Common Rail Fuel Injection System
1 - high pressure fuel pump; 2 - fuel filter; 3 - a fuel tank with a preliminary fuel filter and a booster fuel pump; 4 - ECU; 5 - control unit for glow plugs; 6 - storage battery; 7 - high pressure accumulator (rail); 8 - pressure sensor; 9 - pressure limiting valve; 10 - fuel temperature sensor; 11 - nozzle; 12 - glow plug; 13 - coolant temperature sensor; 14 - crankshaft speed sensor; 15 - camshaft position sensor; 16 - temperature sensor of air entering the engine; 17 - boost pressure sensor (BPS); 18 - air flow meter; 19 - turbocharger; 20 - EGR positioner; 21 – a combination of devices; 22 - accelerator pedal position sensor; 23 - brake contacts; 24 - switch on the clutch pedal; 25 - speed sensor; 26 - vehicle speed control unit; 27 - air conditioning compressor; 28 - air conditioning control unit; 29 - diagnostic tool with connector
1 - fuel tank; 2 - preliminary fuel filter; 3 - booster fuel pump; 4 - fuel filter; 5 - low pressure fuel lines; 6 - high pressure fuel pump; 7 - high pressure fuel lines; 8 - high pressure accumulator (rail); 9 - nozzles with electromagnetic control, screwed into the cylinder head; 10 - return fuel line; 11 - ECU
Common rail fuel system includes low pressure fuel stage and high pressure fuel stage and ECU (11).
Low pressure fuel supply
Common rail low pressure fuel delivery includes:
- fuel tank with pre-filter;
- booster fuel pump;
- fuel filter;
- low pressure fuel lines.
Priming fuel pump
The electric fuel priming pump with fuel pre-filter continuously supplies a certain amount of fuel from the fuel tank to the high pressure fuel pump. The pump not only supplies fuel, but, within the limits of the safety system, must stop the fuel supply in the event of an accident, i.e. with the ignition on and the engine stopped.
The fuel pump consists of three main elements:
- pump;
- electric motor;
- covers.
Fuel filter
Insufficient fuel cleaning can result in damage to high pressure fuel pump assemblies, delivery valves, and injector nozzles. The fuel filter cleans the fuel before it enters the high pressure fuel pump and thus prevents premature wear in the sensitive parts of the pump.
Diesel fuel may contain water or in a bound form (emulsion), or in free form (e.g. condensation of water vapor when the temperature changes). If water gets into the injection system, this can lead to corrosion of the injection system elements, so a warning alarm is installed that turns on a warning lamp in the instrument cluster if it is necessary to drain the water from the fuel filter.
High pressure fuel supply
Common rail high pressure fuel delivery includes:
- high pressure fuel pump with pressure control valve;
- high pressure fuel lines;
- high pressure accumulator (rail) with pressure sensor, pressure limiter, flow limiter, nozzles;
- return fuel line.
High pressure fuel pump
High pressure fuel pump (schematic representation of a longitudinal section)
1 - drive shaft; 2 - eccentric cam; 3 – pump element with pump plunger; 4 - compartment of the pump element; 5 - suction valve; 6 - exhaust valve; 7 - seal; 8 - high pressure connection to the pressure accumulator; 9 - ball valve; 10 – fuel return; 11 - fuel supply from the booster fuel pump; 12 - safety valve with a throttle hole; 13 - fuel supply under low pressure to the pump element
High pressure fuel pump (schematic representation of a cross section)
1 - drive shaft; 2 - eccentric cam; 3 – pump element with pump plunger; 4 - suction valve; 5 - exhaust valve; 6 - input
The high pressure fuel pump delivers fuel at a pressure of 1350 bar through the high pressure fuel lines to the high pressure accumulator.
The high pressure fuel pump is located at the boundary between the low and high fuel pressure stages. Under all operating conditions, the service life of the fuel pump corresponds to the service life of the vehicle.
The fuel pump is lubricated with diesel fuel. The fuel is compressed by three pistons set radially at an angle of 120°to each other. The pump supplies three portions of fuel per revolution of the crankshaft. For a diesel engine with a displacement of 2.0 liters, operating at a rated speed of the crankshaft and a generated pressure of 1350 bar, a power of 3.8 kW is required to drive the pump, taking into account mechanical efficiency. approximately 90%.
