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Technical Paper

Virtual Optimization of Race Engines through an extended Steady State Lap Time Simulation Approach

2018-04-03
2018-01-0587
Minimizing the lap time for a given race track is the main target in racecar development. In order to achieve the highest possible performance of the vehicle configuration the mutual interaction at the level of assemblies and components requires a balance between the advantages and disadvantages for each design decision. Especially the major shift in the focus of racecar powerunit development to high efficiency engines leads to a development of lean boosted and rightsized engines. In terms of dynamic engine behavior the time delay from demanded torque to provided torque could reveal an influence on lap time performance. Therefore maximizing the full load behavior as sole objective is insufficient to gain minimal lap time. By means of continuous predictive virtual methods throughout the whole development process the influence on lap time by dynamic power lags i.e. caused by the boost system, can be recognized efficient even in the early concept phase.
Technical Paper

Virtual Full Engine Development: 3D-CFD Simulations of Turbocharged Engines under Transient Load Conditions

2018-04-03
2018-01-0170
The simulation of transient engine behavior has gained importance mainly due to stringent emission limits, measured under real driving conditions, and the concurrently demanded vehicle performance. This is especially true for turbocharged engines, as combustion engine and turbocharger form a complex system in which the components influence each other causing for example the well-known turbo-lag. Because of this strong interaction, the components cannot be analyzed separately during a transient load case as they mutually determine their boundary conditions. Three dimensional simulations of full engines in stationary operating points have become practicable several years ago and will remain a valuable tool in virtual engine development; however the next logical step is to extend this approach into the transient domain.
Technical Paper

A simulative study for post oxidation while scavenging on turbo charged SI engines

2018-04-03
2018-01-0853
Fulfilling exhaust emissions regulations and meet customer performance needs mainly drive the development. Turbocharging system plays a key role for that. Currently turbocharging should provide highest engine power density at high engine speed by also allowing a very responsive performance in low engine speed. This represents a trade-off in turbocharger development. A large scaled turbine allows having moderate exhaust gas back pressure for peak power region but leading to loss of torque in low engine speed. In the last years of engine development scavenging helped to get away a bit from this trade-off as it increases the turbine mass flow and also reduces cylinder internal residual gas at low engine speed. The mostly in-use lean strategy runs air fuel ratios of closed to lambda=1 in cylinder and global (pre catalyst) of lambda=1.05 to lambda=1.3. This will be out of the narrow air fuel ratio band of lambda=1 to ensure NOx conversion in the 3-way-catalyst.
Technical Paper

A Two-Stage Knock Model for the Development of Future SI Engine Concepts

2018-04-03
2018-01-0855
At specific operating conditions, the auto-ignition in the unburnt mixture that precedes the occurrence of knock in conventional SI engines happens in two stages. In a previous publication (SAE 2017-24-0001), the authors demonstrated that the low-temperature heat release significantly influences the auto-ignition behavior of the mixture, thus severely impairing the prediction capabilities of the Livengood-Wu integral that the majority of the commonly used 0D/1D knock models are based on. Consequently, a new two-stage auto-ignition prediction approach for modeling the progress of the chemical reactions was introduced. It was demonstrated that the proposed auto-ignition model predicts the occurrence of two-stage ignition and accurately considers the significant influence of low-temperature heat release on the mixture’s auto-ignition behavior at various operating conditions.
Technical Paper

Reaction Kinetics Calculations and Modeling of the Laminar Flame Speeds of Gasoline Fuels

2018-04-03
2018-01-0857
For many aspects of engine development, 0D/1D-simulation has evolved into an important tool to obtain reliable results at passable effort, especially for transient operations. Based on the necessary simplification of the three-dimensional reality to one-dimensional models, 1D-simulation heavily depends on the quality of the used sub-models. For internal combustion engines, adequate modelling of combustion chamber processes is of essential importance. Quasi-dimensional approaches to describe burn rates of SI-engines base on the modelling of laminar flame speeds. However, direct measurements of laminar flame speeds are usually taken in the air-fuel equivalence ratio (AFR) range of 0.7 to 1.7 and pressures of only several bar. Common approaches then extrapolate to unsurveyed ranges, which causes contradictory data for laminar flame speed values.
Technical Paper

A Quasi-Dimensional Charge Motion and Turbulence Model for Diesel Engines with a Fully Variable Valve Train

