Oldřich Vítek

New Approach to Turbocharger Optimization using 1-D Simulation Tools

The paper deals with the investigation of turbocharger optimization procedures using amended 1-D simulation tools. The proposed method uses scaled flow rate/effficiency maps for different sizes of a radial turbine together with a fictitious compressor map. The compressor pressure ratio/efficiency map depends on compressor circumference velocity only and predicts the both compressor specific power and achievable efficiency. At the first stage of optimization, it avoids the problems of reaching choking/surge limits. It enables the designer to find a suitable turbine type under realistic unsteady conditions (pressure pulses in exhaust manifold) concerning turbine flow area. Once the optimization of turbine/compressor impeller diameters is finished, the specific compressor map is selected. The proposed method provides the fast way to the best solution even for the case of a VGT turbine. Additional features have been developed for the representation of scaled turbine and compressor maps. They are based on application of versatile regression functions. No recalculation of normalized curves is needed. The method is presented for the model of a one-loop hot gas stand (steady performance compressor/turbine optimization) and for a modelled engine with different compressors. The potential of optimized turbocharger is evaluated at simulated typical operation points for a passenger car equipped with a 4-cylinder 2 dm engine.

Optimization of 2‑Stage Turbocharged Gas SI Engine Under Steady State Operation

Abstract The proposed paper deals with an optimization of a highly-turbocharged large-bore gas SI engine. Only steady state operation (constant engine speed and load) is considered. The paper is mainly focused on theoretical potential of 2-stage turbocharging concept in terms of performance and limitation. The results are obtained by means of simulation using complex 0-D/ 1-D engine model including the control algorithm. Different mixture composition concepts are considered to satisfy different levels of NOx limit - fresh air mixed with external cooled EGR is supposed to be the right approach while optimal EGR level is to be found. Considering EGR circuit, 5 different layouts are tested to select the best design. As the engine control is relatively complex (2-sage turbocharger group, external EGR, compressor blow-by, controlled air excess), 5 different control means of boost pressure were considered. Each variant based on above mentioned options is optimized in terms of compressor/turbine size (swallowing capacity) to obtain the best possible BSFC. The optimal variants are compared and general conclusions are drawn.

Statistical Analysis of Detailed 3-D CFD LES Simulations with Regard to CCV Modeling

Abstract The paper deals with statistical analysis of large amount of detailed 3-D CFD data in terms of cycle-to-cycle variations (CCVs). These data were obtained by means of LES calculations of many consecutive cycles. Due to non-linear nature of Navier-Stokes equation set, there is a relatively significant CCV. Hence, every cycle is slightly different – this leads to requirement to perform statistical analysis based on ensemble averaging procedure which enables better understanding of CCV in ICE including its quantification. The data obtained from the averaging procedure provides results on different space resolution levels. The procedure is applied locally, i.e., in every cell of the mesh. Hence there is detailed CCV information on local level – such information can be compared with RANS simulations. Next, volume/mass averaging provides information at specific locations – e.g., gap between electrodes of a spark plug. Finally, volume/mass averaging of the whole combustion chamber leads to global information which can be compared with experimental data or results of system simulation tools (which are based on 0-D/1-D approach).

Knowledge Transfer from Detailed 3-D CFD Codes to System Simulation Tools – CCV Modeling in SI Engine

Abstract The paper deals with CCV knowledge transfer from reference data (either experiments or 3-D CFD data) into system simulation SW tools (based on 0-D/1-D CFD). It was verified that CCV phenomenon can be modeled by means of combustion model perturbations. The proposed methodology consists of two major steps. First, individual cycle data have to be matched with the 0-D/1-D model, i.e., combustion model parameters are varied to achieve the best possible match of in-cylinder pressure traces. Second, the combustion model parameters (obtained in previous step) are statistically evaluated to obtain PDFs and cross-correlations. Then such information is imposed to the 0-D/1-D tool to mimic pressure traces CCV. Good correspondence with the reference data is achieved only if both PDFs and cross-correlations are imposed simultaneously. Different engine operating points were evaluated to draw some general conclusions in terms of CCV. It was confirmed that turbulence properties and initial flame kernel development are the dominant factors. However, these factors are neither independent nor random – they seem to be correlated. Operating points with high CCV are more organized in terms of the statistics – they exhibit strong cross-correlations of combustion model parameters.

