Crossing the combustion modes in diesel engines
Crossing the Combustion Modes in Diesel Engines Current and upcoming legislations in the engine community force the development of cleaner combustion systems. Modern diesel engines all apply electronically controlled high pressure fuel injection into a high density, high temperature environment of variable chemical composition. Due to the flexibility of the engine hardware new combustion concepts emerge. Typically these exploit this flexibility to operate engines with earlier injection timings
... injection timings compared to the conventional set-point. As such the number of parameters influencing the cylinder processes in new engine concepts is increasing rapidly and the significance of numerical models to comprehend the processes is growing. This thesis presents two types of approaches with different complexity levels to model engine combustion. The first one is a phenomenological multi-zone approach. In this model, the cylinder geometry is divided into a number of homogeneous zones. The complicated interaction between the flow and the fuel injection process that determines the important fuel stratification is not modeled. This drawback is minimized by introducing a CFD model to obtain a realistic level of charge stratification . The interaction between the zones during the simulation is simulated via the diffusive mixing and expansion. As such the computational effort is reduced considerably enabling a direct implementation of detailed reaction mechanisms. In this work, it is observed that 10 zones are sufficient to approximate the main processes in a reliable manner. The study shows that the results are very sensitive to the turbulent diffusivity and the applied reaction mechanism. The main conclusion is that a multi-zone model is a viable alternative for new combustion studies. These models are practical for qualitative trend studies but the quantitative match with experimental results, which depends on many parameters, is quite demanding. In the second approach, the Flamelet Generated Manifold (FGM) method is implemented within the CFD framework, here a commercial package: STAR-CD. A CFD setup consists of large number of cells preventing a direct utilization of detailed chemical models. Detailed chemistry information is still required to capture phenomena like auto-ignition or exhaust emissions thus different chemical reduction methods exist. In the FGM approach, combustion phenomena are preprocessed by solving a simpler system within certain assumptions. From these results a look-up database is generated where thermo-chemical properties are stored as a function of controlling variables, namely the mixture fraction and the progress variable. The FGM method is studied in a high pressure constant volume combustion chamber.