The use of Compressed Natural Gas as an alternative fuel in urban transportation is nearly established and represents an efficient short and medium term solution to face with urban air pollution. However, in order to completely exploit its potential, the engine needs to be specifically designed to operate with this fuel. In the latest years, the authors have investigated the performances of a Heavy Duty Turbocharged CNG fuelled engine both experimentally and by using some analytical tools specifically developed by them which have been used for the engine optimisation. In the present paper the simulation approach has been enlarged by means of a co-operative use of a CFD code and experimental analysis on the actual engine. The numerical simulation of combustion process has, in fact, been used, to interpret series of pressure cycles, aiming to analyse how cyclic fluctuations influence engine behaviour in terms of combustion efficiency and temperature and pollutant distribution.
Combustion evolution has been simulated using an improved version of KIVA 3V [1,2]. To take properly into account one of the major sources of the combustion instability a detailed spark ignition and initial flame kernel formation model [3] has been implemented in the fluid-dynamics 3D model. By varying the value of some input parameters in a stochastic manner around their measured values, it has been then possible to simulate series of pressure cycles with the same statistical properties of the measured ones. The obtained cycles have been then used to understand how the different pressure history, as observed for sequentially acquired pressure cycles, influences the distribution of thermodynamic and physical variables and, consequently, the pollutant formation processes.