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The Basics of Common Rail Injection Systems Explained

Common rail injection system

In the common rail system, the fuel, pressurized up to 1800 bar (180MPa), is maintained in the common rail (fuel tube) and injected into the cylinder through the injector with a solenoid valve that controls the nozzle back pressure of the fuel according to the signal sent from the ECM. Similar to gasoline ECCS engine systems, diesel injection volume and timing are fully controlled by the ECM based on the various information signals.

The supply pump draws the fuel from the fuel tank and pumps the high-pressure fuel to the rail. The fuel pressure in the rail is generated by the cam, plunger, and fuel metering operation of the SCV (Suction Control Valve) and controlled optimally by the feedback system using the pressure sensor. In this regard, the common rail system is quite different from that of the conventional diesel engine in regard to the fuel delivery system.

Fuel Flow
Common rail injection system

Common rail injection system

Common Rail System Diesel Component Parts

Common rail injection system

The fuel supply pump consists primarily of the pump body (eccentric cam, ring cam, and plungers), SCV (Suction Control Valve), fuel temperature sensor, and feed pump.

Feed Pump
Common rail injection system

The trochaic type feed pump, which is integrated into the supply pump, draws fuel from the fuel tank and feeds it to the two plungers via the fuel filter and the SCV (Suction Control Valve). The feed pump is driven by the drive shaft. With the rotation of the inner rotor, the feed pump draws fuel from its suction port and pumps it out through the discharge port.

Suction Control Valve
Common rail injection system

A linear solenoid type valve has been adopted. The ECM controls the duty ratio (the length of time that the current is applied to the SCV), in order to control the quantity of fuel that is supplied to the high-pressure plunger. Because only the quantity of fuel that is required for achieving the target rail pressure is drawn in, the actuating load of the supply pump decreases.

By turning the SCV ON/OFF, an amount of fuel corresponding to the actuation duty ratio is supplied and fuel is discharged by the pump plungers.

Long duty cycle ON => small valve opening => minimum intake quantity  Short duty cycle ON => large valve opening => maximum intake quantity
Common rail injection system

Operation of the fuel supply pump

The two plungers are positioned vertically on the outer ring cam for compactness. The engine drives the supply pump at a ratio of 1: 2. As the drive shaft rotates, the eccentric cam rotates in the eccentric state, and the ring cam moves up and down while rotating. Then, they pressurize the fuel sent to the plunger chamber and send it to the rail. The quantity of fuel supplied to the rail is controlled by the SCV, using signals from the ECM. The SCV is a normally opened type (the intake valve opens during de-energization).

Common rail injection system

As shown in the illustration below, the rotation of the eccentric cam causes the ring cam to push plunger A upward. Due to the spring force, plunger B is pulled in the opposite direction from plunger A. As a result, plunger B draws in fuel, while plunger A pumps it to the rail.

Common rail injection system

This injector replaces the conventional injection nozzle, and it achieves optimal injection by effective control in accordance with the signals from the ECM. Signals from the ECM determine the length of time and the timing while the current is applied to the solenoid. This, in turn, determines the quantity, rate, and timing of the fuel that is injected from the injector. Each injector is individually calibrated.

No Injection
When no current is supplied to the solenoid, the spring force is stronger than the hydraulic pressure in the control chamber.
Common rail injection system

Thus, the solenoid valve is pushed downward, effectively closing the outlet orifice. For this reason, the hydraulic pressure that is applied to the command piston causes the nozzle spring to compress. This closes the nozzle needle, and as a result, fuel is not injected.

When the current is initially applied to the solenoid, the attraction of the solenoid pulls the TWV (Two-Way Valve) up, effectively opening the outlet orifice and allowing the fuel to flow out of the control chamber. After the fuel flows out, the pressure in the control chamber decreases, pulling the command piston up. This causes the nozzle needle to rise and injection to start.

Common rail injection system

The fuel that flows past the outlet orifice flows to the leaking pipe and below the command piston. The fuel that flows below the nozzle needle lifts it upward, which helps to improve the nozzle’s opening and closing response.

When the current continues to be applied to the solenoid, the nozzle reaches its maximum lift, where the injection rate is also at the maximum level. When the current to the solenoid is turned OFF, the TWV falls, causing the nozzle needle to close immediately and the injection to stop.

Injector Adjustment Value Registration
During manufacturing, each injector is checked for its individual flow rate, and a calibration code is etched on the top surface of each injector. When replacing the fuel injector(s) or ECM, injector adjustment value registration must be performed.

Common rail injection system

Fuel Rail Pressure Sensor
This sensor detects the fuel pressure in the rail and sends a signal to the ECM. It is a semi-conductor type pressure sensor that utilizes the characteristic whereby electrical resistance changes when the pressure is applied.

