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Pressure Control Of Common Rail Injection Systems

Posted on 1/4/2016 11:27:53 AM

Abstract: There are many methods to control the common rail pressure.First methods is to offer more fuel than needed in the common rail and use the pressure control valve to spill the excess fuel back to the fuel tank.The second method is to minimize the amount of fuel pressurized to the rail pressure by metering the fuel at the high pressure pump.According to the engine operating conditions,we could choose which pratical common rail control methods to use.

Production common rail fuel systems are armed with a closed-loop high pressure control-system to enable 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 rail pressure,guided by the a pressure sensor,is maintained by the pump through the delivering fuel to the common rail continuously.And the difference between the nominal rail pressure value and the measured one is the input signal for the controller. 
We define the rail pressure is the system output and the actuator used to control the rail pressure to be 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.

The other method is to meter the high pressure pump fuel which assure the required the amount of fuel could be delivery to the common rail.Some pump metering methods 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. We could control the amount of fuel entering the pump and avoiding compression of excess fuel to high pressure carefully,then we could improve the fuel injection systems hydraulic efficiency and also avoid the generation of excessively high fuel temperatures.But there is still possibility that we can't avoid the need for a high pressure regulator.But we still can use the pressure regulator to provide some trimming of the rail pressure.

2.Pressure Control Valve

We could locate the pressure control valve (PCV) for controlling rail pressure in the one rail extremity (pump-external PCV), Figure 1, or at the pump outlet (pump-integrated PCV), Figure 2.Pump manufacturing costs could be reduced by the pump-external PCV while 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.

We could find that the proximity of the system input (PCV) and system output (rail pressure sensor) could lead to rail pressure control with a PCV fast.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.
Common Rail Diesel Fuel Injection System with Pressure Control Valve located on the Rail
Bosch CP1 Pump with Integrated Pressure Control Valve

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