Pump operation
The booster fuel pump delivers fuel through a filter with a water separator to the inlet and safety valve of the high pressure fuel pump. Fuel through the throttle opening of the safety valve lubricates the moving parts of the pump, and also cools it. A drive shaft with eccentric cams moves the three pump plungers up and down according to the shape of the cam. As soon as the supply pressure exceeds the opening pressure of the relief valve (0.5... 1.5 bar), the booster pump forces the fuel through the high pressure fuel pump inlet valve into the pump element compartment, the piston of which moves down (intake stroke). The inlet valve closes when the pump piston passes through BDC and since fuel cannot flow out of the pump element chamber, it is compressed regardless of supply pressure.
The increasing pressure opens the exhaust valve and as soon as a pressure equal to the pressure in the accumulator is reached, the compressed fuel enters the high pressure circuit. Pump piston continues to deliver fuel until it reaches TDC (injection stroke), after which the pressure decreases and the exhaust valve closes. The fuel remaining in the pump element compartment expands and the pump piston moves down. As soon as the pressure in the pumping element compartment decreases below the pressure created by the booster pump, the inlet valve opens and the process is repeated.
Since the pump capacity exceeds the fuel consumption of the engine, the excess high pressure fuel is returned to the fuel tank through the pressure control valve. This leads to unnecessary heating of the fuel and a decrease in overall efficiency.
High pressure accumulator (rail)
1 – high pressure accumulator; 2 - input from the high pressure fuel pump; 3 - pressure sensor in the accumulator; 4 - fuel return to the fuel tank; 5 - to the fuel injector
The pressure generated by the high pressure fuel pump is distributed through the accumulator and fuel lines to the injector. At the same time, due to the volume of fuel in the accumulator, the fuel pressure fluctuations created by the high pressure fuel pump and the opening injectors are reduced. The compressibility of the fuel as a consequence of the high pressure is used to achieve an accumulator effect. Fuel pressure is measured by a gauge and maintained at the desired level by a pressure control valve.
High pressure fuel lines
High pressure fuel lines are designed to transfer fuel from the high pressure accumulator to the injectors and must withstand the high frequency pressure fluctuations that occur during engine operation. The fuel lines are made of steel and have an outer diameter of 6 mm and an inner diameter of 2.4 mm. All high pressure fuel lines must be the same length. The difference in distance between the battery and each fuel injector is compensated for by bending the fuel lines.
Pressure meter
1 - electrical contacts; 2 - printed circuit board and circuit circuit; 3 – diaphragm with sensor element; 4 - high pressure connection; 5 - sensor thread
The pressure sensor transmits a signal to the ECU that corresponds to the actual pressure in the pressure accumulator.
The pressure sensor consists of the following elements:
- integrated sensor element welded to the body;
- printed circuit board with electric circuit;
- sensor housing with electrical connector.
Pressurized fuel through the hole acts on the sensor diaphragm, on which the sensor element is installed (semiconductor device) converting pressure into an electrical signal. Through the connector pins and the electrical circuit, the generated and amplified signal is transmitted to the ECU. The sensor works as follows: when the shape of the diaphragm changes, the electrical resistance of the layers glued to the diaphragm changes. A change in pressure of 1500 bar results in a change in the shape of the diaphragm by 1 mm.
Depending on the applied pressure, the output voltage of the sensor varies from 0 to 70 mV and, after amplification, is 0.5–4.5 V. Accurate measurement of pressure in the accumulator is necessary for the correct functioning of the fuel injection system. In the operating range, the measuring accuracy must be within±2%. If the pressure sensor fails, the pressure control valve switches to «diaphragm» and injection system, using a spare (soft) function, takes a predetermined pressure value.
Pressure limiting valve
The pressure relief valve performs the same function as the overpressure valve. In case of overpressure, the valve, by opening, limits the pressure in the accumulator. The opening pressure of the pressure limiting valve is 1500 bar.
The pressure relief valve is a mechanical device that includes the following elements:
- housing with external thread for screwing into the pressure accumulator;
- connection of the fuel return pipe to the fuel tank;
- movable plunger;
- spring.
