Common Rail System Of Fuel Injection

The common rail system of fuel injection was used in marine diesel engines during the early and mid-19th century. These systems where well known in a particular type of engine famously called the "Doxford" engines or opposed piston engines. These engines are hard to find these days as they have been replaced by more efficient engines.

The common rail system of fuel injection is a simple and efficient system. The recent trend in the automobile industry is the usage of common rail direct fuel injection systems in some modern vehicles. Some of the engines in the automobile sector introduce this common rail system of injection not only in diesel engines, but also in petrol/gasoline engines. Lets discuss the operation of a marine diesel engine with a common rail fuel injection system.

The common rail system has one or more common high pressure, multiple plunger, fuel pump/pumps. The fuel is discharged into a manifold preferably called a "rail" which is maintained at a very high pressure. From this common rail, fuel is supplied to all the fuel injectors in the various cylinder units. Between the rail and the injector or injectors for a particular cylinder is a "timing valve" which determines the timing and extent of fuel delivery. Spill valves are connected to the manifold or rail to release excess pressure and accumulator bottles are included to dampen out pump pressure pulses. The injectors in a common rail system are often referred to as fuel valves.

The timing valve in the common rail system is operated by a cam and a lever as shown in the figure below. When the timing valve is lifted by the cam, the lever also is lifted, thus ensuring that the valve in the timing valve block allows the high pressure fuel oil to reach the fuel injectors. Also, the timing valve operating lever is fixed to a sliding rod. This sliding rod is in turn fixed with the manoeuvring lever, which in turn governs the amount (quantity) of the fuel to be injected into each cylinder unit, meeting the load demands on the engine.

As mentioned earlier, the system has a two or more common fuel pumps, which are driven by the main engine itself. They deliver high pressure fuel oil into one common line, thus called a "common rail" fuel injection system. This high pressure oil in the common rail tends to experience a pressure surge (shock), when the engine load changes suddenly or continuously over a period of time. To dampen these pressure surges, an accumulator bottle is provided on the high pressure line (common rail). It is specially marked in dark blue in the diagram below.

Also the volume capacity of the high pressure common rail is sufficient to suit sudden load changes. When compared with the individual pump engines, these common rail engines do not require a great deal of concentration at the drive shaft. As the fuel pumps supply fuel oil at a high pressure, it is stored against an accumulator pressure of around 400 to 550 atmospheres. This pressure is sufficiently maintained by the compressed air operated "spill valve." If the pressure in the common rail increases suddenly, the spill valve will open up and drain the fuel oil to the service tank or the fuel oil drain tank as per the design. Then the timing valve controls the timing of start/stop of fuel injection.

For the initial start-up of the system, the fuel priming pump is provided, which can build up a pressure of about 140 atmospheres. This priming pump is usually pneumatically driven.