Highway Diesel Services - Understanding Common Rail
Highway Diesel Services -
Understanding Common Rail
Please see our warning when working on common rail diesels or looking for leaks.
Diesel is now more powerful, responsive and environmentally friendly than ever.
Thanks largely to the development of the diesel 'common rail' system, the days of noisy, unresponsive tractor style engines are long gone. Instead of only being fitted to trucks, tractors and 4WD, diesel engines now find applications in small city vehicles such as the Volkswagen Golf, Ford Focus and Holden Astra through to high end luxury vehicles such as the Mercedes Benz E class, BMW X5, and Volvo XC70. In addition, common rail diesel is the preferred choice for commercial vehicles such as the Mercedes Benz Sprinter and Vito, Iveco Daily and others because of the low down torque and improved fuel economy.
So what is common rail, how does it work, what are the advantages, what types of Australian vehicle's is it fitted to and what does it mean for you? This website will endeavour to answer these questions in a basic and easy to understand way. It is by no means a detailed analysis of this complicated diesel injection system.
What is Common Rail
As it's name suggests there is a common rail that acts as an accumulator to feed fuel to all of the injectors. Filtered fuel is supplied to the rail by means of a high pressure pump.
1 = High Pressure Pump
2 = High Pressure Rail
3 = Electronic Control Unit
4 = Injector
Unlike various other diesel fuel injection systems where the injector only receives fuel pressure once the injection pump has metered an amount of fuel for that cylinder, the common rail system stores a constant amount of high pressure fuel at the rail and right up to the injectors.
The main advantage of this system is to vary injection pressure and timing over a broad scale leading to many benefits in engine design, improved driveability and greatly reducing it's environmental impact.
Therefore the heart of the common rail system is the injectors and electronic software and hardware.
How does Common Rail work?
Filtered diesel fuel is delivered in a low pressure condition (between 6-7 bar or 85-100psi) from the diesel fuel tank to the high pressure pump. This can be done by either an electric fuel pump, gear pump or both.
The high pressure pump uses mechanical plungers to increase this fuel pressure to pressures up to and including 2000 bar (over 29,000psi) and delivers this to the common rail.
This pressure is regulated and monitored by the electronic diesel control by means of a rail pressure sensor. Due to the safety concerns of a diesel leak at 2000 bar, there is also the facility for a pressure-relief valve or pressure maintaining valve.
A common rail injector comes in 2 basic types: the solenoid-valve injector and the piezo-inline injector.
You have probably used Piezo crystals before on your BBQ or gas stove without even realising. When the button is pressed on your BBQ lighter the movement of the Piezo crystals create a high voltage spark. The reverse is the case when these crystals are installed in a common rail injector. A voltage is sent to the injector, which creates movement in the Piezo crystals and allows the delivery of fuel.
The Piezo injector allows the implementation of very short and rapid fuel delivery characteristics. By avoiding mechanical forces acting on the needle, such as a pushrod, it allows consistent and accurate delivery of diesel with up to 7 injection cycles per engine combustion cycle.
All of these components are interconnected by means of the Electronic Diesel Control Unit. It gathers a seemingly endless source of information by means of input sensors such as:
Throttle Position (including idle and kick down switch)
Engine Speed
Vehicle Speed
Top Dead Centre engine reference
Start of Injection
Rail Pressure
Charge-air pressure
Oil pressure
Charge-air temperature
Engine temperature
Fuel temperature
Air Mass Meter
Calculating all these inputs against the manufactures mapping it can control actuators such as:
Injectors
High Pressure Pump metering unit
Exhaust Gas Recirculation
Boost Pressure actuators (on variable vane turbos)
All of these capabilities allows the common rail diesel injection system to be one of the fastest growing segments in Australian vehicles, and the choice of most manufactures.
WHAT ARE THE ADVANTAGES COMMON RAIL?
Increased Power & Torque
Improved Fuel Economy
Environmentally friendly - cleaner exhaust emissions
Reduced engine noise
Improved driveability
