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Technical Paper

A Consistent Flamelet Model to Describe the Interaction of Combustion Chemistry and Mixing in the Controlled Auto Ignition Regime

In internal combustion engines operating in Controlled Auto Ignition (CAI) mode, combustion phasing and heat-release rate is controlled by stratification of fuel, fresh air, and hot internally recirculated exhaust gases. Based on the Representative Interactive Flamelet (RIF) model, a two-dimensional flamelet approach is developed. As independent parameters, firstly the fuel mixture fraction and secondly the mixture fraction of internally recirculated exhaust gases are considered. The flamelet equations are derived from the transport equations for species mass fraction and total enthalpy, employing an asymptotic analysis. A subsequent coordinate transformation leads to the phase space formulation of the two-dimensional flamelet equations. By the use of detailed chemical reaction mechanisms, the effects of dilution, temperature, and chemical species composition due to the internally recirculated exhaust gases are represented.
Technical Paper

A Reduced Kinetic Reaction Mechanism for the Autoignition of Dimethyl Ether

A reduced kinetic reaction mechanism for the autoignition of dimethyl ether is presented in this paper. Dimethyl ether has proven to be one of the most attractive alternatives to traditional fossil fuels for compression ignition engines. It can either be produced from biomass or from fossil oil. For dimethyl ether, Fischer et al. (Int. J.Chem. Kinet. 32 ( 12 ) (2000) 713-740) proposed a detailed reaction mechanism consisting of 79 species and 351 elementary reactions. In the present work, this detailed mechanism is systematically reduced to 31 species and 49 reactions. The reduced mechanism is discussed in detail with special emphasis on the high temperature thermal decomposition of dimethyl ether and on the fuel specific depleting reactions, which produce the methoxymethyl radical. In addition, a reaction pathway analysis for low temperature combustion is applied, where hydroperoxy-methylformate is found to be the dominating parameter for the low temperature regime.
Technical Paper

Combined Simulations and OH-Chemiluminescence Measurements of the Combustion Process using Different Fuels under Diesel-Engine like Conditions

The influence of different fuels and injection pressures on the flame lift-off length (LOL), as well as the combustion structure under quiescent conditions in a heated high-pressure vessel were experimentally investigated using OH chemiluminescence measurements. This data was used to validate the newly developed G-equation coupled with MRIF (G-MRIF) model, which was designed to describe the lifted Diesel combustion process. The achieved results are very promising and could be used as a tool to apply this combustion mode into Diesel engines. Furthermore these measurements were used to validate the approach of a new combustion model, which was developed using former OH chemiluminescence measurements by the authors. Based on this approach the LOL is mainly determined by auto-ignition and therefore highly dependent on the cetane number. This model is presented in more detail within this work.
Journal Article

Fuel Cell System Development: A Strong Influence on FCEV Performance

In this article, the development challenges of a fuel cell system are explained using the example of the BREEZE! fuel cell range extender (FC-REX) applied in an FEV Liiona. The FEV Liiona is a battery electric vehicle based on a Fiat 500 developed by FEV. The BREEZE! system is the first applied 30 kW low temperature polymer electrolyte membrane (LT PEM) fuel cell system in the subcompact vehicle class. Due to the highly integrated system approach and dry cathode operation, a compact design of the range extender module with a system power density of 0.45 kW/l can be achieved so that the vehicle interior including trunk remains completely usable. System development for fuel cells significantly influences performance, efficiency, package, durability, and required maintenance effort of a fuel cell electric powertrain. In order to ensure safe and reliable operation, the fuel cell system has to be supplied with sufficient amounts of air, hydrogen, and coolant flows.
Technical Paper

Functional Safety for Hybrid and Electric Vehicles

Hybrid and electric vehicles present a promising trade-off between the necessary reductions in emissions and fuel consumption, the improvement in driving pleasure and performance of today's and tomorrow's vehicles. These hybrid vehicles rely primarily on electronics for the control and the coordination of the different sub-systems or components. The number and complexity of the functions distributed over many control units is increasing in these vehicles. Functional safety, defined as absence of unacceptable risk due to the hazards caused by mal-function in the electric or electronic systems is becoming a key factor in the development of modern vehicles such as electric and hybrid vehicles. This important increase in functional safety-related issues has raised the need for the automotive industry to develop its own functional safety standard, ISO 26262.
Technical Paper

Injection Rate Shaping Investigations on a Small – Bore DI Diesel Engine

So far, the effect of injection rate shaping on the diesel combustion in small-bore DI diesel engines has not been extensively investigated, especially at high part load conditions with high EGR rates. The benefit of injection rate shaping is already verified for heavy duty engines at high load conditions with and without EGR. For this investigation, single cylinder engine investigations were conducted at the VKA / RWTH Aachen University. In order to meet the future NOx legislation limits like US-Tier2Bin5 it is crucial to reduce NOx especially at the high load points of the certification cycles, as FTP75 or US06. For the single cylinder investigations two part load points were chosen, which have relevance for the mentioned certification cycles. The experimental work focuses on different rate shapes as rectangular (Common-Rail type), ramp and boot shape at high EGR rates.
Technical Paper

