Hydraulically intensified medium pressure common rail (MPCR) electronic fuel injection systems are an attractive concept for heavy-duty diesel engine applications. They offer excellent packaging flexibility and thorough engine management system integration.Two different concepts were evaluated in this study. They are different in how the pressure generation and injection events are related. One used a direct principle, where the high-pressure generation and injection events occur simultaneously producing a near square injection rate profile. Another concept was based on an indirect principle, where potential energy (pressure) is first stored inside a hydraulic accumulator, and then released during injection, as a subsequent event. A falling rate shape is typically produced in this case. A unit pump, where the hydraulic intensifier is separated from the injector by a high-pressure line, and a unit injector design are considered for both concepts.Details of the injection process, including hydraulic and mechanical characteristics, were analyzed and compared. A one dimensional fuel injection simulation program was used. A summary of the mathematical model and program structure is provided and the advantages and disadvantages are outlined.Due to the different rate shapes, the two Fuel Injection System (FIS) concepts result in different calculated combustion and emission characteristics. A CFD model for three-dimensional analysis of chemically reactive flow with sprays, KIVA, was used to assess the combustion and emission characteristics of the different systems used in the study.The effect of a short injection duration on combustion and emissions was also explored. The calculated results showed an increase in the duration of heat release and soot emissions in this case. This may be attributed to the local fuel rich zones created.Each FIS concept was composed of identical major components including a hydraulic control valve, hydraulic intensifier, check valves, and nozzle. Starting from a 'generic' design, a fair comparison was possible.This paper discusses the approaches, methods, and results of the evaluation study. The final objective of the project is to design and build a prototype FIS and demonstrate its capability in a single cylinder research engine.