Two major technologies to reduce CO2 emissions are the use of high EGR levels to increase the compression ratio and reduce the knock sensitivity and the stratified combustion. A limiting parameter for these combustion modes is the stability of the ignition. The ignition requirements are very much dependent on the combustion system considered. The size and the duration of the spark and the temporal evolution of the energy transferred into the gas are major parameters to be controlled.
To develop a high performance ignition system at acceptable cost for the automotive industry, it is important to avoid strong technology changes by conserving the spark based system. For this it is important to understand the energy path from the power electronics through the arc until the initiation of the combustion. This knowledge remains insufficient today as well in the industrial environment as within fundamental research teams. <br />A number of ignition models used today are limited to a simple energy deposit and even the most sophisticated models do not take in account all physical effects and its interactions, missing even the most important effects for the engine application. <br />The most difficult part is the modeling of the hot plasma and the interaction plasma - ignition kernel. Starting with the state-of-the-art and our own experience we need to understand the physical and chemical mechanisms that determine the minimum energy deposit into the ignition volume to reach the right temperature to initiate the chemical reactions. Moreover, once the chemical reactions started, we need to guarantee the propagation of the flame front taking in account the heat losses on the surface of the kernel. <br />The main roadblock can be defined as: « What need to be the characteristics of the plasma arc to allow an optimal initiation of the combustion of highly dilutes mixtures ?« <br />In consequence, FAMAC is a fundamental research project which aims the understanding of the physics of the initial arc, of the heat and radical transfer into the ignitable gas and the initialization of the chemical combustion process using an approach integrating different levels of modeling, simulation and experimental validation. At the end the project should allow to identify the physical key parameters for the design of future ignition systems.
The project approaches the technologic/scientific bottlenecks in the following way:
• Combine fundamental research institutes (with fundamental, aeronautics and automotive backgrounds) with an applied automotive research institute and an industrial automotive supplier.
• Develop a methodology integrating detailed plasma modeling with the simulation of the turbulent flow and detailed chemistry to better understand the energy transfer mechanisms and the initiation of the turbulent flame kernel.
• Combine the simulation at different level of details (plasma, standard chemistry, LES, DNS) with sophisticated experimental investigations for a rigorous validation of each modeling step:
- Emission spectroscopy characterises the vibrational and rotational temperature of the plasma
- Spontaneous Raman diffusion measures the vibrational and rotational temperatures in the periphery of the arc and during post-discharge.
- Fast Schlieren visualization characterizes the shock wave and the rapid hot gas expansion
- Emission spectroscopy in the spark determines the concentrations of the different species produced in the plasma, the electron density and the temperature at the end of the discharge
• The project integrates the steps from detailed local plasma physics until the ignition in the combustion engine by linking specific simulation methods and validates each step experimentally: DNS of the plasma towards DNS with complex chemistry, DNS with simplified chemistry and LES in a complex realistic geometry with simplified chemistry.
• Extract from this analysis the most important requirements for an innovative ignition system for highly diluted combustion systems.
The project includes 3 highly linked tasks, each with a simulation and an experimental approach:
- Characterization and validation of the ignition requirements in a transparent single cylinder engine
- Fundamental research using a simplified geometry
- Fundamental research using a more complex geometry
The main results are so far:
• The characterization of the thermodynamic and aerodynamic conditions during the ignition phase in the engine
• The evaluation of different ignition systems in terms of ignition stability for two different engine operating points with a variation of air and EGR dilution
• The documentation of the fact that the main parameter limiting the dilution for these operating points is the total energy supplied independent of the spark duration (ns to ms) and the number of spark breakthroughs
• The supply of a database for the thermodynamic and transport properties at extremely high temperatures (<50000K) for air and the ignitable mixture
The experimental part on the engine dynamometer produced interesting results concerning the comparison of different ignition systems and is used to well define the detailed combustion bomb experiments.
