Common rail high pressure fuel injection system
Diesel engines often use lean combustion with excess air compared to gasoline engines, resulting in very low CO and HC emissions, but high NOx and PM (particulate matter) emissions. To address these issues, it is necessary to improve the combustion state and shorten the combustion time, and it is effective to increase the fuel injection pressure to promote fuel atomization. As a solution to this problem, fuel supply systems using high-pressure fuel injection devices have become mainstream.
A common rail high pressure fuel injection system is a type of fuel supply system for diesel engines that uses a common rail (piping) that stores fuel at high pressure and injects it into each cylinder of the engine. In this system, the fuel is first compressed by a high pressure pump and then sent to each injection nozzle while maintaining a constant high pressure in the common rail. The use of a common rail makes it possible to precisely control the injection pressure, injection timing, and injection amount according to the engine’s operating conditions.
This precise fuel injection control improves combustion efficiency, reduces emissions and optimizes engine performance. It reduces emissions such as NOx and PM, improving fuel economy and increasing power while complying with environmental regulations. Common rail high pressure fuel injection systems are widely used in modern diesel engines due to their flexibility and efficiency.
table of contents
Common rail high pressure fuel injection system and its features
A schematic diagram of a common rail high pressure fuel injection system is shown below.
Figure 1: Common rail high pressure fuel injection system
To roughly explain how a common rail high-pressure fuel injection system works, as shown in Figure 1, the fuel in the fuel tank is sucked up by a feed pump integrated with the supply pump, then the fuel is filtered out of impurities by a fuel filter and sent to the supply pump.
The fuel is then pressurized by a plunger in the supply pump and pumped to the common rail, after which it is sent to the injectors and injected into the combustion chambers according to a signal from the electronic control unit (ECU).
If the pressure in the supply pump exceeds a specified value, the overflow valve opens, and if the pressure in the common rail exceeds a specified value, the pressure limiter opens to return fuel to the fuel tank so that the pressure does not exceed the specified value.
In addition, any fuel that leaks from the injectors is returned to the fuel tank.
Features of the common rail high pressure fuel injection system
- With conventional mechanical fuel injection systems, the maximum injection pressure from the injection nozzle is affected by engine speed and engine load, making it difficult to obtain good injection pressure across the entire range of engine speeds. However, with a common rail fuel injection system, pressurized fuel can be stored in the common rail, ensuring a constant stable injection pressure.
- Compared to mechanical fuel injection systems, having the ECU calculate the amount and timing of fuel injection enables more precise control.
- By dividing the fuel injection into multiple stages and optimizing the injection timing by the ECU, it is possible to reduce vibration and noise from the engine, and also to suppress black smoke and NOx emissions.
Supply Pump
- What is a supply pump?
The supply pump generates the fuel pressure in the common rail, as shown in Figure 2-(1). There are two types: a discharge volume control type, in which the fuel discharge volume is controlled by a discharge volume control valve when the supply pump plunger rises, and an intake volume adjustment type, in which the fuel discharge volume is controlled by a intake volume control valve when the plunger descends, as shown in Figure (2).
Figure 2: Difference in valve arrangement between discharge volume control type and suction volume adjustment type supply pumps
- Supply pump body
The supply pump body is composed of a plunger, camshaft, feed pump, and discharge or suction volume control valve. As shown in Figure 3, there are vertical types in which the plungers are arranged in series, and radial types in which the plungers are arranged radially perpendicular to the camshaft.
Figure 3: Vertical supply pump
1. Feed pump
There are three types of feed pumps: trochoid, gear, and vane. All of them are built into the supply pump and are driven by a camshaft built into the pump via gears from the engine body. They draw fuel up from the fuel tank and send it through a fuel filter to the pressure section inside the supply pump.
2. Discharge volume control valve
The discharge amount control valve used in a discharge amount controlled supply pump adjusts the pressure inside the common rail, and controls the discharge amount from the supply pump by turning the discharge amount control valve on and off.
