Injection process simulation methods were developed for both the unit injector (UI) system and the pump-line nozzle (PLN) system consisting of an in-line injection pump, fuel line, and nozzle. Simulation results agreed well with measured ones. With regard to the shape of injection rate and the peak injection pressure change at various engine speeds, the injection characteristics of the UI system are better than those of the PLN system. Simulation results showed that similar injection characteristics can also be obtained with the PLN system by using a concave cam with a carefully designed cam profile for a sleeve-controlled in-line injection pump and by changing the prestroke according to the operating conditions. Engine test results demonstrated the possibility of improving the trade-off between NOx and fuel consumption by shaping the injection rate.The shape of injection rate plays an important role in diesel combustion(1,2)*, affecting exhaust emissions and also combustion noise. High-pressure injection is thought to be necessary to reduce soot and particulate emissions, but it normally leads to increase in NOx emission(3). Then initial rate of injection should be kept small to reduce NOx, and steep pressure drop at the end of injection is also required for minimizing smoke. Accordingly, a delta-shaped injection rate is ideal for reducing both NOx and particulate emissions. Injection pressure should be high at low speeds but should not exceed a certain limit at the rating speed because the durability of an injection system is limited. Because unit injectors can produce this ideal injection-rate shape and have appropriate injection pressure characteristics, they have been used in many heavy-duty diesel engines meeting the very stringent US94 emission standards. For production engines, it is difficult to substitute unit injectors for in-line injection pumps because of the high investment necessary for changing the existing facility.Two-spring injectors reduce the initial rate of injection, resulting in a reduction of discharge velocity from nozzle holes at the initial stage of injection. Air entrainment into fuel spray plume is thereby deteriorated. We were eager to make the injection characteristic of a pump-line nozzle (PLN) system the same as those of a unit injector (UI) by designing an appropriate cam profile for an in-line injection pump. So we developed two injection process simulation models - for both UI and PLN systems - so that we could study the characteristics of injection process through injection process simulation.