Model-based pre-ignition diagnostics in a race car application

Since 2014, Formula 1 engines have been turbocharged spark-ignited engines. In this scenario, the maximum engine power available in full-load conditions can be achieved only by optimizing combustion phasing within the cycle, i.e., by advancing the center of combustion until the limit established by the occurrence of abnormal combustion. High in-cylinder pressure peaks and the possible occurrence of knocking combustion significantly increase the heat transfer to the walls and might generate hot spots inside the combustion chamber. This work presents a methodology suitable to properly diagnose and control the occurrence of pre-ignition events that emanate from hot spots. The methodology is based on a control-oriented model of the ignition delay, which is compared to the actual ignition delay calculated from the real-time processing of the in-cylinder pressure trace. When the measured ignition delay becomes significantly smaller than that modeled, it means that ignition has been activated by a hot spot instead of the spark plug. In this case, the presented approach, implemented in the electronic control unit (ECU) that manages the whole hybrid power unit, detects a pre-ignition event and corrects the injection pattern to avoid the occurrence of further abnormal combustion

Canonical sampling through velocity-rescaling

We present a new molecular dynamics algorithm for sampling the canonical distribution. In this approach the velocities of all the particles are rescaled by a properly chosen random factor. The algorithm is formally justified and it is shown that, in spite of its stochastic nature, a quantity can still be defined that remains constant during the evolution. In numerical applications this quantity can be used to measure the accuracy of the sampling. We illustrate the properties of this new method on Lennard-Jones and TIP4P water models in the solid and liquid phases. Its performance is excellent and largely independent on the thermostat parameter also with regard to the dynamic properties

Nuclear quantum effects in solids using a colored-noise thermostat

We present a method, based on a non-Markovian Langevin equation, to include quantum corrections to the classical dynamics of ions in a quasi-harmonic system. By properly fitting the correlation function of the noise, one can vary the fluctuations in positions and momenta as a function of the vibrational frequency, and fit them so as to reproduce the quantum-mechanical behavior, with minimal a priori knowledge of the details of the system. We discuss the application of the thermostat to diamond and to ice Ih. We find that results in agreement with path-integral molecular dynamics can be obtained using only a fraction of the computational effort.Comment: submitted for publicatio

Arboriculture Biologique : 11 années d'expérimentation en vergers de pêchers et pommiers

Un programme de recherche a été conduit en arboriculture biologique pendant 11 ans (1994-2004) à l'unité expérimentale INRA (Institut National de la Recherche Agronomique) de Gotheron dans le Sud-Est de la France, en partenariat avec Agribiodrôme (F-26150 Die), asssociation de développement de l'Agriculture Biologique (AB) en Drôme, et le GRAB (Groupe de Recherche en Agriculture Biologique, F-84911 Avignon). Les objectifs étaient : (i) acquérir des références en arboriculture biologique ; (ii) identifier et analyser les verrous techniques de la production de pêches et pommes biologiques, afin d'optimiser la conduite des vergers AB et/ou de proposer de nouvelles expérimentations ; (iii) évaluer à long terme l'impact du mode de production pour le sol et la communauté d'arthropodes du verger. Le suivi longitudinal réalisé dans les 3 vergers expérimentaux de pêchers et pommiers a permis de souligner certaines contraintes en culture pérenne AB, notamment pour la conduite du sol (entretien de la fertilité, enracinement limité par le travail du sol sur le rang pour limiter les adventices) et la protection du verger. Probablement en rapport avec une fertilisation modérée et une conduite de la protection favorable à des régulations naturelles, certains ravageurs ne se sont pas développés : acariens phytophages, pucerons du pêcher,... D'autres bio-agresseurs ont pu être contrôlés par l'utilisation de méthodes biotechnique ou microbiologique : confusion sexuelle (tordeuse orientale), virus de la granulose (carpocapse),... En revanche, la maîtrise de certains bio-agresseurs a nécessité l'utilisation d'importantes quantités de matières actives présentant des effets non intentionnels (cuivre contre la cloque du pêcher, soufre contre la tavelure du pommier) et/ou a été insuffisante (puceron cendré du pommier, maladies de conservation du pêcher). Le suivi de la teneur en nitrates de la solution du sol à différentes profondeurs a permis d'établir que le risque de lessivage, et donc de pollution par les nitrates, était nul pour notre stratégie de fertilisation et notre situation. Enfin, l'étude de la communauté des arthropodes dans le verger de pommiers AB (2001-2003) a mis en évidence une biomasse plus élevée et un cortège d'auxiliaires de structure différente (prédominance de prédateurs polyphages) par rapport à d'autres vergers conduits en conventionnel. En raison d'une efficacité limitée et/ou d'effets non intentionnels de la lutte directe contre les bio-agresseurs en verger, il est donc important de concevoir et de gérer le verger AB afin de limiter le recours aux intrants phytosanitaires : variétés, distances de plantation, architecture de l'arbre, aménagement de l'environnement du verger, prophylaxie,..