Application of Les, Pans and Rans to a Case of Intake Channel Steady Flow Test Bench

Shrnutí Članek se zabyva vyhodnocenim průtokovych vlastnosti (průtokovy součinitel, součinitel přične a tečne rotace) saciho kanalu užitim jak 3-D CFD simulace, tak měřeni. Byla provedena citlivostni studie různych parametrů simulace. Nejdůležitějši jsou vlastnosti sitě a použite modely turbulence (uvažovane modely: LES, PANS a RANS). Co se tyka sitě, tak kritickym parametrem je konfigurace v těsne blizkosti stěny - tloušťka a počet vrstev těsně u stěny maji velky vliv na průtok, avšak součinitele přične a tečne rotace na tom teměř nezavisi. Odtud plyne, že tyto parametry sitě lze považovat za ladici parametry CFD modelu. Co se tyka velikosti oka sitě, 2 velikosti byly testovany (0.6 a 0.3 mm). Ukazalo se, že ani jedna z těchto siti neni dostatečně jemna pro korektni aplikaci LES. A tedy PANS funguje spiše jako RANS. Z hlediska modelů turbulence je PANS jevi jako nejlepši možnost, protože kombinuje vyhody jak LES, tak RANS. Obecně je možne konstatovat, že lze dosahnout relativně dobre shody na průtoku, ale součinitele přične a tečne rotace jsou problematicke, neboť byla dosažena pouze kvalitativni shoda. Abstract The paper deals with evaluation of intake channel flow properties (discharge coefficient, tumble and swirl ratio) using both 3-D CFD simulation and measurement. Sensitivity of different calculation parameters was tested. The most important ones are mesh parameters and applied turbulence models (considered models: LES, PANS and RANS). Concerning the mesh, the critical parameter is a mesh configuration near a wall - a thickness of mesh boundary layer and an amount of these layers have significant impact on mass flow rate while tumble/swirl ratio is unaffected by that. Based on that, these mesh parameters can be considered as tuning constants for fine-tuning the CFD model. Regarding the mesh cell size, 2 different values were tested (0.6 and 0.3 mm). It was found out that neither mesh is fine enough to perform proper LES. Hence, PANS performance is close to RANS. Concerning the turbulence models, PANS seems to be the best one as it combines advantages of both LES and RANS. Generally speaking, mass flow rate prediction is relatively good while swirl/tumble one is more problematic as only qualitative agreement can be achieved.

Application of a Radial Turbine 1-D Model

Shrnutí Članek, na několika přikladech, popisuje simulace s využitim 1-D modelu radialni turbiny turbodmychadla. Cilem je popsat současny stav a ukazat potencial 1-D přistupu v oblasti modelovani turbodmychadla. Pro možnost testovani modelů turbiny a kompresoru byly vyvinuty rozmanite simulačni nastroje. Model turbinoveho testovaciho stavu umožňuje provozovat turbinu v široke provozni oblasti. Model zkušebniho stavu turbodmychadel se spalovaci komorou je vhodny pro přiřazeni kompresoru turbině. Članek uzaviraji vysledky fungovani modelu turbiny ve spojeni s modelem virtualniho čtyřtaktniho zažehoveho motoru. Abstract The paper presents the simulation results of several applications of a radial turbine 1-D model. The aim is to describe the current situation and to demonstrate the potential of this approach for turbocharger modelling. Various simulation tools have been developed for testing turbine and compressor performance. The test bed allows the turbine to be operated across a broad working range. The model of the hot gas stand is suitable for turbine-compressor matching. The results of turbine behaviour in conjunction with the model of a virtual four-stroke SI engine are discussed.

Pressure wave supercharger 0-d model

SHRNUTI Přispěvek představuje využiti a potencial 0-D algebraickeho modelu tlakoveho vyměniku. Tlakovy vyměnik je nestacionarni zařizeni, ktere využiva tlakovych vln ke stlačovani tekutin v serii axialnich kanalků. Ke stlačovani dochazi přenosem energie z vysokotlakeho do nizkotlakeho proudu bez odděleni pevnymi stěnami. 0-D model vychazi z fyzikalniho popisu procesů uvnitř tlakoveho vyměniku. Zjednodušeny popis procesů je korigovan kalibračnimi koeficienty určenymi pomoci 1-D modelu. Kalibračni koeficienty jsou do 0-D modelu přenesny regresnimi funkcemi. ABSTRACT This paper is aimed at presenting the role and potential of a 0-D algebraic model of a pressure wave supercharger (PWS - ComprexR or HyprexR). The PWS is a non-steady flow device that uses shock waves to pressurize fluids by transferring energy from a highpressure flow to a low-pressure flow without separation by physical walls in a series of axial cells. The 0-D model aims at offering a simple description of processes inside a PWS based on physics, and enables close to real time simulation. The simplified processes are corrected with calibration coefficients determined using a 1-D model and transferred to the 0-D model using regression functions.