Pressure Control of Common Rail Injection System

Introduction

Production common rail fuel systems are equipped with a closed-loop high pressure control-system that stabilizes the rail pressure within a relatively small margin to 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. This pressure is monitored by a pressure sensor and the difference between the nominal rail pressure value and the measured one is the input signal for the controller. In control terminology, the rail pressure is the system output while the position of the actuator used to control the rail pressure is the system input.

There are a number of approaches to control the pressure in the common rail. One way is to supply more fuel than is needed to the common rail and use a high pressure regulator—commonly referred to as a pressure control valve—in the high-pressure circuit to spill the excess fuel back to the fuel tank. In this approach, the pressure control valve position is the control system input. While this approach was used exclusively in some early fuel injection systems such as those with Bosch CP1 pumps (Figure 1 and Figure 2), poor efficiency and an excessively high fuel return temperatures can result.

Another approach is to meter the fuel at the high pressure pump to ensure that only the amount of fuel required by the injectors is supplied to the common rail. A number of pump metering approaches are possible. One common approach is to meter the fuel drawn into the pump (inlet metering) with some type of inlet metering valve (IMV)—sometimes also referred to simply as a fuel metering valve (FMV). Another approach is to allow the pump to draw in an uncontrolled amount of fuel and meter the pump’s discharge flow (outlet metering) with a valve such as an outlet metering valve (OMV). Another means is to vary the effective displacement of the high pressure pump. By carefully controlling the amount of fuel entering the pump and avoiding compression of excess fuel to high pressure, the fuel injection system hydraulic efficiency can be improved and generation of excessively high fuel temperatures can be avoided. It should be noted, however, that metering the fuel at the injection pump may not avoid the need for a high pressure regulator. A pressure regulator can still be used to provide some trimming of the rail pressure.

Pressure Control Valve

A pressure control valve (PCV) for controlling rail pressure can be located at one rail extremity (pump-external PCV), Figure 1, or at the pump outlet (pump-integrated PCV), Figure 2. The pump-external PCV leads to lower pump manufacturing costs but the proximity of the regulator to the injectors can introduce additional disturbances in injector dynamics. In the pump-integrated PCV solution, the fuel throttled by the control valve joins the leakage flow from the pumping chambers as well as the fuel flowing in the pump’s cooling and lubrication circuits. This combined flow is discharged from the pump to return to the fuel tank.

[schematic]
Figure 1. Common rail diesel fuel injection system with pressure control valve located on the rail(Source: Bosch)
[schematic]
Figure 2. Bosch CP1 pump with integrated pressure control valve(Source: Bosch)

Rail pressure control with a PCV is inherently fast because of the proximity of the system input (PCV) and system output (rail pressure sensor). In other words, the system does not include the delay resulting from fuel passing through the high pressure pump as would be the case for some of the pump metering approaches.

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