In order to meet the increasingly stringent worldwide emissions legislation with the lowest possible fuel consumption, DAF Trucks N.V. will be fitting an electronically-controlled pump-pipe-nozzle (PLD) fuel injection system on its new heavy-duty diesel engines. In this system, the pump elements are grouped together in a separate housing, the UPEC (Unit Pump Electronically Controlled) housing, that will take the position of the current mechanical in-line pump.This results in an injection pipe length comparable with the pipe length of conventional FIE systems, which is rather extreme for PLD systems. Early calculations based on a one-dimensional hydraulic model of a single PLD element have shown that the performance characteristics of the UPEC system can match those of a system with shorter pipes, whilst retaining the present layout of the cylinder block. This also maintains the flexibility to adapt to the FIE requirements for the following steps in emission legislation.First bench tests nevertheless demonstrated that the hydraulic behaviour of the system was sensitive to small variations in the UPEC housing geometry that could not be described with the available models. To gain a better understanding of the hydraulic phenomena that play a role in both the high pressure part as well as the low pressure part of the system, a new way of simulating the PLD system has been developed. This simulation model contains six PLD elements and injectors as well as the UPEC housing, taking into account the interaction between all system parts.With this model, the significance of many design parameters like system volume, restrictions, supply pump capacity, system pressure, pipe length, etc. has been studied and validated. It appears that a careful optimization of all these variables can contribute to a more stable system. However, with the pipe length used in the UPEC system, a fully insensitive system can only be obtained by using snubber valves between the pump elements and the injection pipes.