The development of combustion engines with direct injection requires a comprehensive knowledge of the in cylinder combustion process as well as the used high pressure injection system. One main characteristic of injection systems is their mass flow over time behavior. For prevalent diesel and gasoline injection valves (injectors) fully developed simulation models as well as test benches are available to analyze the injection process. Besides the established engines a trend towards compressed natural gas (CNG) engines in passenger cars is recognized. Due to the small injection duration of a few milliseconds, the flow rate measurement is particularly challenging and requires highly dynamic measuring. The existing test benches are designed and optimized for liquid fuels and are only partly suitable for the evaluation of gaseous fuels such as CNG. A typical test method is to inject fuel into a long tube in which a pressure wave propagates. Based on the pressure signal the mass flow of the injected fuel is approximated. For gaseous fuels the correlation of mass flow and pressure propagation is only known for specific test cases and therefore the method is not directly applicable to gaseous fuels.
This paper presents a newly designed measurement device to evaluate the mass flow rate as well as the injector needle displacement during an injection process of gaseous fuels. The test bench is designed to operate in a fully equipped injection system including gas lines, common rail and injection valves, to also investigate the interaction of the individual system components.
The design is based on a closed test chamber in which the pressure rises during the injection. To overcome the influence of propagating pressure waves inside the chamber on the measurement, different chamber designs are evaluated. An optimized design, separating the chamber into two volumes which are connected by a damping sleeve, is presented. The injection itself is carried out in a first volume and the measurement is conducted in a second damped volume.
Based on the measured pressure the mass flow rate through the injection valve is approximated, utilizing the equations of thermodynamics.
