Simplified Presentation on Diesel Engine Electronic Common Rail Fuel Injection System

In the common rail system, fuel is distributed to the injectors from a high pressure accumulator, called the rail. The rail is fed by a high pressure fuel pump. The pressure in the rail, as well as the start and end of the injection in each cylinder are electronically controlled. Advantages of the common rail system include flexibility in controlling both the injection timing and injection rate. Stable pilot injections which can be delivered by the common rail have proven to lower the engine noise and the NOx emissions.

Common Rail System Layout

The common rail system includes the following components :

• High pressure fuel pump
• Rail for fuel storage and distribution
• Electrohydraulic injectors
• Electronic control unit (ECU).

An electric camshaft driven low pressure lift pump takes the fuel from the fuel tank, pumps it Common Rail Fuel Injection through a fuel filter and feeds the high pressure pump. A solenoid operated metering valve controls the amount of fuel entering the high pressure pump. The high pressure pump is driven by the engine and delivers fuel at a constant pressure via a pressure regulator to the rail. A pressure sensor installed in the rail monitors the fuel pressure. The signal is used by the ECU to control the rail pressure by acting on both the pressure regulator and the inlet metering valve. Excess fuel is returned from the pressure regulator to the fuel tank (the temperature of the return fuel in the CR system is higher than in conventional fuel injection systems and often exceeds 100°C—an important consideration for the use of fuel additives or emulsified fuels in CR engines).

The rail serves as a fuel accumulator. The fuel volume in the rail also dampens pressure oscillations caused by the high-pressure pump and the injection process. From the rail, the fuel is supplied at constant pressure to the injectors via high pressure pipes. The ECU generates current pulses which energize each injector solenoid valve in sequence and define the start and the end of each injection event per engine cycle. The common rail can generate more than one injection and gives more flexible control of the rate of injection compared to the conventional rotary pump injection systems.

Common Rail

The rail is a thick walled tube designed to supply the full fueling without significant pressure drop. The volume of the rail varies from only a few cubic centimeters in passenger cars, to as much as 60 cm3 in heavy-duty applications. A solenoid operated metering valve at the inlet to the pump controls the high pressure fuel delivery to the rail. The rail pressure can be controlled in the range between 20 and 250MPa.

Just as it was the case with P-L-N systems, common rail is also prone to effects related to wave dynamics in the rail and in fuel lines. Waves generated by sudden changes in pressure in one part of the system, such as when injection needle valve is opened, may become reflected at rigid terminations in the system and return to their origins, causing unwelcome consequences, such as reduced injection pressure.

Injectors

The injectors have to be designed to achieve high accuracy in both fuel quantity and start of injection timing. Common rail systems use electrohydraulic injectors which incorporate a small control valve with hydraulic balancing. A conventional needle valve is operated by adjusting the pressure differential between the seat (lower side) and the top of the valve, utilizing a small, solenoid operated control valve mounted above the needle. In some common rail systems, the solenoid is replaced by a piezo actuator

The control valve may be two- or three-way in different designs. The injector is characterized by small overall size, with many passenger car designs fitting an envelope of 17 mm in diameter. The low moving mass and small dimensions of components have the advantage of reducing the impact force on the nozzle seat when the nozzle closes. Another advantage of this type of design is its operation with low control current.The CR injection nozzle is being opened and closed by movements of the nozzle needle. The needle is balanced by pressures at the nozzle seat and at the needle control chamber (a space located at the opposite end of the needle). When the solenoid valve is energized by the ECU, it opens the chamber to the return fuel line. The pressure in the needle control chamber drops, the needle is lifted, the nozzle opens and the injection begins. When the current in the solenoid valve drops to zero, the control valve returns to its seat.The pressure in the needle control chamber increases and becomes equal to the pressure at the nozzle seat. The needle closes due to a nozzle spring force and stops the injection. Optimal control of the system is achieved by calibrating several orifices in the hydraulic assembly of the injector. Injection quantity is determined by rail pressure, hydraulic flow of the nozzle, and the pulse length from the ECU. In the common rail system, the injector needle is always exposed to high pressure. As soon as the needle opens, the injection begins. This creates a vulnerability of the common rail to accidental fuel leaks. If the needle is not totally closed, fuel under high pressure will leak to the combustion chamber after the injection. It may happen, for example, when a particle of dirt becomes trapped in the nozzle seat. The leakage leads to an abnormal combustion, which may manifest itself as engine knock.

Functions of CR Injection System

The common rail fuel injection system, in integration with the engine control module, performs a number of important functions to meet the requirements of modern powertrains. These functions could be classified as follows:

• Fuel delivery and injection timing
• Injection rate modulation (pilot injections)
• Post (secondary) injections
• Speed control
• Cold start procedure

Fuel delivery and timing are the basic functions of any type of fuel injection system, which affect the engine torque and power output, fuel economy, emissions, and noise. The fuel delivery is a function of the injection pressure and duration. In the CR system, both parameters, as well as the injection timing, can be controlled to tighter limits than in conventional fuel systems.

One of the reasons behind the trend toward common rail system was to minimize the pump torque requirement. Another important decision factor in favor of common rail was noise quality. DI engines are characterized by higher peak combustion pressures and, thus, by higher noise than the IDI engines. It was found that improved noise and low NOx emissions were best achieved by introducing pilot injection(s). This was best realized in the common rail system, which was capable of stable deliveries of small pilot fuel quantities in the entire load/speed range of the engine.

The common rail system was also compatible with variable geometry turbochargers (VGT). If a VGT is used, conventional injection systems become difficult to match with the engine, as the maximum available injection pressure in the P-L-N system increases with engine speed. Common rail, where the injection pressure was controlled independently from engine speed, could be matched to these engine turbocharger combinations providing more torque at low speeds and, thus, better fuel economy.

In the common rail system the fuel is distributed to the injectors from an accumulator, called the rail. The rail is fed by a high pressure fuel pump. The pressure in the rail, as well as the start and end of the injection in each cylinder are electronically controlled. The system is characterized by the following advantages:

Fuel pressure does not depend on the engine speed and load conditions allowing for flexibility in controlling both the injection rates and timing. This feature differentiates the common rail from conventional injection systems, where injection pressure increases with engine speed.

High injection pressures and good spray preparation are possible even at low engine speeds and loads.

Capability to deliver stable, small pilot injections can be used for decreased NOx emissions and noise.

Option for a post injection may be used together with such emission control technologies as particulate filters, lean NOx catalysts, or NOx adsorbers.

Fuel pump operates with low drive shaft peak torque.

For most engines common rail systems can replace conventional injection systems without requiring major engine modifications.

Spark-ignited and diesel engines may use virtually the same crankcase, the same assembly structure and the same crankshaft/camshaft sensor technologies.

A drawback of the common rail is its potential vulnerability to accidental fuel leaks due to the fact that injector needle valves are always exposed to high fuel pressures. It is an important—but not trivial—function of the control system to detect fault conditions related to fuel leaks.