Figure 4: Discharge volume control valve
Discharge control valve operation
Figure 5: Operation of discharge volume control valve
Figure 5(1): Operation of discharge volume control valve (suction stroke)
- Inhalation process
As shown in Figure 5 (1), when the plunger is descending, the discharge control valve is OFF (open) and low-pressure fuel is sucked into the pumping section (plunger chamber) via the discharge control valve.
Figure 5(2): Operation of discharge volume control valve (non-pressure feed stroke)
- Prestroke
Even if the plunger goes into the upward stroke as shown in Figure 5 (2), as long as the discharge control valve is OFF (valve open), the drawn-in fuel passes through the discharge control valve and is returned without being pressurized.
Figure 5(3): Operation of discharge volume control valve (pressure feed stroke)
- Pressure stroke
As shown in the figure, when the discharge volume control valve is turned ON (closed) at the timing appropriate for the required discharge volume, the return passage is cut off and the pressure inside the plunger chamber is increased (pressure rises).
3. Intake volume control valve
The suction volume control valve used in suction metering type supply pumps adjusts the pressure in the common rail and linearly adjusts the amount of fuel sent to the pressure delivery section (plunger chamber) by controlling the duty ratio, thereby controlling the amount of fuel discharged from the supply pump to the common rail.
Figure 6: Operation of suction volume control valve
Duty ratio
The duty ratio is the percentage of time that a signal is in the “ON” state within a certain period. 0% means that it is always OFF, and 100% means that it is always ON. It is often used in power control, signal processing, etc.
Intake volume control valve operation
The suction volume control valve is driven by duty ratio control, and the total oil volume of the supply pump is high when no power is applied (duty 0%) and decreases as the duty ratio becomes larger (approaching 100%).
Figure 6(1): Maximum oil feed (duty ratio: 0%)
- Maximum oil flow rate
When the power is off, the needle valve and piston valve are pressed by a spring, so the intake port and exhaust port are connected, as shown in Figure 6 (1). As a result, all of the fuel pumped by the feed pump is sucked into the plunger chamber.
Figure 6(2): Intermediate oil feed (duty ratio: 0%~99%)
- Intermediate Region
As shown in Figure 6 (2), when the drive duty ratio from the ECU increases, the needle valve begins to rise, and the piston valve narrows the passage area of the intake port and exhaust port, reducing the amount of intake into the plunger chamber.
Figure 6(3): Non-pressure feed (duty ratio: 100%)
- No pressure feed
As shown in Figure 6 (3), when the drive duty ratio increases further and the needle valve rises, the slit in the piston valve becomes blocked, resulting in no pressure being fed.
Figure 7: Suction volume characteristics
FIG. 7 shows the inhalation amount characteristic.
Common Rail
The common rail system accumulates fuel pressurized by a supply pump and distributes it to the injectors of each cylinder. The fuel pressure of this system is detected by a pressure sensor installed in the common rail. The ECU performs pressure feedback control so that the actual pressure value matches the preset pressure value according to the engine speed and the amount of fuel injected.
In addition, the common rail is fitted with a flow damper, a pressure limiter and a common rail pressure sensor, as shown in Figure 8.
Figure 8: Common rail
Flow Damper
Flow dampers are mainly used in engines for large vehicles, and consist of a piston, spring, body and stopper as shown in Figure 9. The flow damper damps the pressure pulsation applied to the common rail.
Figure 9: Flow damper
Operation
Figure 10: Operation of flow damper
- Normal
Under normal circumstances, the piston moves slightly to the right as shown in Figure 10(1) due to the flow of fuel, causing it to float, and the spring force dampens the pulsation (to maintain balance).
- In abnormal situations
In the event that a fuel leak occurs beyond the flow damper, excess fuel will pass through the flow damper, so the piston moves from the state shown in Figure 10(1) to the state shown in Figure 10(2), causing the tip to hit the main body seat surface and block the fuel passage.
