While high-pressure injection systems such as radial piston pumps and unit injection systems have greatly improved the precision and efficiency of diesel injection, they still suffer a fundamental limitation. The actual injection process into each engine cylinder is limited to a single event for each power stroke. That is, for each power stroke, the injector opens (at the optimum timing) and stays open until the ‘correct’ fuel charge for the current load on the engine has been delivered. It then closes until the next power stroke.
To further improve the emission control and fuel efficiency of a CI engine it becomes desirable to spread the injection process over longer periods of the power stroke. This means being able to open and shut each injector several times during each delivery cycle. Modern Common Rail (CR) injection systems allow this.
How Does it Work?The “Common Rail” part of the system is simply a pipe full of very high-pressure diesel fuel. This is known as a fuel manifold or accumulator, or fuel ‘rail’ in automotive jargon. It has a relatively large diameter steel pipe (around 20mm in diameter) running along the cylinder head of the engine, quite close to the actual injectors. A common high-pressure pump keeps the rail continuously supplied with fuel at pressures up to 2000 bar. A short high-pressure pipe then supplies each injector. As far as each injector is concerned, the rail represents an ‘infinite’ source of high-pressure fuel. It simply needs to open and shut at the appropriates times to deliver this fuel to the combustion chamber.
Bosch released the first CR system in 1997, capable of nozzle pressures up to 1350 bar. This was increased to 1600 bar with the second generation system in 2001. However, the real revolution arrived with the third generation systems in 2003.