Michal Pasternak

A PDF-Based Model for Full Cycle Simulation of Direct Injected Engines

In one-dimensional engine simulation programs the simulation of engine performance is mostly done by parameter fitting in order to match simulations with experimental data. The extensive fitting procedure is especially needed for emissions formation - CO, HC, NO, soot - simulations. An alternative to this approach is, to calculate the emissions based on detailed kinetic models. This however demands that the in-cylinder combustion-flow interaction can be modeled accurately, and that the CPU time needed for the model is still acceptable. PDF based stochastic reactor models offer one possible solution. They usually introduce only one (time dependent) parameter - the mixing time - to model the influence of flow on the chemistry. They offer the prediction of the heat release, together with all emission formation, if the optimum mixing time is given. Hence parameter fitting for a number of kinetic processes, that depend also on the in cylinder flow conditions is replaced by a single parameter fitting for the turbulent mixing time. In this work a PDF based model was implemented and coupled to the full cycle engine simulation tool, WAVE, and calculations were compared to engine experiments. Modeling results show good agreement with the experiments and show that PDF based Dl models can be used for fast and accurate simulation of Dl engine emissions and performance.

Aspects of 0D and 3D Modeling of Soot Formation for Diesel Engines

In this work, zero-dimensional (0D) and three-dimensional (3D) models were applied to the same engine experiment, investigating aspects of 0D and 3D modeling of combustion and soot formation for diesel engines. The 0D simulations were carried out using a direct injection stochastic reactor model (DI-SRM), which is built on a probability density function (PDF) approach. The 0D model allows for the use of detailed chemistry for calculation of combustion, emission formation, and interaction between chemistry and turbulent flow. The 3D computational fluid dynamics (CFD) simulations were performed using a PDF-time scale combustion model and a flamelet library soot source term model. The DI-SRM results demonstrate the applicability of the flamelet model for the combustion process and also elucidate the limitations of the interactive flamelet model when calculating emission formation. The emission results, if plotted in mixture fraction space, show a dispersion for species such as NO and CO, but a flamelet structure for species such as C2H2 and OH, which makes the latter ones applicable for calculation of the source terms of soot formation in mixture fraction space. The CFD calculations were used to verify assumptions made in the DI-SRM and the DI-SRM results were used to verify the assumptions inferred by using tabulated chemistry. It is demonstrated that the DI-SRM can be used for soot modeling under diesel engine conditions and that the flamelet library approach for modeling of soot formation in CFD is sound.

Gasoline engine simulations using zero-dimensional spark ignition stochastic reactor model and three-dimensional computational fluid dynamics engine model

A simulation process for spark ignition gasoline engines is proposed. The process is based on a zero-dimensional spark ignition stochastic reactor model and three-dimensional computational fluid dynamics of the cold in-cylinder flow. The cold flow simulations are carried out to analyse changes in the turbulent kinetic energy and its dissipation. From this analysis, the volume-averaged turbulent mixing time can be estimated that is a main input parameter for the spark ignition stochastic reactor model. The spark ignition stochastic reactor model is used to simulate combustion progress and to analyse auto-ignition tendency in the end-gas zone based on the detailed reaction kinetics. The presented engineering process bridges the gap between three-dimensional and zero-dimensional models and is applicable to various engine concepts, such as, port-injected and direct injection engines, with single and multiple spark plug technology. The modelling enables predicting combustion effects and estimating the risk of knock occurrence at different operating points or new engine concepts for which limited experimental data are available.

Soot Source Term Tabulation Strategy for Diesel Engine Simulations with SRM

In this work a soot source term tabulation strategy for soot predictions under Diesel engine conditions within the zero-dimensional Direct Injection Stochastic Reactor Model (DI-SRM) framework is presented. The DI-SRM accounts for detailed chemistry, in-homogeneities in the combustion chamber and turbulence-chemistry interactions. The existing implementation [1] was extended with a framework facilitating the use of tabulated soot source terms. The implementation allows now for using soot source terms provided by an online chemistry calculation, and for the use of a pre-calculated flamelet soot source term library. Diesel engine calculations were performed using the same detailed kinetic soot model in both configurations. The chemical mechanism for n-heptane used in this work is taken from Zeuch et al. [2] and consists of 121 species and 973 reactions including PAH and thermal NO chemistry. The engine case presented in [1] is used also for this work. The case is a single-injection part-load passenger car Diesel engine with 27 % EGR fueled with regular Diesel fuel. The two different approaches are analyzed and a detailed comparison is presented for the different soot processes globally and in the mixture fraction space. The contribution of the work presented in this paper is that a method which allows for a direct comparison of soot source terms - calculated online or retrieved from a flamelet table - without any change in the simulation setup has been developed within the SRM framework. It is a unique tool for model development. Our analysis supports our previous conclusion [1] that flamelet soot source terms libraries can be used for multi-dimensional modeling of soot formation in Diesel engines.

