engine noise | Lower engine noise compared to conventional diesel systems |
| Engine heating | Requires very short to no heating-up time |
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What You'll Learn
Injecting fuel as smaller droplets
The high-pressure pump is the heart of the fuel system. This is where diesel fuel pressure is increased to over 2,000 bar, which is far higher than the pressure in a common car tyre, which is around 2.5 to 3.5 bar. This high-pressure fuel is stored in a reservoir, or the 'common rail', until it is required by the injectors.
The high pressure results in the diesel being atomized into small droplets. This atomization has several benefits. Firstly, it improves combustion as it provides a much higher ratio of surface area to volume. This, in turn, improves the vaporization of the fuel droplets, allowing for more efficient combining of atmospheric oxygen with vaporized fuel, delivering more complete combustion.
The atomization of the fuel also helps to lower engine noise and reduce emissions. This is further improved by the ability of the engine's electronic control unit (ECU) to inject a small amount of diesel just before the main injection event ("pilot" injection), thus reducing its explosiveness and optimising injection timing and quantity.
The common rail system also allows for flexibility in controlling both the fuel injection quantity and injection timing, which enables better spray penetration and mixing, even at low engine speeds and loads. This is in contrast to other injection systems, where injection pressure increases with engine speed.
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Injector control
The injectors in a common rail system are typically controlled by the Engine Control Module (ECM) or the Engine Control Unit (ECU). These units receive feedback from various sensors in the system, such as the fuel temperature sensor, and adjust the pressure and fuel injection accordingly. The ECM/ECU ensures that the injection process is accurately controlled down to the split second, optimizing power delivery while minimizing mechanical stress and noise.
The injectors themselves are electrically activated, opening a hydraulic valve that consists of a nozzle and a plunger. This allows the fuel to be sprayed into the cylinders at the desired pressure. The injectors work in parallel with the high-pressure fuel pump, which can generate pressures exceeding 2,000 bar. This high pressure is a key advantage of the common rail system, as it results in the fuel being atomized into smaller droplets, leading to improved combustion and reduced emissions.
The flexibility offered by the common rail system's injector control is significant. It allows for multiple injections at any pressure and time (within predefined limits), which can be optimized for variations in fuel quality, cold starting, and emission control. This flexibility also enables better spray penetration and mixing, even at low engine speeds. Some advanced common rail fuel systems can perform up to five injections per stroke, with pre-injection and post-injection cycles further reducing noise and improving emission control.
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High-pressure pump
The high-pressure pump is a critical component of a diesel fuel rail system. It is responsible for increasing the pressure of the diesel fuel to levels exceeding 2,000 bar or 200 MPa (approximately 29,000 psi). This high-pressure fuel is then stored in a reservoir, known as the common rail, until it is required by the injectors.
The pump's function is to ensure that the fuel is consistently delivered to the common rail at the desired pressure. This is achieved through various methods, including mechanically or electronically controlled means. One common approach is to supply more fuel than necessary to the common rail and utilise a pressure control valve to spill the excess fuel back into the fuel tank. This helps maintain the target pressure within the system.
The high-pressure pump's role is essential in achieving the desired fuel injection pressure, which is independent of engine speed. This feature is a significant advantage over older systems, where injection pressure was directly tied to engine speed. By separating these factors, the common rail system allows for better control of fuel injection quantity and timing, resulting in improved spray penetration and mixing, even at low engine speeds.
The design of the high-pressure pump can vary, with single-plunger pumps being a popular choice due to their simplicity and effectiveness. However, it's important to select a pump that is compatible with the engine type and its specific requirements, as different applications have varying needs in terms of fuel flow rate, pressure, and quality.
Overall, the high-pressure pump plays a vital role in the diesel fuel rail system, enabling the delivery of highly pressurised fuel, which ultimately contributes to improved combustion, greater fuel economy, reduced emissions, and quieter engine operation.
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Fuel injection timing
In a common rail diesel system, the timing of fuel injection is controlled by the Engine Control Module (ECM) or the Engine Control Unit (ECU). These electronic control systems take input from various sensors, including fuel temperature sensors, to adjust the pressure and timing of fuel injection accordingly. The ECM/ECU ensures that the injection pressure remains high and relatively constant throughout multiple injection events, resulting in improved combustion and fuel efficiency.
The timing of fuel injection is crucial in diesel engines because it determines when combustion occurs. Injecting diesel fuel before the piston reaches the top dead center (BTDC) allows the air-fuel mixture to burn completely, creating maximum pressure in the cylinder. This, in turn, forces the exhaust to push the piston down with maximum force during the power stroke. However, if the injection timing is too advanced, it can cause detonation, where the mixture pushes on the pistons as they move upward, potentially damaging the engine.
To optimize performance and emissions, manufacturers set specific injection timings for each engine model. These timings aim to balance power output and legal emission limits. However, adjusting injection timing, also known as spill timing, can be done to fine-tune engine performance and meet specific power requirements. Advancing the injection timing can provide benefits such as increased power and improved fuel efficiency, but it must be done carefully to avoid engine damage.
Modern diesel engines, particularly common rail systems, have improved injection timing control compared to older systems. The use of high-pressure pumps, solenoid valves, and piezoelectric injectors allows for more precise fuel delivery and timing. This results in better fuel atomization, improved combustion, and reduced engine noise and emissions.
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Improved fuel filtration
The performance of diesel engines relies on their fuel filtration system. Diesel fuel filtration is critical to maintaining the reliability and performance of diesel engines. As diesel engines evolve, their designs require cleaner and drier fuel.
The introduction of the "Tier" system for lower emissions diesel engines has increased the need for high-pressure fuel injections in diesel fuel engine designs. This change in diesel engine design to meet required emission controls has created a significant need for better fuel quality. Cleaner fuel is required because common rail fuel systems, which are standard for Tier III and Tier IV engines, have much tighter tolerances and are extremely sensitive to water and particulate contamination.
The World Wide Fuel Charter specifies an ISO rating of 18/16/13, which means that per mL, diesel fuel can have 1,300–2,500 particles of 4 microns and up. However, fuel injector OEMs recommend stricter standards of an ISO level of 12/9/6, which reduces the maximum number of particles per mL to less than 1 for particles of 14+ microns.
To improve fuel filtration, bulk fuel conditioning equipment can be added to the fuel offloading and forwarding lines, removing the burden of contamination removal from on-engine filters. This not only provides fewer emergency maintenance incidents and extends vehicle operational time but also makes the fuel much cleaner, offering more protection to critical and sensitive diesel engine components like fuel injectors and control valves.
Additionally, fuel tanks can be kept free of water with continuous fuel tank circulation and conditioning systems, and the use of the proper on-engine filter is critical to the vehicle's success.
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Frequently asked questions
A diesel fuel rail is a component of a common rail direct fuel injection system. It is a reservoir that stores highly pressurised fuel, which is then passed into the injectors for injection.
The diesel fuel rail works by storing high-pressure fuel, which is then supplied to the injectors. The injectors are electrically activated, opening a hydraulic valve and spraying fuel into the cylinders at the desired pressure.
The use of a diesel fuel rail allows for better combustion, greater economy, lower emissions, and quieter running of the engine. The high-pressure injection delivers improved power and fuel consumption benefits.
Almost all common rail diesel problems are expensive. Issues can include contamination from water and debris, which can lead to a poor seal and reduced performance. Other common problems include engine cranking, starting and stalling, and low or slow build-up of fuel rail pressure.
Preventative maintenance is key to avoiding issues with diesel fuel rails. It is important to regularly inspect and replace components, such as the fuel filter, to ensure system cleanliness and longevity.
