5.2.3. Characteristics of system of injection
Characteristics of usual system of injection
In usual systems of injection of fuel with the distributing device and the pump of direct injection the system of injection includes only the main phase of injection – without control and late injection. On the distributive fuel pump with electromagnetic management of action happen to approach to a control phase of injection of fuel. In usual systems of operation of creation of pressure and adjustment of amount of injectable fuel are connected with each other due to interaction of a cam and a plunger of the fuel pump. It exerts the following impact on characteristics of injection of fuel:
– pressure of injection increases together with increase in speed and amount of the air coming to the engine;
– during the actual process of injection pressure of injection increases and then decreases up to the final pressure of injection.
As a result of it the following consequences take place:
– the smaller amount of fuel with lower pressure, than amount of the air coming to the engine is injected;
– peak pressure makes more than doubled average value of pressure of injection;
– according to requirements for effective combustion the unloading speed curve actually is triangular.
Peak pressure is decisive for mechanical loading of elements of the fuel pump and its drive. On usual systems of injection of fuel it is decisive for quality of formation of mix in the combustion chamber.
|
|
|
Characteristics of system of injection of "Common Rail" (accumulator fuel system)
In comparison with usual systems of injection for obtaining the ideal characteristic of injection it is necessary to fulfill the following requirements:
– each operational mode of the engine requires division of the knot creating pressure and fuel injection knot;
– at the initial moment of injection the amount of injectable fuel has to be low as far as it is possible (i.e. the lag effect between the initial moment of injection and the beginning of ignition has to be considered).
These requirements are fulfilled by system of injection "Common Rail" – accumulator fuel system – with its control and main phases of injection.
"Common Rail" represents block system and, in fact, the following knots are responsible for the characteristic of injection of fuel:
– the nozzles with electromagnetic management screwed in a head of cylinders;
– accumulator of the high pressure (rail);
– fuel pump of high pressure.
|
|
|
Also the following knots are necessary for functioning of system:
– electronic control ECU device;
– sensor of frequency of rotation of a bent shaft;
– sensor of position of the camshaft (phase sensor).
In cars for creation of pressure the radial piston pump of high pressure is used, at the same time pressure is created irrespective of fuel injection process. Productivity of the pump is directly proportional to the frequency of rotation of a bent shaft of the engine. In comparison with usual systems of injection the actual forcing of fuel is uniform, i.e. in Common Rail the fuel pump of high pressure not only has the smaller sizes, but also its drive is to a lesser extent subject to peak loadings.
Fuel nozzles are connected to a lath short pipelines and, in essence, include sprays and the solenoidal valve operated by ECU. After interruption in supply of tension on the solenoidal valve the injection of fuel stops. Assuming the constant pressure, amount of injectable fuel in direct ratio to an interval of time during which the solenoidal valve is open. This process is completely independent of the frequency of rotation of a bent shaft of the engine and frequency of rotation of the pump (injection of fuel depending on time).
High-speed switching of the solenoid is reached due to use of high voltage and current. It means that it is necessary to use specially developed solenoidal valve providing the platform at the time of opening. The moment of injection is defined by EDC control system (Electronic Diesel Control – Electronic control the diesel engine) which uses the sensor of frequency of rotation of a bent shaft and the sensor of position of the camshaft for phase definition of a running cycle.