Pressure Limiter
Figure 11: Pressure limiter
The pressure limiter in Figure 11 has the role of opening the valve when the pressure in the common rail becomes abnormal for some reason, and regulating the pressure by releasing the fuel into the fuel tank.
Figure 12: Pressure limiter operation characteristics
Fig. 12 shows the operating characteristics of the pressure limiter, and when the pressure in the common rail rises and reaches the opening valve threshold, the pressure limiter opens, fuel is returned to the fuel tank, and the pressure in the common rail starts to drop. However, although the valve should normally close when the valve closes, in the figure, the valve closes when the pressure in the common rail exceeds the closing valve threshold.
This property is called hysteresis, and the operation of the pressure limiter can be explained as having a hysteresis property.
- Hysteresis
Hysteresis refers to the delayed reaction of a physical system to external forces or changes in conditions, specifically, the property of a system not changing in exactly the same way from one state to another and vice versa.
Injector
The injector is activated by an electrical signal from the ECU to spray the high-pressure fuel distributed from the common rail into a fine mist and then injects it directly into the combustion chamber.
The ECU calculates the optimal injection timing, injection amount, and injection rate for each instance based on the data (electrical signals) input from each sensor that detects the condition of the engine and driving conditions, and sends the results to the injector as an electrical signal.
There are two types of injectors: solenoid and piezoelectric. Here we will explain the solenoid type injector.
Figure 13: Solenoid type injector
As shown in Figure 13, the injector is composed of a nozzle section, nozzle spring, command piston, control chamber, orifice, solenoid valve, etc., and is attached to each cylinder of the engine.
The solenoid valve controls the injection amount, injection timing, and injection rate by controlling the pressure in the control chamber.
The nozzle orifice is multi-hole, and the injector body is often held in place with a clamp. An O-ring is installed at the part of the cylinder head where the injector is inserted to prevent engine oil from entering the injector mounting hole in the cylinder head.
Injector operation
Figure 14: Injector operation
- No injection
In Figure 14 (1), when no current is applied to the solenoid coil, the valve closes the orifice due to the force of the valve spring, and high-pressure fuel from the common rail flows into the control chamber and nozzle chamber under the same pressure.
At this time, the cross-sectional area (inner diameter) of the command piston in the control chamber is larger than the cross-sectional area (inner diameter) of the nozzle needle, so due to the pressure-receiving area (the larger the receiving area is, the higher the pressure will be under the same pressure), the force on the command piston acts in the direction of pushing down the nozzle needle, the nozzle hole closes, and no fuel injection occurs.
- injection
In FIG. 14(2), when electricity is applied to the solenoid coil, electromagnetic force is generated, the valve overcomes the force of the spring and moves in the upward direction, opening the orifice.
As a result, the fuel in the control chamber gradually flows out, reducing the pressure in the control chamber. When the pressure in the control chamber falls below the pressure applied to the underside of the nozzle needle, the pressure difference causes the nozzle needle to rise and fuel injection begins.
- End of injection
When the solenoid coil is de-energized, the valve begins to move down due to the force of the valve spring and closes. At this time, high-pressure fuel from the common rail flows back into the control chamber and nozzle chamber, causing the nozzle needle to rapidly move down, returning to the position shown in Figure 14 (1), closing the nozzle hole and ending fuel injection.
Sensors
The sensors detect the engine operating conditions and vehicle operation conditions. The detected information is converted into electrical signals and input to the ECU.
This article explains the sensors used to control common rail high pressure fuel injection systems.
Sensor Index
1.Air Flow Meter
The air flow meter is a sensor that is installed between the air cleaner and the inlet manifold and detects the amount of intake air into the engine as an electrical signal and inputs this information to the ECU.
The ECU uses this electrical signal to calculate the amount of intake air and uses it to control the fuel injection amount, EGR, etc.
Figure 15: Hot wire air flow meter
The “hot wire” type is generally used for air flow meters.
