Common Rail injection system

Common Rail is a modern fuel injection system used mainly in diesel engines, but also in some gasoline engines (direct injection systems).


How does the common rail system work?

1. Common Rail:

   • This is a high-pressure fuel tank or pipe that supplies the injectors with a uniform and constant fuel pressure. The high pressure is achieved using a special high-pressure pump.

2. High pressure pump:

   • The pump compresses the fuel to a very high pressure (typically up to 2000-2500 bar or more in diesel engines). The fuel is then fed into a common tank.

3. Electronic injectors:

   • Fuel injection is carried out via electronically controlled injectors, which inject fuel into the combustion chamber at precisely the right time and in exactly the right amount.

4. Control system:

   • The engine control unit (ECU) regulates the pressure in the common rail and controls the opening and closing of the injectors to ensure optimal fuel consumption and power.
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The structure of the vehicle fuel system and its main components (the diagram shows a diesel engine common rail system):


Figure. Fuel tank and fuel pump.

• Fuel tank: A container designed to store fuel, where the fuel is protected against leakage and evaporation.
• Fuel pump (also known as the high-pressure pump, high-pressure fuel pump, or injection pump – external high-pressure pump):Transports fuel to the engine while maintaining stable pressure.
• Fuel supply pump (in-tank low-pressure electric pump): Supplies fuel from the tank to the fuel system at low pressure.
• Fuel filter: Removes contaminants, water, and particles from the fuel.
• Fuel injectors: Spray the fuel into the engine as a fine aerosol to ensure efficient mixing with air.
• Fuel pressure regulator: Ensures correct pressure and fuel flow within the system.
• Electronic Control Unit (ECU – Engine Control Unit): Controls fuel quantity and injection timing according to engine requirements. The ECU is the main control unit of the engine, responsible for managing and coordinating multiple engine systems, including the fuel system, air intake, ignition (in gasoline engines), and emissions control. It acts as a central computer, using data from sensors to control actuators and optimize engine performance. In gasoline engines: controls fuel injectors and ignition timing. In diesel engines: manages the EDC system.
• EDC Control Unit (Electronic Diesel Control): A dedicated control system specifically designed for diesel engines. It focuses mainly on the fuel injection process and other aspects of diesel engine performance.
• Engine speed sensors (crankshaft and camshaft sensors): Monitor engine rotational speed and position.
• Fuel temperature sensor: Measures the temperature of the fuel.
• Fuel pressure sensor: Monitors fuel pressure in the system.
• Fuel pressure control valve: Regulates fuel pressure within the system.
• Shut-off valve / stop valve / solenoid (fuel cut-off): A component typically found in diesel engines that controls fuel flow to the engine by allowing or stopping fuel delivery. It plays a key role in safely and effectively shutting down the engine when required. The shut-off valve is often part of the fuel injection pump system and works together with other control mechanisms.
• Fuel Delivery Module (FDM) / Fuel Pump Module: An integrated unit, usually located in the fuel tank, that combines the fuel pump and related components for fuel supply.
• Fuel pressure regulator: Maintains the required fuel pressure within the fuel system. 

Advantages of the common rail system:

1. Fuel efficiency:
   • Precise fuel metering and high pressure improve combustion efficiency, reducing fuel consumption.
2. Low emissions:
   • Uniform and complete combustion helps reduce harmful exhaust gases.
3. Smooth and quiet operation:
   • High pressure and precise atomization reduce noise and vibration typical of diesel engines.
4. Power and torque:
   • The system allows for better power delivery and engine responsiveness.

 

Problems with the common rail system:

1. Fuel quality sensitivity:
   • Poor quality fuel (e.g. contamination or water) can damage the high-pressure pump and injectors.
2. High cost of components:
   • System parts such as the high pressure pump and injectors are expensive and require precise maintenance.
3. Repair sensitivity:
   • Diagnostics and repair of a system require specialized equipment and knowledge.

 

Summary

Common Rail is a fuel system for modern diesel engines that allows for greater fuel efficiency, lower emissions and improved engine performance. The success of this system lies in its ability to deliver fuel accurately and at high pressure under all operating conditions.

If you are planning to repair a Common Rail system, you can find the necessary tools here!