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

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

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.

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