As shown in Figure 15, the hot wire air flow meter is installed in the intake passage and operates at a constant temperature of several hundred degrees.It consists of a heating resistor (heat wire) and a temperature compensation resistor.
- Electrical signal detection of intake air volume
As shown in FIG. 15, the air flow meter takes in a portion of the intake air as a bypass flow, and this bypass flow cools the heating resistor (hot wire) inside the sensor.
The mechanism is that the degree of cooling changes depending on the amount of bypass flow (intake air amount) that cools the heating resistor, and as a result, the electrical resistance value of the heating resistor changes.
This allows the fluctuations in the current flowing through the entire air flow meter circuit to be converted into an electrical signal.
- Heating resistor (heat wire)
In a heating resistor, the temperature and resistance are proportional. In other words, the lower the temperature, the smaller the electrical resistance, and the higher the temperature, the higher the resistance. The temperature here refers to the temperature of the heating resistor itself, not the temperature of the intake air.
Therefore, when the amount of intake air is small, the amount of heat dissipated by the heating resistor is small (the temperature of the heating resistor is high) and the electrical resistance increases, so Ohm’s law shows that the current in the circuit is smaller for the same voltage.
Conversely, when the amount of intake air is large, the amount of heat dissipated by the heating resistor is large (the temperature of the heating resistor becomes lower) and the electrical resistance becomes smaller, so again according to Ohm’s law, the current in the circuit becomes larger at the same voltage.
The ECU converts the change in current from this electrical signal into a change in voltage and detects it as the amount of intake air.
Figure 16: Hot wire air flow meter circuit diagram
Figure 16 shows the circuit inside the air flow meter. When the amount of intake air changes, a bridge circuit connecting four resistors controls the current to the heating resistor (R1) so that the temperature difference between the heating resistor (R1) and the temperature compensation resistor (R2) is always kept constant.
Heating resistors and temperature compensation resistors
As an example of a circuit, when the amount of intake air increases, the resistance value of the heating resistor (R1) decreases, so
R1 * R4 < R2 * R3 、VM ≠ VK …➀
However, VM and VK are the resistance values at that time.
When the control unit in Figure 16 detects this state, it increases the current flowing from the power supply to VB (heating R1).
R1 * R4 = R2 * R3 In other words, control so that VM = VK.。…➁
The temperature compensation resistor (R2) is a resistor in this circuit that is used to compare the resistance values of VM and VK. It improves measurement accuracy by enabling control such as result ➁ by comparing the electrical resistance with the heating resistor (R1) as shown in ➀.
2.Boost Pressure Sensor
The boost pressure sensor detects the boost pressure produced by the turbocharger by detecting the pressure inside the intake pipe.
In the boost pressure sensor shown in FIG. 17, when pressure is applied to the silicon chip in the sensor unit due to an increase in pressure in the intake pipe, the resistance value of the silicon chip changes.
Figure 17: Boost pressure sensor
Figure 18: Circuit diagram
Figure 19: Output voltage characteristics
Fig. 18 is a circuit diagram of the boost pressure sensor, and Fig. 19 shows the output voltage characteristics of the boost pressure sensor.
As shown in the circuit diagram of Figure 18, the boost pressure sensor has a bridge circuit made up of four variable resistors inside a sensor unit kept in a vacuum, with pressure from the intake pipe acting on one side of the bridge circuit.
When pressure is applied to this sensor, the silicon chip is subjected to stress caused by the pressure difference with the vacuum chamber on the opposite side, causing four resistance values to change.
The potential difference caused by this resistance change is amplified by the IC and input to the ECU as an electrical signal.
The input electrical signal is converted into a digital signal by an A/D converter and then input into the microcomputer inside the ECU for calculations.
19 shows the output voltage characteristic of the boost pressure sensor. This characteristic shows that the output voltage increases in proportion to the increase in pressure in the intake pipe.
