Computer simulation of the hydraulic and mechanical functions has been used extensively in the development of the Company's Electronic Unit Injector for truck engines: the EUI-200. The program was developed in house as part of a long tradition of simulating diesel fuel injection equipment.The paper describes the role of simulation in the product development, from testing the feasibility of design concepts, through the development phases, and into serial production. Specific examples show the sizing and optimization of some of the key design details. Sensitivity analyses revealed the critical manufacturing processes and were helpful in diagnosing and solving a problem with one particular batch of prototypes. Two stage and electronic pilot injection developments are described, as well as some fundamental analyses of the injection performance, through the identification of cavitating and non-cavitating flow regimes in the nozzle spray.Overall, the use of simulation is shown to bring understanding, order and insight into the essential behaviour of the system, and the interaction between component parts. Typical test rig and engine results are provided in unprocessed form for comparison with the computed results, which shows the level of accuracy achieved by the simulation.