The experimental activity planned to characterize the initial arc phase created a number important technical challenges due to the extremely short timescale which requires data acquisition in the ns time scale. A first not expected difficulty was the discovery of a temporal jitter of the ignition system of the order of a micro s, 1000 more than the needed precision. This made us choose the CORIA ns ignition system having sufficient precision as reference system for the simulation and the experimental validation.
The plasma simulation was developed meanwhile on the basis of data from literature, unfortunately not complete allowing only a partial validation today
G. L. Pilla, L. de Francqueville, Stabilization of Highly Diluted Gasoline Direct Injection Engine using Innovative Ignition Systems, 1. SAE Birmingham
M. Castela, B. Fiorina, A. Coussement, O. Gicquel, N. Darabiha, C. Laux “Direct numerical simulations of pulsed discharges assisted combustion in quiescent and turbulent flow conditions” International Journal of energetic materials and chemical propulsion
C.O. Laux, D. Rusterholtz, D.A. Xu, M. Simeni, D. Pai, G. Pilla, D. Lacoste, G. Stancu, “Nanosecond Repetitively Pulsed Discharges for Plasma-Assisted Combustion,” 8th International Conference on Reactive Plasmas and 31st Symposium on Plasma Processing
The FAMAC program is proposed in a context of CO2 emissions reduction, in order to get an average of 95g CO2/km in the European fleet by 2020. This project is part of the EURO 6c reduction of pollutant emissions from 2017 and in respect of the forthcoming EURO 7 to be defined for after 2020.
The downsizing of spark ignition engines associated with the diluted combustion is one of the main areas identified for gains in CO2. The implementation of high EGR rates at medium and high load allows to increase the compression ratio and to push the knock limit in order to improve engine performance over the entire operating range. Stratified combustion is an additional lever to envisage the high rates of EGR down to very low loads, while maximizing energy efficiency.
The state of the art shows that the stability of combustion initiation is the limiting factor for these combustion modes, greatly reducing the expected gains in CO2. Continental's research confirms the difficulty of initiation and demonstrates that the needs in terms of initiating combustion are very different and depend on the combustion system considered. It also shows the potential of arc-initiated combustion. The size and duration of the spark are as relevant as the temporal evolution of the energy transmitted by the spark to the gas.
In order to develop an efficient ignition system at an acceptable cost for the automotive industry, it is important to avoid any technological breakthrough by maintaining arc systems. It is therefore essential to understand the energy chain of power electronics through the arc until initiation of combustion. Currently, this knowledge remains insufficient both in industry in basic research.
FAMAC is a fundamental research project dedicated to the understanding of the arc’s physics, the heat transfer to the flammable gas and the combustion initiation, in an approach that integrates different levels of modeling, simulation and experimental validation. This project consists of three main tasks strongly related to one another:
- Characterization and validation of an efficient ignition system requirements;
- Fundamental research in simplified geometry;
- Fundamental research in complex geometry.
Some innovative activities of fundamental research conducted as part of a PhD project impose of 4-year project. The total FAMAC budget is $ 3.6M€.
The association of an industrial partner (Continental) with an applied research institute (IFPEN) and key experts of plasma and ignition in France (LAPLACE, EM2C, CORIA, CERFACS) guarantees to reach the final deliverables of the program, meaning significant progress in:
- the detailed modeling of the arc, the plasma (in-and off-equilibrium) and the mechanisms of transfer to gas (heat and radicals) versus time;
- the understanding of the key factors in the optimization of the ignition functions in automotive engines.
Monsieur Hans NUGLISCH (Continental Automotive France SAS) – firstname.lastname@example.org
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
IFPEN IFP Energies Nouvelles
CERFACS Centre Européen de Recherche et de Formation Avancée en Calcul Scientifique
CNRS - EM2C Laboratoire d'Energétique Moléculaire et Macroscopiq, Combustion
LAPLACE LAboratoire PLAsma et Conversion d’Energie
CNRS - CORIA CNRS - COmplexe de Recherche Interprofessionnel en Aérothermochimie
CAF Continental Automotive France SAS
Help of the ANR 1,590,924 euros
Beginning and duration of the scientific project: September 2012 - 48 Months