A Theoretical Investigation on the Effects of Combustion Chamber Geometry and Engine Speed on Soot and NO x Emissions A. De Risi, D. F. Manieri and D. Laforgia Università degli Studi di Lecce Dipartimento di Ingegneria dell'Innovazione Via Arnesano, 73100 Lecce - Italy ABSTRACT The objective of the present investigation is to assess the influence of the combustion chambers geometry and engine speed on the velocity flow fields, temperature distribution and NO x and soot emissions mechanism of formation. The investigation has been carried out both experimentally and by numerical simulations. A modified version of the computational fluid dynamics (CFD) Code KIVA-3V has been used for modeling combustion process and engine emission. In particular five different combustion chamber geometries were investigated, using as basic shape a mexican-hat piston bowl, and introducing later changes to the piston cavity and to the fitting radius to the piston crown. Key-words: Diesel Engine, Combustion Chamber Geometry, Emissions. INTRODUCTION Engine manufacturers, facing the more and more stringent emission standards concerning environmental regulations, are producing their efforts to reduce both soot and nitrogen oxides (NO x ) emissions. One of the feasible emission control strategies consists in reducing the engine out pollutant concentration by optimizing engine combustion both with an improved combustion chamber design and an adequate injection strategy. Unfortunately, previous studies have shown that NO x and soot emissions are related, thus strategies developed to reduce particulate tend to produce an increase of the amount of released NO x . The development of a procedure able to control simultaneously both these pollutants requires the knowledge of where NO x and soot are formed as well as of their mechanisms of formation. Many studies have been carried out to clarify the effect of exhaust gas recirculation, injection pressure and strategy (multiple injection), combustion chamber geometry on NO x and soot mechanisms of formation, . In particular, in [1] has been shown that the extended Zel'dovich mechanism was able to well model the NO x production in diesel engines. In the same study was also found that soot mechanism of formation could be appropriately modeled by a single-step competition between the soot mass formation rate, as predicted by Hiroyasu's model, and oxidation rate predicted with Nagle and Strickland-Constable model. In the present study experimental results and three dimensional simulations are used to asses the effect of the combustion chamber geometry on the velocity flow fields and high temperature domains, which are decisive factors in the formation and the evolution of NO x and particulate. Simulations were carried out by using a modified version of KIVA-3V code. NUMERICAL MODELS The numerical models are based on the KIVA-3V code [2, 3]. The modified RNG κ−ε turbulence model proposed by Han and Reitz [4] was adopted in the present investigation. This model differs from the standard RNG κ−ε for the presence of an extra term in the dissipation equation, in order to account for the compressibility of the flow. The modified RNG κ−ε model has been shown [4] to reproduce the large- scale flame structures in a more realistic way improving the prediction of the high temperature domains and, in concert, the accuracy on the prediction of NO x and soot emissions. Injection was modeled by using the “Blob” injection model, described in [5] and the TAB breakup model [6].The multistep Shell ignition model [7], was used in conjunction with the laminar and turbulent characteristic time combustion model [8] to describe the entire process. A temperature threshold of 1000 K to switch from ignition chemistry (T<1000 K) to combustion chemistry (T>1000K) was chosen. The soot emission model adopted in this study is the Hiroyasu formation model [9] and the Nagle and Strickland-Constable oxidation model [10]. Soot concentration is predicted by a single step equation which considers the temporal rate of change of the soot mass due to the rates of formation and of oxidation: dt dM dt dM dt dM oxid soot form soot soot − = (1) Where the rate of formation has the following expression: