Multi-zone Modeling of PCCI Combustion with CFD Coupling for Stratification

PCCI combustion is a viable alternative for diesel combustion. The concept of PCCI combustion is associated with early injection of the fuel whilst applying high EGR levels and operation with a highly lean mixture such that ignition is (well) after the injection event. In this way, a premixed mixture is formed and the operation is performed at relatively lower temperatures. Thus, it is possible to reduce soot and NOx emissions to very low values. PCCI combustion is analyzed using a multi-zone model. In the multi-zone model, complicated transport equations are ignored so that much more detailed chemical mechanisms compared to CFD models can be introduced. The model is still coupled to a CFD model to estimate the fuel distribution which is important to improve the quality of the model. The effects of different chemical mechanisms and CFD coupling timings (i.e. different fuel distributions) are studied. For the analysis, dedicated model fuel experimental results are used to evaluate the quality of the modeling results. In the multi zone model, 10 zones are sufficient to describe the stratification with sufficient resolution. The analysis shows that emission trends are mainly predicted qualitatively similar to those of experiments with respect to the injection timing. This is generally correct for different fuel distributions which have a big influence on the emissions but not on the combustion phasing

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

Premixed charge compression ignition (PCCI) is a clean and efficient alternative for classical diesel combustion. The concept of PCCI combustion is associated with early injection of the fuel whilst applying high exhaust gas recirculation (EGR) levels and operation with a highly lean mixture such that ignition takes place (well) after the injection event. Thus, it is possible to reduce soot and oxides of nitrogen (NOx) emissions simultaneously. PCCI combustion is analyzed using a multi-zone model. In the multi-zone model, chemical mechanisms which are much more detailed compared to those used in computational fluid dynamics (CFD) approaches can be introduced directly. The CFD 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, a basic inter-zonal diffusive mixing is applied. The level of mixing is estimated with a sensitivity study. When the inter-zonal mixing is included, emission results become much less sensitive to the crank angle (CA) 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 NO xemission 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 of PCCI combustion with FGM tabulated chemistry

Premixed Charge Compression Ignition (PCCI) is a new combustion concept aiming a simultaneous reduction of oxides of nitrogen and soot emissions. Therefore the operation focuses on improved fuel–air mixing before ignition and lower maximum in-cylinder temperatures during the complete engine cycle. In the PCCI-regime, the injection and ignition events do not overlap due to the longer ignition delay timings. As such ignition is not influenced by the injection event like in the conventional operation and it is essentially governed by chemical kinetics. Numerical methods should incorporate flow and chemistry models in an accurate way. However, the computational demand for modeling these phenomena is high and the researchers work on several reduction techniques to achieve a practical computational efficiency. In this work the Flamelet Generated Manifold method is applied within the Computational Fluid Dynamics (CFD) framework to study PCCI combustion. In FGM, thermo-chemical properties are preprocessed by solving canonical systems (here, Igniting Counter-flow Diffusion Flamelets and Homogeneous Reactors) and stored in a manifold as a function of controlling variables. Since ignition control is difficult in the aforementioned combustion concept, the accurate prediction of ignition phenomena is significant. Simulations are performed with three different mesh settings, where the course grid proves to be sufficiently accurate. Later the effect of multiple pressure levels is investigated using both canonical systems and the study shows that a number of three pressure levels is sufficient to capture the ignition phasing with Homogeneous Reactors based FGM tables. Finally, the sensitivity of ignition with respect to injection timing is shown to be predicted precisely

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 uncooledexhaust gases led to significant reduction in wall wetting. This is combined with IMEP improvement. In-house sprayand ignition modeling was used to gain insight into the measured trends

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 uncooledexhaust gases led to significant reduction in wall wetting. This is combined with IMEP improvement. In-house sprayand ignition modeling was used to gain insight into the measured trends

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

Predicting auto-ignition characteristics of RCCI combustion using a multi-zone model

The objective of new combustion concepts is to meet emission standards by improving fuel air mixing prior to ignition. Since there is no overlap between injection and ignition, combustion is governed mainly by chemical kinetics and it is challenging to control the phasing of ignition. Reactivity Controlled Compression Ignition (RCCI) combustion aims to control combustion phasing by altering the fuel ratios of the high- and low octane fuel and injection timings. In this study the dual fuel blend is prepared with gasoline and diesel fuels. The applied injection timings of the diesel are very early (90 to 60° CA bTDC). In the detailed reaction mechanism, n-heptane and iso-octane represent diesel and gasoline fuel, respectively. A multi-zone model approach is implemented to perform RCCI combustion simulation. Ignition characteristics are analyzed by using CA50 as the main parameter. In the experiments for the early direct injection (DI) timing advancing the injection time results in a later ignition. Qualitatively, the trend effect of the diesel injection timing and the effect of the ratio gasoline/diesel are captured accurately by the multi-zone model