Common rail system explained…

This post is written on demand from one of my fellow engineer, as he requested a proper explanation of what exactly the common rail system is it and, its impact and how it works. The following explanation is based on the main marine engines manufacturer’s released information about their development and research on this subject.

Electronically controlled low speed diesel engines, which do not require a camshaft, are quickly making their way into service, as evidenced by the large number of engines that have been ordered.
For the most part, the new marine main engines are based on a standard low-speed two-stroke marine diesel engine, except that instead of the camshaft and its gear-driven fuel injection pump and exhaust valve actuator pump, the engine is equipped with a common-rail system for fuel injection and exhaust valve actuation, and full electronic (computer) control of engine functions.

MAN ME common rail engine. Source and credit: Supreme_Engineer
Wartsila RT-Flex common rail engine. Source and credit: Brian Johannesen

The fuel oil is supplied to a common rail by the fuel supply pumps which are driven from the crankshaft by a gear system (Wartsila engines) or from hydraulically driven fuel booster pumps (MAN engines). The pumps deliver pressurized fuel oil to a collector which then supplies the common fuel rail and all parts of the high pressure fuel system are sheathed in order to prevent high pressure fuel from entering the engine room spaces.

Because classic cam-driven plunger-type fuel pumps compress the fuel by the upward motion of the plunger raised by the rotation of the cam, optimization of injection timing and injection pressure control was challenging. Common rail type engines, on the other hand, do not increase the pressure every time fuel is injected, but instead always maintain the internal pressure of the common rail and automatically open/close fuel valves for the desired duration for fuel injection, and also allow for the flexible setup for the filled and sustained fuel pressure in the common rail.

Wartsila common rail system
MAN common rail system

As the plunger rises, so does the injection pressure for cam-type engines, which means that low injection pressure can be achieved at low engine speeds and high injection pressure can be achieved at high engine speeds, and this is because the cam rotational speed and engine rotational speed are directly related to each other. Since the engine’s rotational speed has a direct effect on fuel injection pressure, it has proven challenging to achieve ideal tuning. It is possible to control the fuel injection pressure for a common rail type electronically-controlled engine without regard to the engine’s rotational speed or the engine’s load. On Wartsila engines, fuel pumps are driven at a frequency around 7-8 times as high as the engine’s rotational speed, so the rate at which fuel is discharged from the fuel supply pumps to the common rail is regulated by rail pressure, while the timing of fuel pumps discharge is not. These engines, on the other hand, have only few fuel pumps, which is less than the number of pumps in a standard cam-type engine. On MAN engines the oil flow pushes the hydraulic piston which is connected to the fuel injection plunger. After the injection has finished, the plunger and piston are returned to their starting position by connecting the hydraulic piston to a drain and driving the injection plunger back by means of the pressure in the fuel supply. The fuel oil pressure booster is then filled and ready for the next injection.

To achieve a quick cut-off in fuel injection for cam-type engines, however, the plunger chamber’s internal pressure had to be released at the end of injection. This resulted in a loss of energy and a decrease in pump efficiency. In addition, countermeasures were needed to prevent the cavitation erosions and pulses in the fuel oil return lines caused by this loss of energy. Electronic control ensures greater efficiency while requiring no countermeasures against cavitation or pulses in the fuel return lines for the fuel pumps used in common rail systems.

Controls of injection timing, total injection amount in one stroke, and fuel injection quantity per unit time (hence referred to as injection rate) are optimized for the common rail engines in accordance with the engine speed and load. A rail valve (high-speed electromagnetic valve) is used to switch the servo oil path and drive the control valve in ICU (Injection Control Unit) according to the command from WECS (Wärtsilä Engine Control System) or FIVA valve (Fuel Injection Valve Actuator) according to the command from ECS (Engine Control System) for the fuel injection controls except for the injection rate. The amount of fuel injected into the engine can be controlled with a wide range of flexibility, regardless of the engine’s speed or load.

