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Technical Paper

Numerical Simulation of the Transient Heat-Up of a Passenger Vehicle during a Trailer Towing Uphill Drive

In the digital prototype development process of a new Mercedes-Benz, thermal protection is an important task that has to be fulfilled. In the early stages of development, numerical methods are used to detect thermal hotspots in order to protect temperature sensitive parts. These methods involve transient full Vehicle Thermal Management (VTM) simulations to predict dynamic vehicle heat-up during critical load cases. In order to simulate thermal control mechanisms, a coupled 1D to 3D thermal vehicle model is built in which the coolant and oil circuit of the engine, as well as the exhaust flow are captured in detail. When performing a transient 3D VTM analysis, the conduction and radiation phenomena are simulated using a transient structure model while the convective phenomena are co-simulated in a steady state fluid model. Both models are brought to interaction at predetermined points by an automatized coupling method.
Technical Paper

Development of Universal Brake Test Data Exchange Format and Evaluation Standard

Brake system development and testing is spread over vehicle manufacturers, system and component suppliers. Test equipment from different sources, even resulting from different technology generations, different data analysis and report tools - comprising different and sometimes undocumented algorithms - lead to a difficult exchange and analysis of test results and, at the same time, contributes to unwanted test variability. Other studies regarding the test variability brought up that only a unified and unambiguous data format will allow a meaningful and comparative evaluation of these data and only standardization will reveal the actual reasons of test variability. The text at hand illustrates that a substantial part of test variability is caused by a misinterpretation of data and/or by the application of different algorithms.
Technical Paper

Investigation on the Performance of a Mechanistic Electric Turbocharger Model for a Vehicular Fuel Cell System

The electric turbocharger is a promising type of air supply unit for future automotive fuel cell drive systems. It comprises of a centrifugal compressor, a variable geometry turbine and a permanent magnet synchronous motor assembled on a single shaft. Compared to other types of vehicular fuel cell air supplies, like for example a screw or roots compressor, it needs less installation space and has lower weight while also causing less noise and vibration. This paper presents a validated mechanistic model of the electric turbocharger. The stationary compressor model is based on a set of aerodynamic loss models with surge and stone wall line prediction capability. Similarly, the stationary variable axial turbine is a detailed station based model derived from aerodynamic losses at the turbine wheel and the stator blades. The aerodynamic losses incorporated in the compressor and the turbine models are implemented under MATLAB/Simulink and show a good correlation with the experimental data.
Technical Paper

Retrospective on Cubic Equation of State for R134a Refrigerant Used in Automotive Application

The need for a consistent and reliable calculation of thermodynamic property of refrigerants has been a topic of research since the past decade. This paper reports a study of various cubic equations of state for a refrigerant being used in automotive air-conditioning applications. The thermodynamic property of refrigerant 1,1,1,2 tetrafluoroethane (commercially known as R134a) is estimated for this purpose. A comparative analysis is made on three sets of equations of state. They are Redlich Kwong equation (RK), Peng Robinson equation (PR) and Patel Teja equation. It is found that the Patel-Teja and Peng-Robinson equations are accurate in the operating region of automotive air-conditioning system. Using these literature based equations and Maxwell correlations, thermodynamic models are developed. They estimate thermodynamic properties of saturated liquid/vapor, sub-cooled liquid and superheated vapor phases.
Journal Article

Air Spring Air Damper: Modelling and Dynamic Performance in Case of Small Excitations

Air spring systems gain more and more popularity in the automotive industry and with the ever growing demand for comfort nowadays they are almost inevitable. Some significant advantages over conventional steel springs are appealing for commercial vehicles as well as for the modern passenger vehicles in the luxury class. Current production air spring systems exist in combination with hydraulic shock absorbers (integrated or resolved). An alternative is to use the medium air not only as a spring but also as a damper: a so-called air spring air damper. Air spring air dampers are force elements which could be a great step for the chassis technology due to their functionality (frequency selectivity, load levelling, load independent vibration behaviour, load dependent damping). Some of their design which avoid dynamic seals by the using of rubber bellows contribute to a better ride comfort.
Technical Paper