Systematic Reduction of Detailed Chemical Reaction Mechanisms for Engine Applications

Copyright

Three-dimensional computational fluid dynamics engine knock prediction and evaluation based on detailed chemistry and detonation theory:

Engine knock is an important phenomenon that needs consideration in the development of gasoline-fueled engines. In our days, this development is supported using numerical simulation tools to further understand and predict in-cylinder processes. In this work, a model tool chain which uses a detailed chemical reaction scheme is proposed to predict the auto-ignition behavior of fuels with different octane ratings and to evaluate the transition from harmless auto-ignitive deflagration to knocking combustion. In our method, the auto-ignition characteristics and the emissions are calculated using a gasoline surrogate reaction scheme containing pathways for oxidation of ethanol, toluene, n-heptane, iso-octane and their mixtures. The combustion is predicted using a combination of the G-equation based flame propagation model utilizing tabulated laminar flame speeds and well-stirred reactors in the burned and unburned zone in three-dimensional Reynolds-averaged Navier–Stokes. Based on the detonation theory by Bradley et al., the character and the severity of the auto-ignition event are evaluated. Using the suggested tool chain, the impact of fuel properties can be efficiently studied, the transition from harmless deflagration to knocking combustion can be illustrated and the severity of the auto-ignition event can be quantified.

Diesel engine performance mapping using a parametrized mixing time model

A numerical platform is presented for diesel engine performance mapping. The platform employs a zero-dimensional stochastic reactor model for the simulation of engine in-cylinder processes. n-Heptane is used as diesel surrogate for the modeling of fuel oxidation and emission formation. The overall simulation process is carried out in an automated manner using a genetic algorithm. The probability density function formulation of the stochastic reactor model enables an insight into the locality of turbulence–chemistry interactions that characterize the combustion process in diesel engines. The interactions are accounted for by the modeling of representative mixing time. The mixing time is parametrized with known engine operating parameters such as load, speed and fuel injection strategy. The detailed chemistry consideration and mixing time parametrization enable the extrapolation of engine performance parameters beyond the operating points used for model training. The results show that the model responds correctly to the changes of engine control parameters such as fuel injection timing and exhaust gas recirculation rate. It is demonstrated that the method developed can be applied to the prediction of engine load–speed maps for exhaust NOx, indicated mean effective pressure and fuel consumption. The maps can be derived from the limited experimental data available for model calibration. Significant speedup of the simulations process can be achieved using tabulated chemistry. Overall, the method presented can be considered as a bridge between the experimental works and the development of mean value engine models for engine control applications.

Suppressing Knocking by Using CleanEGR – Better Fuel Economy and Lower Raw Emissions Simultaneously

The use of external Exhaust Gas Recirculation (EGR) is a common technology available for the reduction of nitrogen oxides (NOx) emitted by internal combustion engines. With regard to gasoline engines, the addition of EGR at higher loads reduces knock tendency and improves fuel economy by reducing the necessity for fuel enrichment. To further maximize these benefits, the recirculated exhaust gases are cooled down that improves engine efficiency by enabling an advanced center of combustion (MFB50). Hereby gasoline engines can be operated at EGR rates up to 20%, which is enabling stoichiometric operation in the entire engine map. On the other hand, cooled EGR is leading to well-known low-temperature issues such as fouling, corrosion and condensation. In response to that challenge, in this work the use of cooled LP- and HP-EGR is analyzed by engine testing for different engine intake temperatures. For dedicated tests a higher compression ratio for improved fuel economy and a coated Gasoline Particulate Filter (CleanEGRTM) for cleaning the EGR gas is investigated as well. As a conclusion, external cooled and cleaned EGR is measure to meet future RDE requirements (stoichiometric operation in entire map) by achieving improvements in engine efficiency and engine out emissions (PN, NOx) simultaneously.

Simulation of the Effects of Spark Timing and External EGR on Gasoline Combustion Under Knock-Limited Operation at High Speed and Load

Combustion in a spark ignition engine operated at high speed and load is investigated numerically with regard to knock behavior. The study focuses on the concurrent impact of spark timing and exhaust gas recirculation (EGR) on the severity of knock. Specifically, the possibility of knock reduction through the lowering of nitrogen oxide (NO) content in the rest-gas is examined. Simulations are carried out using a stochastic reactor model of engine in-cylinder processes along with a quasi-dimensional turbulent flame propagation model and multicomponent gas-phase chemistry as gasoline surrogate. The knock-limited conditions are detected using the detonation diagram. By lowering the NO content in the external EGR the end-gas auto-ignition is suppressed. This prevents a transition to knocking combustion and enables advancing of spark timing that yields better combustion phasing. As a result, fuel economy is improved and the potential benefits of cleaning the EGR are indicated.