Characteristics of conventional injection system
In conventional fuel injection systems with distributor and direct injection pump, the injection system includes only the main injection phase - without pilot and post injection. On a solenoid-controlled fuel injection pump, actions take place as the fuel injection control phase approaches. In conventional systems, the operations of pressurizing and adjusting the amount of injected fuel are connected to each other through the interaction of the cam and the fuel pump plunger. This has the following effect on fuel injection performance:
- injection pressure increases along with an increase in speed and the amount of air entering the engine;
- during the actual injection process, the injection pressure increases and then decreases to the final injection pressure.
As a result, the following consequences take place:
- less fuel is injected at a lower pressure than the amount of air entering the engine;
- the peak pressure is more than twice the average injection pressure;
- in accordance with the requirements for efficient combustion, the discharge rate curve is actually triangular.
The peak pressure is decisive for the mechanical loading of the fuel pump components and its drive. On conventional fuel injection systems, this is decisive for the quality of mixture formation in the combustion chamber.
Discharge rate curve for conventional fuel injection system
Characteristics of the injection system «Common Rail» (common rail system)
Compared to conventional injection systems, the following requirements must be met in order to obtain an ideal injection characteristic:
- for each operating mode of the engine, it is necessary to separate the pressure generating unit and the fuel injection unit;
- at the start of injection, the amount of fuel injected should be as low as possible (those. the inertia between the initial moment of injection and the start of ignition must be taken into account).
These requirements are fulfilled by the injection system «Common Rail» - common rail system - with its control and main injection phases.
«Common Rail» is a block system and, in fact, the following nodes are responsible for the fuel injection characteristic:
- solenoid-controlled injectors screwed into the cylinder head;
- high pressure accumulator (rail);
- high pressure fuel pump.
Unloading speed curve for fuel injection system «Common Rail»
The following nodes are also required for the system to function:
- electronic control device ECU;
- crankshaft speed sensor;
- camshaft position sensor (phase sensor).
In passenger cars, a high-pressure radial piston pump is used to generate pressure, and pressure is generated independently of the fuel injection process. The performance of the pump is directly proportional to the engine speed. Compared to conventional injection systems, the actual injection of fuel is homogeneous, i.e. in common rail, the high pressure fuel pump is not only smaller, but its drive is less subject to peak loads.
The fuel injectors are connected to the rail by short pipes and essentially include the injectors and the ECU controlled solenoid valve. After the voltage supply to the solenoid valve is cut off, fuel injection stops. Assuming constant pressure, the amount of fuel injected is directly proportional to the length of time the solenoid valve is open. This process is completely independent of engine speed and pump speed (fuel injection according to time).
The high speed switching of the solenoid is achieved by using high voltage and current. This means that a specially designed solenoid valve must be used to secure the platform at the moment of opening. Injection timing is determined by the EDC control system (Electronic Diesel Control - Electronic diesel engine control), which uses a crankshaft speed sensor and a camshaft position sensor to phase sense the duty cycle.