Understanding Diesel Fuel Rail Pressure: Performance And Safety

The pressure in a diesel fuel rail is an important factor in the performance of a diesel engine. Common rail direct fuel injection systems, found in both petrol and diesel engines, feature a high-pressure fuel rail that feeds individual solenoid valves, with pressures ranging from 2,000 bar (200 MPa or 29,000 psi) to as high as 2,500 bar (250 MPa or 36,000 psi). This high-pressure fuel rail is supplied by a mechanical high-pressure pump, which ensures that diesel fuel is always available for the engine, even at low revs. The pressurised fuel is stored in a reservoir, or common rail, and is then released into the injectors for combustion. This high pressure results in better atomisation of the fuel, leading to improved combustion, greater fuel economy, reduced emissions, and quieter engine operation.

Injector tip pressure

The common rail is a high-pressure fuel rail that feeds solenoid valves, delivering fuel to the injectors. The pressure in the common rail can reach over 2,000 bar (200 MPa or 29,000 psi) in these systems. This high pressure enables the fuel to be injected as a larger number of smaller droplets, resulting in improved vaporization and more efficient combustion.

The injector tip pressure is crucial for successful fuel injection. It must be higher than the pressure in the combustion chamber at the time of injection. This principle of physics ensures that fuel can flow from areas of high pressure to lower pressure. For example, if the combustion chamber has a pressure of 250 PSI when the injector opens, the injection pressure must exceed 250 PSI for fuel to be injected.

Different engine types may have varying injector tip pressures. For instance, a 3126 HEUI engine typically has a fuel rail pressure of 60-70 PSI, but it utilizes high-pressure oil to fire the injector at around 4,000 PSI. In contrast, common rail systems have higher fuel pressure and do not rely on high-pressure oil.

The higher injector tip pressures in common rail systems primarily serve the purpose of finer atomization of the fuel upon injection. This results in a more even burn within the cylinder, contributing to the overall efficiency of the engine.

Atomisation

The pressure in the common rail system can reach over 2,000 bar (200 MPa or 29,000 psi) and, in the third generation of common rail diesels, up to 2,500 bar (250 MPa or 36,000 psi). This high-pressure fuel is stored in a reservoir called the common rail until it is required by the injectors. The injectors are controlled by the engine control unit (ECU), which injects the diesel fuel into the engine in very small amounts.

The atomisation process involves breaking up the diesel fuel into smaller droplets, which have a much higher ratio of surface area to volume. This improves vaporisation from the surface of the fuel droplets, allowing for a more efficient combining of atmospheric oxygen with vaporised fuel, resulting in more complete combustion. This, in turn, leads to better combustion, greater economy, lower emissions, and quieter running of the engine.

The high-pressure pump is the heart of the fuel system. It is engine-driven and can generate pressure exceeding 2,000 bar. To put this into perspective, a common car tyre pressure would be around 2.5 to 3.5 bar. The high-pressure pump stores the fuel in the common rail, which is simply a pressure accumulator where the fuel is stored 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 the excess fuel back into the fuel tank. A more efficient approach is to meter the fuel at the high-pressure pump to minimise the amount of fuel pressurised to the rail pressure.

The pressure in the common rail is stabilised by a closed-loop high-pressure control system, which ensures the pressure remains within a small margin of the nominal value specified by the ECU 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 higher pressures developed by common rail systems are used mainly for finer atomisation of the fuel upon injection. This atomisation is a critical aspect of modern common rail diesel systems, as it allows for improved engine performance and efficiency.

Fuel efficiency

Fuel rail pressure is a critical component of an engine's fuel system, directly impacting performance, efficiency, and reliability. The pressure at which fuel is delivered to the engine's injectors is typically measured in pounds per square inch (PSI) or kilopascals (kPa). In modern fuel-injected engines, fuel rail pressures can reach as high as 60 PSI (414 kPa), ensuring proper atomization and distribution for efficient combustion and optimal performance.

The benefits of high-pressure common rail (HPCR) fuel systems are well-documented, particularly in diesel engines. HPCR systems provide improved fuel atomization, resulting in cleaner, more powerful, and complete combustion. This leads to enhanced fuel efficiency and reduced emissions. The high-pressure injection delivers fuel as a larger number of smaller droplets, increasing the surface area-to-volume ratio. This improved vaporization and mixing of fuel and oxygen result in more efficient combustion.

The HPCR system consists of a high-pressure fuel rail common to all injectors, with fuel supplied by a high-pressure supply pump. The pressure in the rail can exceed 30,000 to 40,000 PSI, and each injector has its own firing mechanism or solenoid. This design delivers better atomization, improving combustion and fuel efficiency.

However, the complexity of electrical components in HPCR systems is a drawback, with numerous sensors, wiring harnesses, and electrical components required. Additionally, HPCR systems are highly sensitive to contaminated fuel, which can cause extensive damage. Water in the fuel, for example, can decrease lubricity, leading to valve damage and distorted spray patterns.

To ensure optimal performance and efficiency, it is crucial to maintain a clean fuel system and regularly change fuel filters. Using clean, high-quality fuel is paramount in preventing costly repairs and maintaining the longevity of diesel engines with HPCR systems.

Injector dynamics

There are several approaches to controlling the pressure in the common rail. One early method was to supply more fuel than necessary and use a pressure control valve to spill the excess fuel back to the fuel tank. However, a more preferred approach is to meter the fuel at the high-pressure pump to minimise the amount of fuel pressurised to the rail pressure. This can be achieved through inlet metering or outlet metering, or by varying the effective displacement of the high-pressure pump.

The location of the pressure control valve (PCV) is an important consideration in injector dynamics. The PCV can be placed at one rail extremity (pump-external PCV) or at the pump outlet (pump-integrated PCV). While the pump-external PCV leads to lower pump manufacturing costs, it can introduce disturbances in injector dynamics due to its proximity to the injectors. 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, resulting in a faster control process.

The common rail system offers several advantages over other injection systems. It provides flexibility in controlling injection timing and rate, allowing for better spray penetration and mixing, even at low engine speeds and loads. Additionally, it minimises the maximum pump torque requirement and improves noise quality by introducing pilot injection(s).

Fuel temperature

The relationship between fuel temperature and pressure in a diesel fuel rail is critical. As fuel temperature increases, so does its pressure. Therefore, maintaining the desired fuel pressure requires careful temperature control. Higher fuel temperatures can lead to increased pressure, which may exceed the system's designed limits. This can result in fuel leaking back into the fuel tank or, in extreme cases, cause damage to the fuel rail or other components.

To regulate fuel temperature and maintain the desired pressure, various approaches are employed. One method involves supplying excess fuel to the common rail and utilising a pressure control valve to release the surplus fuel back into the fuel tank. This technique, used in early fuel injection systems, often resulted in poor efficiency and elevated fuel return temperatures.

A more modern and preferred approach is to carefully meter the fuel at the high-pressure pump, ensuring that only the required amount of fuel is supplied to the common rail. This method helps minimise the amount of fuel pressurised to rail pressure and reduces the risk of excessive temperature and pressure build-up.

Additionally, some common rail implementations combine both approaches, adapting the strategy based on the engine's operating conditions. This hybrid method allows for greater flexibility and optimisation of the fuel injection system.

Frequently asked questions