5.2.8. Fuel Common Rail system
System of injection of Common Rail fuel
The fuel Common Rail system includes a step of supply of fuel under low pressure and a step of supply of fuel under the high pressure and ECU (11).
Supply of fuel under low pressure
Supply of fuel under the low pressure of the Common Rail system includes:
– the fuel tank with the preliminary fuel filter;
– the pumping-up fuel pump;
– fuel filter;
– fuel-supply lines of low pressure.
The pumping-up fuel pump
The electric pumping-up fuel pump with the preliminary fuel filter continuously gives a certain amount of fuel from the fuel tank to the fuel pump of high pressure. The pump not only gives fuel, but within work of a security system has to stop supply of fuel in case of accident, i.e. at the included ignition and the stopped engine.
The fuel pump consists of three basic elements:
– electric motor;
Insufficient purification of fuel can lead to damage of knots of the fuel pump of high pressure, delivery valves and sprays of nozzles. The fuel filter purifies fuel before its receipt in the fuel pump of high pressure and thus prevents premature wear in sensitive knots of the pump.
Diesel fuel may contain water or in the connected form (emulsion), or in a free form (for example, condensation of vapors of water at change of temperature). If water gets to system of injection, it can lead to corrosion of elements of system of injection therefore the precautionary alarm system which includes a control lamp in combinations of devices is installed if it is necessary to merge water from the fuel filter.
Supply of fuel under high pressure
Supply of fuel under the high pressure of the Common Rail system includes:
– the fuel pump of high pressure with the valve of adjustment of pressure;
– fuel-supply lines of high pressure;
– the accumulator of the high pressure (rail) with pressure sensor, pressure limiter, the stream limiter, nozzles;
– returnable fuel-supply line.
Fuel pump of high pressure
Fuel pump of high pressure (schematic image of longitudinal section)
Fuel pump of high pressure (schematic image of cross section)
1 – power shaft
4 – the soaking-up valve
The fuel pump of high pressure through fuel-supply lines of high pressure gives fuel under pressure of 1350 bars to the accumulator of high pressure.
The fuel pump of high pressure is located on border of steps of low and high pressure of fuel. At all operational modes service life of the fuel pump corresponds to car service life.
The fuel pump is greased with diesel fuel. Fuel compresses three pistons installed radially at an angle 120 ° to each other. The pump gives three portions of fuel for one turn of a bent shaft. The power of 3,8 kW taking into account a mechanical k.p.d is necessary for the diesel engine with a working volume of 2,0 l working with the nominal frequency of rotation of a bent shaft and with a created pressure of 1350 bars for the drive of the pump. about 90%.
Functioning of the pump
The pumping-up fuel pump gives fuel via the filter with a water separator to an entrance and the safety valve of the fuel pump of high pressure. Fuel through a throttle opening of the safety valve greases mobile elements of the pump, and also cools it. The power shaft with eccentric cams moves three plungers of the pump according to a cam form up and down. As soon as pressure of giving exceeds pressure of opening of the safety valve (0,5... 1,5 bars), the pumping-up pump forces fuel to pass through the inlet valve of the fuel pump of high pressure in a compartment of a pump element which piston moves down (an admission step). The inlet valve is closed when the piston of the pump passes through NMT and as fuel cannot flow out from a compartment of a pump element, it compresses irrespective of giving pressure.
The increasing pressure opens the final valve and as soon as pressure equal to pressure in the accumulator is reached, the compressed fuel is included into a contour of high pressure. The piston of the pump continues to deliver fuel, does not reach yet VMT (the delivery course) then pressure decreases and the final valve is closed. The fuel remaining in a compartment of a pump element extends and the piston of the pump moves down. As soon as pressure in a compartment of a pump element decreases lower than pressure created by the pumping-up pump, the inlet valve opens and process repeats.
As productivity of the pump exceeds fuel consumption by the engine, excess fuel under high pressure via the valve of adjustment of pressure comes back to the fuel tank. It leads to unnecessary heating of fuel and decrease in the general efficiency.
Accumulator of the high pressure (rail)
1 – accumulator of high pressure
4 – return of fuel to the fuel tank
Pressure created by the fuel pump of high pressure extends via the accumulator and fuel-supply lines to a nozzle. At the same time, at the expense of fuel volume in the accumulator the fluctuations of pressure of fuel created by the fuel pump of high pressure and the opening nozzles decrease. Compressibility of fuel as a result of high pressure is used for achievement of effect of the accumulator. Pressure of fuel is measured by the sensor and supported at the required level by the valve of regulation of pressure.
Fuel-supply lines of high pressure
Fuel-supply lines of high pressure are intended for transfer of fuel from the accumulator of high pressure to nozzles and have to resist to the high-frequency fluctuations of pressure arising during the operation of the engine. Fuel-supply lines are made of steel and have the outer diameter of 6 mm and internal diameter of 2,4 mm. All fuel-supply lines of high pressure have to have identical length. The distance difference between the accumulator and each fuel nozzle is compensated for the account of bending of fuel-supply lines.
1 – electric contacts
4 – connection of high pressure
The sensor of pressure transmits a signal of ECU which corresponds to actual pressure in pressure accumulator.
The sensor of pressure consists of the following elements:
– the integrated element of the sensor welded on the case;
– the printed circuit board with an electric contour;
– sensor cases with the electric socket.
Fuel under pressure through an opening influences a sensor diaphragm on which the sensor element (the semiconductor device) transforming pressure to an electric signal is established. Through contacts of the socket both an electric chain the generated and strengthened signal is transmitted by ECU. The sensor works as follows: at change of a form of a diaphragm the electric resistance of the layers pasted to a diaphragm changes. Change of pressure upon 1500 bars leads to change of a form of a diaphragm on 1 mm.