Simulation Process of the Heat Protection of a Full Vehicle

In this paper the latest status of the Vehicle Thermal Management (VTM) simulation at the Mercedes-Benz Car Group is shown. First of all VTM is nowadays a routine simulation application and secondly it is embedded in a standard process which starts with the CAD data collection and ends with standard reporting of the simulation results and thirdly VTM is now an integrated simulation application in terms of VTM includes the classical underhood-underbody analysis, the analysis of electric/electronic components, the brake temperature analysis and last not least the thermal comfort of passengers. There is also a close link to the tests of vehicle hardware. Beside the operational simulation process there is a process installed which guarantees good quality of the results.
Technical Paper

Modeling of Injected Diesel Fuel Conversion and Heat Release in Oxidation Catalyst: 3D-CFD & 1D Channels Approach

A system for controlled heat generation in exhaust pipeline is studied, consisting of fuel injector and oxidation catalyst (plus connecting pipes). A 3D-CFD software (StarCD) coupled with a tailored 1D model of catalytic monolith channel (XMR) are employed for simulations of realistic, fully 3D system geometry. Exhaust gas flow, fuel injection, and distribution at the catalyst inlet is solved by 3D-CFD, while the processes inside individual representative channels are simulated by the effective 1D model. The 3D-CFD software calls iteratively the 1D channel model with proper boundary conditions and solves 3D temperature profile over the monolith, utilizing local enthalpy fluxes (including gas-solid heat transfer and reaction enthalpy) calculated by the 1D channel model. Seven representative hydrocarbons are used for characterisation of Diesel fuel composition with respect to catalytic oxidation kinetics.
Technical Paper

Low-speed Boom Noise - Escalating Relevance According to CO2- Targets and High Torque Engines

The increasing shift of drive operation towards efficient engine operation points at very low engine speeds demands a concerted design and tuning of engine, drive-train, assembly attachment and body to avoid annoying low speed boom noise. An additional challenge in this area of conflict is the increasing torque of modern engines at low engine speeds. As an example for a standard passenger car, the modes of operation, which may lead to low speed boom noise, are described. Setting levers along the complete chain of effect are characterised - from cylinder pressure up to the radiating surfaces of the interior. To achieve challenging NVH-targets the application of nonlinear simulation systems is indispensable, in particular in the concept phase of a vehicle. The use of multi-body simulation is presented for a concentrated NVH-optimisation of powertrain and rear axle vibration behaviour to reduce low-speed boom noise. On entire vehicle level hybrid simulation models are useful.
Journal Article

Predicted Roughness Perception for Simulated Vehicle Interior Noise

In the past the exterior and interior noise level of vehicles has been largely reduced to follow stricter legislation and due to the demand of the customers. As a consequence, the noise quality and no longer the noise level inside the vehicle plays a crucial role. For an economic development of new powertrains it is important to assess noise quality already in early development stages by the use of simulation. Recent progress in NVH simulation methods of powertrain and vehicle in time and frequency domain provides the basis to pre-calculated sound pressure signals at arbitrary positions in the car interior. Advanced simulation tools for elastic multi-body simulation and novel strategies to measure acoustical transfer paths are combined to achieve this goal. In order to evaluate the obtained sound impression a roughness prediction model has been developed. The proposed roughness model is a continuation of the model published by Hoeldrich and Pflueger.
Technical Paper

Using High-Fidelity Multibody Vehicle Models in Real-Time Simulations

Digital or virtual prototyping by means of a multibody simulation model (MBS) is a standard part of the automotive design process. A high-fidelity model is built and often correlated against test data to increase its accuracy. Once built the MBS model can then be used for high fidelity analysis in ride comfort, handling as well as durability. Next to the MBS model, current industry practice is to develop a reduced degree of freedom model for the design and validation of control or intelligent systems. The models used in the control system design are required to execute in hardware-in-the-loop (HIL) simulations where it is necessary to run real-time. The reason for the creation of the reduced degree of freedom models so far has been that the high-fidelity or off-line model does not execute fast enough to be used in an HIL simulation.
Journal Article

