Search Results

With the ability of modern high pressure diesel injectors to deliver accurate, closely coupled multiple pulse injections, it is possible to minimize engine combustion noise without negative effect on exhaust emissions. Literature shows that, splitting the cycle heat release into several parts helps to lower peak heat release rate and combustion noise. The charge cooling caused by fuel vaporization can be effectively used to influence ignition delay and achieve lower noise, emissions and fuel consumption. With the traditional pilot-main injection scheme, researchers have shown that, the injection dwell time between the pilot and main is primarily responsible for noise reduction. The current objective is to analytically explore the fundamental physics behind the experimentally observed noise reduction phenomena with multiple injections. This computational study was conducted at a key part-load operation (2000RPM and 5Bar BMEP) with five injection pulses.

High-pressure diesel sprays were simulated with an Eulerian-Lagrangian Spray and Atomization (ELSA) model, based on a multidimensional engine computational fluid dynamics (CFD) code KIVA-3V. The atomization of the dense liquid core in the near-nozzle region was modeled with turbulent mixing of the diesel fuel with the ambient gas. Under the continuum assumption of a fuel-air mixture in this region, two transport equations were solved for the liquid mass fraction and liquid surface area density. At a certain downstream location where the spray became dilute, a switch from the Eulerian to the Lagrangian approach was made to benefit from the advantages of the conventional Lagrangian droplet models, such as droplet collision and turbulent dispersion modeling. The droplet size and velocity to be initialized at this switch were determined by the local CFD cell properties.

In the Premixed-Charge Compression Ignition (PCCI) combustion mode, fuel is injected fairly early before top-dead-center (TDC) of compression compared to the conventional near-TDC injection combustion mode. Early fuel injection into a low temperature in-cylinder environment results in long ignition delay and high peak heat release rate. Since the onset of ignition occurs after the end of injection, importance of spray and bowl induced flow field and mixing is not so obvious. In the present work, computational analysis is used to investigate the effects of spray-bowl interactions on PCCI combustion and emissions at a light-load (4Bar BMEP) operation of a medium-duty, direct injection diesel engine. Multidimensional CFD code KIVA-3V coupled with detailed chemical kinetics is used to perform combustion simulations.

A three-dimensional homogeneous equilibrium model (HEM) has been developed and implemented into an engine computational fluid dynamics (CFD) code KIVA-3V. The model was applied to simulate cavitating flow within injector nozzle passages. The effects of nozzle passage geometry and injection conditions on the development of cavitation zones and the nozzle discharge coefficient were investigated. Specifically, the effects of nozzle length (L/D ratio), nozzle inlet radius (R/D ratio) and K or KS factor (nozzle passage convergence) were simulated, and the effects of injection and chamber pressures, and time-varying injection pressure were also investigated. These effects are well captured by the nozzle flow model, and the predicted trends are consistent with those from experimental observations and theoretical analyses.

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.