Investigation of Spray-Bowl Interaction Using Two-Part Analysis in a Direct-Injection Diesel Engine

The purpose of this study is to investigate the effect of spray-bowl interaction on combustion, and pollutants formation at one specific high-load point of a single-cylinder small-bore diesel engine through computational analysis. The simulations are performed using Representative Interactive Flamelet (RIF) model with detailed chemical kinetics. Detailed chemistry-based soot model is used for the prediction of soot emissions. The simulations are performed for five different injection timings. Model-predicted cylinder pressure and exhaust emissions are validated against the measured data for all the injection timings. A new method - Two-part analysis - is then applied to investigate the spray-bowl interaction. Two-part analysis splits the volume of the combustion chamber into two, namely the piston bowl and the squish volume. Through analysis, among others the histories of soot, carbon monoxide (CO) and nitric oxide (NO ) emissions inside both volumes are shown.
Technical Paper

Laminar Burning Velocities of Nitrogen Diluted Standard Gasoline-Air Mixture

To understand how laminar burning velocities of standard unleaded gasoline-air-mixtures change by varying the concentration of oxygen in the combustible mixture, experimentally and numerical investigations are conducted in this work. Experiments were performed using a heatable pressure vessel which enables optical access. A monochromatic high-speed Schlieren cinematography measurement system combined with a high-speed CCD camera were used to track the propagating spherical flame fronts in the vessel. Numerically, freely propagating one dimensional laminar steady flame calculations were conducted for Primary-Reference-Fuel Air Mixtures (PRF87 or RON87), corresponding for standard gasoline combustible mixtures. Two combustible mixtures were investigated: (1) with air as oxidizer; (2) oxidizer consisting of 15% O2 and 85% N2 by mole fractions. The initial temperature for all investigated mixtures was 373 K.
Technical Paper

Laminar Spherical Flame Kernel Investigation of Very Rich Premixed Hydrocarbon-Air-Mixtures in a Closed Vessel under Microgravity Conditions

In this work very rich premixed laminar spherical flame kernels of hydrocarbon-air combustible mixtures were experimentally and numerically investigated under microgravity conditions. These microgravity combustion experiments were carried out in the Drop Tower of Bremen University. The Closed-Vessel-Bomb-Method (CVBM) was applied for all experimental investigations combined with a monochromatic Helium-Neon-Schlieren Measurement Technique. Images of the propagating spherical flames were tracked with a High-Speed-Camera. The pressure vessel enables optical access and contains a volume of approx. half a litre. Combustible Mixtures were investigated at initial pressures up to 30 bar and initial temperatures were 420 K for all experiments, whereas the equivalence ratio for investigated N-Pentane-Air-Mixtures was Φ=3.0, N-Hexane-Air-Mixtures was Φ=3.3, N-Heptane-Air-Mixtures was Φ=3.5 and the equivalence ratio for investigated Isooctane-Air-Mixtures was Φ=3.9 for all experiments.
Technical Paper

Measurements of Laminar Flame Velocity and Markstein Length for Standard Gasoline and a Corresponding Reference Fuel Mixture (PRF87)

An experimental and numerical investigation of commercial Gasoline (octane number = 90) with a reference fuel (PRF87) were accomplished. Laminar Flame Velocities and Markstein Numbers of these fuel air mixtures were investigated and compared with each other and with numerical results. PRF87 is presented as a reference fuel for Gasoline defined as 87 percent Iso-Octane and 13 percent N-Heptane by volume at ambient conditions. Spherical flames of Gasoline- and PRF87-Air-Mixtures at initial temperature of 373 K, initial pressure range from 10 bar to 25 bar and equivalence ratios from ϕ = 0.7 to ϕ = 1.2 were experimentally investigated using the Constant Volume Bomb Method.
Technical Paper

Modeling of HCCI Combustion Using Adaptive Chemical Kinetics

In this paper an online method for automatically reducing complex chemical mechanisms for simulations of combustion phenomena has been developed. The method is based on the Quasi Steady State Assumption (QSSA). In contrast to previous reduction schemes where chemical species are selected only when they are in steady state throughout the whole process, the present method allows for species to be selected at each operating point separately generating an adaptive chemical kinetics. The method is used for calculations of a natural gas fueled engine operating under Homogenous Charge Compression Ignition (HCCI) conditions. We discuss criteria for selecting steady state species and the influence of these criteria on the results such as concentration profiles and temperature.
Technical Paper

On the Potential of Low Heat Rejection DI Diesel Engines to Reduce Tail-Pipe Emissions

Heat transfer to the combustion chamber walls constitutes a significant portion of the overall energy losses over the working cycle of a direct injection (DI) diesel engine. In the last few decades, numerous research efforts have been devoted to investigating the prospects of boosting efficiency by insulating the combustion chamber. Relatively few studies have focused on the prospects of reducing emissions by applying combustion chamber insulation. A main purpose of this study is to assess the potential of reducing in-cylinder soot as well as boosting aftertreatment performance by means of partially insulating the combustion chamber. Based on the findings from a conceptual study, a Low Heat Rejection (LHR) design, featuring a Nimonic 80A insert into an Aluminum piston, was developed and tested experimentally at various loads in a single-cylinder Hatz-engine.
Technical Paper