Experimental and Numerical Investigation of the Under Hood Flow with Heat Transfer for a Scaled Tractor-Trailer

Aerodynamic design and thermal management are some of the most important tasks when developing new concepts for the flow around tractor-trailers. Today, both experimental and numerical studies are an integral part of the aerodynamic and thermal design processes. A variety of studies have been conducted how the aerodynamic design reduces the drag coefficient for fuel efficiency as well as for the construction of radiators to provide cooling on tractor-trailers. However, only a few studies cover the combined effect of the aerodynamic and thermal design on the air temperature of the under hood flow [8, 13, 16, 17, 20]. The objective of this study is to analyze the heat transfer through forced convection for a scaled Cab-over-Engine (CoE) tractor-trailer model with under hood flow. Different design concepts are compared to provide low under hood air temperature and efficient cooling of the sub components.
Technical Paper

A Numerical Methodology to Compute Temperatures of a Rotating Cardan Shaft

In this paper a new numerical methodology to compute component temperatures of a rotating cardan shaft is described. In general temperatures of the cardan shaft are mainly dominated by radiation from the exhaust gas system and air temperatures in the transmission tunnel and underbody. While driving the cardan shaft is rotating. This yields a uniform temperature distribution of the circumference of the shaft. However most simulation approaches for heat protection are nowadays steady-state computations. In these simulations the rotation of the cardan shaft is not considered. In particular next to the exhaust gas system the distribution of the temperatures of the cardan shaft is not uniform but shows hot temperatures due to radiation at the side facing the exhaust gas system and lower temperatures at the other side. This paper describes a new computational approach that is averaging the radiative and convective heat fluxes circumferentially over bands of the cardan shaft.
Technical Paper

Challenges and Opportunities of Numerically Simulating the Idle Load Case for Vehicle Thermal Management

Collective life-cycle data is needed when developing components like elastomer suspension mounts. Life-time prediction is only possible using thermal load frequency distributions. In addition to current extreme load cases, the Idle Load Case is examined at Mercedes-Benz Car Group as a collective load case for Vehicle Thermal Management (VTM) numerical simulations in early development stages. It combines validation opportunities for HVAC, cooling and transmission requirements in hot-country-type ambient conditions. Experiments in climatic wind tunnels and coupled 3D CFD and heat transfer simulations of the Idle Load Case have been performed. Measurements show steady conditions at the end of the load case. Decoupling of the torque converter, changes in ambient temperature and the technical implementation of a wind barrier for still air conditions exhibit influence on component-level results. Solar load, however, does not significantly change the examined component temperatures.
Technical Paper

Virtual Transfer Path Analysis at Daimler Trucks

As for passenger cars, the overall noise and vibration comfort in commercial trucks and busses becomes an increasingly important sales argument. In order to effectively reduce the noise and vibration levels it is required to identify possible NVH issues at an early stage in the vehicle development process. For this reason a so-called “Virtual Transfer Path Analysis” (VTPA) method has been implemented which combines the results obtained from the conventional multi-body simulation and finite element method approaches. The resulting VTPA tool enables Daimler Trucks to systematically investigate and predict the complex interaction between powertrain excitation and the resulting vehicle response well before hardware prototypes become available. An overview of the theory is presented as well as the practical application and outcome of the technique applied in a past product development.
Technical Paper

Numerical Simulation of the Flow through an Alternator inside an Engine Compartment of a Passenger Car

In this study the numerical simulation of the flow through an alternator inside an engine compartment of a passenger car is investigated. Specifically the interaction of the flow through the alternator with the flow through the engine compartment is explored in detail. The results are compared with a corresponding numerical simulation of an alternator in a surrounding of a test facility and with a numerical simulation of the flow through an engine compartment without taking into account the internal flow through the alternator. Finally the air temperature near the alternator and also the temperature of some components inside the alternator are compared with experimental values measured during a typical load case used for the thermal protection of the passenger car.
Journal Article