Common rail injection system

Pressure Limiter
The pressure limiter relieves pressure by opening the valve if abnormally high pressure is generated. The valve opens when the pressure in the rail reaches approximately 2000 bar (200 MPa) and closes when the pressure falls to approximately 500 bar (50 MPa). Fuel leaked by the pressure limiter returns to the fuel tank.

Common rail injection system

Intake Air Control Valve
Vibration caused by stopping the engine can be controlled by cutting the intake air just before the fuel is cut. The intake air control valve solenoid controls the ON-OFF function ENG0659 of the intake air control valve actuator. After the engine has stopped, the intake air control valve solenoid will return to the fully open position.

Common rail injection system

Barometric Pressure Sensor
The barometric pressure sensor is built into the ECM. The sensor detects the ambient barometric pressure and sends the voltage signal to the microcomputer for fuel injection quantity control.

Fuel Pump Learning Value Clearing
In order to keep the optimum fuel pressure in the fuel rail at all times, the ECM controls the fuel pump with high precision by monitoring the signal of the fuel rail pressure sensor. Accordingly, the ECM always learns the characteristic value of the fuel pump. Fuel Pump Learning Value Clearing is an operation that clears the value of fuel pump learning. It must be performed after the fuel pump is changed. When the ECM is replaced with a new one, performing Fuel Pump Learning Value Clearing is not necessary. If the replacing ECM has been used once, performing Fuel Pump Learning Value Clearing is necessary after the ECM has been replaced.

Electronically controlled EGR volume control valve
• Crankshaft position sensor
• Camshaft position sensor
• Engine coolant temperature sensor
• Vehicle speed sensor
• Fuel pump temperature sensor
• Accelerator pedal position sensors 1 and 2
• Mass airflow sensor
• Intake air temperature sensor
• Turbocharger boost sensor
• Refrigerant pressure sensor
• Park/neutral position (PNP) switch
• Heat-up switch
• Glow plug and glow relay
• Cooling fan relay
• Turbocharger boost control solenoid valve

Replacement of high-pressure injection tubes
The high-pressure injection tubes from the common rail to the injectors and to the fuel supply pump must be renewed whenever they are removed.

Common rail injection system

(1) Fuel Injection Quantity Control
The fuel injection quantity control function controls the fuel injection to an optimal injection quantity based on the engine speed and the accelerator position signals. There are three types of fuel injection quantity control such as normal control, idle control, and start control.

Start Control
For better startability under the cold engine conditions, the lower the coolant temperature becomes, the greater the amount of fuel that is injected. The amount of fuel injected at the engine start is a preset value in the ECM.

Common rail injection system

Common rail injection system

Idle Control
When the ECM determines that the engine speed is at idle, the fuel injection system is adapted for the idle control. The ECM also provides the system with a fast idle control in response to the engine coolant temperature signal.

Common rail injection system

Normal Control
The fuel injection data, predetermined by the correlation between various engine speeds, accelerator pedal positions, and the fuel rail pressure are stored in the ECM memory, forming a map. The ECM determines the optimal amount of fuel to be injected using the sensor signals in comparison with the map.

Common rail injection system

Maximum Amount Control
The maximum injection amount is controlled to the optimum by the engine speed, intake air amount, engine coolant temperature and the accelerator opening in accordance with the driving conditions. This prevents oversupply of the injection amount caused by decreased air density at a high altitude or during a system failure.

Deceleration Control
The ECM sends a fuel cut signal to the fuel injector and the suction control valve during deceleration for better fuel efficiency. The ECM determines the time of deceleration according to the signals from the accelerator pedal released position switch and the crankshaft position sensor (TDC).

(2) Fuel Injection Timing Control
The target fuel injection timing in accordance with the engine speed and the fuel injection amount are recorded as a map in the ECM beforehand. The ECM determines the optimum injection timing using sensor signals in accordance with the map data.

(3) Air Conditioning Cut Control
When the accelerator pedal is fully depressed, the air conditioner is turned OFF to improve the drivability for a few seconds. Also, when the engine coolant temperature becomes excessively high, the air conditioner is turned OFF. This continues until the engine coolant temperature returns to normal.

(4)Fuel Cut Control (At No Load and High Engine Speed)
If the engine speed is above the specification in the no load condition, the fuel will be cut off after a short period of time. The exact time when the fuel is cut off varies based on the engine speed. The fuel cut will occur until the engine speed reaches the specified level, and then the fuel cut is finished.

(5) Glow Control
(6) EGR Volume Control
(7) Turbocharger Boost Control
(8) Cooling Fan Control
(9) Intake Air Control Valve Control
(10)On Board Diagnostic System

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