Simulation of the Low-Temperature Combustion in a Heavy Duty Diesel Engine 2007-01-0904
Early injection strategies in the case of part-load conditions are offering the possibility to enhance mixing and evaporation. Due to the early injection, ignition and evaporation are separated in time and space for that less rich pockets from where soot is formed are occurring. For reducing NOx, cooled EGR is a method to dilute the intake charge. The combustion is shifted to lower temperatures and less NOx is formed. More, the cooling of the intake charge and the higher heat capacity enhance the evaporation time for that ignition starts at later times and combustion is retarded.
For the simulation of such engine cases using high rates of EGR with an early fuel injection, a CFD (Computational Fluid Dynamics) code is coupled interactively with the flamelet model that will be applied here as combustion model. That approach, known as RIF (Representative Interactive Flamelet) model, requires a re-evaluation of the chemical reaction mechanism. The diesel fuel is replaced by a two-component surrogate fuel that is a mixture of 70% n-decane and 30% α-methylnaphthalene (by liquid volume), known as IDEA fuel. That fuel is a good choice and replaces Diesel fuel even for high levels of EGR. The existing chemical reaction mechanism is evaluated and validated by data from shock tube experiments. By comparison with results from engine experiments for a parameter variation of EGR from 40 to 70%, it is shown that the reaction mechanism and the RIF combustion model are suitable in the low-temperature combustion regime.