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Engine stability at idle is an important factor that influences the behaviour of an I.C., S.I. engine, in terms of fuel efficiency, exhaust gas emissions and customer comfort. In particular, the increasing daily use of vehicles in urban traffic bestows more and more importance on the engine idle quality. The engine idle quality is perceived by the user as the constancy of noise tone, low vibrations level and absence of sudden speed drop, noticeable on the steering wheel, gear shift lever, and seat [1]. Combustion characteristics play an essential role on the overall engine quality level at idle. It is important to have an estimation of the engine behaviour in this condition during the engine pre-development phase. While the fluid-dynamic calculation codes mean the engine performance at full load can be predicted, but the modelling of part load and idle behaviour is very difficult.

The intake system of a wide commercial spread spark-ignition engine has been modeled by using a 3-D code. The present configuration of the inlet manifold and inlet port does not generate any organized charge rotation, especially at low rotational speed. Objective of this paper is the research of new solutions, able to produce higher turbulence levels of the in-cylinder flow, without lowering the engine volumetric efficiency, in order to shorten the combustion duration and improve the energy conversion quality. A three-dimensional model for the calculation of the inlet port and valve performance under steady conditions has been developed. First, the normal production intake system has been modeled to the aim of validating the model set-up. The inlet valve discharge coefficient in a steady flow has been calculated. The results obtained showed a good agreement to the measured data and encouraged the authors to use the model for the development of new intake solutions.

As U.S. and European regulation of automotive emissions is getting more stringent, great interest is growing around new solutions for future emission standards. Pollutant reduction can be achieved improving both engine out emission and aftertreatment system efficiency. Engine out emission can be reduced improving combustion process especially during warm-up, friction and the engine management system. In any case engine out emission reduction involves engine sophistication increasing costs, which must be accurately evaluated, especially for small displacement large mass production engine. Since, as it is well known, 80 - 90 per cent of HC and CO emissions are produced during the first 100s of NEDC cycle, great improvement could be achieved reducing the catalyst light-off time. Different configurations of exhaust gas after treatment system have been tested to improve conversion efficiency during warm-up phases.