Common Rail Fuel Injection System Components

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Abstract: The components of a common rail fuel injection system include the rail, a high pressure pump and fuel injectors. Radial, unit and in-line pumps are used in commercial common rail systems. High pressure pump designs are evolving to achieve higher efficiency of the fuel injection system and to facilitate accurate rail pressure control. Several types of injectors can be used in common rail systems, including servo controlled electrohydraulic injectors and direct acting injectors.

Piping System and Rail

In modern common rail systems, the injector supply pipe dimensions and rail volume are critical parameters that can affect injection system dynamic performance. The sizing of these components has a significant impact on critical fuel injection variables such as the dwell time between multiple injections and the minimum fuel injection quantity. With increased use of multiple injections and the need to accurately control small fuel injection quantities starting at about the Euro 4 phase, manufacturers have paid more attention these seemingly mundane components.

The rail is a thick walled tube designed to act as an accumulator to prevent significant pressure drop at the full fueling rate by providing hydraulic capacitance to the high pressure circuit. The volume of the rail varies from only a few cubic centimeters in passenger cars, to as much as 60 cm³ in heavy-duty applications. In most cases, a metering valve at the high pressure pump controls the high pressure fuel delivery to the rail. The rail pressure can be controlled to a value that depends on the needs of any particular engine operating condition. In some cases, rail pressures can reach 300 MPa.

Just as is the case with P-L-N systems, common rail systems are also prone to effects related to wave dynamics in the rail and in fuel lines. Waves generated by sudden changes in pressure in one part of the system, such as when injection needle valve is opened, may become reflected at rigid terminations in the system and return to their origins, causing unwelcome consequences such as reduced injection pressure and variations in injection quantity.

In order to better control the pressure at the injector nozzle, some common rail injectors include an additional accumulator volume in the injector.

Injector Inlet Pipe Effects. The occurrence of high amplitude/low frequency pressure waves during the injection event represents one of the most important challenges in reducing the dwell time between multiple injections. Reducing the amplitude of these oscillations is an important objective of fuel injection system designers. A significant attenuation of pressure oscillations can be achieved by selecting the appropriate dimensions for the injector inlet pipe [2977][2193].

The energy stored in pressure waves induced by injection events with the same injection duration and rail pressure remains almost constant when the geometrical parameters of the injector supply pipes are modified. Hence, owing to the fact that the energy stored in a sinusoidal pressure-wave train increases with the square of both its amplitude and frequency, hydraulic layout modifications leading to increased pressure-oscillation amplitudes should yield reduced frequencies and vice versa [2978].

Since the frequency of the pressure waves is strictly related to the geometric features of the high-pressure circuit, the focus is on designing the circuit in order to maximize the frequency of the waves. Physical modeling systematically shows that this frequency increases with the injector inlet pipe aspect ratio, that is the ratio of the length to the internal diameter, and this is confirmed by experiments. Modulating pressure-wave oscillations in this way is considered an active damping strategy.

Alternatively, the introduction of orifices at the rail to pipe connections or inside the injector can be used. This is considered a passive damping strategy. For a particular injection duration and rail pressure, an orifice will generally decrease the injected fuel quantity when compared to a hydraulic layout without an orifice. The relative reduction is variable, but typically is less than 10%. An orifice will also reduce injection system hydraulic efficiency.

Rail Volume Effects. A relatively large volume accumulator has traditionally been considered fundamental to dampen the pressure fluctuations caused by the fuel pulses delivered by the pump and the fuel-injection cycles in common rail systems. However, studies with a fuel injection system for light vehicles has shown that the progressive reduction in the accumulator volume from 20 to 3 cm3 has no impact on the amplitude of these pressure fluctuations and little negative impact on injector performance [2978][2979]. The high-pressure control capability of the system in these studies resulted from the synergic action of both the system high-pressure hydraulic capacitance and the pressure control device. Although the duty cycle of either the pressure control valve (PCV) or the fuel metering valve at the pump inlet (FMV) depended on the rail size, the high-pressure control system was capable of keeping the pressure level adequately close to the nominal value for the range of accumulator volumes studied. This finding has been applied to the design of newer generation common rail systems for passenger cars which use smaller rail volumes then in the past.

This finding also opens the door to the possibility of removing the rail entirely from the high-pressure circuit. In fact, such a system concept, referred to as Common Feeding, has been developed [2979]. It uses a small hydraulic accumulator volume integrated in the pump which is then connected directly to the injector feed lines, Figure 1. The pressure sensor, PCV and FMV are also pump integrated. The resulting injection system has low hydraulic inertia that gives rise to fast dynamic response during transients and reduced production costs. Furthermore, this system matches the requirements of easy installation on the engine. While there are some notable differences in the system behavior compared to a common rail, these can be accommodated in the design and calibration. Some of the differences include a larger pressure drop and variations in the frequency of free pressure waves in the high pressure circuit. For pilot-main injections, orifices are required at the accumulator outlets to dampen the pressure waves triggered by the pilot injection and minimize its effect on main injection quantity. First commercial applications were targeted for the Chinese market [4556].

It should be noted that a minimum accumulation volume is required in the high-pressure circuit to avoid an excessive decrease in the pressure level during the injection event. Effective monitoring of the pressure in the high pressure circuit also requires a minimum volume to ensure pressure control system stability. The minimum volume for these functions is about one order of magnitude lower than the standard rail volume [2979].