3.Temperature Sensor
The temperature sensor includes an intake air temperature sensor that detects the intake air temperature, a coolant temperature sensor that detects the engine coolant temperature, and a fuel temperature sensor that detects the fuel temperature (diesel).
Since the structures of these sensors are almost the same, we will use a water temperature sensor as an example here.
Figure 20: Water temperature sensor
Figure 21: Circuit diagram
Figure 22: Characteristics diagram
The water temperature sensor has a thermistor built in as shown in Figure 20, and is connected to the ECU as shown in the circuit diagram in Figure 21.
This circuit converts the change in temperature experienced by the thermistor into a change in resistance value, which is then converted into a change in voltage value that can be detected by the microcontroller inside the ECU.
In Figure 21, the microcontroller in the ECU detects the voltage at point A shown in the figure, that is, the voltage obtained by dividing 5V by a resistor and the resistance of the water temperature sensor.
FIG. 22 shows the change in resistance of the thermistor due to the temperature and the voltage value at that time.
4.Rotation Sensor
Rotation sensors include crank angle sensors that detect the engine rotation speed and crank position, cam sensors that detect the rotation speed of the camshaft, and TDC sensors that detect which cylinder is at the top dead center position of the piston in order to control fuel injection timing.
Since the structure of each of these sensors is almost the same, the crank angle sensor will be used as an example here.
The crank angle sensor is attached to the flywheel housing or the crankshaft timing gear, and as shown in Figure 23, there are various types of sensors, such as a pickup coil type and a magnetic resistance element type.
The pickup coil type consists of a magnet (permanent magnet) and an IC with a built-in magnetic resistance element.
Figure 23: Crank angle sensor
Pickup coil type operation
When the engine rotates, the signal rotor shown in Figure 24 rotates. When the protrusions of the signal rotor pass through the magnetic field generated by the magnet, the distance between the signal rotor and the core changes, causing a change in the amount of magnetic field passing through the pickup coil, generating an AC voltage in the pickup coil.
This AC voltage is input as a signal to the ECU, and the ECU converts this analog waveform signal into a digital waveform using a waveform shaping circuit, as shown in Figure 25.
Figure 24: Principle of signal generation
Figure 25: Waveform shaping circuit
Magnetoresistance element type operation
As shown in Figure 26(1), when there is a protrusion of the signal rotor in front of the magnetoresistive element, the magnetic flux emitted from the magnet passes through the magnetoresistive element, increasing the resistance value of the magnetoresistive element.
On the other hand, when there is no protrusion on the signal rotor, as shown in Figure 26(2), the magnetic flux from the magnet does not pass through the magnetoresistance element, and the resistance value of the magnetoresistance element decreases.
This change in resistance value also changes the voltage applied to the crank angle sensor, so the internal IC converts this voltage change into a digital signal as shown in Figure 26 (3) and inputs it to the ECU.
Figure 26: Principle of signal generation
5.Accelerator Position Sensor
The accelerator position sensor is attached to the accelerator pedal as shown in Figure 27, and converts the accelerator pedal depression angle into an electrical signal and inputs it to the ECU.
There are two types of accelerator position sensors: contact type and Hall element type. Here, we will explain the Hall element type as an example.
Figure 27: Accelerator position sensor
Hall element operation
As shown in Fig. 28, a Hall element type accelerator position sensor is made up of a stator fixed inside the accelerator position sensor case, Hall ICs (2 pieces), a magnet that moves with the accelerator pedal, and a yoke.
The “Hall effect” is used to detect the accelerator pedal depression angle.
Figure 28: Internal configuration
Hall effect
Figure 29: Principle of Hall effect
The Hall effect is a phenomenon in which, when a magnetic flux is applied perpendicular to a current flowing through a Hall element as shown in Figure 29, an electromotive force is generated in both the current and the magnetic flux; the electromotive force increases as the applied magnetic flux density increases.
A Hall IC is a combination of a Hall element and an amplifier for amplifying the signal.
