Extensive Investigation of a Common Rail Diesel Injector Regarding Injection Characteristics and the Resulting Influences on the Dual Fuel Pilot Injection Combustion Process

摘要

Natural gas and especially biogas combustion can be seen as one of the key technologies towards climate-neutral energy supply. With its extensive availability, biogas is amongst the most important renewable energy sources in the present energy mix. Today, the use of gaseous fuels is widely established, for example in cogeneration units for combined heat and power generation. In contrast to conventional spark plug ignition, the combustion can also be initialized by a pilot injection. In order to further increase engine efficiency, this article describes the process for a targeted optimization of the pilot fuel injection. One of the crucial points for a more efficient dual fuel combustion process, is to optimize the amount of pilot injection in order to increase overall engine efficiency, and therefore decrease fuel consumption. In this connection, the injection system plays a key role. In the following, a detailed description is given for the systematic investigation and validation of a common rail diesel injector. At first, the injection rate as well as the injector’s spray pattern are measured and analyzed, especially at very short energizing times. The results indicate a very stable and repeatable injector operation, even with very short energizing durations. Moreover, the correlation between energizing time and injected fuel mass remains linear. Secondly, the interference between in-cylinder flow and pilot injection at various injection timings and pressures is investigated by means of three-dimensional CFD simulations. According to the numerical results, the in-cylinder charge motion is not likely to significantly impede the ignition of the pilot injected fuel mass. Even at very low injection pressure and short injection duration conditions, the spray formation remains controllable. Finally, experimental investigations verify the detailed determination of the pilot injection process under real engine conditions. The substitution rate could be maximized to approximately 97.8 %, yet in this case the emissions of unburned hydrocarbons are considerably higher. Moreover, the results clearly indicate, that the actual pilot injection timing has to be carefully adjusted according to the specific engine.