2018-04-03
2018-01-0165
With the increasingly strict emission regulations and economic demands, variable valve trains are gaining in importance in Diesel engines. The valve control strategy has a great impact on the in-cylinder charge motions, turbulence level, thus also on the combustion and emission formation. In order to predict in-cylinder charge motions and turbulence properties for a working process calculation, a zero-/quasi-dimensional flow model is developed for the Diesel engines with a fully variable valve train. For the purpose of better understanding the in-cylinder flow phenomena, detailed three-dimensional CFD simulations of the intake and compression strokes are performed at different operating conditions with various piston configurations. In the course of model development, global in-cylinder charge motions are assigned to idealized flow fields.
Technical Paper

Evaluation of engine-related restriction for the global efficiency by using a rankine cycle based waste heat recovery system on heavy duty truck by means of 1D-Simulation

2018-04-03
2018-01-1451
As a promising concept to improve efficiency of a long-haul heavy duty truck with diesel engine, organic rankine cycle (ORC) based waste heat recovery system (WHR) by utilizing the exhaust gas-es from internal combustion engine will be paid increased attention in recent years. The greatest achievable global efficiency may be however restricted by the engine. On the one hand influences the engine operating conditions the exhaust gas temperature and quantity as waste heat source, and on the other hand limits the engine cooling system the heat rejection from condenser of WHR system. This paper aims to evaluate the impacts of the varied engine applications considering the affects of WHR system on the global efficiency and engine emissions. A complex 0D/1D-simulation model for a turbocharged production heavy duty engine with low-/ high-temperature cooling cir-cuit and a WHR system with ethanol as working fluid have been in GT-Suite established.
Technical Paper

Simulation of Autoignition, Knock and Combustion for Methane-Based Fuels

2017-10-08
2017-01-2186
Abstract Engine Knock is a stochastic phenomenon that occurs during the regular combustion of spark ignition (SI) engines and limits its efficiency. Knock is triggered by an autoignition of local “hot spots” in the unburned zone, ahead of the flame front. Regarding chemical kinetics, the temperature and pressure history as well as the knock resistance of the fuel are the main driver for the autoignition process. In this paper, a new knock modeling approach for natural gas blends is presented. It is based on a kinetic fit for the ignition delay times that has been derived from chemical kinetics simulations. The knock model is coupled with an enhanced burn rate model that was modified for Methane-based fuels. The two newly developed models are incorporated in a predictive 0D/1D simulation tool that provides a cost-effective method for the development of natural gas powered SI engines.
Technical Paper

Fuel Injection Analysis with a Fast Response 3D-CFD Tool

2017-09-04
2017-24-0103
Abstract Main limiting factor in the application of 3D-CFD simulations within an engine development is the very high time demand, which is predominantly influenced by the number of cells within the computational mesh. Arbitrary cell coarsening, however, results in a distinct distortion of the simulation outcome. It is rather necessary to adapt the calculation models to the new mesh structure in order to ensure reliability and predictability of the 3D-CFD engine simulation. In the last decade, a fast response 3D-CFD tool was developed at FKFS in Stuttgart. It aims for a harmonized interaction between computational mesh, implemented calculation models and defined boundary conditions in order to enable fast running simulations for engine development tasks. Their susceptibility to errors is significantly minimized by various measures, e.g. extension of the simulation domain (full engine) and multi-cycle simulations.
Journal Article

In-Situ Measurements of the Piston and Connecting Rod Dynamics Correlated with TEHL-Simulation Techniques

2017-09-04
2017-24-0157
Abstract High combustion pressure in combination with high pressure gradient, as they e.g. can be evoked by high efficient combustion systems and e.g. by alternative fuels, acts as broadband excitation force which stimulates natural vibrations of piston, connecting rod and crankshaft during engine operation. Starting from the combustion chamber the assembly of piston, connecting rod and crankshaft and the main bearings represent the system of internal vibration transfer. To generate exact input and validation values for simulation models of structural dynamic and elasto-hydrodynamic coupled multi-body systems, experimental investigations are done. These are carried out on a 1.5-l inline four cylinder Euro 6 Diesel engine. The modal behaviour of the system was examined in detail in simulation and test as a basis for the investigations. In an anechoic test bench airborne and structure-borne noises and combustion pressure are measured to identify the engine´s vibrational behaviour.
Journal Article

Two-Stage Ignition Occurrence in the End Gas and Modeling Its Influence on Engine Knock

2017-09-04
2017-24-0001
Abstract The most significant operation limit prohibiting the further reduction of the CO2 emissions of gasoline engines is the occurrence of knock. Thus, being able to predict the incidence of this phenomenon is of vital importance for the engine process simulation - a tool widely used in the engine development. Common knock models in the 0D/1D simulation are based on the calculation of a pre-reaction state of the unburnt mixture (also called knock integral), which is a simplified approach for modeling the progress of the chemical reactions in the end gas where knock occurs. Simulations of thousands of knocking single working cycles with a model representing the Entrainment model’s unburnt zone were performed using a detailed chemical reaction mechanism. The investigations showed that, at specific boundary conditions, the auto-ignition of the unburnt mixture resulting in knock happens in two stages.
Technical Paper

Wall Heat Transfer in a Multi-Link Extended Expansion SI-Engine

2017-09-04
2017-24-0016
Abstract The real cycle simulation is an important tool to predict the engine efficiency. To evaluate Extended Expansion SI-engines with a multi-link cranktrain, the challenge is to consider all concept specific effects as best as possible by using appropriate submodels. Due to the multi-link cranktrain, the choice of a suitable heat transfer model is of great importance since the cranktrain kinematics is changed. Therefore, the usage of the mean piston speed to calculate a heat-transfer-related velocity for heat transfer equations is not sufficient. The heat transfer equation according to Bargende combines for its calculation the actual piston speed with a simplified k-ε model. In this paper it is assessed, whether the Bargende model is valid for Extended Expansion engines. Therefore a single-cylinder engine is equipped with fast-response surface-thermocouples in the cylinder head. The surface heat flux is calculated by solving the unsteady heat conduction equation.
Journal Article

Development of an Innovative Combustion Process: Spark-Assisted Compression Ignition

2017-09-04
2017-24-0147
Abstract In the competition for the powertrain of the future the internal combustion engine faces tough challenges. Reduced environmental impact, higher mileage, lower cost and new technologies are required in order to maintain its global position both in public and private mobility. For a long time, researchers have been investigating the so called Homogeneous Charge Compression Ignition (HCCI) that promises a higher efficiency due to a rapid combustion - i.e. closer to the ideal thermodynamic Otto cycle - and therefore more work and lower exhaust gas temperatures. Consequently, a rich mixture to cool down the turbocharger under high load may no longer be needed. As the combustion does not have a distinguished flame front it is able to burn very lean mixtures, with the potential of reducing HC and CO emissions. However, until recently, HCCI was considered to be reasonably applicable only at part load operating conditions.
Technical Paper

Resonance Charging Applied to a Turbo Charged Gasoline Engine for Transient Behavior Enhancement at Low Engine Speed

2017-09-04
2017-24-0146
Abstract Upcoming regulations and new technologies are challenging the internal combustion engine and increasing the pressure on car manufacturers to further reduce powertrain emissions. Indeed, RDE pushes engineering to keep low emissions not only at the bottom left of the engine map, but in the complete range of load and engine speeds. This means for gasoline engines that the strategy used to increase the low end torque and power by moving out of lambda one conditions is no longer sustainable. For instance scavenging, which helps to increase the enthalpy of the turbine at low engine speed cannot be applied and thus leads to a reduction in low-end torque. Similarly, enrichment to keep the exhaust temperature sustainable in the exhaust tract components cannot be applied any more. The proposed study aims to provide a solution to keep the low end torque while maintaining lambda at 1.
Journal Article

Influence of Binary CNG Substitute Composition on the Prediction of Burn Rate, Engine Knock and Cycle-to-Cycle Variations

2017-03-28
2017-01-0518
Abstract Since 0D/1D-simulations of natural gas spark ignition engines use model theories similar to gasoline engines, the impact of changing fuel characteristics needs to be taken into consideration in order to obtain results of higher quality. For this goal, this paper proposes some approaches that consider the influence of binary fuel mixtures such as methane with up to 40 mol-% of ethane, propane, n-butane or hydrogen on laminar flame speed and knock behavior. To quantify these influences, reaction kinetics calculations are carried out in a wide range of the engine operation conditions. Obtained results are used to update and extend existing sub-models. The model quality is validated by comparing measured burn rates with simulation results. The benefit of the new sub-models are utilized by predicting the influence the fuel takes on engine operating limits in terms of knocking and lean misfire limits, the latter being determined by using a cycle-to-cycle variation model.
Technical Paper