Accelerator position sensor operation
Figure 30: Accelerator position sensor operation
Figure 31: Output voltage characteristics diagram
The operation of the accelerator position sensor is as shown in Figure 30 (1) when the pedal is not depressed, as no magnetic flux is applied perpendicular to the current flowing through the Hall element, so no electromotive force is generated.
When the accelerator pedal is depressed as shown in Figure 30(2), the angle of the magnetic flux applied to the stator changes, and a perpendicular magnetic flux is applied to the Hall element as well. This generates an electromotive force in the Hall element, and the sensor voltage rises as shown in Figure 31.
The detection circuit has two systems to deal with the unlikely event of a malfunction.
6.Common rail pressure sensor
The common rail pressure sensor shown in FIG. 32 is a sensor that detects the fuel pressure in the common rail.
It has the same structure as a boost pressure sensor, and is a semiconductor sensor whose electrical resistance changes when pressure is applied to the internal strain detection section.
Figure 32: Common rail pressure sensor
Figure 33: Output voltage characteristics
FIG. 33 shows the output voltage characteristics of a common rail pressure sensor.
ECU(Electronic Control Unit)
The ECU processes information from various sensors and switches, controls the injectors, supply pump, and various relays, and is also equipped with a self-diagnosis system.
The self-diagnosis system is equipped with a function to notify the driver or mechanic in the unlikely event of an abnormality, as well as a fail-safe function and backup function that continues control to minimize the impact on engine performance and functionality.
It also has the function of memorizing the vehicle’s condition when a breakdown occurs.
As shown in FIG. 34, the ECU employs a digital control system that is composed of a CPU that performs arithmetic processing, RAM and ROM that constitute data storage units, and a data input/output circuit unit.
Figure 34: ECU
Injection volume control
Injection amount control is a replacement for the governor function used in injection pumps, and basically controls the injector to optimize the injection amount based on signals of engine speed and throttle opening.
Injection pressure control (common rail pressure control)
Injection pressure control involves controlling the fuel pressure in the common rail. The ECU performs pressure feedback control so that the value of the common rail pressure sensor is equal to the target value calculated from the engine speed and injection amount, and controls the pressure in the common rail by adjusting the discharge volume of the supply pump.
Injection timing control
The injection timing control replaces the timer function used in injection pumps, and basically controls the injector to optimize the timing based on the engine speed and injection amount.
Injection rate control (split injection control)
Higher injection pressure and faster injector response have made it possible to freely select injection timing, so that the fuel required in one cycle can be injected before and after the main injection depending on the operating conditions, as shown in Figure 35, making it possible to improve exhaust gas emissions and reduce combustion noise.
Figure 35: Split injection
From FIG. 35, the outline of each injection is as follows:
- Pilot injection
By injecting at a timing significantly advanced from the main injection and burning the injected fuel in a pre-mixed state with air, it is possible to suppress a sudden increase in combustion pressure, which contributes to reducing combustion noise and improving combustion stability. It also contributes to improving drivability and reducing PM.
- Pre-injection
By injecting it prior to the main injection, the ignition delay of the main injection is shortened, which has the effect of reducing NOx and combustion noise.
- After-injection
By injecting close to the main injection, it is possible to reduce the PM generated by the main injection, which activates diffusion combustion, while at the same time contributing to the activation of the catalyst by increasing the exhaust gas temperature and improving the efficiency of the exhaust gas aftertreatment device.
- Post injection
By injecting at a timing that is significantly delayed from the main injection, it is possible to increase the temperature of the exhaust gas and supply reducing components, thereby activating the catalyst and assisting the operation of the exhaust gas aftertreatment device.
Cylinder-by-cylinder injection amount correction control
In the cylinder-by-cylinder injection amount correction control, rotation fluctuations caused by variations in the combustion state of each cylinder are detected by a rotation sensor, and the injection amount between the cylinders is corrected to reduce the rotation fluctuations.