Fuel injection for future high-speed engines

Diesel engine manufacturers in both propulsion and genset applications are concerned with the development aim of low fuel consumption, reliability, and long service life. Other important issues are low soot, NOx, CO, particle emissions, and good dynamic characteristics; noise levels are also becoming increasingly important. To achieve these requirements more accurate control is required of the timing, quantity and shape of the fuel injection is required.
Modern design has moved towards the use of electronics to achieve this.

Unit Injector

Add caption

Conventional injection systems with mechanical action include inline pumps, unit pumps with long HP fuel lines and injectors. A cam controls the injection pressure and timing, while the fuel volume is determined by the fuel rack position. The need for increased injection pressures in more modern designs means that the variable time lag introduced by distortion of the pipework and compressibility of the fuel cannot be accounted for. Therefore this type of design is losing favour
Unit Injector

A comparison between unit pump and unit injector systems has been made assuming the unit injector drive adopts the typical camshaft/pushrod/rocker arm principle. With the aid of simulation calculations the relative behaviour of the two systems was investigated for a specified mean injection pressure of 1150bar in the injector sac. The time-averaged sac pressure is a determining factor in fuel mixture preparation, whereas the frequently used maximum injection pressure is less meaningful.
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 the unit pump. To generate 1150 bar the unit injector needed 3.5kW-some 6% more power. During the ignition delay period, 12.5% of the cycle related amount of fuel was injected by the unit pump as against 9.8% by the unit injector. The former is, therefore, the overall more stiffer system.
Translating the pressure differential at the nozzle orifice and the volume flow into mechanical energy absorbed, the result was a higher efficiency of 28% for the unit pump, compared to 26% for the unit injector.
From the hydraulic aspect, the unit pump offers benefits in that there is no transfer of mechanical force between the pushrod drive to the cylinder head and less space is needed for the fuel injector which gives 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 requires the appropriate engineering effort. The effort increases considerably if the injection timing is done mechanically.

Unit pump with solenoid valve control.

The engineering complexity involved in being able to freely select fuel injection and timing can be considerably reduced by using a solenoid valve to effect time-oriented 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 , are effected by a microprocessor controlled engine control unit (ECU). Optional adjustment of individual fuel injection calibration and injection timing is thus possible with the injection period being newly specified and realised 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 dependant on the pump speed and the amount of fuel injected. For engines with high meps in the lower speed and low-load ranges, this characteristic is disadvantageous to the atomisation process as the injection pressure drops rapidly. Adjusting the injection timing also influences the in-system pressure build-up, e.g. if timing is advanced, the solenoid valve closes earlier, fuel compression starts at lower injection pressures which, in turn, is detrimental to 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 involves a compensating amount of engineering effort regarding the dimensioning of the camshaft and the gear train, and may even require a vibration damper.
So while the solenoid valve controlled system has a number of advantages it retains the disadvantages of the conventional systems. In the search for a flexible injection system this system only represents a half step.

Common rail injection system (CRIS)

With the CRIS the HP pump delivers fuel to the rail which is common to all cylinders. Each injector is actuated in sequence by the ECU as a function of the crankshaft angle. The injector opens when energised and closes when deenergised. The amount of fuel 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. For rapid load shedding the pressure regulator restricts the HP pump to a maximum of 1330bar, compared to the specified 1200bar
With this 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 deenergised as additional hydraulic valve is activated which ensures rapid closure of the needle valve and. Therefore minimum smoke index. With this servo-assisted injector the opening and closing characteristics can be adjusted individually and effected extremely precisely. It is capable of extremely high reaction speeds for controlling minimum fuel quantities during idle operation or pilot injection.
Compared to a conventional system the pumping force is considerably lower, with pressure generation accomplished by a multi-cylinder, radial-piston pump driven by an eccentric cam. Pressure control is realised by restricting the supply flow. Locating the high-pressure pump on the crankcase presents no problems while the fuel injection control cams are deleted from the camshaft which can therefore be dimensioned accordingly. Fuel quantity injected is determined by the ECU and is a function of desired and measured engine speed. The CRIS allows very fast response times in the region of 10ms
In the event of single injector failure the injector is shut off via the shut off valve. This allows the remaining cylinders to be operated in a get you home mode.

The CRIS system offers the best characteristics and costing for future High speed engine injection systems.