Warstila ICU (Injection control unit)
MAN FIVA valve (Fuel Injection and Valve Actuation)

Large marine engines employ heavy fuel oil, which has a wide range of properties including viscosity, heating value, and contaminants, making it impossible to use directly as servo oil. Lubricant oil with stable qualities when pressurized is used in the ICU’s or FIVA’s, which also has a fail-safe function in case of high-pressure pipes or fuel valves malfunctioning. The ICU and FIVA control the fuel injection on the engine. As a result of this innovation, large marine engines with stringent dependability requirements can now use common rail fuel injection systems.

For the W-X engines, the successors of the RT-flex engines, some of the ICU functions have been relocated to the fuel valves in order to improve the delay in fuel injection.

During low output operation, the fuel valve will open for a brief period of time due to the reduced fuel injection quantity. With conventional engines, since a fuel pump was installed in each cylinder, it was unable to manage the individual fuel valves separately, making it difficult to achieve a good atomization of the fuel oil being injected.

To achieve a good atomization with a very low output, an electronically controlled engine can lower the number of operational fuel injection valves from the full number of installed fuel valves, this resulting in a more stable combustion. The injecting fuel valves of an electronically controlled engine can also be used in a sequence after a predetermined interval to maintain the homogeneity of thermal load in the combustion chamber.

Wartsila schematic chart of fuel injection. Source and credit: Wartsila

Picture above depicts the injection process sequentially. This is a means of controlling the various fuel valves in a single cylinder to inject at different times. Combining this with the previously described fuel injection pressure control allows for a more exact control of heat release rate.

In general, for diesel engines, it is difficult to simultaneously lower fuel consumption and NOx emissions because of the trade-off concerns. The increased thermal efficiency will be achievable with the tuning of combustion to complete while the volume of combustion chamber is small when the piston is near the Top Dead Center (TDC) for reducing the fuel consumption rate; however, the higher operating gas temperature in the combustion chamber will increase NOx. In part, this is because NOx, which is created by N2 and O2 reactions in the atmosphere, has a temperature-dependent producing speed. For example, at a flame temperature of 2 400 K, the quantity of NOx generated is 10 times greater than that generated at a temperature of 2 200 K if combustion lasts around 10 ms.

In order to reduce fuel consumption and NOx emissions, it is necessary to achieve good combustion with an impeded rise in combustion temperature by elaborated control. In order to achieve this form of combustion, the modern engines use this excellent common rail electronically controlled systems, where the sequential fuel injection controls the amount of heat release per unit time (hereinafter called heat release rate) with the fuel injection rate control. The diesel engine adopts a form of combustion where the self-ignition of fuel takes place by fuel injection in the high temperature air filled in the cylinder and compressed by the piston. The high heat release rate will contribute to the increased flame temperature, and increase the generated quantity of NOx as mentioned above. The higher heat release rate in the early stage of combustion is explained by the pre-mixing combustion due to the ignition delay, therefore the reduction in injection quantity in the early stage of fuel injection is effective for suppressing heat release in the early stage.

Using numerous fuel valves in a sequential injection system, the injection rate can be lowered in the first half of combustion without affecting the total amount of fuel injected, which can lower the early heat release rate.

Fuel pumps installed in each cylinder of cam-type engines, simultaneously drive several fuel valves, making individual fuel valve control impossible. To reduce heat emission in the early stages of combustion, this limitation reduces fuel injection rate by lowering the injection pressure in early stages of fuel injection. However, there is a risk of faulty combustion and afterburning in large marine engines that use heavy fuel oil if poor quality fuel is bunkered, and this method is not without its drawbacks.

So, it is obvious that common rail electronically controlled marine engines have shown their advantages of compatibility to the exhaust regulations and the higher efficiency over all the operational output range. Additionally, they greatly contribute to the improvement of automated troubleshooting supporting function based on feedback information from electronically controlled components.

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