Mtu Common Rail System For High Speed Engines

Development work on common rail fuel injection systems by MTU from 1990 benefited the German designer's Series 4000 high speed model (see Chapter 30) which was introduced in 1996 as the first production engine in its class to feature such a system (Figure 8.19). The group's Stuttgart-based specialist subsidiary L'Orange Einspritzsysteme was responsible for developing system components.

Research and development progressed from a pilot project on single-cylinder engines and an eight-cylinder demonstrator engine. The positive results encouraged MTU to develop the Series 4000 engine from the outset with a common rail injection system so that its benefits could be realised not just in terms of injection but in the overall design. The system embraces the following key elements:

• The injectors in the cylinder heads.

• The high pressure pump, driven by a gear system.

• The fuel rail (fuel supply line running along the engine length).

• The fuel lines between the fuel rail and the injectors.

One fuel rail is provided for the injectors and their respective solenoid valves on each bank of cylinders. The fuel pressure is generated by a high pressure pump driven via a gear train at the end of the crankcase. The electronic control system controls the amount of fuel delivered

Figure 8.19 A common rail fuel system was an innovation on the MTU Series 4000 high speed engine

to the injectors by means of the solenoid valves, and the injection pressure is also adjusted to the optimum level according to the engine power demand. Separate injection pumps for each cylinder are eliminated and hence the need for a complicated drive system for traditional pumps running off the camshafts (which, due to high mechanical stresses, calls for reinforced gearing systems or even a second system of gears on larger engines). Fewer components naturally foster greater reliability.

Another benefit cited for the common rail system is its flexible capability across the engine power band, and the fact that it is able to deliver the same injection pressure at all engine speeds from rated speed down to idling. In the case of the Series 4000 engine that pressure is around 1200 bar, corresponding to 1600-1800 bar on a conventional system.

Success is reflected in the fuel consumption (below 195 g/kWh) and emission characteristics of the engine. Future fuel injection systems must be distinguished by extremely flexible quantity control, commencement of injection timing and injection rate shaping: capabilities achieved only with the aid of electronic control systems.

Conventional injection systems with mechanical actuation include in-line pumps, unit pumps with short high pressure (HP) fuel lines and unit injectors. A cam controls the injection pressure and timing while the fuel volume is determined by the fuel rack position. For future engines with high injection pressures, however, the in-line pump system can be ignored because it would be hydraulically too 'soft' due to the long HP lines, MTU asserts.

A comparison has been made between unit pump and unit injector systems, assuming the unit injector drive adopts the typical camshaft/ push rod/rocker arm principle. Using simulation calculations, the relative behaviour of the two systems was investigated for a specified mean injection pressure of 1150 bar in the injector sac. This time-averaged sac pressure is a determining factor in fuel mixture preparation, whereas the frequently used maximum injection pressure is less meaningful, MTU explains. The pressure in a unit pump has been found to be lower than in a unit injector but, because of the dynamic pressure increase in the HP fuel line, the same mean injection pressure of 1150 bar is achieved with less stress in the unit pump. With the unit injector, the maximum sac pressure was 1670 bar (some 60 bar higher than in the unit pump). To generate 1150 bar the unit injector needed 3.5 kW, some 6 per cent more power. During the ignition delay period 12.5 per cent of the cycle-related amount of fuel was injected by the unit pump compared with 9.8 per cent by unit injector. The former is, therefore, overall the stiffer system. Translating the pressure differential at the nozzle orifice and the volume flow into the mechanical energy absorbed, the result was a higher efficiency of 28 per cent for the unit pump compared with 26 per cent for the unit injector.

From the hydraulic aspect, MTU reports, the unit pump offers benefits in that there is no transfer of mechanical forces from the push rod drive to the cylinder head, and less space is required for the fuel injector (yielding better design possibilities for inlet and exhaust systems). With conventional systems, the volume of fuel injected is controlled by the fuel rack; and matching the individual cylinders dictates appropriate engineering effort. The effort is increased significantly if injection timing is effected mechanically.

The engineering complexity involved in enabling fuel injection and timing to be freely selected can be reduced considerably by using a solenoid valve to effect time-orientated control of fuel quantity. To produce minimum fuel injection quantity extremely short shift periods must be possible to ensure good engine speed control. Activation of the individual solenoid valves and other prime functions, such as engine speed control and fuel injection limitation, is executed by a microprocessor-controlled engine control unit (ECU). Optional adjustment of individual cylinder fuel injection calibration and injection timing is thus possible with the injection period being newly specified and realized for each injection phase. Individual cylinder cut-out control is only a question of the software incorporated in the ECU.

With cam-controlled injection systems, the injection pressure is dependent on the pump speed and the amount of fuel injected. For engines with high mean effective pressures in the lower speed and low load ranges, this characteristic is detrimental to the atomization process as the injection pressure drops rapidly.

