Detailed modeling of common rail fuel injection process

Modeling of fuel injection equipment is a tool that is used increasingly for explaining or predicting the effect of advanced diesel injection strategies on combustion and emissions. This paper reports on the modeling of the high-pressure part of a research type Heavy Duty Common Rail (CR) fuel injection system. More specifically, it reports on the observed dynamics of the injection system and the capability of the model to capture this. For that reason, the total high-pressure part of the injection system, i.e. the fuel pump, rail and injector, has been modeled using the AMESim code (Imagine S.A., 2004). The reliability of the resulting hydraulic model is tested through a comparison between numerical results and actual injection measurements. This detailed comparison is based on measurements of injected mass flow rate, needle lift and pressure oscillations in the injection duct for a series of single injection events. It is shown that the hydraulic model is able to accurately simulate the injection rate, needle lift and injection pressure for different rail pressure levels. For accurate numerical results, it is vital that the stiffness of the injector needle assembly and the discharge coefficients of the different flow restrictions in the injector (e.g. nozzle holes) are correctly modeled. Assuming a rigid injector needle results in a too early start of injection. Discharge coefficient values found in literature shows a wide spread. This makes it very difficult to simulate the injected mass flow rate accurately on the basis of literature data. Using the measured injected mass of fuel to tune the discharge coefficient, together with the inclusion of the elasticity of the injector needle, results in a good approximation of the injection rate.