In the context of limiting the environmental impact of transportation, this paper reviews new directions which are being followed in the development of more predictive and more accurate detailed chemical kinetic models for the combustion of fuels. In the first part, the performance of current models, especially in terms of the prediction of pollutant formation, is evaluated. In the next parts, recent methods and ways to improve these models are described. An emphasis is given on the development of detailed models based on elementary reactions, on the production of the related thermochemical and kinetic parameters, and on the experimental techniques available to produce the data necessary to evaluate model predictions under well defined conditions. A IntroductionEven with the expected development of new and cleaner sources of energy, it is still believed that the combustion of liquid fuels will remain the main source of energy for transportation for the next 50 years 1 . It is therefore of the highest importance to limit the environmental impact of using these fuels during this transition period. As traditional fossil fuels are considered to be largely responsible for causing important atmospheric degradations 2 such as global warming 3 , acid rain, and tropospheric ozone increase, an important effort has been made by industry to develop both more efficient types of engines and cleaner types of fuels. A good example is the development of Homogeneous Charge Compression Ignition (HCCI) engines. The HCCI engine is characterised by the fact that the fuel and air are mixed before combustion starts and the mixture auto-ignites as a result of the temperature increase in the compression stroke 4 . This new type of engine has been proposed not only because of its high efficiency compared to that of diesel engines, but also because of its very low emissions compared to gasoline engines with a catalytic converter. With regard to fuels, there is an increasing interest to shift from hydrocarbon fossil fuels to biofuels (particularly bioethanol and biodiesel) 5,6,7,8,9 .While it is especially important to lower the emissions of CO 2 through the use of more efficient powertrains and through an increased biomass derivative content in traditional fuels, the emission reduction of other pollutants should certainly not be neglected. A paper of Wallington et al. 10 describes the parts of automotive engines which could be sources of

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The RETROSYN system, a program (written in LISP) for retrosynthetic planning is introduced. Emphasis is placed on its use of reaction classes. The source of information for the reaction classes is a database of reactions (represented simply as a list of reactants and products), and a means by which this information can be extracted, organized, and then used in RETROSYN is elaborated. The organization of the reaction classes involves establishing a hierarchy of classes and subclasses. Further uses and organizations of reaction classes for use in synthetic analysis are given. INTRODUCTIONReaction databases are in widespread use in the organic chemistryThe quality of these systems depends not only on the chemical information contained within the databases but also on the retrieval system supplied with the databases.M The creation of user-friendly systems which allow for high-level queries are of vital importance. To achieve this, AI techniques can be effectively employed. A typical search in a reaction database using standard systems now available is describcd.Standard Existing Technique for Reaction Search. Given a target molecule, the search for a set of suitable retrosynthetic' reactions serving as suggestions to be actually tried in the laboratory normally consists of several stages of interaction with a database system. The first stage is to ask whether the target is a product in any of the reactions. If not, then the target must be retrosyntheticaiiy analyzed for important groups and reactive centers. This analysis, done by hand, consists of determining the substructures around one of the reactive centers within the target which are involved in a key step in the synthesis. These substructures are the chemist's representation of the class of reactions that are needed to synthesize the molecule. The result of the search for these substructures in the database system is the set of specific reactions in the literature corresponding to this class. Finding a suitable substructure that yields a reasonable number of suggestions is a trial and error process. If too many "hits" are found, then the substructure must be expanded to include more of the target so that less reactions will be included. If no hits are found, then groups of atoms must be eliminated from the substructure to make the search more general.Retrosynthetic Search Using RETROSYN. RETROSYN is a system developed at RISC-Linz for reaction database search [the long term goal is a complete CAOS (computeraided organic synthesis) systems8]. The RETROSYN module is part of an cxpcrt chemical system, written in LISP, which is used to extract, calculate, and organize chemical information (with emphasis on organic synthesis) from the raw information contained in chemical databases9 The system is now in usable prototypc form.The following is a description of a how a single step of a retrosynthctic search can be made by using the RETROSYN module and thc results that can be obtained:User Input: Thc input is given through a menu-driven graphic display using a mouse. T...

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