Adjusting the injection timing also influences the in-system pressure build-up. For example, if timing is advanced the solenoid valve closes earlier, fuel compression starts at a lower speed and thus leads to lower injection pressures which, in turn, undermines mixture preparation.

To achieve higher injection pressures, extremely steep cam configurations are required. As a result, high torque peaks are induced into the camshaft which dictates a compensating degree of engineering effort on the dimensioning of the camshaft and gear train, and may even call for a vibration damper.

While the solenoid valve-controlled system has a number of advantages, MTU argues, it retains the disadvantages of the conventional systems; and in the search for a new flexible injection system it represents only half a step. A full step is only achieved when the pressure generation, fuel quantity and injection timing functions can be varied independently by exploiting the common rail injection system (CRIS).

In the CRIS configuration for a V16-cylinder engine (Figure 8.20) the HP pump delivers fuel to the rail which is common to all cylinders.

Rail Fuel stop valve

Rail Fuel stop valve

Mtu Engine 4000 Series Fuel Circuit
Figure 8.20 Common rail injection system arranged for a V16-cylinder high speed engine (MTU)

Each injector is actuated in sequence by the ECU as a function of the crankshaft angle. The injector opens when energized and closes when de-energized. The amount of fuel injected per cycle is determined by the time differential and the in-system pressure. The actual in-system pressure is transmitted to the control unit via a pressure sensor, and the rail pressure is regulated by the ECU via the actuator in the fuel supply to the HP pump.

In such a system the injector incorporates several functions. The nozzle needle is relieved by a solenoid valve and thus opened by the fuel pressure. The amount of fuel injected during the ignition delay period is regulated by the nozzle opening speed. After the control valve is de-energized an additional hydraulic valve is activated to secure rapid closure of the nozzle needle and, therefore, a minimum smoke index. This servo-assisted injector allows the opening and closing characteristics to be adjusted individually and effected very precisely. It is capable of extremely high reaction speeds for controlling minimum fuel quantities during idle operation or pilot injection, MTU claims.

Compared with a conventional injection system, the pumping force is considerably lower: pressure generation is accomplished by a multi-cylinder, radial piston pump driven by an eccentric cam. Pressure control is realized by restricting the supply flow. Locating the HP pump on the crankcase presents no problems, while the deletion of the fuel injection control cams from the camshaft allows that component to be dimensioned accordingly.

The rail system is required to supply all injectors with fuel at an identical pressure, and its design is based on criteria such as minimum fuel injection quantity deviations between cylinders, faster pressure build-up after start and minimum pressure loss from the rail to the nozzle sac. Simulation exercises have shown that pressure fluctuations can be very low.

The ECU determines the engine speed and calculates the amount of fuel to be injected, based on the difference between actual and preset engine speeds. The individual injectors are energized as a function of crankshaft angle and firing order.

Control of the common rail in-system pressure is also carried out by the ECU. The advantages inherent in the solenoid valve-controlled conventional injection system (regarding speed control due to the improved actuator dynamic) can also be applied to the same extent to CRIS: at the moment of each injection the governor-generated data can be immediately processed. Pressure regulation is highly effective, says MTU. Load shedding (required, for example, when a waterjet propulsion unit emerges from the sea during rough conditions) can be effected from full to zero load in just 10 ms. While this leads to a rapid rise in engine speed the governor reduces fuel injection to zero which, in turn, causes a pressure increase in the rail system as the individual injectors become inactive. The pressure regulator reacts and restricts the HP pumps to a maximum of 1330 bar (a pressure excess of 130 bar over the specified value of 1200 bar). Additionally, the ECU assumes the control and monitoring functions that are standard for MTU engines, including sequential turbocharging control.

With common rail injection the complete system is permanently subjected to extremely high pressures. In the event of failure of a single injector the engine is protected in that the injector is cut off from the fuel supply by the fuel stop valve: no fuel can enter the combustion chamber. The engine can, however, still be operated at reduced (get-you-home) power.

A mechanical pressure relief valve is actuated if, due to a pressure regulating system malfunction, the pressure rises to an unacceptable level. Leaks in the system are identified by the pressure monitoring system.

During tests, MTU reports, the unit pump system displayed the best characteristics due to the high specified injection pressures. Greater flexibility, improved dynamics, reduced design effort and, as a result, lower costs are offered by the unit pump system with solenoid valve control. Only a common rail injection system, however, makes it possible to achieve injection rate shaping over the complete operating range and thus to underwrite the specifications for future diesel engines, MTU asserts.

The high pressure pump for the Series 4000 fuel system was changed from a four- or eight-cylinder radial piston pump to a four-cylinder inline unit to increase its performance and achieve a higher delivery rate for the V20-cylinder engine model.

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Responses

  • leena tallberg
    How is the fuel injection system of mtu 4000 engine?
    11 months ago
  • Adrian
    What are the works inJE in civil engineering in railway?
    9 months ago
  • estella
    What is pressure limiter in mtu engine?
    3 months ago

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