Soot Simulation under Diesel Engine Conditions Using a Flamelet Approach

The subject of this work is 3D numerical simulations of combustion and soot emissions for a passenger car diesel engine. The CFD code STAR-CD version 3.26 [1] is used to resolve the flowfield. Soot is modeled using a detailed kinetic soot model described by Mauss [2]. The model includes a detailed description of the formation of polyaromatic hydrocarbons. The coupling between the turbulent flowfield and the soot model is achieved through a flamelet library approach, with transport of the moments of the soot particle size distribution function as outlined by Wenzel et al. [3]. In this work we extended this approach by considering acetylene feedback between the soot model and the combustion model. The model was further improved by using new gas-phase kinetics and new fitting procedures for the flamelet soot library.
Journal Article

Numerical Simulation of DOC+DPF+SCR systems:DOC Influence on SCR Performance

A numerical model for a diesel oxidation catalyst (DOC) is presented. It is based on a spatially 1D, physical and chemically based modeling of the relevant processes within the catalytic monolith. A global reaction kinetic approach has been chosen to describe the chemical reactions. Water condensation and evaporation was also considered, in order to predict the cold start behavior. Reaction kinetic parameters have been evaluated from a series of laboratory experiments. A correlation between the kinetic parameters and the noble metal loading was developed. The model was used in combination with a SCR-Model to study the influence of changes of noble metal loading and DOC volume on the overall transient NOx performance of a DOC+DPF+SCR system.
Technical Paper

Flow Maldistribution Effects on DPF Performance

This paper focuses on some of the DPF system design issues where 3-dimensional modeling is necessary. The study is based on an existing 3-dimensional DPF model (axitrap) which is coupled to a commercial CFD code (Star-CD, CD-Adapco). The main focus is the effect of the inlet pipe geometry on soot distribution in the filter during loading and regeneration mode. The results show that due to the self-balancing effect, the resulting soot distribution in the filter under typical loading modes with low flow rates is quite uniform. With the assumption of adiabatic inlet pipe, the effect of non-symmetric inlet pipe is also negligible even during regeneration. However, under the realistic assumption of a non-adiabatic inlet pipe, the effect of inlet pipe geometry becomes very significant. Especially, for the case of a bent-shaped inlet pipe, the risk of impartial regeneration of the filter increases significantly.
Technical Paper

Using Simulation to Verify Diagnosis Algorithms of Electronic Systems

In modern vehicles the architecture of electronics is growing more and more complex because both the number of electronic functions – e.g. implemented as software modules – as well as the level of networking between electronic control units (ECUs) is steadily increasing. This complexity leads to greater propagation of failure symptoms, and diagnosing the causes of failure becomes a new challenge. Diagnostics aims at detecting failures such as defect sensors or faulty communication messages. It is subdivided into diagnosis algorithms on an ECU and algorithms running offboard, e.g. on a diagnostic tester. These algorithms have to complement each other in the best possible way. While in the past the diagnosis algorithm was developed late in the development process, nowadays there are efforts to start the development of such algorithms earlier – at least in parallel to developing a new feature itself. This would allow developers to verify the diagnosis algorithms in early design stages.
Technical Paper

Soot Model Calibration Based on Laser Extinction Measurements

In this work a detailed soot model based on stationary flamelets is used to simulate soot emissions of a reactive Diesel spray. In order to represent soot formation and oxidation processes properly, a calibration of the soot reaction rates has to be performed. This model calibration is usually performed on basis of engine out soot measurements. Contrary to this, in this work the soot model is calibrated on local soot concentrations along the spray axis obtained from laser extinction chamber measurements. The measurements are performed with B7 certification Diesel and a series production multihole injector to obtain engine similar boundary conditions. In order to ensure that the flow and mixture field is captured well by the CFD-simulation, the simulated liquid penetration lengths and flame lift-off lengths are compared to chamber measurements.