Understanding Common Rail Fuel Pressure: Performance And Efficiency

what is the fuel pressure in the common rail

The fuel pressure in a common rail system is the pressure at which fuel is distributed to the injectors from a high-pressure accumulator, known as the rail. This pressure can reach over 2,000 bar (200 MPa or 29,000 psi) in common rail direct fuel injection systems, with third-generation common rail diesels featuring piezoelectric injectors capable of fuel pressures up to 2,500 bar (250 MPa or 36,000 psi). The pressure in the rail is electronically controlled and monitored by a pressure sensor, which sends the difference between the nominal rail pressure value and the measured value as an input signal to the controller.

Characteristics Values
Fuel pressure Over 2,000 bar or 200 MPa or 29,000 psi
Fuel pressure (according to some sources) Over 1,000 bar or 100 MPa or 15,000 psi
Fuel pressure in the fuel injection chamber 2050 bar or 29,732 psi
Injection duration 1-2 milliseconds
Fuel velocity 2000 km/h
Valve needle clearance 0.02 mm
Fuel pressure independent of Engine speed and load conditions
Fuel pressure in common rail systems Controlled as a function of engine speed and load

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Injector control pressure too low

Injector control pressure that is too low is a common issue with common rail diesel (CRD) engines. This issue can lead to a range of symptoms, including no start, engine cranking, starting and stalling, rough running, low or slow-building fuel rail pressure, and erratic fuel rail pressure.

The root cause of this problem is often related to fuel contamination, which can damage fuel system components such as the high-pressure injection pump, fuel injectors, fuel pressure sensor, and control components. Fuel contamination can occur due to the presence of water, gasoline, or diesel exhaust fluid (DEF) in the fuel.

To diagnose injector control pressure that is too low, it is recommended to inspect the fuel system for any signs of contamination. This includes checking for metal debris, which can indicate damage to components due to fuel contamination. It is also important to check the quality of the fuel by allowing it to rest and reach room temperature, as water contamination will begin to separate from the fuel.

If contamination is found, the entire fuel system will need to be repaired or replaced, including draining and cleaning the fuel tank, disposing of contaminated fuel, and refilling with clean, high-quality diesel fuel.

To prevent issues with injector control pressure, it is recommended to use high-quality, fresh fuel from well-known truck stops and to regularly check fuel filters and water separators. Additionally, using water-eliminating fuel additives can help maintain the health of the fuel system.

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High-pressure common-rail (HPCR) fuel system

HPCR fuel systems can provide a cleaner and more fuel-efficient combustion process with improved performance and particulate matter reduction. They improve engine performance through more efficient combustion and can be more fuel-efficient than other systems. Due to stable pilot injections, engines with HPCR systems are quieter and produce fewer nitrogen oxides (NOx), reducing the after-treatment requirements to meet various global emission standards.

HPCR systems have been around long enough that most original equipment manufacturers (OEMs) and operators are largely aware of the problems caused by dirt and other hard particulates in the fuel. There is a general understanding of the importance of effective filtration both on the vehicle and on bulk fuel tanks. However, hard particles are not the only fuel-related threat to these engines. Research by Donaldson Filtration Solutions showed that water can play a major role in HPCR system reliability, especially in regions with high humidity and/or poor fuel quality.

To ensure the longevity of HPCR systems, it is important to prevent water from entering the fuel. Water can cause damage in several ways, including rusting components, causing abrasive wear, dissolving certain contaminants, and creating harmful deposits in the fuel stream. In colder climates, water in the fuel can turn to ice, causing operational issues. Therefore, it is crucial to prioritize effective filtration and water removal techniques to maintain the performance and longevity of HPCR fuel systems.

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Fuel contamination

Common rail direct fuel injection systems are built around a high-pressure fuel rail, with pressures reaching up to and above 2,000 bar (200 MPa or 29,000 psi). This high-pressure injection delivers greater power and improved fuel consumption compared to earlier lower-pressure fuel injection systems.

Due to the extreme speeds and pressures in common rail systems, it is crucial that the fuel is clean and free from contamination. Fuel contamination can lead to engine damage, fuel system failure, and even fires. Water, particulates, and non-diesel liquids are the most common causes of fuel contamination.

Water in the fuel can cause corrosion and will erode injector nozzles. It can negatively affect the combustion process, reduce lubrication, and damage other system components. Water can enter the fuel from storage tanks or through condensation caused by cooling temperatures.

Particulates and debris can enter the fuel when it is transferred between storage tanks or when exposed to the atmosphere. They can disrupt engine combustion, block the fuel system, and cause wear on fuel injection equipment.

Improper maintenance is a significant cause of fuel contamination. For example, failing to drain the fuel tank before replacing a fuel system component can allow fuel to get trapped and become contaminated. Dirty hands and tools can also transfer debris between fuel components.

To prevent fuel contamination, it is important to use high-quality, clean fuel and to maintain the fuel system properly. Regularly changing the fuel filters and improving the filtration system can help reduce the risk of contamination.

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Fuel injection system pressure control

Fuel injection is the introduction of fuel into an internal combustion engine, most commonly automotive engines, via a fuel injector. The fundamental functions of a fuel injection system are to determine the appropriate amount of fuel to be supplied and to control the fuel flow to deliver this amount.

