LPG, liquified petroleum gas, is one of the most popular alternative fuels for internal combustion engines. Currently, it is quite easy to adapt a gasoline Multipoint Injection (MPI) engine's fuel system to LPG supply. There are several types of systems that adapt the engine in this way, and many manufacturers, which provide different structural solutions. Usually the LPG control unit is based on signals from the original gasoline supply system. This method is more complicated for direct injection engines, where the fuel dose depends not only on the injection duration but also on variations in injection pressure. In this study the relationship between LPG fuel and gasoline injection duration for gasoline direct injection engines was investigated. This relationship took gasoline injection pressure as a variable. The analysis was based on simple equations of incompressible and compressible steady flow. It was improved by an experiment which allowed the evaluation of injectors' flow characteristics. The results were also compared with the data provided by the manufacturer of LPG systems for MPI engines. (C) 2015 Elsevier Ltd. All rights reserved.