Nozzles
Nozzle: A - nozzle closed (immobility); B - nozzle open (fuel injection)
1 - fuel return 2 - electrical connector 3 - starting element (solenoid valve) 4 - fuel inlet from pressure accumulator 5 - ball valve 6 - leakage hole 7 - supply hole 8 - valve control compartment 9 - valve control plunger 10 - fuel supply channel to the atomizer 11 - atomizer needle
The nozzle ensures that the right amount of fuel is supplied to the combustion chamber. At exactly the right moment, the ECU sends an excitation signal to the injector solenoid, which means the start of fuel delivery. The amount of injected fuel is determined by the opening period of the nozzle and the pressure in the system. Fuel returning from the pressure control valve and low pressure stage is fed into the manifold along with the fuel that lubricated the high pressure fuel pump.
The nozzle consists of the following units:
- atomizer;
- hydraulic system;
- solenoid valve.
Fuel from the high pressure threaded connection is fed through the channel to the atomizer and through the supply hole to the valve control compartment. The valve control compartment is connected to the fuel return line through a leak hole connected to the solenoid valve. When closing the leak hole, the hydraulic force applied to the valve control plunger exceeds the force from pressure on the conical end of the spray needle. As a result, the atomizer needle goes down and hermetically shuts off the high pressure fuel supply to the combustion chamber.
Opening the injector solenoid valve opens a leak hole, causing the pressure in the valve control section to decrease, which also reduces the hydraulic pressure on the plunger. As soon as the hydraulic force becomes lower than the force from the pressure on the conical end of the atomizer needle, the atomizer needle opens and fuel is injected into the combustion chamber. This indirect control of the atomizer needle using a hydraulic force boost system is used because the forces required to quickly open the needle cannot be generated directly by the solenoid valve. The so-called amount of pilot fuel required to open the atomizer needle is supplied in addition to the amount of fuel that actually needs to be injected into the cylinder, and it is fed into the fuel return line through a leak hole connected to the solenoid valve.
In addition to the amount of fuel to control, there is also a loss of fuel in the valve lifter guides and atomizer needle.
The action of the injector during engine operation and pressurization by the high pressure fuel pump is divided into the following four stages:
- nozzle closed (with high pressure application);
- nozzle opens (start of fuel injection);
- the nozzle is fully open;
- nozzle closing (end of fuel injection).
When the engine is off and there is no pressure in the pressure accumulator, the atomizer spring closes the nozzle.
Nozzle closed
When stationary, the injector solenoid valve is not energized and is therefore closed. The leak hole is closed and the valve spring presses the ball against the seat of the leak hole. The high pressure from the pressure accumulator increases in the valve control chamber and is simultaneously present in the volume of the needle chamber of the atomizer. The pressure from the pressure accumulator applied at the end face of the control plunger, together with the spring force of the atomizer needle, holds the needle in the closed position against the opening forces applied in the pressure stage.
Nozzle opens
The nozzle is in a stationary position. The solenoid valve is energized by a current that allows the valve to open quickly. Immediately, the large current supplied to the solenoid is reduced to a current sufficient to hold the solenoid valve in the open position. When the leak hole opens, fuel flows out of the valve control box into the cavity above the valve and from there through the return line to the fuel tank.
The force generated by the solenoid exceeds the force of the spring and the leak hole opens, resulting in a decrease in pressure in the valve control chamber, which also reduces the hydraulic pressure on the plunger. As soon as the hydraulic force becomes lower than the force from the pressure on the conical end of the atomizer needle, the atomizer needle opens and fuel is injected into the combustion chamber.
The opening speed of the atomizer needle is determined by the difference in flow velocity through the leak hole and the feed hole. The control plunger reaches the top position where there is a cushion of fuel formed by fuel flow between the fuel leakage and delivery ports. In this position, the injector nozzle is fully open and fuel is injected into the combustion chamber at a pressure equal to the pressure in the pressure accumulator.
Nozzle closure
After the solenoid valve is de-energized, the valve spring moves the armature down and the ball closes the leak hole. The anchor consists of two parts. However, although the armature plate is controlled by the shoulder when moving down, it may «spring back» with a return spring so that there are no forces acting downward on the armature and ball.
When closing the leak hole, the hydraulic force applied to the valve control plunger exceeds the force from pressure on the conical end of the spray needle. As a result, the atomizer needle goes down and hermetically shuts off the high pressure fuel supply to the combustion chamber. The speed of the atomizer needle is determined by the flow through the feed port.