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No abstract

A multicomponent fuel evaporation model was developed to describe diesel fuels. This model is based on the principles of continuous thermodynamics. In contrast to conventional. models, each component is not described by discrete relations for energy and mass, but the whole mixture is characterized by a distribution function. In most CFD (computational fluid dynamics)-applications the droplet interior is assumed to be well mixed, this means that composition and temperature are spatially constant inside the droplet. But this assumption is only correct in the case of turbulent mixture with high velocity gradients between spray and surrounding gas, e.g. the area near the nozzle. Due to areas with low relative velocities this formulation is inaccurate and demands a more detailed description of the processes inside the droplet. For these areas the use of a model based on a droplet consisting of several shells is advantageous, because composition and temperature are not spatially constant inside the droplet. The model divides the droplet into a finite number of layers that interchange energy and mass. The implementation of these two models into the CFD-code KIVA-3V implifies the effects of the multi-component character of the fuel by describing the different areas of the spray more correctly. Both models are compared in single droplet calculations and show a detailed description of evaporation.

Different synthetic fuels have been investigated within a variety of optical experiments at a rapid compression machine using diverse optical set-ups. The experiments have been carried out to determine the fuel requirements for good homogenisation and a controlled ignition and heat release for HCCI combustion. A directly actuated piezo injection system, which allows a flexible multiple injection strategy has been used to inject the fuel at different times during the compression stroke. Mie-scatter and Schlieren optics have been applied to investigate the different behaviour of the synthetic fuels concerning evaporation and mixture formation. The auto ignition behaviour of the different fuels has been investigated using an intensified relay optics and combustion chamber probes utilising the two-colour-method and a photo multiplier analysis systems. A multiple injection strategy and a 13 hole injection nozzle for HCCI operation mode with diesel-like fuels have been designed and optimised using CFD simulation prior to the experimental work. The experimental results using synthetic fuels will then be used to verify advanced 3D CFD models for multi component fuels and their behaviour concerning mixture formation and HCCI two-stage ignition.