Development and Experimental Investigation of a Two-Stroke Opposed-Piston Free-Piston Engine

2016-11-08
2016-32-0046
Abstract The proposed paper deals with the development process and initial measurement results of an opposed-piston combustion engine for application in a Free-Piston Linear Generator (FPLG). The FPLG, which is being developed at the German Aerospace Center (DLR), is an innovative internal combustion engine for a fuel based electrical power supply. With its arrangement, the pistons freely oscillate between the compression chamber of the combustion unit and a gas spring with no mechanical coupling like a crank shaft. Linear alternators convert the kinetic energy of the moving pistons into electric energy. The virtual development of the novel combustion system is divided into two stages: On the one hand, the combustion system including e.g. a cylinder liner, pistons, cooling and lubrication concepts has to be developed.
Technical Paper

Presenting a Fourier-Based Air Path Model for Real-Time Capable Engine Simulation Enhanced by a Semi-Physical NO-Emission Model with a High Degree of Predictability

2016-10-17
2016-01-2231
Abstract Longitudinal models are used to evaluate different vehicle-engine concepts with respect to driving behavior and emissions. The engine is generally map-based. An explicit calculation of both fluid dynamics inside the engine air path and cylinder combustion is not considered due to long computing times. Particularly for dynamic certification cycles (WLTC, US06 etc.), dynamic engine effects severely influence the quality of results. Hence, an evaluation of transient engine behavior with map-based engine models is restricted to a certain extent. The coupling of detailed 1D-engine models is an alternative, which rapidly increases the model computation time to approximately 300 times higher than that of real time. In many technical areas, the Fourier transformation (FT) method is applied, which makes it possible to represent superimposed oscillations by their sinusoidal harmonic oscillations of different orders.
Technical Paper

Transient Simulation of Nitrogen Oxide Emissions of CI Engines

2016-04-05
2016-01-1002
Abstract This paper presents a quasi-dimensional emission model for calculating the transient nitric oxide emissions of a diesel engine. Using conventional and high-speed measurement technology, steady-state and transient emissions of a V6 diesel engine were examined. Based on measured load steps and steady-state measurements a direct influence of the combustion chamber wall temperature on the nitric oxide emissions was found. Load steps to and from, as well as steady-state measurements down to almost stoichiometric global combustion air ratios were used to examine the behavior of nitric oxide formation under these operating conditions. An existing emission model was expanded in order to represent the direct influence of the combustion chamber wall temperature on the nitric oxide emissions as well as enabling the forecasting of nitric oxide emissions at low global combustion air ratios: Both particularly important aspects for the simulation of transient emissions.
Technical Paper

Investigation of the Gas Exchange (Scavenging) on a Single-Scroll Turbocharged Four Cylinder GDI Engine

2016-04-05
2016-01-1024
Abstract For scavenging the combustion chamber during the gas exchange, a temporary positive pressure gradient between the intake and the exhaust is required. On a single-scroll turbocharged four cylinder engine, the positive pressure gradient is not realized by the spatial separation of the exhaust manifold (twin-scroll), but by the use of suitable short exhaust valve opening times. In order to avoid any influence of the following firing cylinder onto the ongoing scavenging process, the valve opening time has to be shorter than 180 °CA. Such a short valve opening time has both, a strong influence on the gas exchange at the low-end torque and at the maximum engine power. This paper analyzes a phenomenon, which occurs due to short exhaust valve opening durations and late valve timings: A repeated compression of the burned cylinder charge after the bottom dead center, referred to as “recompression” in this paper.
Technical Paper

Numerical and Experimental Studies on Mixture Formation with an Outward-Opening Nozzle in a SI Engine with CNG-DI

2016-04-05
2016-01-0801
Abstract CNG direct injection is a promising technology to promote the acceptance of natural gas engines. Among the beneficial properties of CNG, like reduced pollutants and CO2 emissions, the direct injection contributes to a higher volumetric efficiency and thus to a better driveability, one of the most limiting drawbacks of today’s CNG vehicles. But such a combustion concept increases the demands on the injection system and mixture formation. Among other things it requires a much higher flow rate at low injection pressure. This can be only provided by an outward-opening nozzle due to its large cross-section. Nevertheless its hollow cone jet with a specific propagation behavior leads to an adverse fuel-air distribution especially at higher loads under scavenging conditions. This paper covers numerical and experimental analysis of CNG direct injection to understand its mixture formation.
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