Depending on the enclosed pressure the output tension of the sensor changes from 0 to 70 mV and after strengthening makes 0,5–4,5 Century. Exact measurement of pressure in the accumulator is necessary for the correct functioning of system of injection of fuel. In the working range measuring accuracy has to be in limits of ±2%. At pressure sensor exit out of operation the valve of adjustment of pressure passes into the diaphragm mode and system of injection, using spare (soft) function, accepts in advance set pressure size.
Valve of restriction of pressure
The valve of restriction of pressure performs the same function, as the valve of excessive pressure. In case of excessive pressure the valve opening limits pressure in the accumulator. Pressure of opening of the valve of restriction of pressure – 1500 bars.
The valve of restriction of pressure – the mechanical device including the following elements:
– the case with an external carving for a vvorachivaniye in pressure accumulator;
– connection of a tube of return of fuel to the fuel tank;
– mobile plunger;
Nozzle: A – the nozzle is closed (a motionless state); In – the nozzle is open (fuel injection)
The nozzle provides giving of the necessary amount of fuel in the combustion chamber. Transmits an excitement signal to the nozzle solenoid to precisely determined moment of ECU that means the beginning of supply of fuel. The amount of injectable fuel is defined by the period of opening of a spray and pressure in system. The fuel which is coming back from the valve of regulation of pressure and a step of low pressure moves in a collector together with fuel which carried out lubricant of the fuel pump of high pressure.
The nozzle consists of the following knots:
– hydraulic system;
– solenoidal valve.
Fuel from the threaded connection of high pressure via the channel moves to a spray and through a giving opening in a compartment of control of the valve. The compartment of control of the valve is connected to a returnable fuel-supply line through the leak opening connected to the solenoidal valve. When closing an opening of leak the hydraulic effort enclosed to a plunger of control of the valve exceeds effort from pressure upon a conical end face of a needle of a spray. As a result the needle of a spray falls down and hermetically blocks supply of fuel under high pressure in the combustion chamber.
When opening the solenoidal valve of a nozzle the leak opening opens that leads to pressure decrease in a compartment of control of the valve therefore the hydraulic pressure upon a plunger also decreases. As soon as the hydraulic effort becomes below effort from pressure upon a conical end face of a needle of a spray, the needle of a spray opens and fuel is injected into the combustion chamber. This indirect management of a spray needle with use of hydraulic system of increase in effort is applied because forces which are required for bystry opening of a needle cannot be generated by directly solenoidal valve. The so-called amount of fuel for management, necessary for opening of a needle of a spray, moves in addition to amount of fuel which needs to be entered actually into the cylinder, and it through the leak opening connected to the solenoidal valve moves in a returnable fuel-supply line.
In addition to amount of fuel for management there is also a loss of fuel in guides of a pusher of the valve and a needle of a spray.
Action of a nozzle during the operation of the engine and creation of pressure is subdivided by the fuel pump of high pressure into the following four stages:
– the nozzle is closed (with application of high pressure);
– the nozzle opens (the beginning of injection of fuel);
– the nozzle is open completely;
– closing of a nozzle (end of injection of fuel).
At the switched-off engine and lack of pressure in pressure accumulator the spring of a spray closes a nozzle.
The nozzle is closed
The solenoidal valve of a nozzle is not excited in a motionless state and therefore it is closed. The opening of leak is closed and the spring of the valve presses a ball to a leak opening nest. High pressure from the accumulator of pressure increases in a compartment of control of the valve and at the same time is present at spray needle compartment volume. Pressure from pressure accumulator enclosed in a face surface of a plunger of management together with force of a spring of a needle of a spray holds a needle in a closed position, counteracting against forces of opening applied in pressure stage.
The nozzle opens
The nozzle is in motionless situation. The solenoidal valve is excited by current which provides bystry opening of the valve. Immediately big current given to the solenoid decreases to the current sufficient for deduction of the solenoidal valve in open situation. When the leak opening opens, fuel follows from a compartment of control of the valve in the cavity located over the valve, and from there via the returnable pipeline in the fuel tank.
The effort created by the solenoid exceeds effort of a spring and the leak opening opens that leads to pressure decrease in a compartment of control of the valve therefore the hydraulic pressure upon a plunger also decreases. As soon as the hydraulic effort becomes below effort from pressure upon a conical end face of a needle of a spray, the needle of a spray opens and fuel is injected into the combustion chamber.
Speed of opening of a needle of a spray is defined by a difference in stream speed by an opening of leak and an opening of giving. The plunger of management reaches the top situation where there is a fuel pillow formed by a fuel stream between openings of leak and supply of fuel. In this situation the spray of a nozzle is completely open and fuel is injected into the combustion chamber under pressure equal to pressure in pressure accumulator.
Closing of a nozzle
After interruption in supply of tension to the solenoidal valve the valvate spring moves an anchor down and the ball closes a leak opening. The anchor consists of two parts. However, in spite of the fact that the plate of an anchor copes to a coat hanger when moving down, it can "otpruzhinit" with a returnable spring so that there will be no forces operating down on an anchor and a ball.
When closing an opening of leak the hydraulic effort enclosed to a plunger of control of the valve exceeds effort from pressure upon a conical end face of a needle of a spray. As a result the needle of a spray falls down and hermetically blocks supply of fuel under high pressure in the combustion chamber. Speed of the movement of a needle of a spray is defined by a stream by a giving opening.