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 objective of the present work is to determine a simple methodology to calculate, during the pre-development phase, a geometrical parameter correlated to the combustion quality at idle. In particular this simple methodology, collecting together the valve lift curves and the valve geometry, means a geometrical parameter is determined which can be considered as a measure of the internal EGR rate within the combustion chamber.
Assuming a specific engine configuration, a good correlation has been found between this parameter, labelled “cam overlap total area” here, and the corresponding combustion quality level at idle, measured with indicating methods.