Partially Premixed Combustion of Gasoline Type Fuels Using Larger Size Nozzle and Higher Compression Ratio in a Diesel Engine

If fuels that are more resistant to auto-ignition are injected near TDC in compression ignition engines, they ignite much later than diesel fuel and combustion occurs when the fuel and air have had more chance to mix. This helps to reduce NOX and smoke emissions at much lower injection pressures compared to a diesel fuel. However, PPCI (Partially Premixed Compression Ignition) operation also leads to higher CO and HC at low loads and higher heat release rates at high loads. These problems can be significantly alleviated by managing the mixing through injector design (e.g. nozzle size and centreline spray angle) and changing CR (Compression Ratio). This work describes results of running a single-cylinder diesel engine on fuel blends by using three different nozzle design (nozzle size: 0.13 mm and 0.17 mm, centreline spray angle: 153° and 120°) and two different CRs (15.9:1 and 18:1).
Journal Article

Performance Plus Range: Combined Battery Concept for Plug‑In Hybrid Vehicles

PlugIn Hybrid Electric Vehicles (PHEV) offer the opportunity to experience electric driving without the risk of vehicle break-down due to a low battery charge state. Thus, PHEV's represent an attractive means of meeting future CO2-legislation. PHEV batteries must fulfill a divergent list of requirements: on the one hand, the battery must supply sufficient energy to ensure it can be driven an appropriate distance in EV-mode. On the other hand, even with a low state-of-charge (SOC), the battery must supply sufficient power to assist the engine in vehicle acceleration or to recuperate on deceleration. This leads to a compromise in terms of cell selection. Fundamentally, high energy cells cannot provide high charge and discharge rates and high power cells cannot provide sufficient energy.
Technical Paper

Simulating the Combustion in a DI Diesel Engine Applying a New Model for the Conditional Scalar Dissipation Rate

The Representative Interactive Flamelet (RIF)-model offers a method of separating the numerical effort associated with solving the governing equations for the turbulent flow field from that of the chemistry. This is possible since the chemical time scales can be considered very small compared to those related to the turbulence. The concept has gained widespread recognition owing to its ability of capturing the essential physics underlying combustion. The objective of this paper is to show how a more accurate description of mainly the soot formation and oxidation processes in a high-speed small-bore Direct Injection (DI) diesel engine can be realized within the framework of the RIF-model. This is achieved by deriving a new model for the conditional scalar dissipation rate, describing the transport in the flamelet.
Journal Article

Super-Knock Prediction Using a Refined Theory of Turbulence

The occurrence of severe events of ‘super-knock’ originating from random pre-ignition kernels which sometimes is observed in turbo-charged spark-ignition engines was recently attributed by Kalghatgi and Bradley [4] to developing detonations which originate from a resonance between acoustic waves emitted by an auto-igniting ‘hot spot’ and a reaction wave which propagates along negative temperature gradients in the fuel-air mixture. Their occurrence depends on the steepness of the local instantaneous temperature gradient and on the length of the region of negative gradient. The theory requires that the temperature gradient extends smoothly over a sufficient length in the turbulent flow field. Then localized detonations may develop which are able to autoignite the entire charge within less than a millisecond and thus cause pre-ignition and ‘super-knock’.
Technical Paper

The Influence of Fuel Boiling Temperature on Common Rail Spray Penetration and Mixture Formation for Ethanol and Propylene-Glycol

An intricate experimental investigation of Common-Rail-Sprays were done using a High Pressure Chamber, a Common-Rail-Injection-System as well as three optical measurement techniques. Ethanol and Propylene-Glycol (of purity for spectroscopic applications >99.9%) were used as fuels. The experimental boundary conditions of the high pressure chamber were up to 5 MPa and 800K. In detail an optical shadowgraph imaging and Mie-scattering technique were used. Liquid and gas phase spray penetration are investigated for fuels with low and high volatility respectively boiling temperature (propylene glycol, ethanol) for a variation of ambient gas phase temperature and density. Spatial information of the mixing process of both fuels is obtained by the 1D spontaneous Raman scattering (1D-RS) technique.
Technical Paper

Traction Battery and Battery Control Unit Development

The performance of high voltage batteries is the key factor for further success of electric vehicles. The primary areas for battery development include high voltage (HV) and functional safety, maximum power and usable energy, battery life, packaging and weight reduction. This paper explains the development of the HV battery and the battery management system for the FEV Liona fleet, a retrofit of a pure electric powertrain into a FIAT 500. The multi-disciplinary process used to develop this program includes electrical, mechanical and functional aspects. The layout of the electrical system includes cell selection, layout of modules and the interconnection of twelve modules to a battery pack. The mechanical design of mounting the battery under the floor addresses the housing issues regarding robustness and sealing, the packaging into the vehicle as well as the positioning of the HV components inside the battery.