The common rail method (accumulator injection system) is the injection method used in the Y 25 DT engine in the Omega-B.
In a common high-pressure rail, a high-pressure pump builds up a high pressure which is optimally adjusted for the respective engine operating state.
Small, electronically control valves, directly at the injectors, provide the cylinder with the quantity of fuel needed at the time in very precisely dosed amounts. In addition, a small amount can be injected in advance (pre-injection) using this system.
This provides the Y 25 DT engine with its smooth running.
| 1 | Injector (injection nozzle with electromagnetic valve) | 4 | High-pressure rail |
|---|---|---|---|
| 2 | Pressure line from high-pressure rail to the injector | 5 | Pressure line from high-pressure pump to the high-pressure rail |
| 3 | Oil leak line | 6 | High pressure pump |
Illustration L0008377 shows the installation position of the high-pressure rail (1), the high-pressure lines (2) from the high-pressure rail to the injectors, the high-pressure pump (3) and the high-pressure line (4) from the high-pressure pump to the high-pressure rail on the removed Y 25 DT engine.
The "common rail" system has a modular design. The following components are primarily responsible for the injection:
- Injectors in the cylinder head controlled by solenoid valves
- High-pressure rail
- High pressure pump.
The following components are also needed to operate the system:
- Electronic control unit
- Crankshaft sensor
- Camshaft sensor.
A radial piston pump is used as the high-pressure pump for creating the pressure.
The speed of the high-pressure pump is coupled to the engine speed via a timing chain.
Due to the almost uniform delivery, the high-pressure pump can be designed to be considerably more compact and with a smaller injection drive torque than in conventional injection systems.
The injectors each comprise one injector nozzle and one solenoid valve.
The engine control unit provides pulses for opening or closing the injectors.
The necessary, short switching times for time-appropriate injection can be achieved by a corresponding configuration of the actuation of the solenoid valves by the engine control unit with high voltages and currents.
Pre-Injection
With pre-injection, a small amount of diesel fuel is injected into the cylinder which improves combustion efficiency and has the following effects:
- Compression is slightly increased by a pre-reaction, i.e. partial combustion, thus shortening the ignition delay of the main injection.
- The combustion pressure increase and the combustion pressure peaks are reduced (soft combustion).
These effects reduce combustion noise, fuel consumption and harmful emissions.
Pre-injection contributes only indirectly to the engine torque build-up via the shortening of the ignition delay.
Depending on when the main injection starts, the specific fuel consumption may increase or decrease.
Illustration L0008275 shows the injection characteristic with common rail compared to conventional injection (illustration L0008274).
| 1 | = | Injection pressure (p) |
|---|---|---|
| 2 | = | Time (t) |
| I | = | Pre-injection |
| II | = | Main injection |
| A | = | Rail pressure (pr) |
Illustration L0008274 shows the injection characteristic of conventional injection compared to common rail (illustration L0008275).
| 1 | = | Injection pressure (p) |
|---|---|---|
| 2 | = | Time (t) |
| I | = | Start of delivery |
| II | = | Start of injection |
| A | = | Spray pressure (ps) |
| B | = | Mean injection pressure (pm) |
Main Injection
With the "common rail" accumulator injection system, the level of the injection pressure remains unchanged at 1350 bar for the entire injection process.
Post-Injection
Post-injection can be used for fuel mixing.
It follows the main injection during the exhaust stroke.
It introduces a precisely dosed quantity of fuel into the exhaust gas.
In contrast to pre-injection and the main injection, the fuel does not combust, but evaporates in the exhaust gas due to the residual heat.
This exhaust gas/fuel mixture is conducted to the exhaust system in the exhaust stroke via the outlet valves.
Some of the fuel is fed into the combustion again via the exhaust gas recirculation system and acts like a very early pre-injection.