U Ulas Egüz

Uncooled EGR as a means of limiting wall-wetting under early direct injection conditions

Collision of injected fuel spray against the cylinder liner (wall-wetting) is one of the main hurdles that must be overcome in order for early direct injection Premixed Charge Compression Ignition (EDI PCCI) combustion to become a viable alternative for conventional DI diesel combustion. Preferably, the prevention of wall-wetting should be realized in a way of selecting appropriate (most favorable) operating conditions (EGR level, intake temperature, injection timing-strategy etc.) rather than mechanical modification of an engine (combustion chamber shape, injector replacement etc.). This paper presents the effect of external uncooled EGR (different fraction) on wall-wetting issues specified by two parameters, i.e. measured smoke number (experiment) and liquid spray penetration (model). Experiments performed in a dedicated heavy-duty direct injected (HDDI) diesel engine suggest that the elevation of intake temperature caused by delivery of external uncooled exhaust gases led to significant reduction in wall wetting. This is combined with IMEP improvement. In-house spray- and ignition modeling was used to gain insight into the measured trends. Copyright

Premixed charge compression ignition combustion modelling with a multi-zone approach including inter-zonal mixing:

Premixed charge compression ignition combustion is a clean and efficient alternative to classical diesel combustion. The concept of premixed charge compression ignition combustion is associated with early injection of the fuel while applying high exhaust gas recirculation levels and operation with a highly lean mixture such that ignition takes place (well) after the injection event. Thus, it is possible to reduce the soot emissions and the nitrogen oxide emissions simultaneously. Premixed charge compression ignition combustion is analysed using a multi-zone model. In the multi-zone model, chemical mechanisms which are much more detailed than those used in the computational fluid dynamics approaches can be introduced directly. The computational fluid dynamics model is still used to predict the initial fuel stratification in the cylinder, which is important to improve the quality of the model. For the analysis, dedicated experiments with n-heptane are used to evaluate the results of the model. In such a multi-zone model, 10 zones prove to be sufficient to describe the stratification with adequate resolution. It is observed that different fuel distributions have a large influence on the emissions when there is no mixing between the zones. To overcome this dependence, basic inter-zonal diffusive mixing is applied. The level of mixing is estimated with a sensitivity study. When the inter-zonal mixing is included, the emission results become much less sensitive to the crank angle at which the charge stratification is sampled and the simulation is initialized.

Multi-zone modelling of PCCI combustion

Early Direct Injection Premixed Charge Compression Ignition (EDI PCCI) combustion is a promising concept for the diesel combustion. Although EDI PCCI assures very low soot and NOx emission levels, the injection is uncoupled from combustion, which narrows down the operating conditions. The main purpose is to analyse the effect of mixing. A multi-zone model is presented with the use of detailed chemical models. The paper presents the effects of parameters, like number of zones and chemical model, on emissions and ignition delay. A dedicated set of experiments is also utilised to assess the quality of the model.

Modeling fuel spray auto-ignition using the FGM approach : effect of tabulation method

The Flamelet Generated Manifold (FGM) method is a promising technique in engine combustion modeling to include tabulated chemistry. Different methodologies can be used for the generation of the manifold. Two approaches, based on igniting counterflow diffusion flamelets (ICDF) and homogeneous reactors (HR) are implemented and compared with Engine Combustion Network (ECN) experimental database for the baseline n-heptane case. Before analyzing the combustion results, the spray model is optimized after performing a sensitivity study with respect to turbulence models, cell sizes and time steps. The standard High Reynolds ( Re ) k-e model leads to the best match of all turbulence models with the experimental data. For the convergence of the mixture fraction field an appropriate cell size is found to be smaller than that for an adequate spray penetration length which appears to be less influenced by the cell size. With the optimized settings, auto-ignition and flame lift-off length are analyzed. In general, both techniques capture the qualitative trend of experimental results. However, typically, the HR tabulation method predicts shorter ignition delay and LOL results than the ICDF method. In a quantitative sense, the ICDF and HR methods give better results in LOL and auto-ignition predictions, respectively.