The common rail system is a direct fuel injection system built around a high-pressure fuel rail (over 2,000 bar or 200 MPa or 29,000 psi) feeding solenoid valves. The term "common rail" refers to the fact that all the fuel injectors are supplied by a common fuel rail, which is a pressure accumulator that stores fuel at high pressure.

There are several approaches to controlling the pressure in the common rail. One early method was to supply more fuel than needed to the common rail and use a pressure control valve to spill excess fuel back into the fuel tank. This approach was used exclusively in some early fuel injection systems, such as those with Bosch CP1 pumps, but it resulted in poor efficiency and excessively high fuel return temperatures.

A more preferred approach is to meter the fuel at the high-pressure pump to minimize the amount of fuel pressurized to the rail pressure. This can be achieved through inlet metering, where the fuel drawn into the pump is metered with an inlet metering valve (IMV), or outlet metering, where the pump draws an uncontrolled amount of fuel and the discharge flow is metered with an outlet metering valve (OMV). Another method is to vary the effective displacement of the high-pressure pump.

Production common rail fuel systems are equipped with a closed-loop high-pressure control system that stabilizes the rail pressure within a small margin of the nominal value specified by the electronic control unit for a given engine operating condition. The pump maintains the rail pressure by continuously delivering fuel to the common rail, and this pressure is monitored by a pressure sensor. The difference between the nominal rail pressure value and the measured value is the input signal for the controller.

In terms of practical rail pressure control, the system input is the position of the actuator used to control the rail pressure, while the system output is the rail pressure itself. Rail pressure control with a pressure control valve (PCV) is inherently fast due to the proximity of the system input (PCV) and system output (rail pressure sensor). The PCV can be located at one rail extremity (pump-external PCV) or at the pump outlet (pump-integrated PCV). The pump-external PCV leads to lower pump manufacturing costs, but the proximity of the regulator to the injectors can introduce disturbances in injector dynamics. On the other hand, the pump-integrated PCV solution combines the fuel throttled by the control valve with the leakage flow from the pumping chambers and the fuel flowing in the pump's cooling and lubrication circuits, and this combined flow is discharged from the pump back to the fuel tank.

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Fuel injection system components

Fuel injection systems are made up of several components, each serving a specific function. Here is a detailed overview of the key components in a fuel injection system:

Fuel Tank

The fuel tank is a critical component as it stores the fuel required for the engine to function. It is designed to maintain the fuel temperature below the flashpoint, preventing any potential ignition. The tank is also corrosion-resistant and leakproof, able to withstand pressures of at least 30 kPa. Additionally, it is equipped with a safety valve to release excess pressure and dissipate heat from the fuel returning from the engine.

Fuel Feed Pump

The fuel feed pump plays a crucial role in delivering fuel from the fuel tank to the injection pump. It utilises a spring-loaded plunger that is actuated by a push rod connected to a camshaft. When the push rod is at its minimum position, the spring force on the plunger creates suction, drawing fuel from the tank into the injection pump. When the camshaft turns to its maximum lift position, the plunger is lifted upwards, forcing the fuel through the outlet valve towards the injection pump.

Injection Pump

The injection pump is responsible for delivering the precise amount of fuel under high pressure to the injector. The pressure typically ranges from 120 to 200 bar, ensuring the fuel is injected at the correct instance into each cylinder head. There are two main types of injection pumps: distributor-type pumps and jerk-type pumps. Distributor-type pumps use a single pumping element to distribute fuel to each cylinder via a rotor, while jerk-type pumps employ a reciprocating plunger operated by the camshaft.

Fuel Injector

The fuel injector is a crucial component in the fuel injection system. It is an electronically controlled mechanical device that sprays the right amount of fuel into the engine, creating an optimal air-fuel mixture for combustion. The injector is located in the combustion chamber, inlet manifold, or throttle body. It atomises the fuel, breaking it down into fine droplets, which enhances subsequent combustion. The injector's nozzle is responsible for injecting the fuel into the cylinder, and it must meet specific requirements, such as proper atomisation, fuel distribution, and preventing direct impingement on the combustion chamber walls.

Governor

Governors are devices used to control the fuel quantity based on the engine's load and speed. They ensure that the amount of fuel supplied is appropriate for the engine's needs. There are two main types of governors: mechanical governors and pneumatic governors. Mechanical governors use centrifugal weights to control the fuel flow, while pneumatic governors utilise a diaphragm actuated by a vacuum, with the accelerator pedal controlling the diaphragm movement.

Electronic Control Unit (ECU)

The ECU plays a pivotal role in the fuel injection system by collecting information from various engine sensors to determine the precise amount and timing of fuel injection. It sends electrical signals to the fuel injector, controlling the duration and timing of fuel injection. The ECU also controls other engine functions, such as ignition timing, to ensure optimal engine performance.

Frequently asked questions

A common rail fuel system is a direct fuel injection system that uses a high-pressure pump to deliver fuel to a common rail assembly, which then delivers the high-pressure fuel to each injector.

The fuel pressure in a common rail system typically ranges from 1,000 bar (100 MPa or 15,000 psi) to over 2,000 bar (200 MPa or 29,000 psi).

The common rail system offers several advantages, including flexibility in controlling injection timing and rate, improved fuel economy, higher torque at low engine speeds, and reduced engine noise and emissions.

Fuel contamination can have significant negative effects on a common rail system. Contaminants can damage fuel injectors, the high-pressure injection pump, fuel pressure sensors, and control components. It is important to use clean and contaminant-free fuel to ensure the optimal performance of a common rail system.

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