Directional translocation resistance of Zika xrRNA

xrRNAs from flaviviruses survive in host cells because of their exceptional dichotomic response to the unfolding action of different enzymes. They can be unwound, and hence copied, by replicases, and yet can resist degradation by exonucleases. How the same stretch of xrRNA can encode such diverse responses is an open question. Here, by using atomistic models and translocation simulations, we uncover an elaborate and directional mechanism for how stress propagates when the two xrRNA ends, 5 \u2032 and 3 \u2032, are driven through a pore. Pulling the 3 \u2032 end, as done by replicases, elicits a progressive unfolding; pulling the 5 \u2032 end, as done by exonucleases, triggers a counterintuitive molecular tightening. Thus, in what appears to be a remarkable instance of intra-molecular tensegrity, the very pulling of the 5 \u2032 end is what boosts resistance to translocation and consequently to degradation. The uncovered mechanistic principle might be co-opted to design molecular meta-materials

Well-Tempered Metadynamics: A Smoothly Converging and Tunable Free-Energy Method

We present a method for determining the free energy dependence on a selected number of collective variables using an adaptive bias. The formalism provides a unified description which has metadynamics and canonical sampling as limiting cases. Convergence and errors can be rigorously and easily controlled. The parameters of the simulation can be tuned so as to focus the computational effort only on the physically relevant regions of the order parameter space. The algorithm is tested on the reconstruction of alanine dipeptide free energy landscape

Accurate sampling using Langevin dynamics

We show how to derive a simple integrator for the Langevin equation and illustrate how it is possible to check the accuracy of the obtained distribution on the fly, using the concept of effective energy introduced in a recent paper [J. Chem. Phys. 126, 014101 (2007)]. Our integrator leads to correct sampling also in the difficult high-friction limit. We also show how these ideas can be applied in practical simulations, using a Lennard-Jones crystal as a paradigmatic case

Colored-noise thermostats \`a la carte

Recently, we have shown how a colored-noise Langevin equation can be used in the context of molecular dynamics as a tool to obtain dynamical trajectories whose properties are tailored to display desired sampling features. In the present paper, after having reviewed some analytical results for the stochastic differential equations forming the basis of our approach, we describe in detail the implementation of the generalized Langevin equation thermostat and the fitting procedure used to obtain optimal parameters. We discuss in detail the simulation of nuclear quantum effects, and demonstrate that, by carefully choosing parameters, one can successfully model strongly anharmonic solids such as neon. For the reader's convenience, a library of thermostat parameters and some demonstrative code can be downloaded from an on-line repository

Interchain interaction and Davydov splitting in polythiophene crystals: An ab initio approach

The crystal-induced energy splitting of the lowest excitonic state in polymer crystals, the so-called Davydov splitting Δ, is calculated with a first-principles density-matrix scheme. We show that different crystalline arrangements lead to significant variations in Δ, from below to above the thermal energy kBT at room temperature, with relevant implications on the luminescence efficiency. This is one more piece of evidence supporting the fact that control of interchain interactions and solid-state packing is essential for the design of efficient optical devices