How does the suction control valve regulate fuel rail pressure?

1. The suction control valve regulates the amount of fuel flowing into the main supply pump.

   
   

2. Then, the main supply pump adjusts the output volume and pressure in the fuel rail.

   ---SCV valve assy, suction| Ningbo Henshine Precision Machinery Co., Ltd---

With the help of the suction control valve (SCV), the supply pump regulates the fuel volume in a more accurate way, that is, the output volume of fuel is just the amount needed to maintain the rail pressure required. This reduces the engine operation load, which assists in emission reduction. In addition, less heat will be generated due to the reduced fuel flow.

Denso Common Rail System – Principle of high-pressure generation

A Pump plunger 1

B Pump plunger 2

C To fuel rail

1 Inlet valve

2 Outlet valve

3 Eccentric cam

4 Eccentric cam ring

5 Fuel metering valve

6 Driveshaft

The rotary movement of the drive shaft is converted to a reciprocating movement by the eccentric cam. The eccentric cam ring transfers the reciprocating movement to the pump plungers.

The pump plungers are offset by 180 degrees. This means, that during a reciprocating movement, pump plunger 1 performs exactly the opposite movement to pump plunger2.

The eccentric cam produces an “upward” stroke:

• Pump plunger 1 moves in the direction of TDC, thus compressing the fuel and delivering it to the fuel rail via the outlet valve. The inlet valve is pressed into its seat by the delivery pressure.

  
  

• Pump plunger 2 is moved by the tension spring force in the direction of BDC. Due to the high pressure in the fuel rail, the outlet valve is pressed into its seat. The pump internal pressure opens the inlet valve and fuel flows into the high-pressure chamber.

  
  

The eccentric cam produces a “downward” stroke:

• The process is the reverse of that previously described.

Calibrating the high-pressure pump (fuel metering valve)

After replacing the high-pressure pump and/or the PCM the fuel metering valve of the high-pressure pump must be calibrated with the aid of WDS.

Denso Common Rail System - suction control valve

A Fuel metering valve opened to maximum

B Fuel metering valve opened to minimum

V Voltage

a Low duty cycle

b High duty cycle

1 Transfer pump

2 Fuel metering valve

3 Fuel flow to the high-pressure chambers

Function SCV control valve

NOTE: 

• The fuel metering valve operates together with the fuel pressure sensor (on the fuel rail) in a closed control loop.

  
  

• The fuel metering valve is fully open when de-energized.

The fuel quantity that passes to the high-pressure chambers of the high-pressure pump is dependent on the opening cross-section of the fuel metering valve.

The opening cross-section is determined by the PCM via the duty cycle of the PWM signal:

• Low duty cycle: large aperture cross-section

• High duty cycle: small aperture cross-section

Effects of faults

In the event of malfunctions: Injected quantity = 0 (engine cuts out or cannot be started.)

SCV Overhaul Kit Diagnosis:

The monitoring system checks:

• the circuit for short circuits and open circuits.

  
  

• the fuel metering valve for correct function; the values of the fuel pressure sensor are used for this purpose. The currently measured values from the fuel pressure sensor are continuously compared with the map data. Slight deviations are indicated as control faults whereupon:

  – the quantity of fuel injected is reduced and

  – the pilot injection is deactivated.

  
  

• whether serious deviations exist. They are indicated as a malfunction whereupon

  – the quantity injected is set to 0 and the engine is stopped.

Note: Control faults do not necessarily mean a defective fuel metering valve or a defective high-pressure pump. A blocked fuel low-pressure system or defective fuel injectors could (among other things) also be the cause.

Emissions-related component (vehicles with EOBD):

• Yes (MIL-active), with control faults

• No (Non-MIL active), with malfunctions.